Wavelet Packet Decomposition (WPD) shows promise as an anti-jamming tool.
The WPD is derived from the wavelet transform, which provides a representation of the signal components in a domain spanned by a set of functions that can be seen as band-pass filters with a bandwidth decreasing as their central frequency increases, thus granting a uniform resolution in the decomposition of the signal under analysis.
A paper by Luciano Musumeci and Fabio Dovis of Politecnico di Torino and James T. Curran of the Joint European Commission’s Research Center, titled “A Comparative Analysis of Adaptive Notch Filtering and Wavelet Mitigation against Jammers Interference,” won the Best Paper in Session award in GNSS Vulnerabilities and Anti-Jamming at the ION-GNSS+ 2015 conference.
The paper compares two interference mitigation techniques at the digital signal processing level for jamming signal removal.
The authors compare the traditional adaptive notch filtering scheme, widely discussed in scientific literature, with a new technique based on the use of the WPD. Both techniques are implemented in software, and their performance has been assessed via the use of a fully software GNSS receiver. Both techniques are first applied to a set of simulated GNSS jammed scenarios.
Preliminary results demonstrate that a significant improvement is achieved at both acquisition and tracking level when the WPD algorithm is employed with respect to the application of the classical adaptive notch filtering. In fact, using the adaptive notch filtering, the effective range of the jamming can be reduced from approximately 474 meters up to 127 meters, while when using the WPD-based algorithm, such a range can be further reduced up to approximately 10 meters. These results are also confirmed by successive test campaigns where performance comparison of both software implemented techniques is assessed considering simulated GNSS data.
The WPD-based technique is characterized by a higher computational complexity with respect to the implementation of notch filtering. This is mainly caused by the several filtering operations needed for the time-scale representation computation. Therefore, the number of decomposition stages and the filter length need to be carefully traded off with the jamming detection and removal capability of such a technique.
However, the availability of a high-performing processor together with a jamming detection based on spectral estimation can potentially lead to a faster WPD computation for future real-time applications.
HarvestMaster, provider of agricultural data collection solutions, has introduced a new field applicator that applies various treatments to specific field plots. The field applicator is easily controlled from within HarvestMaster’s Mirus field data-collection software using a software plugin.
Using the Mirus field applicator plugin, users can select which treatments to apply to individual plots from within the Mirus dashboard, and can choose to control the field applicator either manually or automatically, based on GPS location. The system significantly reduces error in treatment applications and improves productivity by streamlining the application process in the field, according to Juniper Systems, parent company of HarvestMaster.
Compatible with a wide range of research spray systems, the Mirus field applicator plugin eliminates the usual bulky and often confusing toggle switch boxes that are typically used to control field applicators. The user imports a file that specifies which formulation is to be applied to each plot, then selects whether to control the applicator either manually or through GPS positioning data, and the system is ready to go.
Windows 10 and a new large display are key features of Juniper Systems’ latest tablet, Mesa 2 Rugged Tablet, released today.
Juniper Systems is a provider of ultra-rugged field data collection solutions.
Featuring the largest display produced by Juniper Systems to date, the Mesa 2 is also Juniper Systems’ first handheld to run on the new Windows 10, which the company said allows for improved decision-making in the field, as well as smooth transitioning from field data collection to office work and back.
With a full Windows 10 operating system, the Mesa 2 provides users with access to a broader range of software options to meet their data collection needs and is powerful enough to use in place of a desktop computer when in the office. The Mesa 2’s 7-inch display strikes a perfect balance between providing ample viewing area for collected data and reducing overall weight for minimal fatigue and superior, all-day comfort, the company said.
The Mesa 2 is designed to perform reliably in harsh environments, and is the only IP68-rated rugged Windows tablet available, providing complete protection against water and dust. It maintains a seal while its ports are in use, while most other tablets on the market are exposed to damage from water and dust if the port cover is not securely in place.
The Mesa 2 also features an extraordinary IllumiView display, providing best-in-class visibility in any lighting conditions, and its chemically-strengthened Dragontrail glass touch screen provides superior durability, reducing haze from surface scratches and cracks normally caused by accidental impact.
The Mesa 2 battery provides users with a full 8-10 hours of runtime, allowing for maximum productivity throughout the workday. Users may also purchase an optional expansion battery from Juniper Systems that provides an additional 4-5 hours of runtime plus hot swap capabilities for those extra-long days where overtime is required.
“The Mesa 2 is in a new sphere relative to our other ultra-rugged devices,” said Nate Holman, Director of Sales and Marketing at Juniper Systems. “While it features the same degree of outstanding quality and ruggedness as other Juniper products, the Mesa 2 provides users with more software options and greater processing capabilities, due to its full Windows 10 operating system and Intel quad-core processor. The Mesa 2 is designed to improve productivity along every point of the data collection process, from the initial planning and gathering of data, to the later data analysis, and finally through the decision-making process. It’s a tablet optimized for efficiency, designed to be ‘your office, anywhere’.”
The Mesa 2 Rugged Tablet will begin shipping in the first quarter of 2016.
Real-time single-frequency precise point positioning for cars and trains
By Peter de Bakker and Christian Tiberius
INNOVATION INSIGHTS with Richard Langley
“IT’S GETTING BETTER ALL THE TIME.”This refrain from the Beatle’s song could well describe precise point positioning or PPP. PPP is a positioning technique that relies on GNSS carrier-phase measurements (in addition to code or pseudorange measurements) from a user’s receiver along with satellite orbit and clock data much more precise (and accurate) than that included in broadcast satellite navigation messages to achieve accuracies down to the centimeter level. It also requires a more sophisticated model of the measurements compared to that used in most consumer GNSS equipment and even some professional devices, including accounting for residual tropospheric propagation delay, carrier-phase windup, and even solid Earth tides.
PPP has been around for more than a decade and ongoing research has gradually improved its capabilities. Until recently, it has been used primarily with dual-frequency GPS observables. However, the technique is not restricted to GPS. It works equally well with observables from other constellations including GLONASS, Galileo and BeiDou. As long as precise orbit and clock products are available (typically from the International GNSS Service or its participating analysis centers), then PPP positioning solutions are possible. And, single-frequency PPP is also possible. The primary advantage of dual-frequency PPP is that the ionospheric propagation delay is almost completely removed by linearly combining the measurements on the two frequencies, taking advantage of the dispersive nature of signal propagation through the ionosphere. But, if good predictions of the ionospheric delay at, say, the L1 GPS frequency are available, then it is possible to do single-frequency PPP. While not as accurate as dual-frequency PPP, the technique is considerably more accurate than typical pseudorange point positioning (the so-called Standard Positioning Service).
