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  • Integrity for Non-Aviation Users: Moving Away from Specific Risk

    Non-aviation users of satellite- and ground-based augmentation systems do not require the conservative level of integrity built into these systems for aviation users. Removing it can produce substantial benefits in terms of smaller error bounds and improved availability.

    By Sam Pullen, Todd Walter, and Per Enge

    Both space-based and ground-based augmentation systems (SBAS and GBAS, respectively) are designed to enhance standalone GNSS navigation to meet the requirements of civil aviation. SBAS and GBAS corrections and integrity information are also available to the non-aviation user population, such as automobiles, buses, and trains on land as well as ships near shore. This much larger user base can benefit as much from the integrity components of SBAS and GBAS as from the increased accuracy obtained from applying SBAS and GBAS pseudorange corrections. However, there are significant differences between the aviation interpretation of navigation integrity and the interpretation that would be natural to most users.

    SBAS and GBAS provide integrity in a multi-step procedure that is laid out in the RTCA Minimum Operational Performance Standards (MOPS) for the FAA versions of both systems: DO-229D for the Wide Area Augmentation System (WAAS) and DO-253C for the Local Area Augmentation System (LAAS). These systems indicate which ranging measurements should be excluded as unsafe to use and provide bounding error standard deviations, or sigmas, for the remaining usable measurements. Each aircraft uses this information to compute vertical and horizontal protection levels that define position-domain error bounds at desired probabilities. This process is straightforward, logical, and is not limited to aviation users. However, the requirements and assumptions underlying it make it very conservative.

    SBAS and GBAS are designed to meet integrity requirements defined in terms of what is known as specific risk. Briefly, this means that all safety requirements must be met for the worst combination of knowable or potentially foreseeable circumstances under which an operation may be conducted. Some variable factors important to safety, such as the user’s satellite geometry, are known by definition. Others, such as receiver thermal noise, are random and unpredictable. But several factors that are critical to GNSS performance, such as multipath and ionospheric errors, are neither completely random nor deterministic. Specific risk typically treats all error sources that are not completely random in a worst-case manner. SBAS and GBAS are designed to mitigate specific risk to support civil aviation, and the resulting conservatism makes SBAS and GBAS less attractive to non-aviation users who expect tighter protection levels relative to nominal system accuracy.

    Fortunately, non-aviation users need not apply all MOPS procedures required of aviation users if their own safety requirements differ. Most users define integrity in average or ensemble terms, meaning that everything not known in practice is treated as random and is probabilistically mixed (or convolved) together. The protection levels valid for these users would be much lower than for aviation users, even though the stated bounding probability is the same. This contrast is illustrated in Figure 1, which shows example bounds on 2-D vertical errors at a probability of 0.95 (the 95th percentile, or 95 percent) for accuracy and a probability of 1–10-7 for integrity. The term VPE stands for vertical position error, while VPL stands for vertical protection level. Analogous terms (HPE and HPL) and a similar picture exist in two dimensions for horizontal errors.

    Only one 95 percent error bound is shown in Figure 1 because this probability can be observed, estimated, and modeled with theory and reasonable amounts of data (hundreds or thousands of independent samples). This is not at all the case at the very small probability of 10-7 that applies to aviation precision approach: it is roughly equivalent to one event in 47.5 years per 150-second precision-approach interval. Both theory and data fall far short of being able to predict such rare-event errors. Extrapolating from available data to 1–10-7 using Gaussian distributions is perilous because the Gaussian distribution almost never applies at such small probabilities. Mixed-Gaussian models, other so-called fat-tailed distributions, and inflation of Gaussian parameters help address this, but the uncertainty regarding the true error distribution results in significantly different error bounds depending on the assumptions that are made. The same is true regarding the effects of faults and anomalies that are more probable than 10-7 but are still rare and poorly understood.

    In the end, different means of assessing these uncertainties and various degrees of user risk aversion result in different 1–10-7 protection levels, as shown in Figure 1. It is this difference that we wish to quantify and exploit in this article.

    Average versus Specific Risk

    The concept of average or ensemble risk is intuitive to those with a background in probability and is one of the key principles of probabilistic risk assessment (PRA). Thus, it helps to examine it first.

    Average risk is the probability of unsafe conditions based upon the convolved (averaged) estimated probabilities of all unknown events. More specifically, probability distributions are derived (based on the best available knowledge) for all unknown parameters relevant to user safety, and these are combined (by probabilistic convolution) to create an overall distribution that represents safety risk as a function of the known parameters. This straightforward, natural interpretation of probability and uncertainty has a major advantage in that it cleanly separates the probabilistic calculation of safety risk from users’ aversion to risk. By keeping risk probability and risk aversion (or severity) separate, a final risk consequence measure can be derived that supports apples-to-apples comparisons of alternatives. One useful result of this is known as the value of information (VOI). By comparing the risk outcomes of two scenarios in which the latter case has additional information (for example, from an additional sensor or integrity monitor), the risk-reduction benefit of the added information can be traded off against the cost and complexity that it introduces to the system. Similar comparisons can be made for any definition of risk, but the definition and use of VOI in an average-risk framework makes the most sense in both theory and practice.

    Turning to specific risk, no single definition exists within the aviation safety community, to our knowledge. This is partially because of the uniqueness and complexity of the concept and partially because multiple inconsistent interpretations appear to exist. Therefore, we provide our own definition: Specific risk is the probability of unsafe conditions subject to the assumption that all credible unknown events that could be known occur with a probability of one (on a risk-by-risk basis).

    To understand how specific risk differs from average risk, it helps to start with a fault-tree representation of risk in which loss of integrity (LOI) can result from any of the nodes of the tree. Figure 2 shows a simplified example of a fault tree for CAT I GBAS. It shows the allocation of the CAT I total integrity risk requirement of 2 × 10-7 per approach to the various possible causes of integrity loss. In specific-risk analysis, each type of failure shown in the tree, if deemed to be a credible failure (meaning, in practice, that its assumed prior probability is larger than compared to its allocation in the fault tree), is assessed that the failure is guaranteed to occur in a worst-case fashion. This means that the variables that describe this particular failure scenario take the values that maximize the hazard to users. In an average-risk analysis, these variables would take many values according to their own probability distributions, and these distributions would be convolved together to provide an overall representation of risk under that scenario. Instead, one scenario drives the specific risk assessment for a particular user class, and it is the worst one possible from that user’s standpoint. (Another user class would be evaluated under a different set of parameters corresponding to the separate worst case for that user.) The improbability of the worst-case combination of parameters is not considered as long as the probability of the failure scenario as a whole is deemed high enough to be of concern.

    Figure 2. Fault tree for CAT I GBAS integrity. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 2. Fault tree for CAT I GBAS integrity.

    Since GNSS augmentation systems contain multiple levels of health monitoring, the worst-case scenario is usually the one that maximizes the probability of an undetected hazardous error for a particular user class. Hazardous error is typically defined as any error that exceeds a pre-defined safety zone known as an alert limit (AL) or any error that exceeds the computed protection level (PL), which allows integrity to be defined separately from the intended application. Both definitions are conservative in that all errors exceeding AL or PL are treated as equally hazardous. In other words, an error just above AL is treated as just as dangerous as an error of 10 × AL. They are also misleading when used in specific-risk analyses because the resulting worst-case conditions are those that give errors just above AL or PL, as these are the generally hardest for monitoring algorithms to detect.

    The use of specific risk in aviation is an evolution of deterministic guidelines for tolerable risk that date back to an earlier era when flying was more dangerous. It remains dominant in aviation safety assessment because it is partly responsible for the development of safer and more reliable air transportation. However, it has several important weaknesses compared to average risk. The first is that the degree of risk aversion preferred for aviation is buried within the hazard probabilities generated by specific risk — it cannot be separated out. This means that specific-risk results do not translate well to other classes of users, as very few users would happen to have the same risk preferences that have evolved within aviation over several decades. In addition, specific risk makes a distinction between unknown events that could be known and those that are both rare and completely unknowable. A very risk-averse value of information is much different than the risk-neutral one built into PRA, as it severely penalizes systems that do not include all potentially-informative sensors. Since each sensor added to a system provides less benefit than the last, almost all cost-effective systems choose to include less than the maximum possible number of sensors.

    The conservatism implicit in specific-risk assessment severely penalizes users. Although PRA would show that the combination of factors (shown in an example induced by extreme ionospheric spatial decorrelation) needed to produce a 40-meter error in a CAT I GBAS system is exceedingly improbable (almost certainly below 10-10 per approach), specific risk forces a significant part of the GBAS risk-mitigation effort to be targeted at this scenario. In this case, since monitoring is not guaranteed to detect the anomaly in time, the only recourse is geometry screening, a cumbersome technique in which the ground system continually evaluates the worst-case error and, if it exceeds a 28-meter tolerable limit at the CAT I decision height, determines which broadcast parameters to inflate such that all satellite geometries causing worst-case errors exceeding 28 meters are made unavailable (because the inflated VPL is larger than the 10-meter CAT I VAL). The result of this procedure is much lower user availability than would be achieved without inflation. SBAS pays a similar penalty, as we will see later. The broadcast grid ionospheric vertical error values that bound worst-case ionospheric errors (and thus the resulting protection levels) are much higher than they would be if the unusual combination of factors needed to create the worst-case error scenario were not the dominant concern.

    To the extent that loss of availability represents a safety issue at the airspace level, the worst-case focus that results from specific risk is not optimal even from a safety standpoint. But this is not the only concern. Specific risk requires a great deal of development and testing to identify and mitigate a handful of very peculiar, non-representative conditions. When schedule and resources are limited, other potential threats that are easier to foresee but seem extremely improbable are often neglected. One example is the treatment of multiple hardware failures. If individual failures are assumed to be statistically independent, the probability of multiple simultaneous failures is very small. However, while statistical independence is a common assumption in probability classes because it makes calculations easier, it rarely applies in the real world. Because satellites and ground receivers are similar, if not identical, the presence of a failure in one unit may suggest a common cause or at least a common vulnerability, meaning that the probability of additional failures is much higher than independence would suggest. Thus, assuming independence by default could lead to neglecting entire categories of risk that are more threatening than the worst-case events that dominate specific risk.

    Maximum WAAS Errors, Protection

    To investigate the conservatism built into SBAS and GBAS specific risk assessment, maximum WAAS horizontal and vertical position errors over time (as measured by the Performance Analysis Network (PAN) maintained by the William J. Hughes FAA Technical Center) have been examined and compared to the protection levels computed when the maximum errors occurred. This study begins with PAN Report #8 (covering January to March 2004 — shortly after WAAS commissioning in mid-2003) and extends through PAN Report #34 (covering July to September 2010). Each PAN report covers three months of observed WAAS performance.

    Figure 3 shows the 38 WAAS reference stations (WRSs) used by the PAN to collect position error and protection level information (some of these stations were not active in 2004 and thus were not used in earlier PAN reports). While measurements from these stations are used to generate WAAS corrections and error bounds, they are also used by the PAN as static pseudo-users that compute WAAS-corrected positions and protection levels according to the aircraft user algorithms specified in the WAAS MOPS. The resulting positions are compared to the known, pre-surveyed positions of each station to derive estimates of vertical and horizontal position errors (VPE and HPE) once per second.

    Figure3 Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 3. WAAS PAN reference station network.

    Figure 3 groups these stations into three sets of stations based on their presumed quality of WAAS coverage. These sets are unofficial and were created for the purposes of this study. The seven stations in the inner set are expected to have good WAAS coverage at all times because they are surrounded by other stations. The 13 stations in the outer set are expected to only have acceptable coverage because s
    ome of them are at the edges of CONUS. The remote stations provide coverage to the inner and outer regions as well as the best possible coverage of their own regions. Because the remote stations extend beyond the primary coverage region of WAAS in CONUS, errors at these stations are not considered here.

    Figure 4 is a 2-D plot of position error versus protection level in the vertical axis (that is, VPE versus VPL) for all epochs and stations during the three months covered by the recent WAAS PAN Report #34 (July 1–September 30, 2010). These results are typical of the entire period since WAAS commissioning in 2003, particularly the last several years. The vertical lines on the plot indicate the 95th-percentile, 99th percentile, and maximum VPEs in this period (1.2, 1.8, and 7 meters, respectively). The maximum VPE occurred at Barrow, AK, which is one of the most remote stations in the WAAS network (see Figure 3). In comparison, the lowest VPLs (intended to be 1–10-7 bounds on VPE) are in the range of 10–15 meters, and values as high as 40 meters are not uncommon. The most demanding approach operation that WAAS supports, LPV-200, allows approaches to a 200-foot minimum decision height and requires that VPL be below a vertical alert limit (VAL) of 35 meters. HPL must also be below a horizontal alert limit (HAL) of 45 meters. When this is not the case, the approach operation is not available; thus these higher VPLs extract a significant cost.

