Cobham AvComm, formerly the Aeroflex AvComm business unit, has introduced the ATC-5000NG NextGen ATC/DME Test Set.
Designed for engineering development, design validation, manufacturing and return-to-service test applications, the ATC-5000NG is the replacement product for the legacy SDX-2000 and the ATC-1400A/S-1403DL. The software defined radio architecture supports more transponder RTCA DO-181E test capability than the legacy products did and has new capability needed to support the Federal Aviation Administration’s NextGen test requirements including ADS-B (RTCA DO-260B) and UAT (RTCA DO-282).
ADS-B is the Automatic Dependent Surveillance-Broadcast for next-generation (NextGen) aircraft navigation. The FAA has mandated that aircraft operating in airspace that now requires a Mode C transponder must be equipped with ADS-B Out by Jan. 1, 2020.
“We are excited to introduce the new ATC-5000NG which offers our customers the most comprehensive test set available in the market today. This will help our customers prepare for new requirements driven by the FAA’s NextGen and Europe’s SESAR projects,” said Ryan Panos, vice president and general manager of Cobham AvComm.
In September 2014, Cobham completed its acquisition of Aeroflex for $1.46 billion.
The Rohde & Schwarz CMW500 is being used to test the ERA-GLONASS system.
The Certification Center Svyaz-Certificate in Russia is now using the R&S CMW500 to certify ERA-GLONASS systems in line with the TR CU 018/2011 technical guideline. The independent Russian test lab is currently the only test lab in Russia accredited to certify these systems. The R&S CMW500 is a wideband radio communication tester.
Equipped with the R&S CMW-KA095 application software, the R&S CMW500 meets all requirements for testing ERA-GLONASS systems and provides reliable, reproducible tests in line with the Russian GOST R 55530 specification, Rohde & Scwarz said. In addition, the R&S CMWrun sequencer software (R&S CMW-KT110) makes the test solution fully automated and user-friendly.
Effective January 1, 2015, all new car models introduced to the Russian market must be equipped with an automatic ERA-GLONASS emergency call system.
Handling the Biases for Improved Triple-Frequency PPP Convergence
By Denis Laurichesse
Precise point positioning (PPP) can be considered a viable tool in the kitbag of GPS positioning techniques. One precision aspect of PPP is its use of carrier-phase measurements rather than just pseudoranges. But there is a catch. Often many epochs of measurements are needed for a position solution to converge to a sufficiently high accuracy. In this month’s column, we look at how using measurements from three satellite frequencies rather than just two can help.
INNOVATION INSIGHTS by Richard Langley
PPP? WHAT’S THAT? This acronym stands for precise point positioning and, although the technique is still in development, it has evolved to a stage where it can be considered another viable tool in the kitbag of GPS positioning techniques. It is now supported by a number of receiver manufacturers and several free online PPP processing services. You might think, looking at the name, that there’s nothing particularly special about it. After all, doesn’t any kind of positioning with GPS give you a precise point position including that from a handheld receiver or a satnav device? They key word here is precise.
The use of the word precise, in the context of GPS positioning, usually means getting positional information with precision and accuracy better than that afforded by the use of L1 C/A-code pseudorange measurements and the data provided in the broadcast navigation messages from the satellites. A typically small improvement in precision and accuracy can be had by using pseudoranges determined from the L2 frequency in addition to L1. This permits the real-time correction for the perturbing effect of the ionosphere. Such an improvement in positioning is embodied in the distinction between the two official GPS levels of service: the Standard Positioning Service provided through the L1 C/A-code and the Precise Positioning Service provided for “authorized” users, which requires the use of the encrypted P-code on both the L1 and L2 frequencies. Civil GPS users will have access to a similar level of service once a sufficient number of satellites transmitting the L2 Civil (L2C) code are in orbit. However, this capability will only provide meter-level accuracy. The PPP technique can do much better than this.
It can do so thanks to two additional precision aspects of the technique. The first is the use of more precise (and, again, accurate) descriptions of the orbits of the satellites and the behavior of their atomic clocks than those included in the navigation messages. Such data is provided, for example, by the International GNSS Service (IGS) through its global tracking network and analysis centers. These so-called precise products are typically used to process receiver data after collection in a post-processing mode, although real-time correction streams are now being provided by the IGS and some commercial entities.
Now, it’s true that a user can get high precision and accuracy in GPS positioning using the differential technique where data from one or more base or reference stations is combined with data from the user receiver. However, by using precise products and a very thorough model of the GPS observables, the PPP technique does away with the requirement for a directly accessed base station.
The other precision aspect of PPP is its use of carrier-phase measurements rather than just pseudoranges. Carrier-phase measurements have a precision on the order of two magnitudes (a factor of 100) better than that of pseudoranges. But there is a catch to the use of carrier-phase measurements: they are ambiguous by an integer multiple of one cycle. Processing algorithms must resolve the value of this ambiguity and ideally fix it at its correct integer value. Unfortunately, it is difficult to do this instantaneously, and often many epochs of measurements are needed for a position solution to converge to a sufficiently high accuracy, say better than 10 centimeters. Researchers are actively working on reducing the convergence time, and in this month’s column, we look at how using measurements from three satellite frequencies rather than just two can help.
“Innovation” is a regular feature that discusses advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering, University of New Brunswick. He welcomes comments and topic ideas. To contact him, see the “Contributing Editors” section on page 6.
While carrier-phase measurements typically have very low noise compared to pseudorange (code) measurements, they have an inherent integer cycle ambiguity: the carrier phase, interpreted as a range measurement, is ambiguous by any number of cycles. However, integer ambiguity fixing is now routinely applied to undifferenced GPS carrier-phase measurements to achieve precise positioning. Some implementations are even available in real time. This so-called precise point positioning (PPP) technique permits ambiguity resolution at the centimeter level.
With the new modernized satellites’ capabilities, performing PPP with triple-frequency measurements will be possible and, therefore, the current dual-frequency formulation will not be applicable. There is also a need for a generalized formulation of phase biases for Radio Technical Commission for Maritime Services (RTCM) State Space Representation (SSR) needs. In this RTCM framework, the definition of a standard is important to allow interoperability between the two components of a positioning system: the network side and the user side.
Classical Formulation
In this section, we review the formulation of the observation equations. We will use the following constants in the equations:
where f1 and f2 are the two primary frequencies transmitted by all GPS satellites and c is the vacuum speed of light. For the GPS L1 and L2 bands, f1 = 154f0 and f2 = 120f0, where f0 = 10.23 MHz.
The pseudorange (or code) measurements, P1 and P2, are expressed in meters, while phase measurements, L1 and L2, are expressed in cycles. In the following, we use the word “clock” to mean a time offset between a receiver or satellite clock and GPS System Time as determined from either code or phase measurements on different frequencies or some combination of them.
The code and phase measurements are modeled as:
(1)
where:
D1 and D2 are the geometrical propagation distances between the emitter and receiver antenna phase centers at f1 and f2 including troposphere elongation, relativistic effects and so on.
W is the contribution of the wind-up effect (in cycles).
e is the code ionosphere elongation in meters at f1. This elongation varies with the inverse of the square of the carrier frequency and is applied with the opposite sign for phase.
Δh = hi – hj is the difference between receiver i and emitter j ionosphere-free phase clocks. Δhp is the corresponding term for code clocks.
Δτ = τi – τj is the difference between receiver i and emitter j offsets between the phase clocks at f1 and the ionosphere-free phase clocks. By construction, the corresponding quantity at f2 is γΔτ. Similarly, the corresponding quantity for the code is Δτp (time group delay).
N1 and N2 are the two carrier-phase ambiguities. By definition, these ambiguities are integers. Unambiguous phase measurements are therefore L1 + N1 and L2 + N2.
Equations (1) take into account all the biases related to delays and clock offsets. The four independent parameters, Δh, Δτ, Δhp, and Δτp, are equivalent to the definition of one clock per observable. However, our choice of parameters emphasizes the specific nature of the problem by identifying reference clocks for code and phase (Δhp and Δh) and the corresponding hardware offsets (Δτp and Δτ). These offsets are assumed to vary slowly with time, with limited amplitudes.
The measured widelane ambiguity, , (also called the Melbourne-Wübbena widelane) can be written as:
(2)
whereNwis the integer widelane ambiguity,μ jis the constant widelane delay for satellite j and μiis the widelane delay for receiver i (which is fairly stable for good quality geodetic receivers). The symbolmeans that all quantities have been averaged over a satellite pass.
Integer widelane ambiguities are then easily identified from averaged measured widelanes corrected for satellite widelane delays. Once integer widelane ambiguities are known, the ionosphere-free phase combination can be expressed as
(3)
whereis the ionosphere-free phase combination computed using the known Nw ambiguity,Dcis the propagation distance,hiis the receiver clock and h jis the satellite clock. N1is the remaining ambiguity associated to the ionosphere-free wavelengthλc(10.7 centimeters).
The complete problem is thus transformed into a single-frequency problem with wavelengthλcand without any ionosphere contribution. Many algorithms can be used to solve Equation (3) using data from a network of stations. IfDcis known with sufficient accuracy (typically a few centimeters, which can be achieved using a good floating-point or real-valued ambiguity solution), it is possible to simultaneously solve for N1 ,hiand h j. The properties of such a solution have been studied in detail. A very interesting property of theh jsatellite clocks is, in particular, the capability to directly fix (to the correct integer value) the N1values of a receiver that was not part of the initial network.
The majority of the precise-point-positioning ambiguity-resolution (PPP-AR) implementations are based on the identification and use of the two quantitiesμ jand h j. These quantities may be called widelane biases and integer phase clocks, a decoupled clock model or uncalibrated phase delays, but they are all of the same nature.