PPP is also traditionally a post-processing technique. That is, data is collected but it is not processed until some later convenient time when the necessary precise products are available. Such an approach is useful for many applications but clearly not for navigation, which requires real-time positioning. But in the past few years, a number of commercial and non-commercial entities have started streaming real-time satellite orbit and clock corrections over the Internet and various radio links, making real-time PPP a reality.
In this month’s Innovation column, we bring together, perhaps for the first time, single-frequency and real-time PPP. Our authors describe a series of experiments they have conducted on roadways and a railway achieving sub-meter horizontal positioning at a 95 percent confidence interval. Such accuracies may already be sufficient for freeway lane and railway track guidance. But we might expect even better accuracies in the future. After all, PPP is getting better all the time.
The single-frequency precise point positioning (SF-PPP) method, developed at Delft University of Technology, was previously demonstrated to provide lane-level position accuracy on a freeway in post-processing mode. Important applications of SF-PPP are lane-level traffic state estimation and lane-level specific driver advice for next-generation car navigation. For a functional system, as well as for advanced experiments in this field, the computed positions have to be available in real time. Therefore, a new real-time implementation of the SF-PPP method was developed as part of the Dutch Dynamic Lane Guidance project. In this article, we outline aspects of the real-time implementation, and we present experimental results from this new implementation collected on a busy freeway in the Netherlands and in a parking lot, as well as results from a railway experiment.
In these experiments, a test vehicle was equipped with a low-end, automotive-type single-frequency receiver with a patch antenna to collect raw GPS observations. A 3G mobile communications link was used to obtain data-correction streams over the Internet using the Ntrip protocol. The SF-PPP processing was performed on a laptop computer onboard the vehicle, in real time. Various forms of ground-truth positions were used to assess the real-time SF-PPP positioning accuracy. For some of our tests, the vehicle was also equipped with high-end GPS antennas and receivers to provide ground truth. The position solutions obtained with the SF-PPP algorithm have been compared to (post-processed) network-RTK solutions using the Netherlands Positioning Service (NETPOS). Additional validation was performed by means of a 5-centimeter-accuracy road-infrastructure map from Rijkswaterstaat, the Dutch Ministry of Infrastructure and the Environment, and by a centimeter-level a priori ground survey.
The new real-time SF-PPP software was tested successfully with performance comparable to our previous post-processing software, and meeting the required accuracy for freeway lane identification. Statistics on the performance are provided, as well as their dependence on a number of external parameters including the number of available satellites.
Precise corrections from both the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt or DLR) and the International GNSS Service (IGS) were used. Delays in the correction streams vary between providers and can increase further in the event of a time-out of the mobile link. The influence of these delays is considered, and an optimal approach for dealing with outages is discussed.
PPP Model and Corrections
The GNSS positioning model is non-linear. The observations are non-linear functions of the unknown parameters plus noise.
To solve for the unknown parameters (including the receiver position coordinates), through least squares estimation, the model must be linearized around an approximate solution.
In our SF-PPP model, the primary observations are, from each satellite, the pseudorange measurement and the carrier-phase measurement. The unknown parameters are the receiver position vector and the receiver clock offset, both of which are involved in the linearization, and also the ambiguity, associated with the carrier-phase measurement, for which the model is already linear.
In the context of PPP, it is important to note that in addition to the linearization around the initial approximate values, the computed observations contain a number of a priori model values for parameters which are not estimated, including:
The precise satellite position and clock offset (including the relativistic effect): The GPS satellite positions and clock offsets are computed from the broadcast products (navigation message) and corrected with real-time data streams via Ntrip. The correction streams of DLR and IGS were used at different times as detailed in Table 1. In post-processing older files, the satellite orbits and clocks are taken from sp3 files, but to keep the processing as close as possible to the real-time functionality, these are first converted to corrections to the broadcast products.
The (neutral) troposphere delay: The troposphere delay is modeled with the a priori Saastamoinen model using the Ifadis mapping function and parameters from the 1976 U.S. Standard Atmosphere.
The ionosphere delay and satellite differential code bias: The ionosphere delay is computed a priori using the one-day predicted Global Ionosphere Maps (GIMs) from the Center for Orbit Determination in Europe (CODE), together with the corresponding differential code biases.
The carrier-phase observations are corrected for the phase wind-up at the receiver and satellite. The user orientation is estimated from the vehicle velocity vector.
TABLE 1. Four SF-PPP field tests.
Besides the primary observations, the ambiguity estimate from the previous epoch can be added to the current epoch as an additional observation per satellite, because it is assumed to be constant in the absence of a cycle slip.
Observations from different epochs are assumed to be uncorrelated, and consequently the ambiguity estimates from previous epochs are uncorrelated to the current observations. Observations to different satellites are also assumed to be uncorrelated.
The carrier-phase ambiguities are the only parameters propagated from a previous epoch to the current epoch. The receiver position coordinates (and receiver clock offset) are estimated each epoch anew — no vehicle dynamics model is involved.
The computed positions are finally corrected for solid Earth tides with an efficient numerical model. Computed positions result in the International Terrestrial Reference Frame (ITRF) 2008 at the epoch of the observations.
In parallel with the positioning filter, statistical hypothesis testing is used to detect errors in the observations or propagated ambiguities (such as those caused by excessive multipath or a cycle slip), based on the detection, identification and adaptation (DIA) procedure. First, an overall model test is run at each epoch to test the validity of the model and observations. If the test is rejected, data snooping is applied to determine which observation is most likely to have caused the problem. If one of the pseudorange measurements is identified, it is removed from the model. If either a carrier-phase measurement or ambiguity is identified, the ambiguity for that satellite is reset; that is, the propagated ambiguity is removed.
Experiments
Four field tests that we have carried out are considered here.
In October 2012, more than 100 laps were driven over a 5-kilometer stretch of the A13 freeway between Delft and Rotterdam. The data collected were reprocessed to validate the new real-time software implementation (but obviously carried out in post-processing mode).
The first real-time tests were performed in December 2014 and later in May 2015 on the same stretch of the A13 freeway.
In May 2015, a third dataset was collected on a recently constructed and nicely outlined parking lot in Delft.
In July 2015, a train carriage was equipped with a GPS receiver and data were collected on a train trip from the center of The Netherlands to the far southern part — a distance of more than 200 kilometers.
Details of the four field tests are collected in Table 1.
Ground Truth
In our earlier experiments, the ground truth for the vehicle positions was computed with measurements from high-end equipment onboard the same vehicle. Both the antenna of the SF-PPP receiver and the high-end antennas were rigidly connected to a wooden beam on the roof rack of the van (positions of the two high-end antennas at both ends of the beam were obtained through network RTK GPS). As our results from this experiment show, the performance, and especially the precision, is very good, but a moderate bias of 17 centimeters in the cross-track direction was observed (see FIGURE 1 and TABLE 2). The suspect cause of this bias was the antenna location, close to the side of the vehicle and not attached to the metal roof itself.