    Figure 4. WAAS vertical protection level versus vertical position error (June–September 2010). Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 4. WAAS vertical protection level versus vertical position error (June–September 2010).

    Figure 5 and Figure 6 (for vertical and horizontal errors, respectively) span the entire period of WAAS PAN Reports used in this study. VPL represents the VPL at the station and time of the maximum VPE; it is not the largest VPL recorded at a particular station. The horizontal errors shown in Figure 6 are defined analogously. Note that the station that observes the largest horizontal error in a given PAN report may differ from the one that observes the largest vertical error.

    Figures 5 and 6 demonstrate that, while both 95 percent and maximum errors are quite low and are within the expected range of each other, the protection levels associated with the maximum errors greatly exceed them. This pattern is clearer in Figure 5 for vertical errors because maximum VPL tends to be more consistent across PAN reports, but it is true for horizontal errors as well.

    Figure 5. WAAS vertical errors and protection levels from 2004–2010. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 5. WAAS vertical errors and protection levels from 2004–2010.
    Figure 6. WAAS horizontal errors and protection levels from 2004–2010. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 6. WAAS horizontal errors and protection levels from 2004–2010.

    Figures 7 and 8 clarify this relationship by plotting the ratio of VPL to VPE and HPL to HPE for the station and time of the maximum error. The mean of this ratio is very high and is about the same in both cases: 5.38 for vertical and 5.21 for horizontal. Figure 7 shows a steady upward trend in the ratio that is mostly due to WRS improvements that resulted in maximum VPE being reduced over time. This trend is clearly visible in Figure 5 and appears to exceed the weaker trend of lowering VPL due to WAAS algorithm enhancements. The same trend is visible in the horizontal Figures 6 and 8 but is weaker due to the greater variability of maximum HPL over time.

    To evaluate the significance of the large PL-to-max-PE ratios in the WAAS PAN database, we need to approximate the number of independent samples from which the maximum errors were derived. As noted before, WAAS protection levels represent error bounds at the 1–10-7 probability level based on specific risk. With one measurement being collected at each operational station every second, a total of about 4.25 billion samples were collected in the PAN reports from January 2004 to September 2010. Note that measurements from remote stations are included in this count, but they are also represented in the conclusions because their PL-to-max-PE ratios are very similar to the ones shown in Figures 7 and 8. Translating this number into the number of statistically independent samples depends on the interval between independent measurements. Because both nominal and rare-event errors affect this interval, it is hard to estimate. Our best guess is a range between roughly 30 and 150 seconds, suggesting that the PAN database contains between 2.8 × 107 and 1.4 × 108 independent samples. Both of these numbers suggest that WAAS protection levels are very conservative from the perspective of average risk.

    Figure 7. Ratio of VPL to VPE from 2004–2010. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 7. Ratio of VPL to VPE from 2004–2010.
    Figure 8. Ratio of HPL to HPE from 2004–2010. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 8. Ratio of HPL to HPE from 2004–2010.

    Adjusting for Average-Risk Users

    Using the above results, a preliminary estimate of the reduced WAAS protection levels that would apply to average-risk users can be made. Figure 9 shows a comparison between the actual 95 percent WAAS VPL and HPL and the adjusted VPL and HPL potentially achievable with WAAS (for the same 1–10-7 bounding probability) for average-risk users. The actual WAAS VPLs are taken from the more recent WAAS PAN Reports starting from #24 (covering January to March 2008) as the period from 2008 to 2010 includes most of the WAAS algorithm improvements introduced since commissioning in 2003. The actual 95 percent VPLs and HPLs represent the largest reported 95th-percentile values among the stations within CONUS for each quarterly period. The lower adjusted VPLs and HPLs are derived by dividing each VPL by a factor of 4.0 and each HPL by a factor of 2.5. These two reduction factors are derived from Figures 7 and 8, respectively, as conservative estimates of the ratio between protection levels and maximum position errors. Note that the factor of 2.5 for horizontal errors does not include the 12-meter error in Cleveland from PAN Report #13, as this is thought to be spurious (that is, not representative of actual WAAS behavior).

    Figure 9. Projected WAAS protection level reductions for average-risk users. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 9. Projected WAAS protection level reductions for average-risk users.

    While projections based on these reduction factors are imprecise, they demonstrate the much lower error bounds that non-aviation users with an average-risk safety perspective could achieve. Most non-aviation users operate on land or sea and will be primarily concerned with horizontal error bounds. Figure 9 suggests that the typical 95th percentile WAAS HPLs of 15–20 meters (for the worst location in CONUS) can be reduced to 6–8 meters and still provide a confident 1–10-7 error bound.

    It is important to emphasize that these preliminary projections for average-risk users are just that. In order to formally establish new integrity requirements and protection levels for existing systems, the hazardously misleading information (HMI) analyses previously done for these systems need to be redone using the principles of PRA and average risk. While the original development of the WAAS and LAAS HMI analyses was lengthy and resource-intensive, almost all of the detailed work is already complete. As long as the original analyses are available, it is a much smaller task to take these results and create PRAs out of them by extracting the original specific-risk assumptions and applying average-risk principles instead.

    LAAS Users. Since the first GBAS ground station design (the Honeywell SLS-4000 LAAS Ground Facility) was certified for CAT I use in 2009 and has not yet been approved for operations at a specific airport, much less data is available to do a preliminary analysis for GBAS similar to the one done for WAAS above. However, the degree of sigma inflation in the parameters broadcast by CAT I LAAS is approximately known, meaning that it can be more-precisely removed from the current LAAS protection levels to estimate what they would be for average-risk users.

    Figure 10 shows the degree of inflation applied to the broadcast σvertical_iono_gradient (or σvig) parameter in order to protect against the worst-case ionospheric anomaly described previously. This result is for the SPS-standard 24-satellite constellation over a 24-hour period at the LAAS installation at Newark Airport, New Jersey (the method used by the Honeywell SLS-4000 is somewhat different). While not all epochs require inflation, a majority cause the nominal σvig value to be increased by a factor of 2 or more, which significantly decreases CAT I availability and currently makes it impossible to take advantage of the Differentially Corrected Positioning Service (DCPS) for non-CAT-I operations.

    Figure 10. Typical σvig inflation factors for CAT I LAAS. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 10. Typical σvig inflation factors for CAT I LAAS.

    Because of the extreme rarity of the worst-case event that dictates this inflation, it would likely not be needed for average-risk users. Figure 11 shows how much the σvig inflation in Figure 10 increases the LAAS VPL at Newark for the standard 24-satellite constellation. The VPL reduction from removing the inflation is not as dramatic as the potential reductions shown for WAAS in Figure 9, but they are significant relative to the 10-meter VAL for LAAS CAT I approaches. Furthermore, the pre-inflated nominal value of σvig for LAAS is 6.4 millimeters/kilometer, which is much higher than the actual one-sigma nominal gradient value of 1–2 mm/km because, under specific risk, the very worst nominal data must be bounded (also, worst-case tropospheric gradients must also be bounded by σvig). Other broadcast parameters that affect VPL, such as σpr_gnd and the ephemeris P-value that bounds worst-case ephemeris failures, would also be reduced significantly by switching to average risk. Overall, it is likely that LAAS protection levels based on average risk would be reduced from the current specific-risk PLs by about the same range of factors (2–5) observed from WAAS data.

    Figure 11. Impact of σvig inflation on LAAS VPL. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 11. Impact of σvig inflation on LAAS VPL.

    User Performance Improvements

    This discussion assumes that most non-aviation users who are not encumbered by the history of aviation standards development will prefer to quantify risk using PRA and the average-risk approach. As noted earlier, average risk better matches most users’ intuitive understanding of uncertainty and has the enormous advantage of separating risk quantification from risk aversion. Regardless of how risk-averse or conservative a given operator is, his or her model of risk aversion can be applied most efficiently to a risk-neutral calculation of risk that fairly represents all aspects of uncertainty. Inserting risk aversion into the calculation of risk, as done in the specific-risk approach, is both inefficient and non-optimal from a safety perspective because extensive focus on a few extreme worst-case events drives attention away from other events.

    The HPL reductions for average-risk users illustrated here would be significant for many classes of ground and marine transportation users. They would allow operations with tighter physical safety margins to be supported. Users who gain no particular benefit from tighter protection levels would still obtain much higher availability of integrity, as a 25-meter HPL could be supported by much poorer satellite geometries than would otherwise be the case. In other words, users that can tolerate 25-meter horizontal error bounds would be able to operate safely a much higher percentage of the time, because the degree of GNSS constellation deterioration needed to exceed this limit would occur much less often. These benefits do not only apply at the 1–10-7 probability level, as they would scale to the higher probabilities (1–10-4 to 1–10-6) that many non-aviation applications would be most concerned with.

    While very few non-aviation users of GNSS today have real-time safety requirements similar to those of civil aviation, the number of such users will likely increase as the coverage of augmented GNSS (and the availability of integrity from standalone receiver-autonomous integrity monitoring, or RAIM) expands. The evolution of standalone civil GPS usage provides a precedent: as basic GPS accuracy improved from tens of meters to several meters, and the cost of user equipment dropped, more and more uses were discovered. A similar, although smaller-scale, trend is likely to occur as the advantages of augmented GNSS become more available and better understood. The primary beneficiaries are likely to be intelligent road-transport systems, train services, and marine transportation in restricted waters.

    One application where tight real-time integrity bounds would be useful is in harbor and marina entry and exit; see Figure 12, taken from a Google map of a marina in San Diego, California. Based on the earlier analysis, two typical 1−10-7 horizontal protection levels are shown: 18 meters using the unchanged WAAS MOPS approach, and 7 meters based upon modifying the broadcast bounding parameters to represent average risk (these HPLs are bounds on error in either direction, positive or negative; thus the 2-D error bounding circle has a diameter of twice the HPL).

    Figure 12. Example of reduced protection levels for harbor/marina access. Source: Sam Pullen, Todd Walter, and Per Enge
    Figure 12. Example of reduced protection levels for harbor/marina access.

    When the resulting error bounds are compared, the relative advantage of the smaller bound for this application is immediately apparent. In general, when HPL is significant compared to potential obstacles, its significance varies with the square of HPL rather than HPL itself, as the area being protected matters more than either linear direction. In this example, the ratio of HPLs being compared is 18/7, or 2.57, but the ratio of HPL-squared is much larger: 182/72 = 6.61.

    When real-time integrity is not needed, augmented GNSS provides an easy means to guarantee or certify vehicle locations after the fact with great precision and reliability, without the need for post-processing. Vehicle and cargo tracking based on standalone GPS is common today, a certification of the correctness of the tracking data to probabilities suitable for legal or commercial guarantees is lacking. For this, error bounds at 1–10-4 to 1– 10-6 probabilities are likely sufficient, and would allow HPLs of below 5 meters from WAAS and below 3 meters from LAAS. In some scenarios, the difference between a 5-meter and a 15-meter guarantee would be minor, but in others, it could make a substantial difference.

    As noted earlier, even for uses where the required HPL (as represented by the safe error limit, or HAL, for a particular application) is satisfied by the existing WAAS and LAAS protection levels, the use of modified average-risk protection levels increases the availability of integrity, which is most often expressed as the probability or percentage of time (over all satellite geometries and othe
    r variable system states) that the integrity requirement is met throughout an operation (in simple terms, that HPL ≤ HAL). For user locations within good WAAS or LAAS coverage, the most variable element over time is satellite geometry. Decreasing HPL by a factor of 2.5 or more substantially increases the margin between HPL and HAL and makes it far less likely that the satellite geometry will degrade to the point where HPL exceeds HAL. For example, if the unmodified WAAS HPL equals HAL at an (un-weighted) HDOP of about 1.5, the resulting satellite availability (an upper bound on overall availability) for the SPS-standard 24-satellite GPS constellation would be roughly 98.5 percent. This means that the satellites in view (in this case, all satellites above 5 degrees elevation at a location in CONUS) would provide HDOP ≤ 1.5 about 98.5 percent of the time. However, the modified average-risk HPL (using the factor-of-2.5 reduction) would roughly translate into a limiting HDOP of about 3.75. This allows the required integrity bound to be satisfied by much poorer GPS geometries and gives a satellite availability of greater than 99.9 percent. Thus, when integrity is needed, this much greater availability of integrity is a major advantage.