A Real-Time PPP-AR Implementation
A PPP-AR technique was successfully implemented by the Centre National d’Etudes Spatiales (CNES) in real time in the so-called PPP-Wizard demonstrator in 2010 and has been subsequently improved. In this demonstrator and in the framework of the International GNSS Service (IGS) Real-Time Service (RTS) and the RTCM, the GPS and GLONASS constellation orbits and clocks are computed. Additional biases for GPS ambiguity resolution are computed and broadcast to the user. The demonstrator also provides an open-source implementation of the method on the user side, for test purposes. Centimeter-level positioning accuracy in real time is obtained on a routine basis.
Limitations of the Bias Formulations. The current formulation works but it has several drawbacks:
The chosen representation is dependent on the implemented method. Even if the nature of the biases is the same, their representation may be different according to the underlying methods, and this makes it difficult for a standardization of the bias messages.
The user side must implement the same method as the one used on the network side. Otherwise, the user side would have to convert the quantities from one method to another, leading to potential bugs or misinterpretations.
It is limited to the dual-frequency case. There are only two quantities to be computed in the dual-frequency case ( and ), but in the triple-frequency case, there are many more possible combinations. For example, one can have (this is a non-exhaustive list) , , ,, , , where the indices refer to different pairs of frequencies, and other ionosphere-free combinations such as phase widelane-only or even phase ionosphere-free and geometry-free combinations are possible.
New RTCM SSR Model
The new model, as proposed by the RTCM Special Committee 104 SSR working group for phase bias messages is based on the idea that the phase bias is inherent to each frequency. Thus, instead of making specific combinations, one phase bias per phase observable is identified and broadcast.
It is noted that this convention was adopted a long time ago for code biases. Indeed, in the RTCM framework, and unlike the standard differential code bias (DCB) convention where code biases are undifferenced but combined, the RTCM SSR code biases are defined as undifferenced and uncombined. The general model for uncombined code and phase biases is therefore:
(4)
Time group delays, τ, and phase clocks, h, in Equation (1) are replaced by code and phase biases (ΔbP andΔbLrespectively). RTCM SSR code and phase biases correspond to the satellite part of these biases. The prime notation denotes the “unbiasing” process of the measurements. Here, the clock definition is crucial. As the biases are uncombined, they are referenced to the clocks. The convention chosen for the standard is natural: it is the same as the one used by IGS, that is,ΔhPin our notation.
This new model can be extended to the triple-frequency case very easily, as it does not involve explicit dual-frequency combinations:
(5)
This new model simplifies the concept of phase biases for ambiguity resolution. This representation is very attractive because no assumption is made on the method used to identify phase biases on the network side. All the implementations are valid if they respect this proposed model. It also allows convenient interoperability if the network and user sides implement different ambiguity resolution methods.
TABLE 1 summarizes the different messages used for PPP-AR in the context of RTCM SSR:
TABLE 1. RTCM SSR messages for PPP-AR.
Bias Estimation in the Dual-Frequency Case. The new phase biases identification in the dual-frequency case is straightforward. There are two biases (, ) to be estimated using two combinations (µ and h). The problem to be solved is described in FIGURE 1.
FIGURE 1. Phase biases estimation in the dual-frequency case.
It can be solved very easily on the network side by means of a 2 × 2 matrix inversion:
(6)
with
Note: All the quantities denote the satellite part of the Δ operator defined above.
Bias Estimation in the Triple-Frequency Case. The triple-frequency bias identification is tricky due to the need, using only three biases, to keep the integer nature of phase ambiguities on all viable ionosphere-free combinations, and in particular combinations that were not used in the identification process. At this level, one cannot make assumptions on what kind of combinations will be employed by a user. The problem to be solved is described in FIGURE 2.
FIGURE 2. Phase biases estimation in the triple-frequency case.
As an example, a naïve solution would be to identify the extra-widelane phase biases,, using the dual-frequency widelane approach, and then identify thebias. Given the large wavelength of the extra-widelane combination, such identification would be very easy. However, the corresponding bias would be only helpful for extra-widelane ambiguity identification, and its noise would prevent its use for widelane 15 (L1/L5) ambiguity resolution or other useful combinations available in the triple-frequency context.
Each independent phase bias can be directly estimated in a filter; however, in order to keep ascending compatibility with the dual-frequency case during the deployment phase of the new modernized satellites, we have chosen to stay in the old framework, that is, to work with combinations of biases. The resolution method is the following:
The widelane biases, that is, the identification of all thebLi – bLj quantities, are solved. For this computation and in order to have an accurate estimate of these biases, the two MW-widelane biasesµ12andµ15are used coupled to an additional phase bias, which is given by the triple-frequency ionosphere-free phase combination with the integer widelane ambiguities already fixed. This last combination using only phase measurements is much more accurate than MW-widelanes. The system to be solved is redundant and the noise of the different equations has to be chosen carefully.
The remaining bias (bLi ) is estimated using the traditional ionosphere-free phase combination of L1 and L2.
This computation has been implemented in the CNES real-time analysis center software, and since September 15, 2014, CNES broadcasts phase biases compatible with this triple-frequency concept on the IGS CLK93 real-time data stream.
Real Data Analysis
To prove the validity of the concept, at CNES, we compute several ambiguity combinations using real data. The process is the following:
Look for good receiver locations having a large number of GPS Block IIF satellites (transmitting the L5 signal) in view for a period of time exceeding 30 minutes, and choose among them, one participating in the IGS Multi-GNSS (MGEX) experiment. The station CPVG (Cape Verde) in the Reseau GNSS pour l’IGS et la Navigation (REGINA) network was chosen for the time span on September 28, 2014, between 19 and 20 hours UTC. During this period, four Block IIF satellites were visible simultaneously (PRNs 1, 6, 9, 30) for a total of 14 GPS satellites in view.
Record a compatible phase-bias stream. The CLK93 stream is recorded during the time span of the experiment.
Perform a PPP solution using the measurements, CLK93 corrections and biases to estimate the propagation distance, the troposphere delay and the receiver clock and phase ambiguity estimates according to Equation (5).
For different ambiguity estimates, compute and plot the obtained residuals.
We present in the following graphs various ambiguity residuals for the four Block IIF satellites in view. The values of each ambiguity are offset by an integer value for clarity purposes.
Melbourne-Wübbena Extra-Widelane.FIGURE 3 represents the MW extra-widelane (between frequencies L2 and L5) ambiguity estimation using our process. The MW extra-widelane ambiguity has a wavelength of 5.86 meters. The noise of the combination expressed in cycles is very low, and the integer nature of ambiguities in this combination is clearly visible.
FIGURE 3. Ambiguity residuals for the extra-widelane 5-2 combination.
Melbourne-Wübbena Widelanes.FIGURE 4 represents the MW-widelanes (the regular 1-2 and 1-5 combinations). Here again, the integer nature of the four ambiguities is clearly visible.
Widelane-Only Ionosphere-Free Phase. In the triple-frequency context, there is a possibility of forming an ionosphere-free combination of the three phase observables. This combination has an important noise amplification factor (>20), but would allow us to perform decimeter-accuracy PPP using only the solved widelane integer ambiguities and if the corresponding phase biases are accurate. In addition, it can be shown that the wavelength of the widelane ambiguity when the extra-widelane ambiguity is solved is about 3.4 meters. It means that the remaining widelane using this combination can be solved if the position is accurate enough (a few tens of centimeters) and the extra-widelane is known. FIGURE 5 shows such a case, that is, the residuals of the widelane ambiguity using this combination and assuming that the extra-widelane is already solved for.
FIGURE 5. Ambiguity residuals for widelane-only 1-2-5 ionosphere free combinations.
Such a case where the solution is the most biasedis shown (the dark blue curve). This behavior is mainly due to the difficulty in estimating the phase biases on this combination accurately using only a few Block IIF satellites. We hope that in the future the increasing number of modernized satellites will help such bias estimation.
N1 Ionosphere-Free Phase.FIGURES 6 to 8 show the three possible ambiguity estimates using the ionosphere-free phase combination with two measurements (we assume that the corresponding widelane has already been solved). In each case, the computed biases allow us to easily retrieve the integer nature of the N1 ambiguity.
FIGURE 6. Ambiguity residuals for the N1 combination using a fixed 1-2 widelane.FIGURE 7. Ambiguity residuals for the N1 combination using a fixed 1-5 widelane.FIGURE 8. Ambiguity residuals for the N1 combination using a fixed 2-5 widelane.
Application to Triple-Frequency PPP
The results presented above show that the integer ambiguity nature of phase measurements is conserved for various useful observable combinations and prove the validity of the model. Another experiment has been carried out to estimate the impact of ambiguity convergence in the triple-frequency context. For that, in order to maximize the observability of the GPS Block IIF constellation and thus the accuracy of the biases, a network of ten stations across Europe has been chosen for the phase biases computation (see FIGURE 9). The station REDU (in green) was the test station to be positioned. The test occurred on January 10, 2015, around 11:00 UTC. At that time, four Block IIF satellites were visible simultaneously (PRNs 1, 3, 6, 9) for a total of 10 satellites in view.
FIGURE 9. Network used for the triple-frequency PPP study.
The PPP-Wizard open source client was used to perform PPP in real time. The advantage of this implementation is that it directly follows the uncombined observable formulation described in Equations (5). The strategy for ambiguity resolution is a simple bootstrap approach.
Convergence of the Widelane-Only Solution. In this test, a PPP solution was performed, but only the fixing of the widelane ambiguities was implemented. As noted in the previous section, the wavelength of the widelane ambiguity when the extra-widelane ambiguity is solved is about 3.4 meters, so it is expected that all the widelanes can be fixed in a very short time. Despite the amplification factor of about 20 of the equivalent unambiguous phase combination, we expect to obtain an accuracy of about 10 centimeters with such a solution.