FIGURE 1. 2D histogram of SF-PPP position errors (with respect to the network RTK GPS solution) in horizontal directions for the 2012 test on the A13 freeway, expressed in local east and north directions (left), and in cross-track and along-track directions (right). The color indicates the number of samples in each bin.TABLE 2. Statistics of the position errors in each direction, for the 100 laps on the A13 freeway.
Therefore, during more recent experiments, the test vehicle was only equipped with a patch antenna for the low-end, automotive-type GPS receiver, and attached directly to the roof of the car, in the middle of the centerline of the vehicle. In this case, the metal roof acts as a ground plane for the antenna, improving the gain and not acting as a source of multipath. However, this setup also has complications for the accuracy assessment. Thus, instead of computing accurate ground truth from the measurements from high-end equipment directly near the test receiver, a number of other ways were used to determine the ground truth.
During the first real-time test on the A13 freeway, a 5-centimeter accurate road infrastructure map from Rijkswaterstaat was used as previously mentioned. This comparison was done both visually and numerically.
For our next experiment, we selected a recently constructed parking lot with a simple, neat rectangular layout. By surveying the corners of the rectangle and using the repetitive pattern, a schematic drawing of the parking lot was made, and used to evaluate the positioning performance in a visual manner. The car was first driven over the lined-up parking spaces in a lengthwise manner, circling round at each end of the parking lot, and changing lanes once each lap at the same point. Then the car was driven along the edges of the rows of parking spaces to and fro over the parking lot.
SF-PPP positions were obtained live in the vehicle while driving. The raw (single-frequency) observations of this experiment were also post-processed with the RTKLib software package using the nearby permanent DLF1 station at the TU Delft GNSS observatory on a very short baseline (less than 1 kilometer). The ambiguity-fixed results could then be used to also numerically assess the SF-PPP positioning performance.
For the test on the train, again the network RTK GPS solution provided the ground truth positions. Two antennas were mounted along the centerline of the carriage at a fixed offset from each other: a patch antenna for the single-frequencyreceiver and a geodetic antenna for the ground truth. With this known offset, and the direction of motion, the ground truth position for the single-frequency receiver was obtained.
The ground-truth positions, either in the European Terrestrial Reference System (ETRS) 89 (from NETPOS or our own survey) or in the local national reference frame Rijksdriehoeksmeting (National Triangulation System) / Normaal Amsterdams Peil (Amsterdam Ordnance Datum) or RD/NAP, have been transformed into ITRF2008, to allow for comparison with the SF-PPP positions.
Computational Performance, Data Rates
The real-time software was used under the 64-bit Windows 8.1 operating system on a moderately fast laptop with i5-4200U CPU running at 1.60 GHz. The software consists of uncompiled Matlab R2014b scripts and functions using timer objects to repeatedly read in new observations, corrections and ephemerides, and to update the position computation. The software can run with data arriving at about 20 Hz in the current state on this platform, but was used with 5-Hz data because of limitations of the receiver to provide raw data and to prevent any overrun. It should be noted that only a few obvious potential computational bottlenecks were targeted; the software was not optimized for efficiency.
The RT SF-PPP implementation relies on a 3G mobile Internet connection for a number of data products. The ionosphere map, which is a predicted product (24 hours ahead), comes as a 200-kilobyte file (and 5 kilobytes for the associated differential code biases), which covers the globe and is valid for 24 hours. The file contains 13 maps at 2-hour intervals, between which interpolation in time is required.
Spatial interpolation is also required for the ionosphere pierce point of each satellite signal, between the grid points in the map (at intervals of 5 degrees in longitude and 2.5 degrees in latitude). The satellite orbit/position corrections (every 60 seconds) and satellite clock corrections (every 10 seconds) are retrieved over the Internet using the Ntrip protocol by means of the Bundesamt für Kartographie und Geodäsie (BKG) Ntrip Client (BNC), which passes these on to Matlab.
The data-rate used by this correction stream is about 1 kilobit per second. The corrections are applied to the broadcast ephemerides (in quasi-Keplerian-element form), which are therefore also required. These satellite ephemerides can be extracted by the GPS receiver itself (from the GPS navigation message), but in our implementation are also collected via Ntrip for convenience only, with a bandwidth consumption of 6 kilobits per second. Note that, much like the software implementation itself, the data stream has not been optimized for any particular bandwidth limitation. For instance, orbit and clock corrections are needed only for those satellites in view, and hence transmitting the data for all satellites of the constellation is not needed.
Results
In this section, we present the results of our tests, followed in the next section with a discussion of important common factors affecting accuracy and continuity of RT SF-PPP.
Road-Test A13 Freeway (100 Laps). Under different conditions, we collected a large amount of data with a van, driving repeatedly the same 5-kilometer stretch of road on the A13 freeway from Rotterdam to Delft. The test amounted to almost a full day of driving.
2D histograms of the results are shown in Figure 1 with corresponding statistics in TABLE 2. Note a small bias in the cross-track direction. The total number of position solutions was 2.0× 105.
Road-Test A13 Freeway (Real Time). The results of the real-time freeway road test are shown in FIGURE 2. The different lanes used by the vehicle are clearly visible in the figure. The number of GPS satellites is indicated by the color bar. Shown is the Delft-Zuid / TU Delft exit of the A13 freeway, roughly a 300 × 300 meter area, taken from the Digitaal Topografisch Bestand (DTB) of Rijkswaterstaat. Note that only the cross-track performance can be assessed in this manner, but fortunately this is exactly the performance aspect that is most interesting for the target application of lane identification. Note also that if the vehicle was not driving exactly in the middle of the lane, which to some extent is unavoidable, this effect cannot be separated from the positioning errors.
FIGURE 2. SF-PPP solution displayed on a 5-centimeter accurate road infrastructure map, on Dec. 18, 2014.
The 95-percent error southbound and northbound is 0.65 meters and 0.58 meters respectively, in the cross-track direction.
Road-Test Parking Lot.FIGURE 3 shows an aerial photograph (left) and schematic drawing (right) of the 3M company parking lot in Delft showing measured positions and driven tracks. The lines in red and yellow represent the measured tracks while driving the same loop over the parking lot again and again (more than 60 times in total), and the purple lines show the track while driving around and following the parking space boundaries with the left front wheel of the test vehicle (4 laps). These lines show both the SF-PPP position error and the driver error. The white parking spaces are each 2 meters wide.
FIGURE 3. Aerial photograph, from Google Earth, (left) and schematic drawing (right) of the parking lot in Delft showing measured positions and driven tracks.
The position errors in local north, east and up directions for part of the first dynamic session, of about 4.5 laps, of the 3M parking lot experiment (lane change 1) are shown in the upper panel of FIGURE 4. We see a clear periodic signal as well as a bias in each direction. The driving direction gives an approximation of the heading (shown in the bottom panel), which confirms that the periodic signal coincides with the driven laps.