    Summary

    SBAS and GBAS broadcasts are freely available to all GNSS users, most of whom will have different definitions of acceptable risk. These users are not optimally served at present and may hesitate to take advantage of SBAS and GBAS as a result.

    Using years of collected data for the FAA WAAS system and analysis of the inflation factors built into the CAT I version of the FAA LAAS system, it appears that average-risk users of WAAS and LAAS would be adequately supported by protection levels that are 2 to 5 times lower than those currently derived by aviation users. The fact that two different approaches used to examine WAAS and LAAS suggest similar levels of over-conservatism lends credence to these estimates. While further validation by full-scale probabilistic risk assessments is necessary, we conclude that non-aviation users willing to accept average risk would obtain much better performance and availability from simple modifications to the existing SBAS and GBAS protection level calculations specified for aviation users.

    Acknowledgments

    We thank the FAA Satellite Navigation Program Office for its support of our research on WAAS and LAAS. However, the opinions expressed here are solely our own. We thank Jim Kelly and Tim Murphy for their explanations of the evolution of today’s SBAS and GBAS integrity requirements. We also thank the FAA Technical Center for its efforts in collecting and publishing WAAS error data over the last decade using its Performance Analysis Network (PAN).

    Sam Pullen is a senior research engineer at Stanford University, where he is the director of the Local Area Augmentation System (LAAS) research effort. He has supported the FAA and others in developing GNSS system concepts, requirements, integrity algorithms, and performance models since obtaining his Ph.D. from Stanford in Aeronautics and Astronautics.

    Todd Walter is a senior research engineer in the Department of Aeronautics and Astronautics at Stanford University. He received his Ph.D. from Stanford and is currently working on the Wide Area Augmentation System (WAAS), defining future architectures to provide aircraft guidance, and on assuring integrity on GPS III.

    Per Enge is a professor of aeronautics and astronautics at Stanford University, where he is the Kleiner Perkins, Mayfield, Sequoia Capital Professor in the School of Engineering. He directs the GPS Research Laboratory and received his Ph.D. from the University of Illinois.

  • Lone Sentinel: Single-Receiver Sensitivity to RF Interference

    By Jenna R. Tong, Robert J. Watson, and Cathryn N. Mitchell, University of Bath

    Using signal-to-noise measurements from a single commercial-grade L1 GPS receiver, it is possible to detect interference or jamming that is above the thermal noise floor and below a power that causes loss of position.

    Interference, intentional or unintentional, is an acknowledged vulnerability of GPS systems. Many of the potential sources of interference are unintentional: interference can caused by harmonics of out-of-band signals, electronic noise, or malfunctioning equipment. The effect, however, is the same independent of intent.

    The presence of high-power interference which causes continual denial of service is fairly easy to detect, but lower power interference may still degrade performance, for example by causing loss of lock on some satellites, thus increasing position dilution of precision, although the receiver continues to output a position. Short periods of denial of service caused by intermittent high-power interference may not be immediately detected depending on the timing and ability of the system in use to deal with temporary loss of signal.

    Therefore, to fully characterize an antenna environment requires a 24/7 system, whether the purpose is to determine whether a location is suitable prior to installation, to identify whether problems at an existing site are due to interference, or to provide warnings of the presence of interference on a continuous basis. In particular, information on timing — for example finding a time of day or day of the week when interference is regularly seen — may assist in determining the source of the interference.

    This research forms part of the GNSS Availability Accuracy Reliability anD Integrity Assessment for timing and Navigation (GAARDIAN) project, which provides a mesh of sensors to monitor the integrity, reliability, continuity, and accuracy of the locally received GPS (or other GNSS) and eLoran signals continuously and to detect anomalous conditions such as local interference, differentiating between possible sources of errors such as interference, multipath, satellite errors, or space weather.

    Here we look at using the signal-to-noise ratio (SNR) values from a single-frequency GPS receiver to detect interference. There are two stages to the algorithm: determining the local environment of the antenna in terms of multipath and interference, and identifying and recording potential interference events.

    Since this method uses values output from a GPS receiver, characterizing the response to interference of the receiver used in the probe is necessary, to indicate what level interference can be detected with the system, as well as ensuring that false positives are not produced, and the effects of interference can be separated from those of multipath and scintillation, which can also cause decreases in SNR.

    We used a commercial, single-frequency receiver, recording this data from NMEA messags for analysis:

    • SNR, in dB, reported as an integer
    • elevation, in degrees, reported as an integer
    • azimuth, in degrees, reported as an integer
    • carrier lock time, in seconds.

    Algorithm. To determine the presence of interference, the normal state of the receiver must first be calculated. Initially it is assumed the receiver is fixed with an unchanging multipath environment. SNR and elevation values from all satellites are accumulated for several hours. To reduce influence of the unknown multipath environment, values from satellites below 10 degrees elevation and from those where the carrier lock time is less than four minutes are removed from the data set.

    A polynomial fit between elevation and SNR is then calculated from the remaining data. A second- or third-degree polynomial generally fits the high-elevation data with deviations from the profile at low elevations being primarily due to multipath where interference is not present.

    The standard deviation of SNR at each elevation is then calculated. The combination of the polynomial and these values of standard deviation characterize the normal environment of the receiver, for the case where interference is not present in the data gathered (Figure 1).

    Tong_1-W Source: Jenna R. Tong, Robert J. Watson, and Cathryn N. Mitchell, University of Bath
    Figure 1. Raw SNR data against elevation, for all satellites in view over a period of 12 hours (blue), and a polynomial fitting to the same data (green).

    To confirm that the threshold values returned by the first stage of the algorithm are valid, a value is calculated for the elevation where the SNR value drops below the polynomial curve by the greatest amount.

    If interference is not present, this is normally found at the point where multipath begins to influence the incoming signal and can be considered as a rough multipath cutoff, used to remove signals that may be influenced by multipath from later stages of the analysis.

    Assuming a well-sited antenna, a value greater than 25 degrees for this value indicates the possible presence of interference in the data used to calculate the polynomial. In cases where this value is high, the data in question would be rejected, and optionally a user may be warned that there may be pre-existing interference. If the antenna-receiver combination has been previously calibrated in a known good environment, it would be also possible to identify interference based on the difference in polynomial and standard deviation values between that environment and the location being tested.

    Figure 2 shows the value of this multipath cutoff (in degrees) for a set of data where interference was known to be present initially, against the start time for the data used to calculate the polynomial and multipath cutoff values, by number of hours from the start of the file.

    Tong_2-W Source: Jenna R. Tong, Robert J. Watson, and Cathryn N. Mitchell, University of Bath

    Once the mask is developed, a threshold value can be set to be n standard deviations below the polynomial, and events are detected by the combination of:

    • At least four satellites with elevations above the multipath cutoff which are below the threshold value or which were above the multipath cutoff previous to losing lock.
    • This status is continuous for more than a set time t.

    Requiring multiple satellites limits the effects of other influences on SNR such as multipath; requiring an extended time period removes very short-term fluctuations.

    The number of false positives and the power of interference required to cause an alarm then depends primarily on the value of the threshold factor n, and on the time period t, which here we kept at a constant of 30 seconds.

    Testing

    To avoid radiating interference, we constructed an RF network to facilitate injection of jamming signals into the GPS signal path. The GPS signal from a roof-mounted choke-ring antenna was passed through an amplifier and attenuator chain to provide 0 dB forward gain, but around 40 dB reverse isolation. An additional stepped attenuator (0–40 dB in 1 dB steps) was also included. The buffered signal from the antenna was then combined with the output of a vector signal generator used to provide the jamming signal.

    The combined signal was then fed into the GPS receiver via a DC-block to remove the antenna bias voltage. The signal generator is capable of producing a wide variety of jamming including matched spectrum wideband noise, CW, and pulsed signals. The adjustment of both the signal generator output power and the signal attenuator a
    llow the replication of a variety of signal-to-noise and jammer-to-noise scenarios.

    With the receiver locked onto a stable position, CW signals at L1 frequency were introduced into the receiver at levels from –125 dBm to –90 dBm in steps of 5 dBm, with at least 15 minutes of buffer time for the receiver to recover between each step (Table 1). Data was logged at 1 Hz throughout. We collected 20 hours of data, to calculate threshold values from data with no known interference.

    Table 1. Source: Jenna R. Tong, Robert J. Watson, and Cathryn N. Mitchell, University of Bath
    Table 1.

    Results

    Twelve hours of data from a period where no known interference was present was used to form the SNR mask, and events longer than 30 seconds were looked for using various values of n for the threshold across all 20 hours of data. A false alarm was considered to be any event where interference was detected while the signal generator was off. Table 2 summarizes the response for different threshold levels.

    Table 2. Source: Jenna R. Tong, Robert J. Watson, and Cathryn N. Mitchell, University of Bath
    Table 2.

    Table2-W Source: Jenna R. Tong, Robert J. Watson, and Cathryn N. Mitchell, University of Bath

    In this test, CW interference of –100 dBm was required before the number of satellites with carrier lock dropped below four even for a single epoch, and –90 dBm was required to cause a sustained loss of lock, but jamming of –105 dBm was still detectable by this system with no false positives returned.

    Decreasing the threshold began to produce false positives without detecting the smaller interference signals. This is not surprising as the thermal noise floor, assuming 2 MHz bandwidth, is about –110 dBm.

    In the raw data from the detected events, a sharp dip in SNR is often seen at the beginning of an event, followed by recovery as the receiver compensates. In this particular case, where the aim is to detect the interference, this could lead to interference going undetected if the initial sharp dip was underneath the time threshold (30 seconds) and the recovery took the SNR of some of the satellites above the SNR threshold (Figure 3).

    Figure 3. Value of polynomial mask (blue) and actual SNR (red) as recorded for four satellites during the period around the injection of the -100 dBm CW signal, showing initial dip and partial recovery. Source: Jenna R. Tong, Robert J. Watson, and Cathryn N. Mitchell, University of Bath
    Figure 3. Value of polynomial mask (blue) and actual SNR (red) as recorded for four satellites during the period around the injection of the -100 dBm CW signal, showing initial dip and partial recovery.

    Conclusion

    Using only SNR values from a low-cost L1 GPS receiver, it is possible to detect CW interference which is above the thermal noise floor and below a power that causes loss of position. Different types of interference are expected to produce a different response, and unintentional interference is likely to be broadband or not directly centered on L1. The antenna used may also have a strong effect. These factors have not been examined here, although in practice the algorithm has run in multiple locations with different antennas, both direct and via splitters.

    Regardless of the precise type of interference, the system would be expected to detect any interfering signal which impacts the SNR of the receiver, and to do so even if the signal strength was below a level which caused denial of service in that area.

    The results are specific to the receiver used and its response to interference, although the algorithm would be capable of using data from any receiver that provided SNR values. Ideally the system used for measurement would have little or no built-in interference rejection.

    Although this data was collected and then examined after the fact for signs of interference, the system works in precisely the same way in real time. Further trials will test the algorithm’s performance in real time and with different jamming scenarios, and compare results from multiple receivers in a single location and the performance of the algorithm with different antennas.

    Acknowledgments

    This work was funded by the Engineering and Physical Sciences Research Council and the Technology Strategy Board.

    Manufacturers

    Single-channel receiver, Chronos Technology CTL430; vector signal generator, Rohde & Schwarz SMIQ03.

    Jenna R. Tong is a postdoctoral researcher in electronic and electrical engineering at the University of Bath. Her Ph.D. in electron tomography is from the University of Cambridge.

    Robert J. Watson received a Ph.D. degree in electronic engineering from the University of Essex, and is senior lecturer in electronic and electrical engineering at the University of Bath.

    Cathryn N. Mitchell is a professor of engineering at the University of Bath and the Director of Invert Centre for Imaging Science. She received a Ph.D. from the University of Wales Aberystwyth.

  • On the Edge: Tracking Slips and Creeps: Earthquake Monitoring Gets Substantial Boost from GPS

    By Tracy Cozzens

    The Earth’s surface is constantly shifting, being deformed as earthquake faults accumulate strain, and slip or slowly creep over time. Not long ago, scientists relied solely on seismometers to monitor the earth’s movements. Today, GPS has taken prominence as an indispensible tool.