FIGURE 10 shows the convergence time of several PPP runs in this context (16 different runs of five minutes are superimposed), in terms of horizontal position error.
FIGURE 10. Widelane-only triple-frequency PPP convergence (horizontal position error).
The extra-widelanes are fixed instantaneously; the remaining widelanes are fixed in about two minutes on average to be below 30 centimeters (this is represented by the different sharp reductions of the errors). This new configuration, available in the triple-frequency context, is very interesting as it provides an intermediate class of accuracy, which converges very quickly and which is suitable for applications that do not demand centimeter accuracy. Another interesting aspect of this combination is the gap-bridging feature. In PPP, gap-bridging is the functionality that allows us to recover the integer nature of the ambiguities after a loss of the receiver measurements over a short period of time (typically a pass through a tunnel or under a bridge). This is done usually by means of the estimation of a geometry-free combination (ionosphere delay estimation) during the gap. Realistic maximum gap duration in the dual-frequency case is about one minute. In the triple-frequency case, the wavelength of the geometry-free combination involving the widelane (if the extra-widelane is fixed) is 1.98 meters. With such a large wavelength, the gaps are much easier to fill, and we can safely extend the gap duration to several minutes. In addition, the widelane combinations are wind-up independent, so there is no need to monitor a possible rotation of the antenna during the gap, as in the dual-frequency case.
Overall Convergence (All Ambiguities). Another PPP convergence test has been carried out with all ambiguities fixing activated (four different runs of 15 minutes are superimposed). Results are shown in FIGURE 11.
FIGURE 11. All ambiguities triple-frequency PPP convergence (horizontal position error).
The centimeter accuracy is obtained in this configuration within eight minutes, which is a significant improvement in comparison to the dual-frequency case. Further improvement of this convergence time is expected with an increase in the number of Block IIF satellites and, subsequently, GPS IIIA satellites.
Convergence Time Comparison Between the Dual- and Triple-Frequency Contexts. Thanks to these new results, a realistic picture for PPP convergence in the dual- and triple-frequency contexts can be drawn. To do so, polynomial functions have been fitted over the data points obtained in the previous studies. Two data sets were used:
Standard dual-frequency convergence (GPS only, 10 satellites in view).
Triple-frequency convergence (GPS only, 10 satellites in view, four Block IIF satellites).
FIGURE 12 represents the comparison between the two polynomials (horizontal error).
FIGURE 12. Realistic PPP convergence comparison between dual- and triple-frequency contexts (horizontal position error).
Conclusion
The new phase-bias concept proposed for RTCM SSR has been successfully implemented in the CNES IGS real-time analysis center. This new concept represents the phase biases in an uncombined form, unlike the previous formulations. It has the advantage of the unification of the different proposed methods for ambiguity resolution, and it prepares us for the future; for example, for a widely available triple-frequency scenario. The validity of this concept has been shown; that is, the integer ambiguity nature of phase measurements is conserved for various useful observable combinations.
In addition, we have also shown that the triple-frequency context has a significant impact on ambiguity convergence time. The overall convergence time is drastically reduced (to some minutes instead of some tens of minutes) and there is an intermediate combination (widelane-only) that has some interesting properties in terms of convergence time, accuracy and gap-bridging for non-demanding centimeter-level applications.
Acknowledgments
The contributions of colleagues contributing to the IGS services are gratefully acknowledged. Geo++ is thanked for useful discussions on the standardization of phase bias representation.
DENIS LAURICHESSE received his engineering degree and a Diplôme d’études appliquées (an advanced study diploma) from the Institut National des Sciences Appliquées in Toulouse, France, in 1988. He has worked in the Spaceflight Dynamics Department of the Centre National d’Etudes Spatiales (CNES, the French Space Agency) in Toulouse since 1992, responsible for the development of the onboard GNSS Diogene navigator. He was involved in the performance assessment of the EGNOS and Galileo systems and is now in charge of the CNES International GNSS Service real-time analysis center. He specializes in navigation, precise satellite orbit determination and GNNS-based systems. He was the recipient of The Institute of Navigation Burka Award in 2009 for his work on phase ambiguity resolution.
“Undifferenced GPS Ambiguity Resolution Using the Decoupled Clock Model and Ambiguity Datum Fixing” by P. Collins, S. Bisnath, F. Lahaye, and P. Héroux in Navigation, Journal of The Institute of Navigation, Vol. 57, No. 2, Summer 2010, pp. 123–135, doi: 10.1002/j.2161-4296.2010.tb01772.x.
“Integer Ambiguity Resolution on Undifferenced GPS Phase Measurements and Its Application to PPP and Satellite Precise Orbit Determination” by D. Laurichesse, F. Mercier, J.-P. Berthias, P. Broca, and L. Cerri in Navigation, Journal of The Institute of Navigation, Vol. 56, No. 2, Summer 2009, pp. 135–149, doi: 0.1002/j.2161-4296.2009.tb01750.x.
“Resolution of GPS Carrier-Phase Ambiguities in Precise Point Positioning (PPP) with Daily Observations” by M. Ge, G. Gendt, M. Rothacher, C. Shi, and J. Liu in Journal of Geodesy, Vol. 82, No. 7, pp. 389–399, doi: 10.1007/s00190-007-0187-4. Erratum: 10.1007/s00190-007-0208-3.
“The CNES Real-time PPP with Undifferenced Integer Ambiguity Resolution Demonstrator” by D. Laurichesse in Proceedings of ION GNSS 2011, the 24th International Technical Meeting of The Satellite Division of the Institute of Navigation, Portland, Ore, September 20–23, 2011, pp. 654–662.
“The RTCM Multiple Signal Messages: A New Step in GNSS Data Standardization” by A. Boriskin, D. Kozlov, and G. Zyryanov in Proceedings of ION GNSS 2012, the 25th International Technical Meeting of The Satellite Division of the Institute of Navigation, Nashville, Tenn., September 17–21, 2012, pp. 2947-2955.
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.
In the latest update of its Motion Tracker product portfolio, Xsens has added active heading stabilization (AHS) to its core sensor fusion algorithms on the MTi 10-series and MTi 100-series. Both series are MEMS-based inertial measurement units (IMU), attitude and heading reference systems (AHRS), and vertical reference units (VRUs).
The AHS algorithm delivers fundamentally improved heading tracking accuracy, Xsens said. The improved robustness in heading tracking is particularly evident in Xsens’ line of vertical reference units (MTi-20 and MTi-200). These products now provide actively stabilized heading tracking, delivering 20x less drift than pure gyroscope dead reckoning for most application scenarios. This means heading tracking drift as low as 1 degree after one hour for many applications, while remaining fully immune to magnetic distortions.
Xsens said this characteristic makes the MTi line of products a highly accurate, but cost-effective solution for robotic/indoor navigation, camera stabilization, satellite communication, directional drilling, borehole/pipeline inspection and pedestrian navigation applications, Xsens said.
“Customers are already choosing our MTis because of their accurate heading tracking capabilities, but this algorithm will bring the accuracy to a whole new level, enabling more applications and creating new markets. The 12 cm2 MTi comes with an easy-to-use library, so that integrating the solution is straight-forward,” said Marcel van Hak, Product Manager of Industrial Applications for Xsens.
AHS is available immediately as a free firmware upgrade to all MTi customers as part of the just-released MT Software Suite 4.3.
The following video shows a demonstration of the Active Heading Stabilization, with the Xsens MTi is mounted on a robotic vacuum cleaner.
Tandem Expansion Fund, a Canadian growth-equity investor, has acquired a majority interest in Averna, a developer of test solutions and services for communications and electronics device-makers, according to a news release from Averna. The transaction provides Averna with the financial resources to “accelerate organic and strategic growth as well as to expand its international presence,” the release said.
Founded in 1999, Averna is a test engineering company that provides test expertise and solutions for tier-one clients in wide-ranging industries around the world, including aerospace and defense, telecom infrastructures, automotive and transportation, consumer electronics and life sciences. Averna has more than 300 employees and offices in 5 countries.
“This is a new chapter for Averna and we are proud to have the support of these strategic and respected partners who share our vision and values,” said André Gareau, Averna’s president and CEO. “Averna is a Montreal-based success story and it is important for us to continue hiring the best local talents. This investment will help us extend our leadership position in each of our key industries as well as continue growing the company locally and internationally.”
“Averna’s unique expertise in test, growing base of customers across the globe, excellent management team and portfolio of solutions position the company at the forefront of the electronic test and quality market,” said André Gauthier, managing partner at Tandem Expansion Fund. “With this investment, Tandem is providing solid support to Averna in the next phase of its development.”
Averna has offices around the world as well as a network of partners such as JOT Automation, Keysight Technologies, and National Instruments. Incorporated in 1999, Averna is a Best in Test award winner and has been honored as one of the Deloitte Fast 500 fastest-growing technology companies in North America.
Analysis of new Galileo signals at an experimental ground-based augmentation system (GBAS) compares noise and multipath in their performance to GPS L1 and L5. Raw noise and multipath level of the Galileo signals is shown to be smaller than those of GPS. Even after smoothing, Galileo signals perform somewhat better than GPS and are less sensitive to the smoothing time constant.
By Mihaela-Simona Circiu, Michael Felux, German Aerospace Center (DLR), and Sam Pullen, Stanford University
Several ground-based augmentation system (GBAS) stations have become operational in recent years and are used on a regular basis for approach guidance. These include airports at Sydney, Malaga, Frankfurt and Zurich. These stations are so-called GBAS Approach Service Type C (GAST C) stations and support approaches only under CAT-I weather conditions; that is, with a certain minimum visibility. Standards for stations supporting CAT-II/III operations (low visibility or automatic landing, called GAST D), are expected to be agreed upon by the International Civil Aviation Organization (ICAO) later this year. Stations could be commercially available as soon as 2018.