FIGURE 4. Position errors (top) in local north, east and up directions and heading (bottom) for part of the first dynamic session, about 4.5 laps, of the 3M parking lot experiment (lane change 1).
The figure shows that the errors in the position solution are on the order of 0.2 meters, and consist of a bias in each of the three directions and a periodic signal with a period equal to the lap-time (confirmed by the driving direction of the vehicle). Since the bias does not depend on the orientation of the vehicle, and given the slow variation over time, the most likely cause is a residual ionosphere error or errors in the satellite products. The repeating pattern, on the other hand, is most probably related to multipath or near-field effects related to the vehicle antenna.
Rail-Test Amersfoort to Simpelveld. The train carriage with the GPS antennas installed was pulled by a 1955-built diesel-electric locomotive. A trip of more than 200 kilometers was made, over the main Intercity Network of Nederlandse Spoorwegen (NS) / ProRail (Dutch Railways). Only the last 20 kilometers were on a local line to a historic railway station.
The overhead power line (about 1 meter above the GPS antennas) and portals seem to have no impact on the SF-PPP positioning performance. An example of the positioning accuracy is shown in FIGURE 5. The figure shows position error scatter for an almost 20-kilometer stretch of nearly straight east-west track through rural and forest areas (Weert to Roermond). The time span of the data is 10 minutes, and the data rate was 5 Hz. SF-PPP positions were compared with NETPOS network RTK GPS solutions. Generally, eight satellites were received and used in the SF-PPP solution. The corresponding error statistics are presented in TABLE 3.
FIGURE 5. Position error scatter for an almost 20-kilometer stretch of nearly straight east-west track through rural and forest areas (Weert to Roermond); 10 minutes of data at 5 Hz.TABLE 3. Statistics of the position errors, over 2994 epochs, in along- and cross-track directions, for the position scatter shown in Figure 5.
A heavy steel-construction bridge along the route at the River Lek near Culemborg, 15 kilometers south of Utrecht, was found to degrade positioning performance considerably. The heavy steel construction of the bridge hampers reception of GPS satellite signals. The positioning performance on the bridge is shown in FIGURE 6. The computed SF-PPP trajectory overlaid on a Google Earth aerial photograph is shown on the left.
FIGURE 6. Positioning performance on the Lek Bridge. Left: measured trajectory overlaid on a Google Earth aerial photograph. The number of satellites available is indicated by the color bar. Right top: SF-PPP positions in local east-north directions. Right bottom: Absolute cross-track offset of position solution with respect to a straight line, as a function of time.
From the positions, one can clearly see the train driving straight on the right-hand track (going south) on the ramp onto the bridge, and on the ramp down from the bridge. However, on the bridge itself, position solutions show considerably larger variations of up to 8 meters. The image shows a 250-meter stretch of the track. Also, the number of satellites available, and used in the position solution, drops considerably (indicated by the color bar) while the train is on the bridge. On the right of the figure at the top, the SF-PPP positions in local east-north coordinates are shown along with a straight line between the first and last epochs, representing the assumed straight track. The plot at bottom right shows the absolute cross-track offset of the position solutions with respect to the straight line, as a function of time, over 250 5-Hz epochs.
Analysis
Two factors significantly affect the performance of our tests: the number of satellites available and the continuity and latency of the corrections.
Number of Satellites.As can be expected, the SF-PPP position accuracy depends to a large extent on the number of satellites used to compute the solution. For the third test, the road-test in the 3M parking lot, the three-dimensional position error (SF-PPP versus RTK GPS) is shown as a boxplot in FIGURE 7 in which various accuracy measures are plotted as a function of the number of satellites for the second and longest dynamic part of the test (lane change 2), consisting of about 12,000 epochs of data. During this session, the available number of satellites varied between 10 and 12. This number was reduced artificially by increasing the elevation mask angle to 15 and to 30 degrees. The red lines show the medians, the boxes show the 25th and 75th percentiles, the dashed lines cover all data points not considered outliers, and outliers are plotted with red plus signs. The graph shows a clear improvement going from six to seven or more satellites.
FIGURE 7. Boxplot of 3D position error vs. the number of satellites for the second and longest dynamic part of the 3M parking lot test (lane change 2).
PPP Correction-Stream Outages.To determine the optimal approach to an interruption in the correction data stream, we studied the variation of the corrections over time. Suppose we lose reception of the correction stream at epoch 0, and we keep using the last-received corrections (simply hold onto them). Then the change in values can be interpreted as the additional error introduced in the positioning algorithm by the outage on the mobile link. The effect is not catastrophic. Only after about 200 seconds do the additional satellite clock errors grow to the decimeter level. The position errors remain even smaller.
However, one might wonder whether this can be improved further by performing a linear extrapolation of the corrections, for example, using a number of previous epochs. We looked at what would happen in this case if 5 minutes of previous data are used. For the clock errors, there is no real benefit — the errors only grow larger. But the position errors do remain smaller during the first 5 minutes of extrapolation. After that time, the errors are larger than those without the linear extrapolation (just holding onto the last corrections). The effect of increasing the order of the polynomial extrapolation was also considered. The polynomials of different order outperform each other at different extrapolation times, and also the number of previous epochs used for the polynomial estimation impacts this. Further optimization to reduce the satellite position errors might well be possible, but may be of marginal value, since, the extrapolated clock error is dominant and polynomial extrapolation does not improve this. Simply using the most recent corrections is thus a straightforward and acceptable approach.
Conclusions
In this article, we outlined a real-time implementation of single-frequency GPS precise point positioning. With a fairly low-cost GPS receiver and reception of a modest correction data stream, it is possible to achieve sub-meter horizontal positioning accuracy, in real-time, live in the vehicle (95-percent error of better than 1 meter). Actual results were shown from four field tests: two tests using a vehicle on a freeway, a vehicle test in a parking lot, and one test on a train.
The number of satellites used in the position solution has a big effect on the positioning performance; seven or more satellites yields a good position accuracy. And up to 5 minutes outage of the satellite position and clock corrections does not seem to pose a serious threat to SF-PPP positioning performance.
Acknowledgments
The Dynamic Lane Guidance project under which the first road test was carried out was funded by the Ministry of Infrastructure and Environment, the Province of Noord-Brabant and the Eindhoven Regional Government in the context of Brabant in-car III. This project was carried out in close cooperation with colleagues in the Transport and Planning Department at TU Delft.
We acknowledge the provision of the Real-Time Clock Estimation (RETICLE) satellite clock products by André Hauschild at DLR for several of our field tests. We are also grateful for the use of the IGS Real-Time Service. Also, we acknowledge the provision of the NETPOS network RTK GPS service as ground truth by Lennard Huisman of Kadaster, the Dutch Land Registry and Mapping Agency. Colleague Hans van der Marel analyzed the NETPOS RTK-GPS solution of the train test. Colleagues of the TU Delft Railway Engineering Department offered the opportunity to carry out the test on the train trip from Amersfoort to Simpelveld.