    PANGA, the monitoring network covering the Pacific Northwest, uses GPS to monitor this movement by measuring the precise position (within 5 millimeters or less) of stations near active faults relative to each other. By determining how the stations have moved, ground deformation can be determined.

    If the plates near the coast or the Cascade Mountains move even a few centimeters, the scientists at PANGA know within seconds. The network is still being built, but eventually it’s expected that PANGA will be able to sense earthquakes faster and more accurately than traditional seismometers, and issue alerts to warn citizens of impending activity.

    “GPS is helpful in distinguishing magnitude 8 from M9 earthquakes quickly,” explained Rex Flake, PANGA. “By design, seismometers only record high-frequency energy that becomes saturated during strong ground motion. Moreover, seismic data ‘clip’ at high magnitudes whereas GPS become more accurate. Seismographs are mainly intended to detect very small to moderately large earthquakes. GPS gives actual ground motions that in theory could be incorporated very quickly into tsunami models and warning systems. That is one of the things we are working on now.”

    Volcano Watch. “A more speculative application is that some (not all by any measure) large earthquakes are preceded by slow creep events,” said Andrew Miner, PANGA. “While not really good enough to predict an earthquake, I think if we saw a very large transient creep event it would at least ring alarm bells. Unfortunately though, earthquakes are by their nature just not very predictable, at least to the level of a day or week that people could reasonably act on. On the bright side, volcanoes are reasonably predictable, and GPS is also an important tool in monitoring them. We work with the Cascade Volcano Observatory on several monitoring projects.”

    PANGA is one of a series of earthquake monitoring networks stretching along the West Coast. The Pacific Northwest Geodetic Array is run by the PANGA Geodesy Laboratory at Central Washington University (CWU) in Ellensburg, and  includes 300 continuously operating, high-precision GPS receivers located throughout the Pacific Northwest. Sixty more stations are expected to be installed this year. Trimble, Leica, Topcon, and Javad are the main receivers used in the region.

    Data from these receivers is continuously downloaded, analyzed, archived, and disseminated. About one third of PANGA’s GPS stations are telemetered in real-time back to CWU, where the data are processed using NASA’s Jet Propulsion Laboratory’s GIPSY/OASIS II software for high-precision data analysis, and Trimble’s RTKNet Integrity Manager software for real-time analysis. The data provide relative positioning of several millimeters across the Cascadia subduction zone and its metropolitan regions. These real-time data are used to monitor and mitigate natural hazards arising from earthquakes, volcanic eruptions, landslides, and coastal sea-level hazards.

    Sagging Bridges. The data are also used to monitor man-made structures such as Seattle’s sagging Alaska Way Viaduct, the State Route 520 and Interstate 90 floating bridges, and dams throughout the Cascadia subduction zone, including those along the Columbia River. For instance, for the S.R. 520 bridge, PANGA teamed up with Washington State Department of Transportation (WSDOT) to monitor movement of the 520 bridges during wind storms and seismic events.

    The receivers continuously monitor and record structural deformation with about a millimeter precision. Raw GNSS satellite phase and pseudorange estimates are acquired and processed continuously into receiver positions estimated every 5 seconds and delivered with 10 and 30-second latencies. Daily-averaged receiver positions computed with predicted and post-processed satellite orbit and clock corrections are provided with 1-6 day latencies.

    GPS_Monument-W
    Seattle’s aging Alaska Way viaduct is one of several major man-made structures being monitored by PANGA’s GPS Network. (photos courtesty of CWU Geodesy Lab.)

    Tremor Slips. The Northwest is at the forefront of earthquake-related GPS research, in large part because the area provides a lot to learn from GPS monitoring, Flake said. “For example, when we started it was strongly suspected but not definitely known that the Cascadia subduction zone was locked over parts of its surface and a major earthquake threat. Thanks to GPS monitoring we now have a pretty good idea not only exactly where it is locked, but also when parts of it do slip or creep.

    “One important discovery made with GPS data, along this line, was that of the Episodic Tremor Slip (ETS) events that occur here in the Northwest U.S.,” Flake said. “Since the time duration of ETS motion takes place on the scale of days to weeks, these earthquake events were unrealized by traditional seismic detection methods.”

    GPS data shed light on this peculiarly predictable earthquake phenomenon. “With these GPS data we can measure strain accumulation within the continental crust (where people live) and calculate the residual that can be expected to rebound in a large subduction zone earthquake,” Flake said.

    “Even more detailed than that, we can use GPS data from past ETS events to constrain the locked zone of the subducting crustal plate by inferring the amount of slip at depth that best reproduces the observed GPS recordings — important in determining possible magnitude and location of the megathrust earthquakes (Mw = 8 to 9) that will someday occur. This is of obvious concern to society and is a major reason that we lead the geodetic applications of GPS research.”

    Data Online. PANGA maintains a website that integrates daily GPS measurements from about 1,500 stations along the Pacific/North American plate boundary, ranging from Alaska to the U.S-Mexico border. Cleaned, network solutions from several arrays are merged and grouped into regional clusters.

    Arrow on a Velocity Field Map of Oregon and Washington represent ground motion as measured by GPS at each particular location. The grey circles are 2 sigma error ellipses (click to enlarge.)
    Arrow on a Velocity Field Map of Oregon and Washington represent ground motion as measured by GPS at each particular location. The grey circles are 2 sigma error ellipses (click to enlarge.) (photos courtesty of CWU Geodesy Lab.)
    The panga team constructs a bedrock drill-brace geodetic monument at Howard Hanson Dam east of Auburn, Washington.
    The PANGA team constructs a bedrock drill-brace geodetic monument at Howard Hanson Dam east of Auburn, Washington. (photos courtesty of CWU Geodesy Lab.)

  • Letters to the Editor: LightSquared Satellite Case Skimpy

    LightSquared Satellite Case Skimpy

    Thank you for the story “LightSquared, FCC Rebuttals Distort Record.” One thing worth clarifying: you state, “It appears that the purpose of Lightsquared’s satellite service is, now, to provide ancillary service in remote areas not covered by the ubiquitous primary terrestrial network, or in the event that the terrestrial network is destroyed — exactly the opposite of what the FCC authorized and the GPS industry had understood and agreed to.”

    But even this can’t work unless they are going to limit the number of subscribers in remote areas. A 4G user will expect decent wireless data throughput, but the subscriber’s connection is shared with other users within a spotbeam covering a large area. All connections must fit within the bandwidth of the single beam.

    Cell networks get around this problem by frequency re-use, possible since each cell covers a small geographic region. The key characteristic of a cellular network is the ability to re-use frequencies to increase both coverage and capacity.

    Admittedly, the LightSquared satellite, SkyTerra-1, is a very sophisticated space vehicle with a record 500 spotbeams. However, it provides a maximum user data rate of 300–400 Kbps, quite a bit short of what you would expect from a 4G-LTE connection. And at 400 Kbps, a 20-MHz spotbeam could still support only 50 to 100 connections.

    Another problem with using the satellite link is that although they have managed to solve the problem of requiring a special user handset, the user will still have to be outdoors and in the open to communicate with the satellite. And it is a geostationary satellite, which means high latency — at least 1/4 second. Using this for voice would create user annoyance.

    LightSquared should stop using the word “ancillary” and stop pretending that their network has a significant satellite component. It is going to have to be all ground-based if they are to provide 4G connectivity. It’s really starting to sound like the satellite is just a way for LightSquared to meet their FCC requirement.

    — Mike Whitehead
    VP Technology, Hemisphere GPS
    [Ed: citations and some discussion ommitted for space; available on request.]

    On Eric Gakstatter’s Survey columns:

    Keep up the good work. I find your e-newsletter columns the best way to stay informed [on the LightSquared issue].

    — editor, survey magazine

    Sounding Off

    When someone comes onto the basketball court with a hockey stick, the referees should not negotiate rule changes. Anything that allows LSQ to proceed either on the high or low ends of their allocated spectrum will in the long run be a blow to PNT users and suppliers worldwide. While this may very well end up as a compromise to the detriment of us all, now is not the time to concede any ground. He who flinches, loses, and this is not the time for engineers to give ground. It will set a precedent that lawyers will use again to decimate the spectrum in the future.

    —Informed particpant

  • Expert Advice: Critical Offshore Applications of SBAS GNSS

    JDL-photo-II-W
    James D. Litton, President/CEO, Litton Consulting Group

    Precise positioning of many different kinds of vessels and other equipment depend upon satellite-based augmentation systems (SBAS) of GNSS, principally GPS and GLONASS at this time. The applications range from exploration to production and delivery of hydrocarbons to shore-based installations and navigation of very large crude carriers, or oil tankers. Decisions and recommendations are strongly needed to keep these services free from interference.

    It is fallacious to think that because LightSquared or similar use of out-of-band high-power terrestrial radiation would be confined to a continental region (physically impossible, in any case), that no harm would accrue to offshore navigation assets. The three principal suppliers of these offshore precise positioning services are Fugro’s Starfix services, C&C Technology’s C-Nav which utilizes John Deere/NavCom’s StarFire systems, and Subsea 7’s Veripos system.

    All of these systems depend upon GNSS reference receivers placed around the world in networks which depend upon corrections that are derived from regionally sited reference stations. The 10-centimeter level of precision now required for many of the most dangerous and valuable applications requires, in turn, centimeter-level accuracy in base stations in the United States and elsewhere in the world.

    Inmarsat frequencies allocated to these applications for delivery of the differential corrections generated by these reference stations have been in use for both the huge number of land applications (agriculture, infrastructure development, river and harbor navigation, seismic exploration, pipeline surveys, etc) and offshore applications. Changing these frequencies is feasible only at great cost to both Inmarsat and the many on- and offshore uses. Inmarsat may be compensated by LightSquared for its costs, but not so the many millions of dollars of expense to offshore and onshore operators in down time, redesign and reprogramming of receivers, and suspension of critical operations.

    The offshore applications outlined here are just a few of the more familiar. No attempt has been made to capture all of these applications in this short memorandum, but operators in this industry, represented by the National Ocean Industries Association (NOIA), have made their position clear in the attached letter to the FCC.

    Major Offshore Applications

    Exploration. Modern seismic exploration depends upon seismic streamers many kilometers long. Several such streamers (containing thousands of hydrophones for capturing reflections from deep beneath the ocean floor and determining the structure and composition of the strata which may contain hydrocarbons) are towed by each ship. The seismic profiles which result are depicted in three dimensions with great precision. Discovery and assessment of such strata depend sensitively upon the positioning accuracy of these streamers, which, in turn, depend sensitively on the position of the vessel with respect to the center of the earth, because the vessel’s trajectory is the reference for the relative positioning of the streamers by magnetic and inertial means, sometimes augmented by GNSS receivers integrated into the seismic streamers.

    Drilling. Increasingly, drill rigs and drill ships are placed and maintained in position by dynamic positioning systems that depend upon augmented GNSS systems for stabilizing the massive structures over the well head. In deep water (more than 5,000 feet), only dynamic positioning through the use of massive thrusters (such as those employed by the Deepwater Horizon vessel of Transocean in the Macondo well disaster, commonly referred to as the BP disaster) is feasible. With as much as 10,000 feet of riser attached to these drill ships between the well head and the ship, safe operation is critically dependent upon very precise positioning of the vessel. Further, down-hole positioning depends upon inertial and wireline systems, which are calibrated by the use of augmented GNSS systems.

    Production. Production platforms range from single sites over a single well to massive platforms with undersea pipelines and risers connecting them to manifolds on the sea floor, which in turn are connected to multiple well heads in an area. This infrastructure is placed, maintained, and monitored with the use of SBAS systems integrated with acoustic systems. Use of remotely operated vehicles and autonomous underwater vessels or vehicles, submarines equipped with sensors that can image and manipulate underwater structures, for these purposes is prevalent.

    Station Keeping. Supply vessels, crew vessels, special-purpose vessels, and helicopters are positioned relative to the drill rig, seismic vessel, production platform, and pipeline-laying vessel by SBAS systems fused with other sensors such as lasers and microwave distance-measuring equipment. A huge drill ship, for instance, moving about in response to ocean dynamics but centered on the well head, cannot be docked to a supply vessel solely with ropes and cables. Each vessel must be free to move but to move synchronously with each other. Because of the huge masses involved, the velocity of each relative to the other must be kept as near zero as possible. Centimeter-level precision is required for this purpose. In all of the applications listed above, at various stages, vessels require station keeping with other vessels to very precise relative distances and velocities.