However, for both GAST C and D, the availability of the GBAS approach service can be significantly reduced under active ionospheric conditions. One potential solution is the use of two frequencies and multiple constellations in order to be able to correct for ionospheric impacts, detect and remove any compromised satellites, and improve the overall satellite geometry (and thus the availability) of the system.
A new multi-frequency and multi-constellation (MFMC) GBAS will have different potential error sources and failure modes that have to be considered and bounded. Thus, all performance and integrity assumptions of the existing single-frequency GBAS must be carefully reviewed before they can be applied to an MFMC system. A central element for ensuring the integrity of the estimated position solution is the calculation of protection levels. This is done by modeling all disturbances to the navigation signals in a conservative way and then estimating a bound on the resulting positioning errors that is valid at an allocated integrity risk probability.
One of the parameters that is different for the new signals and must be recharacterized is the residual uncertainty attributed to the corrections from the ground system (σpr_gnd). A method to assess the contribution of residual noise and multipath is by evaluating the B-values in GBAS, which give an estimate of the error contribution from a single reference receiver to a broadcast correction. Independent data samples over at least one day (for GPS) are collected and sorted by elevation angle. Then the mean and standard deviations for each elevation bin are determined.
Here, we evaluate the E1 and E5a signals broadcast by the operational Galileo satellites now in orbit. In the same manner as we did for GPS L5 in earlier research, we determine the σpr_gnd values for these Galileo signals. As for GPS L5, results show a lower level of noise and multipath in unsmoothed pseudorange measurements compared to GPS L1 C/A code.
DLR GBAS Facility
DLR has set up a GBAS prototype at the research airport in Braunschweig (ICAO identifier EDVE) near the DLR research facility there. This ground station has recently been updated and now consists of four GNSS receivers connected to choke ring antennas, which are mounted at heights between 2.5 meters and 7.5 meters above equipment shelters. All four receivers are capable of tracking GPS L5 (in addition to GPS L1 and L2 semi-codeless) and Galileo E1 and E5a signals. Figure 1 gives an overview of the current ground station layout, and Table 1 gives the coordinates of the antennas.
Figure 1. DLR ground facility near Braunschweig Airport, also shown in opening photo at left.Table 1. Ground receiver antenna coordinates.
Smoothing Techniques
The GBAS system corrects for the combined effects of multiple sources of measurement errors that are highly correlated between reference receivers and users, such as satellite clock, ephemeris error, ionospheric delay error, and tropospheric delay error, through the differential corrections broadcast by the GBAS ground subsystem. However, uncorrelated errors such as multipath and receiver noise can make a significant contribution to the remaining differential error. Multipath errors are introduced by the satellite signal reaching the antenna via both the direct path from the satellites and from other paths due to reflection. These errors affect both the ground and the airborne receivers, but are different at each and do not cancel out when differential corrections are applied.
To reduce these errors, GBAS performs carrier smoothing. Smoothing makes use of the less noisy but ambiguous carrier-phase measurements to suppress the noise and multipath from the noisy but unambiguous code measurements.
The current GBAS architecture is based on single-frequency GPS L1 C/A code measurements only. Single-frequency carrier smoothing reduces noise and multipath, but ionospheric disturbances can cause significant differential errors when the ground station and the airborne user are affected by different conditions. With the new available satellites (GPS Block IIF and Galileo) broadcasting in an additional aeronautical band (L5 / E5), this second frequency could be used in GBAS to overcome many current limitations of the single-frequency system.
Dual-frequency techniques have been investigated in previous work. Two dual-frequency smoothing algorithms, Divergence Free (Dfree) and Ionosphere Free (Ifree), have been proposed to mitigate the effect of ionosphere gradients.
The Dfree output removes the temporal ionospheric gradient that affects the single-frequency filter but is still affected by the absolute difference in delay created by spatial gradients. The main advantage of Dfree is that the output noise is similar to that of single-frequency smoothing, since only one single-frequency code measurement is used as the code input (recall that carrier phase noise on both frequencies is small and can be neglected).
Ifree smoothing completely removes the (first-order) effects of ionospheric delay by using ionosphere-free combinations of code and phase measurements from two frequencies as inputs to the smoothing filter. Unlike the Dfree, the Ifree outputs contain the combination of errors from two code measurements. This increases the standard deviation of the differential pseudorange error and thus also of the position solution.
Noise and Multipath in New GNSS Signals
GBAS users compute nominal protection levels (H0) under a fault-free assumption. These protection levels are conservative overbounds of the maximum position error after application of the differential corrections broadcast by the ground system, assuming that no faults or anomalies affect the position solution. In order to compute these error bounds, the total standard deviation of each differentially corrected pseudorange measurements has to be modeled. The standard deviation of the residual uncertainty (σn, for the nth satellite) consists of the root-sum-square of uncertainties introduced by atmospheric effects (ionosphere, troposphere) as well as of the contribution of the ground multipath and noise. In other words, these error components are combined to estimate σn2 as described in the following equation:
(1)
The ground broadcasts a value for σpr_gnd (described later in the section) associated with the pseudorange correction for each satellite. These broadcast values are based on combinations of theoretical models and actual measurements collected from the ground receivers that represent actual system characteristics. Unlike the ground, σpr_air is computed based entirely on a standardized error model. This is mainly to avoid the evaluation of multipath for each receiver and each aircraft during equipment approval.
In addition to the characteristics of nearby signal reflectors, multipath errors are mainly dependent on signal modulation and other signal characteristics (for example, power, chip rate). In earlier research, we showed that the newly available L5 signals broadcast by the GPS Block IIF satellites show better performance in terms of lower noise and multipath. This mainly results from an increased transmitted power and a 10 times higher chip rate on L5 compared to the L1 C/A code signal.
In this work, we extend this evaluation to the new Galileo signals and investigate their impact on a future multi-frequency, multi-constellation GBAS. Characterization of these new signals is based on ground subsystem measurements, since no flight data with GPS L5 or Galileo measurements are available at the moment. We assume that the improvements observed by ground receivers are also applicable to airborne measurements. This assumption will be validated as soon as flight data are available.
The measurements used were collected from the DLR GBAS test bed over 10 days (note that Galileo satellite ground track repeatability is 10 sidereal days) between the December 14 and 23, 2013. In that period, four Galileo and four Block IIF GPS satellites were operational and broadcast signals on both aeronautical bands E1 / L1 and E5a / L5.
In Figure 2, the suppression of multipath and noise on the Galileo signals can be observed, where the code multipath and noise versus elevation for GPS L1 C/A BSPK(1), Galileo E1 (BOC (1,1)) and Galileo E5a (BPSK(10)) signals are shown. The code multipath and noise was estimated using the linear dual-frequency combination described in equation (2), where MPi represents the code multipath and noise on frequency i, ρi the code measurement, and ϕi,and ϕj represent the carrier-phase measurements on frequencies i and j, respectively. Carrier phase noises are small and can be neglected.
(2)
Figure 2. Raw multipath function of elevation for GPS L1, Galileo E1 (BOC (1,1)) and Galileo E5a (BPSK(10)) signals.
The multipath on the Galileo E1 (BOC(1,1)) signal (the magenta curve) is lower than the GPS L1 C/A (BPSK(1))(black curve), especially for low elevation, where the advantage of the E1 BOC(1,1) is more pronounced. The lower values can be explained by the wider transmission bandwidth on E1 and the structure of the BOC signal. Galileo E5a (green data in Figure 2) again shows a better performance than Galileo E1. This was expected due to the higher chip rate and higher signal power. A comparison of the raw multipath and noise standard deviations for GPS L1, L5 and Galileo E1, E5a signals is presented in Figure 3.
Figure 3. Ratios of the multipath and noise standard deviation function of elevation.
The curves there show the ratios of the standard deviations for each elevation bin. The values for GPS L1 are almost 1.5 times larger than those for Galileo E1 BOC(1,1) (green curve) for elevations below 20°. For high elevations, the ratio approaches 1.0. This corresponds to the observations in the raw multipath plot ( Figure 2). With the same signal modulation and the same chip rate, E5a and L5 have very similar results (red curve), and the ratio stays close to 1.0 for all elevations.
The blue and the purple curves in Figure 3 show the ratio of GPS L1 C/A (BPSK(1)) and GPS L5 (BPSK(10)), and Galileo E1 (BOC(1,1)) and Galileo E5a (BPSK(10)), respectively. The ratio of GPS L1 to GPS L5 (blue curve) increases with elevation from values around 2.5 for low elevations, reaching values above 3.5 for elevations higher than 60°. As Galileo E1 performs better, the ratio between Galileo E1 and Galileo E5a (purple curve) is smaller, from a value of 1.5 for elevations below 10 degrees to a value of 3.0 for high elevations.
Until now, we have presented the evaluation of raw code noise and multipath. However, in GBAS, carrier smoothing is performed to minimize the effect of code noise and multipath. The value that describes the noise introduced by the ground station is represented by a standard deviation called σpr_gnd and is computed based on the smoothed pseudoranges from the reference receivers. In the following section, we focus on the evaluation of σpr_gnd using different signals and different smoothing time constants. Note that, in this study, σpr_gnd contains only smoothed multipath and noise; no other contributions (for example, inflation due to signal deformation or geometry screening) are considered.