Manufacturers
The vehicle receivers used for the tests were u-blox AG TIM LP and 7P modules in evaluation kits fed by a Tri-M Technologies Inc. Big Brother SM-66 or Taoglas Dominator AA.161 antenna. A Trimble Navigation R7 receiver with a Zephyr Geodetic antenna was used to establish ground truth for some tests.
PETER DE BAKKER is a researcher in the Faculty of Civil Engineering and Geosciences at Delft University of Technology (TU Delft). He recently finished his Ph.D. dissertation on user algorithms for GNSS precise point positioning, and is working on localization for automotive applications, including autonomous vehicles.
CHRISTIAN TIBERIUS is an associate professor in the Faculty of Civil Engineering and Geosciences at TU Delft. He has been involved in GNSS positioning and navigation research since 1991, currently with an emphasis on data quality control, satellite-based augmentation and precise point positioning.
Further Reading
• Earlier Work on Single-Frequency Precise Point Positioning
“Lane Identification with Real Time Single Frequency Precise Point Positioning: A Kinematic Trial” by R.J.P. Van Bree, P.J. Buist, C.C.J.M. Tiberius, B. van Arem and V.L. Knoop in Proceedings of ION GNSS 2011, the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation Portland, Ore., Sept. 19–23, 2011, pp. 314–323.
“Real Time Satellite Clocks in Single Frequency Precise Point Positioning” by R.J.P. Van Bree, C.C.J.M. Tiberius and A. Hauschild in Proceedings of ION GNSS 2009, the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation, Savannah, Ga., Sept. 22–25, 2009, pp. 2400–2414.
“Single-frequency Precise Point Positioning with Optimal Filtering” by A.Q. Le and C. C. J. M. Tiberius in GPS Solutions, Vol. 11, No. 1, 2007, pp. 61–69, doi: 10.1007/s10291-006-0033-9.
• Single- vs. Dual-Frequency Precise Point Positioning
Improved Convergence for GNSS Precise Point Positioning by S. Banville, Ph.D. dissertation, Department of Geodesy and Geomatics Engineering, Technical Report No. 294, University of New Brunswick, Fredericton, New Brunswick, Canada. Recipient of The Institute of Navigation 2014 Bradford W. Parkinson Award.
“Variations of Box Plots” by R. McGill, J.W. Tukey and W.A. Larsen in The American Statistician, Vol. 32, No. 1, Feb. 1978, pp. 12–16, doi: 10.2307/2683468.
The Defense Advanced Research Projects Agency (DARPA) is holding a Proposers Day on Feb. 1 to inform potential contractors about the Atomic Clock with Enhanced Stability (ACES) program.
ACES is a potential $50 million program that seeks to develop battery-powered atomic clocks that work to provide warfighters with synchronization and precision timing capabilities during navigation, communications, electronic warfare and reconnaissance missions in the event of a GPS shutdown.
The registration deadline for the Proposers Day is 5 p.m. EST on Jan. 25. The Proposers Day will be held Feb. 1 from 9:30 a.m. to 5 p.m. EST at the DARPA Conference Center, 675 N. Randolph Street, Arlington, Virginia 22203.
The host is Robert Lutwak, ACES program manager at DARPA. In 2012, GPS World awarded Lutwak its Leadership Award for Products.
The meeting will provide information and promote additional discussion on the ACES program, address questions from potential proposers, and provide an opportunity for potential proposers to share their capabilities and ideas for teaming arrangements.
The ACES Proposers Day will include overview presentations by government personnel, technical presentations by potential proposers and collaborators, and an open poster session to facilitate interaction and teaming.
According to the Department of Defense (DoD), “Precision timing and synchronization is essential to DoD communications, navigation, reconnaissance, and electronic warfare systems. The requirements for timing precision and stability have grown increasingly demanding as DoD systems have evolved towards distributed engagement and surveillance architectures, and this trend is expected to continue for the foreseeable future.
“The ACES program aims to develop portable, battery‐powered atomic clocks with stability, repeatability, and environmental sensitivity approaching that of laboratory‐grade cesium beam frequency standards. This will be accomplished through research, development and integration of reduced SWaP components and technologies for advanced atomic physics interrogation techniques. These include, but are not limited to, laser‐cooled and magneto‐optically trapped atomic samples, and RF‐trapped ion samples, as well as interrogation of less environmentally‐sensitive microwave and optical transitions.”
Northrop Grumman Corporation has been awarded an order to support embedded GPS/inertial navigation system (INS) pre-Phase 1 modernization efforts.
The Military GPS User Equipment (MGUE) program is developing M-code-capable GPS receivers, which are mandated by Congress after fiscal year 2017 and will help to ensure the secure transmission of accurate military signals.
Under the cost-plus-fixed-fee order valued at $4.8 million from the Joint Service Systems Management Office, Northrop Grumman will evaluate new GPS receivers’ modes of performance, including M-code and Selective Availability Anti-spoofing Module (SAASM).
Additionally, the company will perform trade studies, assess the state of development of MGUE for upcoming applications, and contribute to architecture development for next-generation GPS/inertial navigation systems.
“We are honored to help shape the next generation of navigation systems that will modernize the GPS infrastructure and keep our warfighters safer,” said Bob Mehltretter, vice president, navigation and positioning systems business unit, Northrop Grumman Mission Systems. “We are committed to using our navigation systems expertise to develop a solution that offers dependable and accurate positioning, navigation and timing information.”
The updated GPS/inertial navigation system will also comply with the Federal Aviation Administration’s NextGen air traffic control requirements that aircraft flying at higher altitudes be equipped with Automatic Dependence Surveillance-Broadcast (ADS-B) Out by January 2020.
ADS-B Out transmits information about an aircraft’s altitude, speed and location to ground stations and to other equipped aircraft in the vicinity.
The modernized system is expected to be available for platform integration starting in 2018.
Pre-GPS techniques actually responsible for the Greenwich shift
By Stephen Malys, John H. Seago , Nikolaos K. Pavlis, P. Kenneth Seidelmann and George H. Kaplan
The historical prime meridian runs through a telescope established in 1851 by Sir George Airy at the Royal Observatory at Greenwich, England. It was adopted as an international standard as the prime meridian for zero longitude in 1884 during the International Meridian Conference held in Washington, D.C.
The observatory’s line in the pavement is a major tourist attraction, but the prime meridian used by satellite navigation systems is located 102 meters east of that historic location (see Figure 1).
Some people mistakenly thought that GPS, or the earlier Navy Navigation Satellite System (Transit), was responsible for this offset. But research, recently published in the Journal of Geodesy, concludes that the zero longitude used by GPS arrived at its current location in 1984, before GPS existed as an operational system.