    Containment and Recovery. When there is a requirement for a flotilla of vessels such as attended the Macondo blow-out event, there are as many as a hundred large and small vessels in a relatively small area, with the need for central control (by the U.S. Coast Gaurd in this case) and collision-avoidance systems. These systems also depend upon having precise GNSS, mostly using SBAS systems.

    Further application details and additional critical applications can be provided upon request.

    Jim Litton is the President of the Litton Consulting Group, Inc. (LCG).  His GPS-related experience includes being the Chief Engineer at Magnavox during the GPS phase I development, contributing to analysis of ionospheric effects and senior vice-president and general manager of the Magnavox Commercial GPS Division before forming the Litton Consulting Group in 1992. He co-founded NavCom Technology in 1994.  He holds the Hays award from the ION for 1996 and is co-inventor on a codeless GPS receiver patent.   

  • Expert Advice: Energy Production Concerns about LightSquared

    RandallLuthi_W
    Randall Luthi, President, National Ocean Industries Association

    By Randall Luthi, President, National Ocean Industries Association

    To: Mr. Julius Genachowski,
    Chairman, Federal Communications Commission
    Ref: LightSquared, Inc.,
    L-Band allocations impacting GPS FCC File No. SAT-MOD-20101 1 IS-00239

    Dear Chairman Genachowski:

    The National Ocean Industries Association (NOIA), which represents approximately 270 member companies involved in outer continental shelf (OCS) energy production throughout the United States, is gravely concerned over the pending allocation of Mobile Satellite Services (MSS) spectrum to LightSquared, Inc. for terrestrial high-powered transmissions. LightSquared’s proposed transmission structure will adversely impact GPS and Inmarsat signals along our coastlines, both of which are critical to marine operations. Specifically, NOIA is concerned that:

    1. Coastal and near shore GPS operations will be impacted even at (promised) reduced LightSquared power levels. While NOIA understands that LightSquared will be required to reduce its tower transmission power along the coastline, their reduced power transmissions will still be many orders of magnitude greater than GPS signals, virtually guaranteeing interference for users in coastal and near-shore areas.

    2. GPS receiver types used by NOIA members will be impacted substantially. NOIA members primarily use high-precision GPS receivers for their high-accuracy coastal and near shore work. High-accuracy GPS receivers require a wide-band front end, which will be seriously impacted by LightSquared transmissions.

    3. Inmarsat-linked DGPS corrections will have interference. Virtually all of the high-accuracy GPS work requires the use of differential GPS corrections transmitted by Inmarsat in “L” band. These corrections will be jammed by the LightSquared signal. Implications are, for example, dredging or excavation work in areas near buried high-pressure natural gas pipelines. This work will become much more dangerous due to inaccurate, intermittent, or unreliable GPS readings.

    4. Offshore oil and gas operations will be impacted because of interference on land. LightSquared interference from their 40,000 proposed transmission sites across the nation will interfere with dozens of high-accuracy DGPS reference stations used to generate differential corrections for offshore use and high-accuracy operations on shore. Because of this land-based interference with high-accuracy GPS reference stations, hundreds and possibly thousands of coastal users will be impacted. High-accuracy differential GPS corrections are used by a wide cross-section of marine users including dynamically positioned drilling rigs, pipeline construction vessels, rig supply vessels and others. Loss of GPS corrections or erroneous differential GPS corrections due to shore-based interference can cause a floating drilling rig to deviate from station resulting in catastrophic blowouts, environmental damage, and fatalities.

    5. LightSquared will cause interference with Inmarsat. NOIA understands that LightSquared has paid Inmarsat, and will continue to pay Inmarsat a fee, to “endure the pain” caused by the interference. However, Inmarsat’s customers, including virtually all NOIA members, will still be required to endure the pain. Isotropic Inmarsat antennas will be impacted the most. NOIA understands that Inmarsat plans to move user frequencies at their cost. However, NOIA cannot be assured that this solution is viable given the financial drivers LightSquared is offering Inmarsat; it is not reasonable to assume that Inmarsat can compensate thousands of users for the costs of making the changes, even if the equivalent frequencies and powers are available.

    6. NOIA is concerned that the FCC was premature in its decision to issue a waiver to LightSquared. Unlike the FCC’s historic test-then-approve, NOIA is concerned that the FCC has fast-tracked the effort and has improperly and unnecessarily implemented an approve-then-test procedure for this applicant. NOIA is concerned that the FCC may have directed findings of the professional staff in its decision making.

    7. NOIA believes that millions of land and airborne GPS and DGPS user groups will be severely impacted by LightSquared transmissions. In conclusion, NOIA and its 270 member companies are extremely concerned that high-end GPS, DGPS, and the associated GPS reference stations will be interfered by LightSquared transmissions in the band previously protected for the very low power signals typical of satellite communications. The real-time GPS positioning needs of NOIA member companies are critical to the safety and success of their operations, and although these operations are at-sea they are totally dependent on shore-based GPS reference receivers, therefore LightSquared’s land-based operations will affect the offshore regions as well. With the marine industry’s giant assets including very specialized 2 vessels of all types completely dependent on GPS, the safety and environmental implications of GPS interference is astronomical. NOIA is concerned that other specialized Inmarsat installation members will also be disrupted.

    Finally, NOIA believes that the FCC is moving too quickly and needs to step back and make its decisions based on sound science, understanding that national wireless coverage is being pursued with all deliberate speed by several knowledgeable industrial groups that have paid for the value received from their frequency allocations. It does not need this asymmetric and competition-reducing spectrum grab by a group without the years of experience and trust of those who are building the infrastructure to accomplish the very laudable outcome that is ostensibly LightSquared’s motivation.

  • On-Site Geo-Referencing of 3D Static Terrestrial Laser Scans

    By Jens-André Paffenholz

    This blog presents an efficient procedure for directly geo-referencing static 3D laser scans. This is a worthwhile way to obtain the required transformation parameters from the local sensor-defined coordinate system to a global system. Therefore, a multi-sensor systems (MSS) is designed with a phase-measuring laser scanner and 3D positional sensors (see Figure 1). By means of at least one eccentrically mounted GNSS antenna on top of the rotating laser scanner one gets a 3D trajectory of the antenna reference point (ARP). The analysis of the resulting trajectory within a recursive state-space filtering approach (e.g., Kalman filter) yields the transformation parameters (position and orientation) and their full variance-covariance matrix. Apart from the geo-referencing of single laser scans the propagation of the transformation parameter variances to the point clouds is possible. Moreover, an improvement of the obtained direct geo-referencing results by means of matching algorithms (like, e.g., Iterative Closest Point (ICP) algorithm) with consideration of the stochastic point cloud information of each single 3D point is feasible.


    Figure 1. Sketch of the MSS (at the Geodetic Institute of the Leibniz Universität Hannover) composed of a phase-measuring laser scanner, GNSS equipment and two single-axis inclinometers.

     

    Overview about the enlisted sensors, their specifications and the algorithm for the transformation parameter estimation

    The main characteristic of the terrestrial laser scanning (TLS) technique for engineering geodesy is the immediate data acquisition in 3D space. This is realised with a high spatial resolution (a few millimetres for mean distances of approx. 25 m), as well as with a very high frequency (up to 50 profiles per second) in a relative or local sensor-defined coordinate system. The TLS technique can be used in a static or a kinematic mode. Static scanning is characterised by one single fixed translation and orientation of the laser scanner in relation to an absolute or global coordinate system. For kinematic scanning, where the data acquisition is commonly reduced to 2D profiles, the translations and orientations are time-dependent. Hence, the transformation parameters for each profile are different in relation to each other as well as to an absolute or global coordinate system. When a combination of several static scans from different stations into one coordinate system (registration) is required, the transformation parameters for each scan have to be determined. For an additional link to an absolute or global coordinate system (geo-referencing), typically control points in a known geodetic datum are necessary. By the direct observation of the required transformation parameters by means of GNSS equipment and arbitrary navigation sensors, one can solve the registration and geo-referencing in one single step without the need of additional control points.

    At the present developmental stage of the MSS (at the Geodetic Institute of the Leibniz Universität Hannover), it is composed of a phase-measuring laser scanner, one eccentrically mounted GNSS antenna and two inclinometers on top of the rotating laser scanner (cf. Figure 1). Hereby, the horizontal rotation of the laser scanner of at least 360 degrees is suitable to derive the position as well as the azimuthal orientation of the laser scanner.

    Currently, the GNSS data processing is done in post processing. In general, real-time processing is possible within the purposed geo-referencing procedure. The practicability within the direct geo-referencing procedure due to expected higher variances for the trajectory points of the ARP has to be investigated in the future. However, the short high frequent trajectory of the ARP makes the GNSS analysis a challenging problem which has to be overcome. The overall duration is about 15 min with up to 20 hz data rate. In this approach the alternating antenna orientation with respect to an earth-centred earth-fixed coordinate system will contribute to the error budget due to the right-hand circular polarisation of the satellite signals and the azimuthally varying phase centre corrections (PCC). In addition, near-field effects caused by the antenna adaption (made from aluminium) on the laser scanner, or possibly multipath from the vicinity surrounding the scanner may contribute to the error budget. Investigations of these GNSS related errors yield to no significant impact of the used antenna adaption within a double difference analysis in the observation domain. As expected, the rotated PCC against the original PCC has an effect of up to 5 mm in the observation domain which corresponds to the horizontal offset components of the used GNSS antenna. The analysis in the coordinate domain also indicates an effect of up to 5 mm. The analysis shows that the PCC effect is dominated by the phase centre offset components. One can conclude that within the currently applied epoch-wise GNSS analysis the effect of rotated PCC has no significant impact on the transformation parameters in the geo-referencing procedure. For further details about these investigations please refer to Paffenholz et al. (2011).

    The analysis of the 3D ARP trajectory (cf. Figure 2) is performed within an adaptive extended Kalman filter (aEKF). This yields the transformation parameters (position and orientation) alongside their full variance-covariance matrix. The benefits of using a closed form algorithm on the basis of a Kalman filter (KF) are the following: Firstly, the KF allows real-time data processing, and secondly, the parameter estimation will be less sensitive to outliers. To deal with non-linearities in the system and measurement equations, an extended KF (EKF) is used to estimate the transformation parameters of the MSS. Another promising approach for a non-linear state estimation is the combination of Sequential Monte Carlo filtering (also known as particle filter) and an EKF, which was proposed by Alkhatib et al. (2011). The main benefit of the proposed approach is the better performance in case of high-nonlinear state-space equations. An improvement of the dynamic model of the EKF can be achieved by augmenting the EKF with adaptive parameters. These parameters are time invariant and system-specific with well-known initial values. For further details please refer to Paffenholz et al. (2010).


    Figure 2. Sample ARP trajectory of a 360 degree rotation of the laser scanner around its vertical axis. Red indicates the original10 hz measurements with a Javad GNSS receiver Delta with Javad GrAnt-G3T antenna. Blue and green indicate the predicted and filtered trajectory within the aEKF approach, respectively.


    Performing the direct geo-referencing by applying the transformation parameters and calculation of the uncertainty measures of the 3D point cloud

    The final step of the purposed direct geo-referencing procedure is to apply the transformation parameters (translation vector as well as at least the azimuthal orientation) to the 3D point cloud. The three spatial rotation parameters can be reduced to the azimuthal orientation in case of a sufficient sensor orientation to the direction of gravity. The left part of Figure 3 shows the transformation result from the local sensor-defined to an absolute coordinate system in the case of two 3D point clouds, each from a different static scanner station (red and blue). The radial distance between the scanner and the object is 15 m and 20 m, respectively. It is obvious, that the two geo-referenced point clouds have a slight misalignment of a few centimetres. Due to the known absolute coordinates of the pillar on the roof of the building (middle part of the figure), one can conclude that the geo-referencing of the blue point cloud is inaccurate. Moreover, the variances for the transformation parameters from the blue station are higher than the variances for the red station. This leads to the conclusion that the estimated transformation parameters for the blue station are not reliable. Nevertheless, this direct geo-referencing can be used as adequate pre-registration for matching algorithms.