B-values and σpr_gnd
B-values represent estimates of the associated noise and multipath with the pseudorange corrections provided from each receiver for each satellite, as described in Eurocae ED-114A and RTCA DO-253C. They are used to detect faulty measurements in the ground system. For each satellite-receiver pair B(i,j), they are computed as:
(3)
where PRCTX represents the candidate transmitted pseudorange correction for satellite i (computed as an average over all M(i) receivers), and PRCSCA(i,k) represents the correction for satellite i from receiver k after smoothed clock adjustment, which is the process of removing the individual receiver clock bias from each reference receiver and all other common errors from the corrections. The summation computes the average correction over all M(k) receivers except receiver j. This allows detection and exclusion of receiver j if it is faulty. If all B-values are below their thresholds, the candidate pseudorange correction PRCTX is approved and transmitted. If not, a series of measurement exclusions and PRC and B-value recalculations takes place until all revised B-values are below threshold. Note that, under nominal conditions using only single-frequency measurements, the B-values are mainly affected by code multipath and noise.
Under the assumption that multipath errors are uncorrelated across reference receivers, nominal B-values can be used to assess the accuracy of the ground system. The standard deviation of the uncertainty associated with the contribution of the corrections (σpr_gnd) for each receiver m is related to the standard deviation of the B-values by:
(4)
where M represents the number of the receivers and N represents the number of satellites used. The final sigma takes into account the contribution from all receivers and is computed as the root mean square of the standard deviation of the uncertainties associated with each receiver (Equation 4).
Figure 4 shows the evaluation of (σpr_gnd) for the Galileo E1, BOC(1,1) signal and the GPS L1 C/A signal for increasing smoothing time constants (10, 30, 60, and 100 seconds). Starting with a 10-second smoothing constant, Galileo E1 shows much better performance than GPS L1. The difference shrinks as the smoothing constant increases due to the effectiveness of smoothing in reducing noise and short-delay multipath. However, even with 100-second smoothing (the purple curves), Galileo E1 BOC(1,1) shows lower values of (σpr_gnd).
Figure 4. σ(pr_gnd) versus elevation for Galileo E1 (dotted lines) and GPS L1 (solid lines for different smoothing constants: red (10s), green (30s), cyan (60s), purple (100s).
A similar comparison is presented in Figure 5, of the performance of GPS L1 and Galileo E5a. The Galileo E5a signal is significantly less affected by multipath, and the difference stays more pronounced than in the Galileo E1 – GPS L1, even with 100-second smoothing. It can be also observed that the Galileo signals have a lower sensitivity to the smoothing constant. The Galileo E1 signal shows an increase of sensitivity for low elevations (below 40°), while on E5a, a smoothing constant larger than 10 seconds has almost no impact on the residual error. Thus, a shorter smoothing constant on Galileo E5a generates approximately the same residual noise and multipath a 100-second smoothing constant on GPS L1.
Figure 5. σ(pr_gnd) versus elevation for Galileo E5a (dotted lines) and GPS L1 (solid lines) for different smoothing constants: red (10s), green (30s), cyan (60s), purple (100s).
The values for (σpr_gnd) are, however, impacted by the number of satellites which are used to determine a correction. Since only a very limited number of satellites broadcasting L5 and Galileo signals are currently available, these results should be considered preliminary. The first evaluations strongly indicate that with the new signals, we get better ranging performance. Based on the performance advantage of the new signals, a decrease of the smoothing constant is one option for future application. This would reduce the time required (for smoothing to converge) before including a new satellite or re-including a satellite after it was lost.
In the current GAST-D implementation, based on GPS L1 only, guidance is developed based on a 30-second smoothing time constant. A second solution, one with 100 seconds of smoothing, is used for deriving the Dv and Dl parameters from the DSIGMA monitor and thus for protection level bounding (it is also used for guidance in GAST-C). During the flight, different flight maneuvers or the blockage by the airframe can lead to the loss of the satellite signal.
Figure 6 shows the ground track of a recent flight trial conducted by DLR in November 2014. The colors represent the difference between the number of satellites used by the ground subsystem (with available corrections) and the number of satellites used by the airborne subsystem in the GAST-D position solution. One of the purposes of the flight was to characterize the loss of satellite signals in turns. In turns with a steeper bank angle, up to 3 satellites are lost (Turns 1, 3, and 4), while on a wide turn with a small bank angle (Turn 2), no loss of satellite lock occurred. It is also possible for airframe to block satellite signals, leading to a different number of satellites between ground and airborne even without turns.
Figure 6. Ground track of a flight trial conducted by DLR. The colors represent difference between number of SVs used by the ground system and number of SVs used by the airborne.
With this in mind, a shorter smoothing constant would allow the satellites lost to turns or to airframe blockage to be re-included more rapidly in the position solution. However, a new smoothing constant would have to be validated with a larger amount of data. Data from flights trials has to be evaluated as well to confirm that similar levels of performance are reresentative of the air multipath and noise.
In a future dual-frequency GBAS implementation, an important advantage of lower multipath and noise is to improve the Ifree position solution. In earlier research, we demonstrated that the error level of the Dfree solution is almost the same as for single-frequency, but an increase in error by a factor of 2.33 was computed for the Ifree standard deviation based on L1 C/A code and L2 semi-codeless measurements.
If the errors on L1 (E1) and L5 (E5a) code and carrier phase measurements are statistically independent the standard deviation of the σIfree can be written as,
(5)
where α=1−f 21 ∕ f 25, and σL1,σL5 represent the standard deviations of the smoothed noise and multipath for L1 (E1) and L5 (E5a), respectively. Considering σpr_gnd,L1(E1)) = σpr_gnd,L5(E5a)) in equation (5), the noise and multipath error on Ifree (σIfree) increases by a factor of 2.59.
Figure 7 shows the ratio σIfree/σL1 using measured data. We observe that the measured ratio (the black curve) is below the theoretical ratio computed based on the assumption of statistically independent samples (the constant value of 2.59). This is explained by the fact that the multipath errors in the measurements are not independent but have some degree of statistical correlation. The standard deviations are computed based on the same data set used in the raw multipath and noise assessment using 100-second smoothed measurements sorted into elevation bins of 10° spacing.
Figure 7. Measured ratio σIfree/σL1 function of elevation.
Conclusion
We have shown how GBAS can benefit from the new signals provided by the latest generation of GPS and Galileo satellites. We have demonstrated improved performance in terms of lower noise and multipath in data collected in our GBAS test bed. When GBAS is extended to a multi-frequency and multi-constellation system, these improvements can be leveraged for improved availability and better robustness of GBAS against ionospheric and other disturbances.
Acknowledgment
Large portions of this work were conducted in the framework of the DLR internal project, GRETA.
Manufacturers
The ground facility consists of four JAVAD GNSS Delta receivers, all connected to Leica AR 25choke ring antennas.
Mihaela-Simona Circiu is is a research associate at the German Aerospace Center (DLR). Her research focuses on multi-frequency multi-constellation Ground Based Augmentation System. She obtained a 2nd level Specialized Master in Navigation and Related Applications from Politecnico di Torino.
MIchael Felux is is a research associate at the German Aerospace Center (DLR). He is coordinating research in the field of ground-based augmentation systems and pursuing a Ph.D. in Aerospace Engineering at the Technische Universität München.
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.
Winners in the 30 categories will be announced in October.
The 2015 European Satellite Navigation Competition (ESNC), an international innovation competition that recognizes the best ideas in satellite navigation, will run from April 1 to June 30. Winners will be announced in October.
There are more than 20 regions participating, and the ESNC will award prizes worth a total of €1 million in 30 categories.
“Satellite navigation is an essential element of modern mobility and a key technology in particular, in the age of a data-driven economy. This is exactly where the European Satellite Navigation Competition comes in. It provides a public platform to the creative community in order to help promising ideas turn into solutions that are commercially mature and generate added value for society,” said Alexander Dobrindt, Germany’s Federal Minister of Transport and Digital Infrastructure (BMVI).
A jury of international research and industry experts will select the year’s overall champion among the winners of the categories, which comes with an additional €20,000 and access to a six-month incubation program in the champion’s preferred region.
International kick-off for the 2015 ESNC is scheduled for April 21 in London.
“As the Galileo satellite constellation continues to expand, efforts to promote corresponding applications will become increasingly important. This is where the ESNC is already playing a key role,” said Matthias Petschke, the European Commission’s director of satellite navigation programs. “As such, the Commission is definitely looking forward to seeing the creative and innovative GNSS-based applications submitted this year.”
This year’s special topic prizes are being sponsored by the European GNSS Agency (GSA), the European Space Agency (ESA), the German Aerospace Center (DLR) and the Ministry of Transport and Digital Infrastructure (BMVI) in cooperation with the German Federal Ministry for Economic Affairs and Energy (BMWi). Entrants may submit prototypes to the GNSS Living Lab Challenge, while the University Challenge specifically addresses students and research assistants.
“Those who enter the ESNC benefit in particular from our global network, which provides them with tailored support in developing their business concepts and bringing them to market,” said Thorsten Rudolph, managing director of Anwendungszentrum GmbH Oberpfaffenhofen.
All of the information on this year’s prizes, partners, and terms of participation is available at the ESNC website.
When subjected to very strong interference, a GNSS receiver can be totally blinded and stop working. This is often the scope of intentional jammers. However, in a number of cases the presence of interference is severe enough to significantly decrease receiver performance, but not so much as to make the receiver lose its lock on the satellite signals or blind the acquisition of the satellite signals.
Such intermediate power values turn out to be the most dangerous cases, because sometimes they cannot be detected, but lead to a worsening of the positioning performance. The accuracy of the position solution depends on, among others, the quality of the pseudorange measurements and/or the phase measurements. Thus, when radio-frequency interference (RFI) degrades the pseudorange and phase measurements or induces cycle slips on the phase measurements, the accuracy of the position solution will decrease.