The orientation of the World Geodetic System 1984 (WGS 84), and therefore the direction of the prime meridian associated with WGS 84, was established when the U.S. Defense Mapping Agency (now part of the National Geospatial-Intelligence Agency) adopted the orientation of an international scientific standard known as BTS 84, the BIH Terrestrial System 1984.
The Bureau International de l’Heure (BIH) was a predecessor to the International Earth Rotation and Reference Systems Service (IERS). Unlike previous terrestrial systems of reference used by the BIH, the BTS 84 was not established by tracking stars with optical telescopes.
BTS 84. BTS 84 was created using geodetic techniques known as satellite laser ranging, lunar laser ranging and very long baseline interferometry. While satellite Doppler tracking (Transit) data was included in the BIH process, this data type did not contribute to or constrain the orientation of BTS 84.
When an optical instrument is leveled, even to the highest accuracy achievable, its orientation is controlled by the gravity field’s vertical direction at that particular location.
The astronomical zenith, or “up” direction realized by a telescope, is perpendicular to the gravity equipotential (level) surface locally, and therefore is in general deflected from the geodetic zenith that is perpendicular to our best-fitting global ellipsoid model of the Earth due to small irregularities of the gravity field (hence the term deflection of the vertical, DoV).
As a consequence, the astronomical meridian plane at an arbitrary location on the Earth does not necessarily contain the center of mass of the Earth (FIGURE 2).
FIGURE 2. Geometry showing why the Greenwich meridian moved.
Astronomical Time. When optical systems were finally retired from Earth-orientation service by the BIH in 1984, the BIH continued the measurement series for Earth’s rotation by modern geodetic techniques, but required continuity in the determinations. These measurements included those for astronomical time, UT1.
Requiring continuity in UT1 was equivalent to requiring that the plane of the prime meridian keep its orientation, relative to the celestial sphere, as a function of time. But now — for the first time — there was also a requirement to pass this plane through the center of mass of the Earth.That requirement, along with the DoV at Greenwich, moved the trace of the prime meridian on the Earth’s surface in the vicinity of Greenwich by 102 meters to the east of Airy’s telescope (Figure 2).
Journey to the Center of the Earth.Thanks to satellite-tracking techniques, we now know the location of the center of mass of the Earth with an accuracy of about 1 centimeter in three dimensions — about the size of a U.S. dime.
In 1984, its location was known with an accuracy of about 1 meter. When Airy set up his special telescope (called a transit circle), knowledge of the size, shape and center of mass of the Earth was limited to several hundred meters.
The trace of the historical astronomical prime meridian established by Airy’s instrument, and the location of zero longitude indicated by GPS receivers, are both consistent with their own conventions, and their offset from each other does not imply an error in either determination.
Stephen Malys and Nikolaos K. Pavlis work for the National Geospatial-Intelligence Agency.
John H. Seago is with Analytical Graphics, Inc.
P. Kenneth Seidelmann is a member of the Astronomy Department, University of Virginia.
George H. Kaplan is a contractor with the United States Naval Observatory.
Topcon Positioning Group has acquired a significant share of holdings a company that assists customers in virtual design and construction (VDC).
Viasys VDC — based in Espoo, Finland — has developed a suite of tools and services to assist customers in building virtual models for infrastructure and site-work projects. Using building information modeling (BIM) technologies, its solutions create VDC models that optimize the construction process throughout the project’s lifecycle, creating enhanced quality, higher efficiencies and reduced costs, Topcon said.
“Viasys VDC solutions allow for the import of virtually any BIM or non-BIM design model, offering seamless interoperability with open design standards currently in the market — which provides the contractor or engineer with full control and visibility of the entire design throughout the entire project,” said Heikki Halttula, CEO and president, Viasys VDC Ltd. “With advanced simulation tools and communication functions, design-build issues can be detected before actual work starts, or at any time during the process.”
Accurate 5D simulation allows for optimal planning and execution, Topcon said in a news release. Other significant features include cloud-based collaboration functions as well as mobile access to models and information on-site.
Topcon currently offers various BIM and remote site management/visibility solutions aimed at many of the markets served by Viasys VDC.
“Now, with our investment in Viasys VDC, we have partnered with the technology leader to allow us to offer an expanded platform for the future generation of advanced Topcon VDC solutions with seamless BIM interoperability for our partners and customers,” said Ewout Korpershoek, Topcon executive vice president for mergers and acquisitions.
“Partnering with Topcon is an exciting step forward to help advance our industry-leading VDC solutions, while also expanding their reach to a global audience,” Halttula said. “With Viasys VDC offices in Finland, California and Vietnam, we are also well positioned geographically to work directly with existing Topcon operations in Europe, North America and Asia.”
In addition to a full suite of BIM-based mobile workforce solutions, Viasys VDC offers an operational asset management solution as a basis for lifetime maintenance of the VDC managed projects.
KVH’s new GEO-FOG 3D inertial navigation system (INS) continuously provides extremely accurate measurements that keep applications operating in challenging conditions. (Image: KVH)
KVH Industries has introduced the GEO-FOG 3D inertial navigation system (INS). The new product offers roll, pitch and heading accuracies of .05 degrees for demanding applications in unmanned, autonomous and manned aerial, ground, marine and subsurface platforms, such as subsea remotely operated vehicles or mining systems.
The GEO-FOG 3D is based on the company’s high-performance fiber optic gyro (FOG) technology combined with centimeter-level precision RTK GNSS receivers and advanced sensor fusion algorithms. The result is a solution that continuously provides fast, ultra-accurate position, velocity and attitude measurements that keep applications operating no matter how challenging the conditions, according to KVH Industries.
The core inertial sensor for the new system is KVH’s 1750 IMU, an inertial measurement unit incorporating three axes of KVH’s DSP-1750 FOG — a high-performance fiber optic gyro — with three axes of advanced accelerometer technology. The 1750 IMU is then fully integrated with a GNSS receiver and a three-axis magnetometer, a barometric pressure sensor and a triple frequency RTK GNSS receiver to deliver reliable, real-time, centimeter-level positioning and orientation measurements.
The system’s sensor fusion algorithms automatically switch from loosely to tightly coupled filtering for improved performance under poor GNSS signal conditions. The system also offers high-speed update rates and rapid north-seeking gyrocompass capabilities for high-accuracy heading in environments when magnetometers and GNSS-aided heading cannot be used.
The GEO-FOG 3D Dual inertial navigation system (INS) is designed for applications that require heading at system startup or in low dynamic conditions. (Image: KVH)
KVH has also introduced a variant, the GEO-FOG 3D Dual, an INS and attitude and heading reference system (AHRS). This product features two GNSS antennas on a fixed RTK baseline that offers the same reliability and performance levels as the GEO-FOG 3D, with increased heading, pitch, and roll accuracy for static and dynamic applications where single antenna systems can be problematic. The GEO-FOG 3D Dual is a superior choice for applications that require heading at system startup or in low dynamic conditions.