    To overcome this misalignment the application of matching algorithms, like the ICP algorithm, is worthwhile. As input for the ICP algorithm the pre-registered 3D point clouds are used. The a-priori alignment (within a few centimetres) of the two point clouds is sufficient for the application of the ICP algorithm to find an adequate amount of corresponding points for a reliable estimation of the transformation parameters. The ICP result is shown in the right part of Figure 3. One can clearly see that the matching of the two point clouds was successful. The recent topic of the ongoing research is the consideration of the uncertainties of each point cloud within the ICP algorithm for a further improvement of the matching results.
       
    Figure 3. Left: Applied transformation parameters to two scans from different stations (red and blue). Right: Result after running the ICP algorithm on the pre-registered 3D point clouds (shown in the left part of this figure).

     

    In the current research work uncertainties for each single point cloud are calculated by variance propagation: Combining the uncertainties of the scanner measurements (e.g., manufacturer values for the angle and range measurement accuracy), and the uncertainties of the direct geo-referencing procedure (variance-covariance matrix of the transformation parameters obtained within the aEKF). As mentioned before, these uncertainties should be considered in the ICP algorithm in the ongoing work for a further improvement of the matching results. Bae et al. (2009) already stated that the consideration of positional uncertainties in the point cloud matching will be a worthwhile approach to improve the matching, as well as the interpretation of 3D point clouds. An example for the result of the variance propagation of the scanner and direct geo-referencing uncertainties is illustrated in Figure 4. The figure depicts a stochastic point cloud of the red station (similar 3D point cloud as shown in Figure 3). As measure for the uncertainty the mean of the coordinate uncertainty in a range of 5 mm up to 30 mm is shown.

    Figure 4. Stochastic point cloud of red station resulting from variance propagation for the uncertainties of the scanner measurements and the direct geo-referencing procedure. Depicted is the mean of the coordinate uncertainty.

     

    Conclusions and Future Work

    This article describes an on-site direct geo-referencing of 3D static laser scans by means of tracking the circular motion of the laser scanner around its vertical axis with 3D positioning sensors. The required transformation parameters from the local to an absolute coordinate system are estimated within a Kalman filter approach. The results show a misalignment for two different static laser scanner stations in a range of a few centimetres. Nevertheless, this is an adequate pre-registration for matching algorithms. Besides the geo-referencing, the uncertainties of the 3D point clouds are calculated by variance propagation. The future work is focused on the consideration of the stochastic point cloud information within matching algorithms (like, e.g., ICP) for an optimal fusion of different (pre-) registered point clouds into one optimal solution.

    References

    Alkhatib, Hamza; Paffenholz, Jens-André; Kutterer, Hansjörg (2011): Sequential Monte Carlo Filtering for nonlinear GNSS trajectories. In: Sneeuw; Novák; Crespi and Sansò (Eds.): Proceedings of the VII Hotine-Marussi Symposium on Mathematical Geodesy, Rome, 6-10 June 2009. International Association of Geodesy (IAG). 1st Edition. Berlin, Heidelberg: Springer, (in press).

    Bae, Kwang-Ho; Belton, David; Lichti, Derek D. (2009): A Closed-Form Expression of the Positional Uncertainty for 3D Point Clouds. In IEEE Trans. Pattern Analysis and Machine Intelligence 31 (4), pp. 577–590.

    Paffenholz, Jens-André; Kersten, Tobias; Schön, Steffen; Kutterer, Hansjörg (2011): Analysis of the Impact of Rotating GNSS Antennae in Kinematic Terrestrial Applications. In: Proceedings of the FIG Working Week 2011. FIG. Marrakech, published on CD only / also available via www.fig.net.

    Paffenholz, Jens-André; Alkhatib, Hamza; Kutterer, Hansjörg (2010): Direct geo-referencing of a static terrestrial laser scanner. In JAG 4 (3), 115–126.

    Jens-André Paffenholz received his Dipl.-Ing. in Geodesy and Geoinformatics at the Leibniz Universität Hannover. Since 2006 he has been research assistant and since 2008 also PhD candidate at the Geodetic Institute at the Leibniz Universität Hannover, respectively. His current interests are: terrestrial laser scanning, industrial measurement systems, and process automation of measurement systems. The present research focus is: precise direct geo-referencing in terrestrial laser scanning applications.

  • LightSquared Goes Global; GLONASS, Galileo May Be at Risk, Too

    Recent events, some of them summarized here, may appear to have dealt setbacks to LightSquared, the boundless opportunist of wireless broadband that just happens to interfere with GPS. But the company has not run out of moves yet. Would you, if you had $20 billion at stake? The latest gambit, led by lawyers and cloaked in jargon, appears to be an end-run around the U.S. government to appeal to the International Telecommunications Union, which has ultimate and international authority over spectrum. Watch out, GLONASS and Galileo — and U.S. troops operating in foreign theaters.

    GPS World has received copies of three “fact sheets” authored by two lawyers and a strategic consultant. The documents are addressed to ITU-R WP 4C, the International Telecommunications Union Working Party that handles mobile satellite services (MSS) and radio determination satellite service (RDSS spectrum) and orbits. One document is titled “ Compatibility between Complimentary Ground Componenet in the 1525–1559 Mhz and 1626.5–1660.5 Mhz Bands and Other Service.” All three documents appear to be cover sheets for longer treatises, and their language and citations are not entirely clear to me, as my legal and regulatory background leaves something to be desired.

    However, they announce their purpose as “to modify and refine the example methodology to calculate aeronautical mobile satellite (route) service spectrum requirements,” and “to address ongoing Integrated Mobile Satellite Service Complimentary Ground Component compatibility matters,” and finally “to update the Integrated Mobile Satellite Service Complimentary Ground Component technical characteristics based upon the most recent information regarding CGC deployment plans in this frequency band.”

    One source familiar with the documents, who did not wish to be named, commented that “One should interpret what LightSquared is doing with ITU as a bellwether indication of intent to use the whole band at the full authorized power, no matter how they spin ‘protect GPS’ in their press releases. 



    “At first blush, the filings look innocuous; let me assure you, they are not. This is the first salvo. Watch what they do, much more than what they say.

    

“These are fact sheets intended to inform the U.S. government that LightSquared intends to develop papers with the intent to get the U.S. government to approve the papers to be sent to the ITU WP-4C, the Working Party that handles MSS and RDSS spectrum & orbits. The ultimate goal is to work internationally to allow LightSquared to allow ancillary terrestrial component (ATC) broadcast globally.”

    The three so-called fact sheets are appended here.

    In other developments, going now in reverse chronological order, from most recent to early June:

    Congressional Activity

    On June 23, the U.S. House of RepresentativesAppropriations Committee approved the fiscal year 2012 Financial Services and General Government Appropriations bill. One amendment to the bill prohibits funding for the Federal Communications Commission (FCC) to remove conditions on or permit certain commercial broadband operations until the FCC has resolved concerns of harmful interference by these operations on GPS devices. The amendment was adopted on a voice vote. More details here.

    Previously, on May 27, the U.S. House of Representatives passed a bill stating that the FCC shall not provide final authorization for LightSquared operations until Defense Department concerns about GPS interference have been resolved. The bill then went to the U.S. Senate for its action.

    The House actions and a letter to the FCC signed by 32 U.S. senators may presage a showdown over the issue between Congress and the president, who has promised increased broadband access. A 4G wireless network providing this access could be facilitated by LightSquared sales of service via its tower transmitters to wireless carriers. LightSquared has already signed a $20 billion, 15-year deal with Sprint.

    Money Talks

    A report on “The Economic Benefits of Commercial GPS Use in the United States and the Costs of Potential Disruption” was presented by during a June 21 webinar sponsored by the Coalition to Save Our GPS.  The report estimates that “the direct economic benefits of GPS technology on commercial GPS users are . . .  over $67.6 billion per year in the United States,” but also that ““the direct economic costs of full GPS disruption to commercial GPS users and GPS manufacturers are estimated to be $96 billion per year in the United States.”

    Final Report Withheld

    At the last minute of a June 15 deadline for the final Working Group report on interference, LightSquared asked for a two-week extension. Federal regulators granted the request, and the final report is now due on July 1.

    A spokesperson for the Coalition to Save Our GPS revealed that “The Working Group results show devastating interference to GPS and no proven method of mitigation. Delay will not change these results. These results are the same results the FCC had had before it granted the waiver.”

    Some Solution. Three days after requesting the delay, LightSquared announced it had solved the problem, by proposing to broadcast only from the lower end of its permitted spectrum band. GPS experts countered that this would still disable the functioning of high-precision receivers.

    “This comes out of the blue, without the knowledge, agreement or consensus of the industry group studying the problem,” riposted the Coalition to Save Our GPS. “That may well be because virtually nothing has actually changed in this “new” proposal relative to what LIghtSquared pledged at the outset of testing. The power levels don’t change. Nor do the frequencies. In fact, the only thing that has changed is the order in which the channels within the band adjacent to GPS would be deployed.

    “LightSquared’s announced “solution” has two components:

    “1. LightSquared acknowledges that “[e]arly test results indicated that one of LightSquared’s 10MHz blocks of frequencies poses interference to many GPS receivers.” LightSquared states that for “the next several years” it would not operate in this band – which is directly adjacent to GPS spectrum and is referred to as the “upper MSS band.” During this period, LightSquared would commence operations in a second 10 MHz block of the MSS band , referred to as the “lower MSS band,” slightly further away from GPS.

    “2. According to the proposal ‘LightSquared will modify its FCC license to reduce the maximum authorized power of its base-station transmitters by over 50 percent. This action will limit LightSquared to the power it was authorized to use in 2005.’

    “This so-called solution is not a solution in any shape, form or fashion,” continues the Coalition. “This is not a move to an alternative frequency band. Nor is it a reduction in power relative to what has been tested from the beginning. The “solution” would cause massive disruption to many critical U.S. economic sectors, initially including public sector users of high precision GPS, later followed – af
    ter “the next several years” — by other GPS users. The only real solution to the LightSquared interference problem is to move out of the MSS band altogether."

    Click here for the full document, “New ‘Solution’ Is a Non-Starter.”

    Air Transport Opposes Waiver

    The Air Transport Association and the Aircraft Owners & Pilots Association told Congress that the only acceptable mitigation is for LightSquared’s operations to be moved outside of the L-band and away from GPS. “With so much of the early evidence showing that LightSquared’s proposed network would potentially endanger nearly every flight operating in U.S. airspace, it seems evident that no further development of this system can be allowed.”

    Military Report Calls for FCC Retreat

    The National PNT Engineering Forum concluded after testing classified and GPS receivers under LightSquared terrestrial transmission conditions: “Significant concerns remain that operation of an ATC integrated service as originally envisioned by the FCC cannot successfully coexist with GPS.”

    The NPEF report calls for rescinding the FCC waiver for LightSquared terrestrial transmissions, conducting more thorough studies on impacts, and revisiting the 2003–2010 authorizations. The group tested a variety of military receivers under classified categorization, also known as “government receivers.”

    Rebuttals Distort Record

    Claims by LightSquared’s Carlisle and FCC chair Julius Genachowski, that the GPS industry knew long ago about LightSquared’s plan for powerful terrestrial transmitters, contradict the truth. Examination of FCC filings show that the GPS industry knew about and agreed to a plan by a previous ownership of the company, for a different purpose, with a different business concept, and employing a completely different technological approach, one that would not have harmed GPS transmissions and disabled GPS users the way the current LightSquared plan does.

    The terrestrial broadband operations first unveiled in November 2010 cannot be described as ancillary to the purpose for which Lightsquared predecessors Motient, MSV, and SkyTerra received their spectrum and licenses — that is, to provide a service that was primarily a mobile satellite service. The November letter to the FCC described a new business model that turns the original concept on its head. LightSquared for the first time revealed plans to build a “nationwide network of 40,000 terrestrial base stations,” and stated that “the capacity of its fully deployed terrestrial network across all base stations will be tens of thousands of times the capacity of either of [its] satellites.”

    The deviations from established policy required to accommodate LightSquared’s new business model are not technicalities. They represent a fundamental change to a complex and interrelated set of rules that were carefully designed to protect GPS users from interference.

    The predecessor companies had to protect their own primary satellite operations from interference. The protection that their own satellite operations required was also sufficient — at that time — to protect GPS receivers. The terrestrial network and powerful signal LightSquared now proposes bear no resemblance to the operations the FCC authorized in 2003.