Impact on the Front End
The front-end filters the incoming signal, demodulating it to the chosen intermediate frequency before performing the analog-to-digital conversion (ADC).We must consider the presence in the front end of the adjustable gain control (AGC) between the analog portion of the front end and the ADC. When the GNSS band is interference-free, AGC gain depends almost exclusively on thermal noise, since the received signal power is below that of the thermal noise floor. When in-band interference is present, the AGC will squeeze the incoming signal to match the maximum dynamics of the ADC, causing a reduction of the amplitude of the useful signal, which may be lost. This may typically happen in the presence of some kind of wide-band interference (WBI) spread over a bandwidth larger than the passband of the front-end filter.
With narrow-band (NBI) or continuous-wave interference (CWI), statistics of the digital signal at the ADC output are also affected. In this case the AGC can still compress the input signal to avoid a stronger saturation, but the following receiver stages will have to deal with a GNSS contribution quantized only on lower levels.
In the presence of stronger interference, even the other components of the front end (filters and amplifiers) may be led to work outside of their nominal regions, generating nonlinear effects or clipping phenomena (in which the signal amplitude exceeds the hardware’s capability to treat them). In both cases, spurious harmonics are generated and mixed with the useful signal in the front end itself.
Impact on the Acquisition Stage
If the interference is not driving the AGC/ADC to full saturation, the acquisition module is still able to perform its task, processing the interfered signal to estimate the code phase and the Doppler shift with respect to the local code. The correlation with the local code can be seen as a spreading operation followed by a filter.
Figure 1. GPS L1 C/A acquisition search space in (a) an interference-free environment and in the presence of (b) –140 dBW in-band CWI; (c) –135 DBW in-band CWI; (d) –130 dBW in-band DWI.
Figure 1 showsthe acquisition search space for different levels of theinterfering power of a CWI from –140 to –130 dBW compared to the interference-free case. The search spaces depicted for the four scenarios are achieved using 1 ms of coherent integration time and three non-coherent accumulations, and the peak-to-noise-floor separation defined as
is considered as a figure of merit. The value of αmean decreases as the interfering power increases, thus increasing the probability of a false alarm. With the increasing power of the CWI, a modulation effect in the search space floor in the Doppler domain dimension can be observed. Such an effect is mainly determined by the new harmonics components generated by the multiplication between the locally generated carrier and received CWI. Such an effect also depends on how the interfering signal and the useful GNSS signal are combined at the entrance to the acquisition block, which in turn depends on the random variables φ0 and θint.
In the presence of WBI, a different effect is observed in the acquisition search space. Considering a band-limited Gaussian white noise spread all over the GNSS useful filtered signal components, the effect on the CAF envelop is an increase in the noise floor. This increases the search space noise floor. The presence of additive band-limited noise causes a uniform increase in the noise floor tin the search space that might mask the correct correlation peak and thus fool the acquisition process.
Impact on the Tracking Stage
Interference impact on the tracking stage has a direct consequence on the quality of the measured pseudorange. Harmful interfering signals increase the variance of the time-of-arrival (TOA) estimate by the discriminator and modify the shape of the S-curve of the code discriminator, thus creating in some cases a bias in the measurements.
Figure 2 depicts outputs of the early-prompt-late correlators. In the presence of in-band CWI and of NBI, the interference is injected 9.3 seconds after the beginning of the tracking stage where the receiver is correctly locked on the received signal. A CWI, shifted 200 kHz with respect to the signal intermediate frequency (in correspondence with a C/A code spectrum line), increases the noise at the correlators outputs and leads to harmonic behavior of the early-prompt-late correlator outputs.
Figure 2. GPS L1 C/A code tracing error comparison: coherent and non-coherent early-late processing (CELP and NELP).
NBI increases the variance of the correlators’ outputs; this directly increases the pseudorange error and the noise on the receiver phase measurements. Additive band-limited noise leads to an overall increase in the carrier phase discriminator output variance over the 3σ threshold, which for a PLL two-quadrant arctangent discriminator is 45 degrees. When in presence of strong CWI, a sudden jump of the phase discriminator output is detected as soon as the CWI is injected onto the received signal.
Impact on the Estimated Signal-to-Noise Ratio
Sticking to the definition of C/N0 as the ratio between the received power and the power spectral density due to thermal noise at the input of the receiver, the presence of interference should not change the value, since the thermal noise is not increasing. However, the C/N0 value provided by the receivers is estimated on the basis of the correlator outputs at the tracking stage. For this reason the estimation is affected by the presence of the additional (nonthermal) noise generated by the interference. The variation of the C/N0 can also be used as observable for interference (or other threats) detection.
Chapters: The Interference Threat; Classification of Interfering Sources and Analysis of the Effects on GNSS Receivers; The Spoofing Menace; Analytical Assessment of Interference on GNSS Signals; Interference Detection Strategies; Classical Digital Signal Processing Countermeasures to Interference in GNSS; Interference Mitigation Based on Transformed Domain Techniques; Antispoofing Techniques for GNSS. The book is intended for members of the engineering/scientific community with pre-existing knowledge of satellite navigation principles and GNSS.
FabIo Dovis holds a Ph.D. in elecronics and communications engineering from Politecnico di Torino, Italy, where he is an associate professor.
Spectracom’s GSG-6 Series multi-frequency GNSS signal simulator. Photo: Spectracom
Spectracom has added capability to simulate India’s global navigation satellite system, IRNSS, and Japan’s regional satellite system, QZSS, to its GSG-6 Series multi-frequency GNSS signal simulator. The simulator is designed to be field upgradeable to simulate all current and future GNSS constellations so current customers can benefit from these features without the need for a factory return in most cases.
“Spectracom understands the need for system developers and integrators to be compatible with various GNSS systems. Support for multiple constellations is a requirement in many markets and additional satellites add signal diversity for improved reliability,” said Spectracom Global Sales and Marketing Vice President Rohit Braggs. “Our easy-to-use, compact and affordable GNSS simulator can now be configured with IRNSS and QZSS capability in addition to the big four: GPS, GLONASS, BeiDou and Galileo. Our customers can buy what they need now and easily upgrade in the future, often times without a hardware upgrade.”
In anticipation of the deployment of new GNSS systems, Spectracom ensures that every GSG simulator that leaves the factory is tested for compliance with all L-band signal frequency and modulation specifications as defined in their ICDs, the company said.
The Series 6 multi-frequency simulator is fully capable of all four bands of any system: L1 / E1 / B1; L2 / L2C; L5 / E5 / B2; and E6 / B3.
“As we have seen with our recent roll-out of Beidou and Galileo signal compatibility, when the need for new signals arise, we will offer those capabilities with a simple upgrade path,” Braggs said. “This ensures our customer’s investment is always protected.”
First and foremost, let’s give a big hand to Adam and Anastasia, the two Galileo FOC satellites that were successfully launched on March 27. Following the not-so-successful Galileo launch in August, it was imperative that this go smoothly.
Although the Double-A launch occurred after the conclusion of this year’s Munich Satellite Navigation Summit, anticipation of the event set the context for the entire convocation. The summit is a fixture on the European and global GNSS calendar. It is always intense, often spectacular and sometimes leaves one with contradictory feelings. This year it took place March 24-26 and sought to determine the future of PNT, encouraging delegates to look into the crystal ball and predict developments.
If we go by the number of times these words were repeated during the three days of the summit, the future will hinge around compatibility and interoperability. The multi-constellation GNSS is already here. The elephant in the room remains, as always, interference, but here integration of alternative sensors and signals should hold the key to continuous and possibly resilient operations.
As usual the summit kicked off with a high-level plenary in the imposing Allerheiligen-Hofkirche (Court Church of All Saints) in the Residenz München, the Bavarian royal palace. The welcoming speeches and presentations were interspersed with some pleasant jazz, and the atmosphere was relaxed.
Into the Crystal Ball
Matthias Petschke, director of EU Satellite Navigation Programmes at the European Commission, admitted that 2014 had been difficult, but he was looking forward to 2015. Clearly the deployment of the Galileo infrastructure — especially the space segment — was critical, and the March 27 launch was very much on his mind. However, he expressed confidence that the launch would be fine and that satellite production was, and would remain, on schedule. In the long view, he stated: “We will make it for 2020,” signifying full operational capability (FOC).
He also talked about stimulating global markets to foster uptake of Galileo and EGNOS, and this was discussed by Carlo des Dorides, executive director of the European GNSS Agency (GSA). The ground infrastructure is very much in place and preparing for the Galileo exploitation phase. A significant milestone in that process would be finding the right partner to lead Galileo operations for the next ten years. A tender was now in process to find that organization or consortium. Des Dorides described the process as a competitive dialogue with the emphasis on finding a partner who can inspire new ideas and provide innovative solutions. The contract is big, worth around 1 billion euros.
Carlo des Dorides, Executive Director of the European GNSS Agency (GSA), discusses the 1 billion euro tender, now in process to find the organization or consortium to lead Galileo operations for the next ten years. Photo: GSA
He also emphasized the successes for EGNOS in the year. Almost 180 airports now benefit from EGNOS-enabled approaches and more than 70 percent of “GNSS-enabled” farmers in EU use the EU’s SBAS.
Johann-Dietrich Wörner, chairman of the German Aerospace Centre (DLR) — and the nominated next Director-General of ESA – highlighted the growing dependence of critical services on GNSS. In this context multiple systems were not a question of competition; it was all about redundancy and safety. Multi-GNSS improves availability, accuracy and reliability.
The view from the United States was given by Harold “Stormy” Martin, Director, National Coordination Office for Space-Based Positioning, Navigation, and Timing in Washington, D.C. The GPS fleet was now 30 strong in orbit including four successful launches in 2014 and he stated the 2014 averaged user range error to be 70 cms — the best ever — and improving year on year.