“KVH’s GEO-FOG 3D and GEO-FOG 3D Dual provide exceptional accuracy and outstanding performance in a single, small package (less than 1.6 pounds), at price points never previously achieved in the industry,” said Jay Napoli, KVH’s FOG/OEM vice president. “And, because KVH controls the entire design and production process, from creating its own optical fiber to packaging its FOGs together with other sensors for advanced applications, these new products — and all of our open-loop FOGs, IMUs and INSs — offer outstanding accuracy and excellent durability at a lower cost than competing systems.”
Reliable, high-accuracy navigation and control are essential to unmanned, autonomous and manned platforms that must operate in conditions that include magnetic interference and the absence of reliable satellite navigation data. The integrated FOG, GNSS and sensor fusion technologies allow the GEO-FOG 3D and GEO-FOG 3D Dual to achieve performance levels that are beyond typical INS- or MEMS-based solutions.
Both the GEO-FOG 3D and GEO-FOG 3D Dual are designed to support current and future satellite navigation systems including GPS, GLONASS, Galileo and BeiDou. Both systems offer data rates up to 1000 Hz, and the ability to output data over a high-speed RS-422 interface or RS-232 interface, which ensures the systems can be easily and readily integrated in a wide range of platforms.
The GPS Directorate at the Space and Missile Systems Center (SMC) continues to look for someone to build 22 GPS III satellites in the near future.
SMC issued a request for proposals on Jan. 8, with rather complicated terms. The first eight GPS III satellites are already under contract, and two have been built, but delivery and launch schedules have dragged. The Air Force incorporated several other payload requirements for the satellites, beyond those of new GPS signals themselves.
This is the Air Force’s third effort to find a builder.
The RFP is for “11+ Phase 1 Production Readiness Feasibility Assessment. “ It covers GPS III space vehicles 11 and beyond. The process, if followed as the Air Force envisions, will award up to three relatively small fixed-price contracts.
Artist’s concept of a GPS III satellite in orbit, courtesy of Lockheed Martin.
According to an Air Force press release, “The scope of this effort includes the current GPS III SV01–08 technical baseline with the addition of redesigned Nuclear Detonation Detection System (NDS), Search and Rescue/GPS (SAR/GPS), and Laser Retroreflector Array (LRA) payloads, Unified S-Band (USB) compliance, Regional Military Protection capability No changes are allowed to the GPS Next Generation Operational Control System (OCX) or Military GPS User Equipment (MGUE) interfaces.”
The first Air Force effort to recompete the contract for future GPS III satellites came in 2014. A 2015 initiative lowered the bar as far as requirements, but also lowered the award very dramatically, from $200 million each for two companies to $6 million each for three companies.
The 2016 announcement appears on the surface to replicate the terms of the 2015 campaign. There has been no official explanation as to why the terms changed between 2014 and 2015, and why they did not between 2015 and 2016.
Microsemi Corporation, a provider of semiconductor solutions differentiated by power, security, reliability and performance, today announced its SyncServer S6xx series of Network Time Protocol (NTP) servers.
The new SyncServers provide a highly secure, accurate and flexible timing and frequency platform for synchronizing network elements and mission-critical electronics systems in enterprise information technology (IT) applications such as Internet protocol telephony and physical security, and government instrumentation applications such as satellite communications and defense operational infrastructure.
“Microsemi’s new SyncServer series is a rock-solid enterprise level time server, interoperating easily with our Domain Time II software,” said Jeffry Dwight, president of Greyware Automation Products, the leading provider of time synchronization, management, and auditing software for Windows. “The new SyncServer raises the bar for accurate time synchronization with hardware-based time stamp support, which we found significantly reduced jitter and latency in time served, without losing accuracy. Installation was also much more flexible than any other GPS/GNSS unit we’ve tested. Anyone needing dependable high-quality NTP timestamps should consider Microsemi’s new SyncServer series.”
The new series features SyncServer S600, a security-hardened NTP time server with Microsemi’s NTP Reflector technology for robust security, accuracy and reliability of network time services, and the SyncServer S650, a highly versatile timing and frequency system with the company’s FlexPort technology for multiport, user definable output signal configuration.
The SyncServer S600 is designed for enterprise IT customers managing corporate networks in industries such as financial services and healthcare, while the SyncServer S650 is designed for electronics system engineers synchronizing mission-critical, system-level instruments.
“Robust security, system agility and flexibility of time services are essential for modern IT networks,” said Sri Purisai, vice president of timing and synchronization business, at Microsemi. “Our innovative SyncServer S6xx series timing platform makes significant advances in the security hardening of timing ports, as well as adaptability to various network topologies and flexibility of timing output configuration. This next-generation offering from Microsemi provides our customers a simple migration path to meet future requirements for faster, more agile and scalable network operations.”
According to the 2014 U.S. State of Cybercrime Survey, organizations use a gamut of security technologies to protect network operations. Time plays a vital role in determining the critical “when” of several key security technologies. Survey respondents cited intrusion detection (62 percent), log monitoring to identify intrusion attempts (49 percent) and security event analysis (40 percent) as technologies used for network protection. Without accurate time synchronization to UTC across the network, the effectiveness of these tools in securing the network becomes marginal.
SyncServer S600
Microsemi’s SyncServer S600 is a network time server with security-hardened NTP Reflector technology, supporting extremely high-capacity and ultra-accurate NTP server operations in a multiport, dedicated network time appliance. Easily integrated into existing, future and cloud network topologies, including software-defined networking (SDN), the SyncServer is designed for IT network administrators and architects who are heavily reliant upon server log files for network management.
SyncServer S600 comes with four 1 Gigabit Ethernet (GbE) local area network (LAN) ports, each port equipped with hardware time stamping, multiplying the network configuration possibilities. All ports are equipped with high-resolution hardware time stamping, and the S600 is NTP and precision time protocol (PTP) ready in a multiport PTP configuration.
A simple software update and license purchase/installation will be available in a future software release. Other benefits include interoperability, ease-of-use, extensive security choices and a modern web interface, Microsemi said.
Additional features:
NTP hardware time stamping standard, with nanosecond accuracy
NTP reflector technology for improved security, NTP throughput and accuracy
Comprehensive suite of security protocols
SyncServer S650
As a superset of Microsemi’s SyncServer S600, the SyncServer S650 provides all the features of the SyncServer S600, as well as additional offerings. Leveraging the company’s FlexPort timing technology, it delivers flexibility in precise time and stable frequency synchronization in a price competitive commercial off-the-shelf (COTS) solution.