    For further commentary in this vein, see LightSquared, FCC Rebuttals Distort Record.

    PNT Advisory Board: Move ATC

    At its June 9–10 meeting, the National Space-Based Positioning, Navigation and Timing (PNT) Advisory Board found that GPS services cannot be assured if the LightSquared plan is approved, and that the only viable option for continued availability of GPS as well as new wireless broadband is to find another spectrum for LightSquared not adjacent to the GPS frequency.

    The formal recommendation reads: “The provision of GPS services cannot be assured if the LightSquared proposal for satellite and terrestrial broadband provision using the MSS L-Band receives final approval.

    “The only reasonable and viable option to continue ubiquitous availability of GPS and the provision of a new 4G wireless broadband capability would be for the FCC to assign an alternate frequency spectrum to LightSquared that has little or no probability of affecting the delivery or utilization of GPS/GNSS services.”

    During its meeting, the Advisory Board heard directly from one representative of LightSquared, the company’s executive vice president, regulatory affairs and public policy, Jeff Carlisle, and from Jim Kirkland, vice president and general counsel, Trimble Navigation, speaking on behalf of the Save Our GPS Coalition.  
"Without knowing otherwise," commented one observer, "one might have thought they were talking about two different sets of FCC actions. Their interpretations of FCC actions were completely orthogonal to each other."

    During the discussion, one Advisory Board member, a former governor of the state of Wyoming, told presenter Jeff Carlisle of LightSquared, “Your definition of mitigation seems more tied to a legal argument than a common-sense argument.”

    
Other speakers on the LightSquared/GPS panel included Dean Bunce, co-chair of the National PNT Engineering Forum (NPEF), which has had responsibility for testing various classified GPS receivers under LightSquared conditions; and Robert Frazier of the Federal Aviation Administration (FAA) Spectrum Planning and International Office. 


    Most of the presentations from the meeting are now posted online.

    Another observer at the Advisory Board meeting opined of the LightSquared presentation and subsequent replies to questions from board members, “I’ve seen weasels before, but not like this. Misinformation, mis-statements, reversals and take-backs, outright lies.”

    Tests Slam Hi-Precision Receivers

    Data from Las Vegas field tests show that wide-bandwidth, high-precision GPS receivers started feeling the effects of the LightSquared transmission about 1,800 meters from the tower. Medium-bandwidth high-precision GPS receivers started feeling the effects of the LightSquared transmission at about 1,200 meters from the tower. In each case, there was about a 200-meter buffer from when the GPS receivers started to feel the effects of the LightSquared transmission to the GPS receiver being jammed, at 1,600 meters and 1,000 meters respectively. For further details, see this article.

    GPS World has received further details of the tests but not an authorization to publish them yet.

    Deere & Company, a major provider of precision agriculture equipment and services, notified the FCC on May 26 of substantial interference with its GPS receivers by the LightSquared signal. Deere receivers registered impact of and interference by the LightSquared signal as far away as 22 miles from a transmitter. Further, the company has found no practicable technical solution to the problem.

     

  • European Commission Awards Final Contracts Making Galileo a Reality

    The European Commission (EC) announced that the final two contracts, out of six, for Galileo, Europe’s global navigation satellite programme will be signed at 16.00 by the European Space Agency on behalf of the EC at the prestigious Le Bourget Aerospace Fair in Paris. The combined valued of the two contracts is €355 million. The contract signed with Thales Alenia Space (FR), for a value of €281 million, ensures the formatting of navigation information for broadcast by the satellites. The contract signed with Astrium (UK), for a value of €73.5 million concerns the "housekeeping" of the satellites including the maintenance and correct positioning of the satellites in orbit. Signature of these contracts is essential for the deployment and provision of three initial services by Galileo in 2014:

    1. The free Open Service basic signal, which everybody can use.
    2. The Public Regulated Service comprising two encrypted signals with controlled access for specific users like governmental bodies.
    3. Search-and-Rescue Service for humanitarian search and rescue activities.

    For Vice President Antonio Tajani, European Commissioner for enterprise and industrial policy, “The award of the contracts to French and UK companies once again underlines the true cross-border European collaboration which is Galileo. Signature of the contracts marks the end of a rigorous procurement process, and the beginning of a new chapter for Galileo. Rigorous – because I personally insist on reducing costs wherever possible throughout the Galileo programme. A new chapter for Galileo – because we are now well and truly on the road to putting in place the infrastructure leading to the provision of vital services to citizens in 2014. We are all looking forward to the launch of the first two operational Galileo satellites on 20th October from French Guiana Space port”.

    According to the announcement, the procurement of services essential for Galileo’s full operational capability is divided into six contracts. In January 2010, three contracts were awarded to ensure system engineering support, satellites and launchers (see IP/10/7) A fourth contract was signed in Brussels in October 2010 with SpaceOpal for operating the space and ground infrastructure (IP/10/1382). Galileo will underpin many sectors of the European economy through its services: electricity grids, fleet management companies, financial transactions, shipping industry, rescue operations, peace-keeping missions will all benefit from the free Open Service, the Public Regulated Service and the Search-and-Rescue service.

    The EC reports that in addition, Galileo will make Europe independent in a technology that is becoming critical, including for such areas as electricity distribution and telecommunication networks. Galileo is expected to deliver €60 billion to the European economy over a period of 20 years in terms of additional revenues for industry and in terms of public and social benefits, not counting the benefit of independence.

    Galileo will provide three early services in 2014/2015 based on an initial constellation of 18 satellites, says the EC: an initial Open Service, an initial Public Regulated Service and an initial Search-and-Rescue Service. Further services to follow later will cover a Commercial Service combining two encrypted signals for higher data throughput rate and higher accuracy authenticated data.

  • Galileo’s Soyuz Launchers Arrive at French Guiana

    The European Space Agency (ESA) announced that two Soyuz launchers which will fly the first four satellites of Europe’s Galileo navigation system into orbit have arrived at Kourou harbour in French Guiana, completing a journey that took them halfway round the world.

    The first two Galileo In Orbit Validation satellites are set to be launched from Europe’s Spaceport on 20 October, with two more following them into orbit by mid-2012. The October launch will be the first flight of a Soyuz rocket from French Guiana.

    The two Soyuz ST-B launchers and their Fregat-MT upper stages were carried across the Atlantic aboard Arianespace vessel MN Colibri, arriving on June 18. The rocket hardware left by train from the Soyuz manufacturing plant in Samara, Russia and the Fregat factory in Moscow to St. Petersburg harbour, where it was loaded for shipment, leaving on June 3 for French Guiana.

    Soyuz_ST-B_launchers_at_Kourou_harbour_node_full_image_2
    Soyuz ST-B launchers at Kourou harbor.

    According to the ESA, the next step will be the Launcher Flight Readiness Review, due to take place on 21 July. Authorisation will then be given to begin assembling the rocket hardware and deployingthe initial Soyuz ST-B launcher for the first Galileo campaign.

    The first two Galileo satellites — known as PFM and FM2, for Protoflight Model and Flight Model 2 – are currently undergoing their final qualification and acceptance tests at Thales Alenia Space in Rome, Italy. Once Satellite Flight Readiness Review has given the green light, both satellites and their ground equipment and launch teams will arrive at the beginning of September for the launch campaign.

    Soyuz ST-B is the most powerful version of the famous Soyuz launcher, while the Fregat-MT is an upgraded version of the Fregat upper stage.

    Other Soyuz hardware is already in storage at Kourou but only the combination of Soyuz ST-B and Fregat-MT was up to the demanding task of conveying the Galileo satellites into their circular 23,222 km orbits. A European dispenser will hold the satellites in place as they share their ride to orbit, and then release them into their final orbits.

    Baseline versions of the reignitable Fregat were previously employed to deliver ESA’s GIOVE-A and -B experimental satellites in 2006 and 2008, which secured the rights to Galileo’s radio frequencies. Fregat-MT carries an additional 900 kg of propellants for its double-satellite load.

    The ESA says that October’s launch will be a historic occasion, the first time that a Soyuz launcher lifts off from a spaceport other than Baikonur in Kazakhstan or Plesetsk in Russia.

    Because French Guiana is so close to the equator each launch will benefit from Earth’s spin, increasing the maximum payload to geostationary transfer orbit from 1.7 tonnes to three tonnes, says the ESA. As a medium-class launcher, Soyuz will complement Ariane and Vega to enhance the flexibility and competitiveness of Europe’s launcher family. Each three-stage rocket will be assembled horizontally in the traditional Russian manner, transferred to the launch site and moved to the vertical so that its payload can be mated onto it from above. A new mobile launch gantry enables this process, while protecting the satellites and the launcher from the humid tropical environment.

    These first four Galileo satellites will form the operational nucleus of the full Galileo satnav constellation, according to the announcement. They are fully representative of the others that will follow them into orbit, combining the best atomic clock ever flown for navigation — accurate to one second in three million years.

  • LightSquared: High-Precision Receivers Are Collateral Damage

    Originally, the LightSquared/GPS Technical Working Group’s (TWG) report was due to the FCC on June 15, 2011. LightSquared requested from the FCC, and received, a two-week extension to submit their report. Three days later, LightSquared announced it found a solution to the GPS interference problem. Its new proposed solution is not good news for the high-precision GPS user community. Instead, it’s a threat directed squarely at high-precision GPS users like you and me. Do you recall what I wrote a month ago? It’s ringing true with the latest LightSquared proposal.

    “I’m going to keep this simple. You, the high-precision GPS user, are likely going to be considered collateral damage.

     

    The military is going to be accommodated in the name of national security. The aviation industry is going to be accommodated in the name of safety-of-life. The auto navigation industry is going to be accommodated because they are high-profile. The high-precision user is going to be thrown under the bus because we are the most difficult to accommodate (technically) and don’t have a high profile nor are perceived as significant enough to accommodate.”

     

    If you recall, the TWG consists of LightSquared and GPS industry representatives tasked with testing the effect that LightSquared’s proposed system may have on GPS. Four of the five sub-teams were ready to file their final report with the FCC on June 15. LightSquared’s sub-team, according to the Coalition to Save Our GPS webinar on Thursday June 16, was only sub-team not ready to file its report.

    Ok, so after the FCC granted LightSquared the two-week extension, I’m thinking we have a two-week hiatus from the LightSquared discussion as LightSquared compiles and prepares their July 1 submission.

     

    Nope, not a chance.

    On Monday, a mere three business days after they requested a two-week extension from the FCC,  LightSquared announced they’ve found a solution to the GPS interference problem and issued a statement titled “LightSquared Solution to GPS Issue Will Clear Way for Nationwide 4G Network.”

    LightSquared’s solution is to temporarily abandon the upper frequency they originally planned to roll out (1550-1555 MHz) and rollout its service using the lower spectrum I refer to as “modified” (1526-1536 MHz) in Figure 1.

    It’s important to remember that the lines separating frequency spectrums are not “brick walls.” There is signal “roll off” that results in a gray area between spectrums. That’s the reason the LightSquared upper frequency at 1550-1555 MHz was slamming GPS. Even though it is apparently separated from GPS L1, the sheer power of the LightSquared signal at 1,500 watts significantly bled into the red RNSS zone (1559-1610 MHz) in Figure 1.

     

                                                  Figure 1: FCC Spectrum Dashboard

    Using only the lower frequency spectrum (1526-1536 MHz), LightSquared claims that they are “largely free of interference issues with the exception of a limited number of high-precision GPS receivers that are specifically designed to rely on LightSquared’s spectrum.” LightSquared’s CEO said that this solution will accommodate 99.5% of the GPS receivers.

    Uh oh, guess who the remaining .5% are? Yes, your high-precision GPS receiver. One half of one percent is about the percentage of high-precision GPS receivers with respect to the total GPS market size in the U.S.

    I’m pretty confident that LightSquared isn’t weighting the receivers, so that means a $2 GPS chip inside a mobile phone carries the same weight as your $15,000 RTK receiver. But obviously the impact on our infrastructure and economy differs by orders of magnitude between the two.

    Remember last month when I wrote that high-precision GPS receivers might be thrown under the bus and considered collateral damage (LightSquared: It’s Worse than You Think)? The latest LightSquared proposal is what I was referring to. High-precision GPS receivers are the most difficult to accommodate, and LightSquared is thinking that if they tell the FCC (and the world) that they’ve taken care of 99.5% of the GPS receivers in the U.S., the other .5% can deal with it.