One major upcoming trend is a realization that there’s a need to establish a U.S.-wide backup coverage for GPS outage due to natural or man-made interference. The U.S. is currently assessing alternatives with a decision likely in summer 2015.
There was a particularly warm welcome from the audience for Michael Khailov, deputy head of Roscosmos and co-ordinator for GLONASS. Last year the Russians were conspicuous by their absence at the Munich Summit, but for 2015, despite the intervening local difficulty in Ukraine, they were back in force. Khailov claimed that the sustainable development of the world depends on GNSS. On more esoteric ground he stated that GLONASS had maintained stable operations in 2014 and three more satellites had bene launched. Further launches would depend on operational circumstances. The user domains for GLONASS were continuously expanding. Continuing the summit text he said that it was better [working] together than separately — in fact separately often doesn’t work at all and therefore we must continue to promote interoperability and the Munich Satellite Summit is a good forum for this.
Jianyun Chen of the China Satellite Navigation bureau also took up the theme of all GNSS together. Sixteen Beidou (pronounced — for the avoidance of doubt — as ‘bay-doe’) had been launched since 2007 and the Chinese had been in discussion with Russia to ensure full interoperability with GLONASS. This process will be repeated with GPS and Galileo.
GNSS Updates
One of the idiosyncrasies of the Munich Summit is its very discreet signage. If you don’t know where it is — and specifically the correct side door that brings you up two floors to the main Max Joseph Saal venue — it is highly likely you’ll miss it! But once you are in it is two full-on days of updates on systems and discussions on a vast range of topics that impinge on the development and implementation of GNSS around the world.
Discreet signage. Photo: GSA
The first two session of the summit proper gave updates on the GNSS systems in operation and under development as well as the regional and augmentation systems. Much of the material was slightly more detailed versions of presentations at the plenary but a few news snippet emerged.
“Stormy” Martin said that a modified battery charge control had been implemented that would extend operational life for some of the fleet by one or two years. He also reiterated the improving accuracy performance of GPS which was now much better that its published standards. He predicted that the first GPS III would be available for launch in 2016 and said that GPS was improving every day.
Eric Chatre from the European Commission reiterated that Galileo was still expecting to start early services in 2016 with full operational capability in 2020. He expected 18 satellites to be launched by 2018. The new Ariane 5 launcher will enable the launch of four satellites at one time and the first launch with this system would be in 2016. In terms of the ground segment only one station in the Pacific was yet to be established.
Sergey Karutin of Roscosmos talked about a four-fold accuracy improvement for GLONASS with the use of new clocks and the introduction of new CDMA signals that will improve accuracy and access. According to Dongfeng Yu of the China Satellite Navigation Office the BeiDou constellation is moving from “regional to global, active to passive” and is aiming for global coverage by 2020.
U.S. SBAS developments were covered by Deborah Lawrence of the Federal Aviation Administration (FAA). The Wide Area Augmentation System (WAAS) now has 100 percent coverage for LPV200 in CONUS. More than 41,000 runway ends are now included, and she predicted full completion in 2016.
Jean-Marc Pieplu of the GSA talked about EGNOS status. The next system release (2.4.1) should be published in Q3 2015 and will include a significant input on ionospheric corrections. Further service evolution includes a plan to declare LPV 200 in Q4 this year and EGNOS coverage will be extended to 72 deg North and ensure full coverage of the 28 EU member states.
The Russian Augmentation system SDCM performs at 0.8 metre accuracy according to Grigory Stupak of JSC / Russian Space Systems. He noted new validated SDCM ground stations had been established in Antarctica and Brazil and stated that global exploitation was a key objective for SDCM as its satellite coverage was very wide. GLONASS and GPS together could ensure complete coverage. He also indicated that work was in hand for SDCM SBAS service certification for LPV 200 and he called for providers of all WAAS to work closely together.
2020 Vision
After lunch we were offered the chance to hear some expert views on the future of GNSS and PNT with Prof Vidal Ashkenazi of Nottingham Scientific Limited asking for their vision of GNSS in 2020. By that year there should be 100-120 GNSS satellites in orbit, multi-constellation receivers would be the norm, but what would be the new applications and what were the challenges?
Jamming and spoofing would still be issues. Pierre Bouniol of Thales thought that in civil aircraft receivers would probably incorporate jamming indicators by 2020 to inform users when signals may be compromised. For Stuart Riley of Trimble the key was integration of other sensor signals to bridge any GNSS signal outage. Gang Mao of Unicore Communications Inc. in China considered multiple frequencies to be a big help in reducing the threat of jamming. Nigel Davies of QinetiQ agreed saying there were a host of technical solutions but key for success would be solutions that use low power, are low cost and feature high usability. He also noted that safety certification of receivers for use in driverless vehicles would be required and this challenging application would need the provision of robust continuous navigation — and sub-metre accuracy.
The future market for GNSS was also discussed in a session that unveiled the GSA’s 4th Issue of its comprehensive GNSS Market Report. With almost four billion GNSS devices used worldwide and all regions experiencing growth, GNSS represents an unprecedented business opportunity. Over the past 15 months the GSA’s team of market monitoring experts has taken a close look at all aspects of the GNSS marketplace with analysis of both hardware and software market opportunities, technology trends and future developments.
Gian-Gherardo Calini, Head of Market Development at GSA, gives highlights of the comprehensive GNSS Global Market report. He will deliver this information in an April 16 webinar hosted by GPS World. Photo: GSA
The top-line results were presented by Gian-Gherardo Calini, Head of Market Development at GSA. GNSS is one of the few growing markets in the world showing 12.7 percent CAGR. It is a very attractive market with volumes and revenues driven by mass market segments: the dominant two being Location-based services and transport applications. This latest edition includes information a new market segment: Timing and Synchronisation. One area that is not included is security and government applications. Mr Calini indicated that this information has been collected by the GSA team but as it is essentially for users of the Public Restricted Service (PRS) it was not included in the open report.
Although the report is very much “Galileo flavored,” its findings are of great importance and value to whole GNSS community and will be the subject of a GPS World webinar with Mr Calini and myself on 16 April. You can register — free — for this informative global perspective now.
A panel discussion followed and covered a range of topics and applications from aviation to agriculture. Again the consensus was that chips would become multi-constellation and quickly. Philippe Prats of STMicroelectronic outlined automotive applications from insurance applications to advanced driver assistance systems (ADAS).
The role of government mandates in establishing markets was seen as positive. The e911 mandate in the states had provided the seed for GPS integration into smartphones. Similarly authentication was also seem as a significant future market driver.
Multi frequency was also showing on industry’s radar and in a couple of years will be a reality thought Philippe Prats with the main motivation being better accuracy. Frank van Diggelen of Broadcom highlighted the recent GPS World feature demonstrating cm accuracy on a smartphone.
Legal Issues
A dedicated session on legal issues was not the best attended part of the conference, which is a shame as it had some serious points to raise and highlighted a gap that is opening up between our technical abilities in GNSS and the legal basis for its use. The Munich Summit is to be commended for its commitment to providing a platform for these issues every year; they are often ignored elsewhere.
Oliver Heinrichs, a partner at BHO Legal in Cologne, emphasised the need to establish a firm regulatory framework and to ensure that any decisions did not cross World Trade Organisation (WTO) provisions and the General Agreement on Tariffs and Trade (GATT). In particular the idea of mandating a specific GNSS for applications such as emergency response systems in cars may well be incompatible with WTO rules.
Amedeo Arena of Universitá degli Studi di Napoli Federico II in Naples noted that all GNSS players were members of the WTO and considered that GNSS services and their trade was definitely “caught by the GATTs” so no favouritism for ‘home’ systems should be allowed.
Another area of controversy is automated vehicles. In discussion after the session I learnt that current international conventions governing the use of motorised vehicles require a human supervisory role at all times. There will need to be some fundamental legal groundwork done before the first driverless vehicles will be allowed out on the road for real.
These are legally complex issues and certainty will only come from test cases. Talking of complexity Aleksey Bolkunov of the Russian Federal Space Agency revealed that the legal, regulatory and standardisation measures governing GLONASS and GNSS in Russia consisted of more than 900 documents originating at various different levels of the state. This clearly gave great scope for “regulatory collisions” and he is involved in work to develop a single regulatory framework that should eliminate the remaining barriers to GNSS use in Russia.
Emerging Applications
Peter Grognard of Galileo Services chaired a final session of the day on emerging applications. Bruno Bougard of Septentrio saw dependable accuracy as key to emerging markets. He thought high precision driven by surveying was becoming more and more mainstream. For autonomous driving the challenge was to provide cost-effective, dependable accuracy at 10-30cm that was safe, reliable, and always available. This would require multi GNSS, multiple signals, highly integrated sensors and transparent and open augmentation.
For Neil Gerein of Novatel the mantra is “Accuracy, availability, assurance.” Users needed availability to their PNT solutions at all times. He also saw future applications integrating GNSS with inertial sensors and correction systems for high accuracy without the need for a base station.
or Neil Gerein of Novatel the mantra is “Accuracy, availability, assurance.” Photo: GSA
Lionel Garin of Qualcomm Inc talked about ADAS. Safety was paramount and he foresaw the need for rigorous design and certification procedures similar to that required for the aviation market. Fortunately the industry has lots of expertise here. Philip Mattos of u-blox UK argued that a volume market is in femtocell and small cell synchronisation in mobile networks where GNSS is the lowest cost solution.
Tom Stansell praised geometry as the most important and unique ingredient supplied by multi constellation GNSS. And the second most important ingredient was interoperability. He doubted users would care where their signals originated and devices would still be generically described as ‘GPS’ into the future. Application growth will be stimulated by the better geometry supplied by multi-GNSS constellations. When the E6 signal became available he predicted that 10cm accuracy would enable reliable lane keeping for ADAS.