FlexPort timing technology efficiently and cost-effectively adds innovative “any signal, any connector” technology through software configuration, eliminating the wasted space inherent with legacy-style fixed-signal modules with fixed-signal types.
Specially designed for system and instrumentation engineers in the electrical, system, metrology, communications and defense markets looking to easily output a variety of accurate and stable time and frequency signal types in a cost-effective manner, the device provides network-based timing features with software upgrades to completely security-harden the system.
The GPS referenced SyncServer S650 is built for modern electronic systems and networks that require synchronization performance adaptable to a wide range of applications. Microsemi’s FlexPort configurations eliminate the need for distribution chassis, saving time and costs, in addition to providing an easy-to-use system, Microsemi said. Other benefits include high accuracy and signal quality, as well as environmental design robustness.
In addition to the features of the SyncServer S600, the SyncServer S650 has:
Clock accuracy typically better than 10 nanoseconds to universal time
Standard timing I/O card that meets most popular timing output requirements, eliminating the need to purchase multiple plug-in modules
FlexPort technology option for any signal, any connector flexibility.
The founders of Chronos and UrsaNav have formed a new collaboration, named Taviga, that will focus on preserving and establishing low-frequency (LF) positioning, navigation and timing (PNT) networks in the United Kingdom, Europe and the U.S., using repurposed Loran-C or purpose-built eLoran technology.
Taviga aims to ensure timing and navigation for critical infrastructure, protecting it from cyber and other threats, and addressing the concern that over-dependence on single systems for PNT increases vulnerability.
According to a joint press release, “Taviga combines the founders’ decades of experience specializing in low-frequency (LF) PNT technology and industrial timing applications at national and international levels. Its objective is to provide a commercially operated assured LF PNT service.”
Partner with Government. Charles Curry of Chronos Technology Ltd. in the UK and Charles Schue of UrsaNav Inc. in the U.S. joined forces to launch Taviga Ltd. and Taviga LLC. Taviga anticipates working in partnership with government agencies and other entities that have a vested interest in reducing the vulnerability and improving the resilience of critical national infrastructure with a dependency on the GPS and other GNSS sources of PNT.
“We have been researching the precise timing capability of eLoran transmissions for over 10 years,” Curry said. “During that time, the system has never failed, and most impressively it has continued to deliver sub-microsecond time accuracy traceable to UTC in very challenging locations, including deep inside buildings.
“Our research program was supported by the UK’s Innovation Agency — Innovate UK through two flagship projects, GAARDIAN and SENTINEL,” Curry continued. “These two projects highlighted the vulnerabilities that threaten GPS signals (and in the future, Galileo) such as jamming, interference and spoofing. They also demonstrated how eLoran is a technically dissimilar source of PNT and not vulnerable to the same types of interference. eLoran is a truly complementary source of PNT, ideal for use in critical infrastructure applications such as telecoms, broadcasting, financial services and power utilities.
“Every government, academic and industrial study has resulted in the selection of the LF technology known as Enhanced Loran, or eLoran, as the best wide-area complement to GNSS,” UrsaNav’s Schue added. “There is no doubt that the combination of GNSS and eLoran provides the PNT resilience that most users require.
“Integrated GNSS-eLoran solutions can provide the proof-of-time and proof-of-position necessary to safeguard national infrastructure and for business continuity of operations,” Schue said. “Adding eLoran to the PNT mix enables or enhances capabilities of regional and purpose-built solutions. Overreliance on a single solution is neither prudent nor safe.”
Tests led by Chronos and using UrsaNav’s eLoran receiver engine have consistently demonstrated positioning accuracies of better than 10 m and timing accuracies of less than 100 nanoseconds in the area of differential eLoran reference sites.
Taviga’s goal is the long-term operation of an eLoran system for at least 10 years, a period that provides the necessary service assurance continuity to enable industrial users to invest with confidence in an eLoran-based timing and navigation service that complements their GNSS solutions. As users become accustomed to the capabilities and resilience of a combined GNSS-eLoran solution, Taviga expects to expand the service footprint worldwide.
Timing
Leap second lives on
The International Telecommunication Union (ITU) World Radiocommunication Conference (WRC-15) decided that further studies are required on the impact and application of a future reference time-scale, including the modification of Coordinated Universal Time (UTC) and suppressing the so-called “leap second.”
Leap seconds are added periodically to adjust to irregularities in the Earth’s rotation in relation to UTC, the current reference for measuring time, in order to remain close to mean solar time (UT1). A leap second was added most recently on June 30 at 23:59:60 UTC. The proposal to suppress the leap second would have made continuous reference timescale available for all modern electronic navigation and computerized systems to operate while eliminating the need for specialized ad hoc time systems.
A report will be considered by the World Radiocommunication Conference in 2023. Until then, UTC shall continue to be applied as described in Recommendation ITU‑R TF.460‑6.
Disconnect from the Sun? As a representative of the Lick Observatory of the University of California stated, “a problem is that simply omitting leap seconds would redefine the meaning of the word “day” so that it is not related to the sun in the sky, nor connected with the rotation of the Earth.”
“Modern society is increasingly dependent on accurate timekeeping,” said ITU Secretary-General Houlin Zhao.“ITU is responsible for disseminating time signals by both wired communications and by different radiocommunication services, both space and terrestrial, which are critical for all areas of human activity.”
Further studies will be coordinated by ITU along with several other international organizations.
Inertial/GNSS
Drone rules, registry
The Federal Aviation Administration announced in December an official rule and registration process for hobbyists who own small unmanned aircraft weighing more than 0.55 pounds and less than 55 pounds, including payloads such as on-board cameras.
The online registration system does not yet support registration of small UAS used for any purpose other than hobby or recreation — for example, using an unmanned aircraft in connection with a business. The FAA is developing enhancements that will allow such online registrations by spring of 2016.
The registration procedure is a statutory requirement that applies to all hobby aircraft. Under this rule, any owner of a small UAS who has previously operated an unmanned aircraft exclusively as a model aircraft prior to Dec. 21, 2015, must register no later than Feb. 19, 2016. Owners of any other UAS purchased for use as a model aircraft after Dec. 21, 2015, must register before the first flight outdoors.
Owners may register through a web-based system or paper-based process. There is a $5 registration fee, but the FAA is offering a full rebate for those who register before Jan. 20, 2016.
Under the rule, each aircraft must be marked with a unique number, although not necessarily the serial number. The goal is to help authorities track down an owner if a drone collides with another aircraft, flies too high or encroaches on an airport.
Owners using the model aircraft for hobby or recreation will only have to register once and may use the same identification number for all of their model UAS. The registration is valid for three years.
“We expect hundreds of thousands of model unmanned aircraft will be purchased this holiday season,” FAA Administrator Michael Huerta said. “Registration gives us the opportunity to educate these new airspace users before they fly so they know the airspace rules and understand they are accountable to the public for flying responsibly.”