    It’s not yet clear how LightSquared broadcasting on 10L (1526-1536 MHz) will affect high-precision receivers. We should see some of those details at the end of the month when all reports are filed with the FCC.

    But either way, it’s clear that LightSquared broadcasting in the 1526-1536 Mhz spectrum would slam OmniSTAR and Deere & Co. Starfire users as you can see in Figure 1.

    Not so fast, say GNSS engineers. What about GLONASS, Galileo, and Compass?

    Russia’s GLONASS satellites are increasingly being used by high-precision receivers. In fact, it’s safe to say that all major manufacturers sell GPS/GLONASS receivers, which is an expensive option on most receivers. However, it’s relatively easy to justify the additional expense due to the productivity gains from the additional GLONASS satellites. Generally speaking, more satellites equals less down-time.

    The problem is that the U.S. government has no vested interest in protecting the GLONASS spectrum.

    The FAA doesn’t care about it. The U.S. military doesn’t care about it. The first-responders don’t care about it. Although GLONASS is starting to show up in consumer GPS chips, it’s not being used in those markets like it is in the high-precision markets such as surveying, engineering, construction, agriculture, GIS, and various machine control applications. Therefore, no GLONASS testing was performed at the Maryland test site (simulator not configured to output GLONASS) and little or no testing was done using GLONASS at the New Mexico or Las Vegas sites unless individual companies took it on themselves.

    Some say that GLONASS will get hammered by LightSquared mobile phones.

    To this point, most of the talk has been about GPS interference from LightSquared transmitters in the 1525-1559 MHz spectrum. We also need to be aware of LightSquared mobile phones, of which they intend to field 250 million — 100 million by the end of 2012. While LightSquared has control over the filtering on their transmitters, it have no control over the filtering used in mobile phones designed to use their system.

    I’ve heard there is some mention of LightSquared mobile phones in the reports that are to be filed with the FCC, but not made public yet. However, no LightSquared mobile phones exist today so it’s only possible to simulate them in a lab environment using a lot of design assumptions.

    The uplink frequency used by LightSquared mobile phones (to talk to the nearest tower) is in the range 1626.5-1660.5 MHz. That frequency is getting close to the top end of GPS and really close to GLONASS L1 which has a range of ~1598-1605.4MHz.

    According to one RF engineer I’ve spoken to, “We already know that Iridium (1616-1626.5 Mhz) and Inmarsat cannot co-exist in the upper band and seeing that the LightSquared handset transmit frequency is in that same spectrum, I think GLONASS in the U.S. is toast.”

    The future of GNSS receivers is definitely trending towards integrating GPS, GLONASS, Galileo, Compass, etc. signal
    s. A section of the NPEF report (mentioned above) succinctly describes the interference issue with GNSS receivers.

    Another approach examined involves limiting the LightSquared transmissions to the lower 5 or 10 MHz channel of their planned deployment. However, while this approach would protect a limited number of GPS applications other applications would still be susceptible to interference. Using this approach it may be possible to protect classes of GPS receivers, primarily those with greater receiver selectivity. However, some classes of GPS receivers would still not be protected under this mitigation technique. Receivers having wider RF front-end characteristics, such as those used for scientific and commercial uses requiring high-precision measurements, and some receivers capable of receiving multiple signals from different GNSS systems (e.g., GLONASS) would remain susceptible. Additionally, the use of only the lower LightSquared channel would provide only a temporary solution to the existing interference problems as 4G LTE levels of service may not be possible. Thus, even if allowed, the FCC’s objectives and service conditions on the LightSquared license would not be met.

    Finally and on a slightly different note, the future GPS L1C signal and L1 signals proposed by Galileo and Compass are a wider band than the current GPS L1 CA, which means they are likely more subject to interference from the LightSquared system.

     

    FCC Chairman Julius Genachowski: “As I have stated previously to Congress, the commission will not permit LightSquared to begin commercial service without first resolving the commission’s concerns about potential widespread harmful interference to GPS devices. Under no circumstances would I put at risk our nation’s national defense or public safety.”

    The FCC has stated on numerous occasions that LightSquared won’t be allowed to begin commercial service until GPS interference issues are resolved, but what does that really mean?

    Chairman Genachowski has also stated that “It should come as no surprise to anyone involved in the LightSquared matter that the company was planning for some time to deploy a major terrestrial network in the spectrum”. He’s implying that all parties involved should have prepared for this moment, and if the GPS industry didn’t, it should bear some of the burden. This is bad news indeed.

    Bottom line: The FCC is not looking out for your interests. The National Broadband Plan is heavy on their minds. I can clearly see the FCC thinking “in the interest of the bigger picture, the high-precision GPS user community can deal with it since its only .5% of the total GPS market.”

    We need to squash this new proposal by LightSquared in a hurry. It’s a threat directed squarely at the high-precision GPS user community.

     

    LightSquared Consultant claim: in the GPS industry’s “insatiable thirst for precision,” it made poor engineering decisions that made GPS receivers more vulnerable to interference from neighboring bands.

    Although it appears the statement is from an independent consulting firm, PRTM consultant Dan Hays is a Harbinger crony so don’t let it fool you into thinking it’s anything but another piece of LightSquared propaganda.

    But, let’s visit the subject for a minute to clarify because LightSquared has also claimed that high-precision GPS receivers are somehow at fault because they “are looking in our spectrum”.

    Jim Kirkland, VP and General Counsel for Trimble Navigation, said it well when I presented Mr. Hays’ statement during the Coalition to Save Our GPS webinar last Thursday. Mr. Kirkland responded:

    “… we’ve engineered our products to use services that are available for payment to LightSquared’s predecessors. That’s a critical point…these precision receivers are designed to receive MSS signals to make the services better and they pay for those services to Skyterra (owned by LightSquared) and Inmarsat (LightSquared vendor). So if that’s a bad design decision that we decided to design our receivers so that our customers could pay money to Skyterra (LightSquared), that’s one of the more absurd things I’ve heard in this whole debate.”

     

    What he’s talking about is that OmniSTAR pays SkyTerra (LightSquared) to lease bandwidth on their satellite to deliver corrections to high-precision GPS users on the ground. Yes, if you pay OmniSTAR for their VBS, HP, or XP service, then a portion of what you pay goes to LightSquared. The irony is as thick as molasses. Furthermore, Deere & Co/Navcom offer a similar service called Starfire in which they lease satellite bandwidth from Inmarsat. LightSquared and Inmarsat are connected. Based on an original agreement signed in December 2007 between LightSquared’s predecessor and Inmarsat, Inmarsat is to receive hundreds of millions of dollars from LightSquared towards “the re-banding and efficient reuse of L-band radio spectrum covering North America.”

    Essentially, what LightSquared is doing is selling the GPS industry their satellite-to-earth wireless services (a la OmniSTAR), but they are complaining that the GPS industry has designed GPS receivers to utilize services in which LightSquared gets paid. Is that a “poorly designed GPS receiver”?

    I’ve even heard, through the grapevine, that some legislators are regurgitating this nonsense of “poorly designed GPS receivers.” Maybe there’s no ill-intent, but it’s either that or a fair amount of ignorance.

    Logically, many of today’s high-precision GPS receivers have OmniSTAR/Starfire capability built into their antennas and RF front-ends to look into the 1525-1559 Mhz spectrum for the OmniSTAR/Starfire signals. They don’t focus on particular frequencies in that band because the frequencies change periodically as OmniSTAR users can attest. Also, even if you have the OmniSTAR/Starfire capability turned off in your receiver, the antenna is still designed to look into that band so there’s no way around it.

    Like I mentioned earlier, even if your receiver isn’t designed to utilize OmniSTAR/Starfire, no one knows yet if it might be affected by the LightSquared 5L/10L signal.


    Where do we go from here?

    There’s a lot more to talk about on this issue. It’s as critical as it’s ever been that you make you concerns known to your state senators and representatives as well as the FCC. Scroll to the bottom of this article for web links and contact information.

     

    Free Webinar – Thursday, June 23. LightSquared: What it means to the GPS Surveying/Mapping User Community

    Thursday, June 23, I will conduct a webinar to discuss the LightSquared issue as it relates specifically to the GPS Surveying/Mapping community (high-precision users). Joining me will be Dr. Mike Whitehead, VP of Technology at Hemisphere GPS. He’s a leading GNSS design engineer and can speak clearly about the technical ramifications of LightSquared interference on high-precision GPS receivers. Click here to sign up for the webinar. Sign up even if you can’t attend the live webinar on Thursday because you’ll be sent an email on how to view the archived presentation that you can listen/view at your convenience.

     

    Light
    Squared coverage at the Esri Survey Summit (July 7-12, San Diego, CA)

    Esri has coordinated comprehensive coverage of the LightSquared issue at this years Survey Summit. Remember, this year the Survey Summit is combined with the ACSM (American Congress on Surveying and Mapping) annual conference so the turnout should be very good.

    On Friday, July 8th @ 2pm at the Survey Summit, I’ll be giving a 60 minute presentation entitled “GPS/GNSS Technology Update” focused on covering the latest developments in GPS/GNSS. I will cover the latest LightSquared news as well as other recent GPS/GNSS technology developments.

    On Saturday, July 9th @ 3:30p and 4:00p respectively, Jeffrey Carlisle from LightSquared and Peter Large from Trimble Navigation will give 30 minute presentations on the GPS/LightSquared interference issue.

    On Sunday, July 10th @ 8:00a-10:00a there will be a GPS/LightSquared discussion panel consisting of myself, Peter Large, Jeff Carlisle, Curt Sumner (ACSM), John Matonich (NSPS), and Dr. Javad Ashjaee. The panel discussion will be moderated by Joe Paiva. This will be the first panel discussion in the industry focused on the high-precision GPS/LightSquared interference issue.

    Following the discussion panel, at 10:30am-Noon,  there will be a strategy session designed to plan actions that surveyors (high-precision users) can take to avoid becoming collateral damage.

    ———————–

    Take Action Now
    The Coalition to Save Our GPS has posted guidance on its website as to how to submit your comments. They are:
    Voice your concerns directly to Congressional Representatives
    To voice your concerns about GPS interference, you can send letters, emails, faxes, call or visit your Congressional representatives’ office in person to explain how you use GPS as a local business and what the impacts of interference would be to the local economy.
    Contact Your Local Senator
    Ask your Senator to support and co-sign the letter from Senators Roberts (R-KS), Nelson (D-NE) and nearly a third of the U.S. Senate: explain how you use GPS in your state and what impact interference or any compromise of the GPS service would have on you and the local economy.
    Write Your Representative
    The Washngton Wire reported this week that “A bipartisan group of 66 House members asked the FCC Tuesday to protect global positioning systems from interference from wireless broadband start-up LightSquared…”
    Please include: “Coalition to Save Our GPS and FCC File No. SAT-MOD-20101118-00239” in your correspondence.
    Send your comments directly to the Federal Communications Commission (FCC)
    Email the FCC: [email protected]
    For your ready reference, below are the actions the Coalition is seeking from the FCC:
    1. The FCC must make clear, and the NTIA must ensure, that LightSquared’s license modification is contingent on the outcome of the mandated study unequivocally demonstrating that there is no interference to GPS. The study must be comprehensive, objective, and based on correct assumptions about existing GPS uses rather than theoretical possibilities. Given the substantial pre-existing investment in GPS systems and infrastructure, and the critical nature of GPS applications, the results of studies must conclusively demonstrate that there is no risk of interference. If there is conflicting evidence, doubts must be resolved against the LightSquared terrestrial system. The views of LightSquared, as an interested party, are entitled to no special weight in this process.
    2. The FCC should make clear that LightSquared and its investors are proceeding at their own risk in advance of the FCC’s assessment of the working group’s analysis. While this is the FCC’s established policy, the Commission’s International Bureau failed to make this explicit in its order.
    3. Resolution of interference has to be the obligation of LightSquared, not the extensive GPS user community of millions of citizens. LightSquared must bear the costs of preventing interference emanating from their devices, and if there is no way to prevent interference, it should not be permitted to operate. GPS users or providers should not have to bear any of the consequences of LightSquared’s actions.
    4. This is a matter of critical national interest. There must be a reasonable opportunity for public comment of at least 45 days on the report produced by the working group and further FCC actions on the LightSquared modification order should take place with the approval of a majority of the commissioners, not at the bureau level.
    Thanks, and see you next time.
    Follow me on Twitter at http://twitter.com/GPSGIS_Eric