And Galileo will supply E6 for free said Ignacio Fernandez Hernandez from the European Commission. Ignacio works on the Galileo Commercial Service design and outlined some significant differentiators of the European system including its broad signal for high accuracy and better multipath resilience, more stable clocks and improved ionospheric modelling compared to GPS.
Lionel Garin sounded a note of caution at the end of the session when he noted that multi constellation ability was good, but he was not sure what was actually gained beyond two, or perhaps three, constellations.
GNSS for Weather
The final day of the conference saw a few fragile heads courtesy of the previous evening’s Summit Space Night 2015 sponsored by Airbus Defence & Space, which took place at the Filmcasino am Hofgarten close to the conference venue. And the first session, chaired by Oliver Montenbruck from the DLR, certainly required a clear focus as we were taken through the use of GNSS in space geodesy, space navigation and reflectometry.
Roland Pail from the Technical University, Munich described results from the satellite gravity missions GRACE and GOCE that looked at mass transport processes on our dynamic Earth. A particularly sobering animation showed the extent of ice mass loss from Greenland over the past decade. But what is role of gnss here? The ability to give precise positioning of the satellites and the fact that the satellite orbits carry information on the gravity field.
Atmosphere sounding using GNSS radio occultation allows precise atmospheric profiles with global coverage in all-weathers. Jens Wickert of the Helmholtz Centre Potsdam said that since 2006 these high vertical resolution profiles had been making a significant impact on the world’s weather forecasting including improved hurricane forecasts. It was also a bias free technique for observing global temperature change. With a multi-GNSS future new missions could be planned as more signals would reduce noise. Combining GNSS and reflectometry could enable accurate tsunami detection from space. Similarly Prof Antonio Rius from Barcelona was using reflected GNSS signals to determine data on the surface of the sea such as surface roughness, extent of sea ice and early warning of a tsunami.
Stefan Sassen of Airbus Defence & Space described the LION GNSS navigation receiver for MEO and LEO platforms. The unit was qualified since 2014 and now 50 were on order. LION is highly flexible with multi-frequency, multi-constellation and multi-antennae configurations possible. It was accurate enough for autonomous orbit raising (a few kms) and or station keeping (to within 100cm).
Finally Manfred Sust of RUAG Space GmbH said that space borne gnss receivers were true enabling technologies for Earth Observation missions as precise orbit determination was key to capturing sharper images.
Alternatives
The second session of the day returned to the practical issues around possible alternative or complementary PNT (APNT) systems. As GNSS becomes ubiquitous many terrestrial PNT systems are being decommissioned (LORAN, VOR), but the potential vulnerability of GNSS signals to interference is highlighting the need for backup. The challenge being to balance functionality and cost in the search for “plan ‘B’ for GNSS” as chairman Michael Meurer from DLR described it.
The FAA’s Deborah Lawrence reiterated her plans for scoping and implementing a backup system to cope with a GPS outage in the US. The FAA is currently engaging with stakeholders to define what the minimum operational target for a GPS outage should be to set the basis for procurement activity. The timeline for a final investment decision was now December 2018.
For Europe Gerhard Berz of Eurocontrol thought there were many potential APNT in place and the topic was in the SESAR 2020 research programme. He thought existing DME could potentially do the job in Europe as it had good coverage, but the challenge is to get good geometry and coverage at low altitudes, in remoter areas and over water.
Prof Per Enge of Stanford University “put the moose on the table” and pointed to the 978 and 1030 ranging frequencies as an existing system that could be used for positioning. But how accurate was it? Airborne experiments had shown good agreement with GPS positioning with an accuracy of around 100m and in turns 300m, which was good enough in an emergency. Further tests using a UAV at spider infested Camp Rogers had demonstrated APNT in flight with 50m error. The UAV itself was specifically developed to navigate using APNT while looking for GNSS jammers.
Wouter Pelgrum of Ohio University discussed the relative merits of eLORAN, which has high power – and therefore difficult to jam – and beyond line of site accuracy of less than 10m, and alternatives such as collocation of pseudolites with mobile phone cell towers. This could also enable high accuracy indoors positioning applications. He believed that APNT will need to be context specific and there was no single solution.
Belabbas Boubeker of the DLR discussed modular APNT concepts while Nick Ward of the UK’s General Lighthouse Authorities indicated there was no coordinated policy on resilient PNT in the European maritime sector at present but his authority and others were exploring the possibility of using eLORAN as a commercial enterprise. Nine transmitters were operational in Europe and the service had been declared in 2014.
Michael Hoppe of Fachstelle der WSV für Verkehrstechniken said resilient PNT was a core element of e-navigation for waterways. A combination of techniques such as medium frequency RF, AIS and eLoran could give good accuracy in areas of highest traffic. First results of trials were encouraging.
Processing Power
The final session of the Summit to grab my full attention was chaired by Frank van Diggelen of Broadcom. He led a wide ranging debate on GNSS receiver architecture trends and more generally the future of chip design and fabrication: are we approaching the end of Moore’s law and if so — what next?
Recently “The balance of power has moved back onto the GNSS chip” to enable lower device power use. To highlight current developments Frank described a couple of Broadcom products: the Broadcom 4773 “location hub” that is at the heart of the Samsung Galaxy 6 “super smart phone” and the 4774 that can access signals from all four GNSS constellations and will be shipping in early 2016 on new smartphones.
In fact earlier this year the 4774 was used to make a first fix using signals from four different GNSS constellations (with signals from one each of GPS, GLONASS, Galileo and BEIDOU satellites) and a significant event in terms of our multi-constellation future.
Greg Turetzky of Intel talked about the benefits and challenges for GNSS in advanced silicon processes. He noted that Intel is now shipping 14nm technology and plans were in hand for the next two generations (10nm and 7nm). Moore’s law has been a great enabler for modern society. If automobiles had taken a similar development in the same timeframe we would all be driving cars with a maximum speed of some 300 000 km/hour that cost us around 4 cents to buy!
The big challenge for GNSS architecture was to take advantage of the smaller geometries while greatly reducing standby power. The integration of multiple radio sources to provide a single location solution was key giving ubiquitous location capability that will improve the experience of every mobile product.
Looking into his crystal ball, Peter Anderson of Integrated Navigation Systems in the UK saw that integrating signals from complimentary technologies and sensors would be important but would lead to a greater demand for digital processing. He predicted that multiband receivers would become standard in consumer devices. He also pointed out that the worst potential source of GNSS jamming for a smartphone was the phone itself! The move to dual frequency would be helpful here.
An overview of the Chinese XIHE system for seamless outdoor and indoor location was given by Dongkai Yang of Beihang University. This Beidou Innovative application provides a LBS system based on gnss and mobile communication networks to give a “fusion of communication and positioning for indoor positioning”. The system is being demonstrated in four areas in China in shopping malls. The target for positioning accuracy in the system is for less than 3 metres indoors and less than 1 metre outdoors.
Franz Kreupl of Munich Technical University gave a sobering view of “life after silicon” – essentially it looks like there isn’t one. He outlined the limits to silicon technology such as tunnelling current and predicted some further progress could be made in reducing interconnect sizes and via circuit design. But new candidate materials for semiconductor electronics from carbon nanotubes to widely hailed 2-D materials graphene and MoS2 all suffer major issues that seem to make them non-starters.
But do we need to keep on miniaturizing? Norbert Schuhmann of Fraunhofer IIS in Nuremberg thought that technology downscaling would have an end in terms of the physics, but especially in terms of reasonable cost. He thought 7nm and 2020 was the end point for the physics but that in fact 28nm should be seen as the actual last node in Moore’s law as from then scaling has no longer also been the path for cost reduction. He saw silicon on insulator technology and monolithic 3-D integration as possible paths forward, but the technology sweet spot — and well suited for GNSS — was 55nm and a format that was already extensively used in automotive applications.
This week nearly all the global navigation satellite systems will push their spatial presence one or two steps further, or higher, if they perform as scheduled. Rarely if ever has there been such a concentrated period of activity in the catapult category. Are we witnessing the real dawn of the multi-GNSS era? GPS, Galileo, BeiDou, and IRNSS all have positioned loaded rockets on the launching pad, destined to heave satnav payloads aloft. Only GLONASS seems stuck in stasis.
Leading the pack, as ever, GPS should send forth the ninth GPS Block IIF satellite (GPS IIF-9) on March 25 at 2:36 in the Eastern U.S. afternoon. Perhaps the event has already occurred by the time you read this.
The seventh and eighth Galileo satellites, Adam and Anastasia, are destined for a double date in space on March 27. After a four-hour flight into orbit 22,300 kilometers high, the duo will spring away from their Fregat fourth stage in opposite directions.
The launch of the fourth satellite for the Indian Regional Navigation Satellite System, scheduled for March 9 but postponed to replace a faulty onboard telemetry transmitter, will now take place on March 29. IRNSS-1D will pass the halfway point in India’s march to a seven-spacecraft regional constellation.
This philatelic first-day cover to commemorate an upcoming BeiDou launch indicates a specific date of March 31, 2015 (circled in red). Credit: BeiDou
There are indications that the first satellite in the BeiDou Phase 3 expansion may be launched by the end of March. Apparently, a BeiDou satellite has been shipped to the Xichang launch site, and tracking ships have left port for the open ocean. Also, a postal stamp first-day cover for the launch — a common Chinese practice — has been issued with a March 2015 inscription. The launch will likely be that of a medium Earth orbit satellite.
A GLONASS-M single-satellite launch from Plesetsk had been expected in the first quarter of this year, but has not materialized. A GLONASS-M triple-satellite launch from Baikonur is expected in the April/May 2015 timeframe. The Russian constellation’s orbit count now stands at 26, fully sufficient for global coverage.
As the Ides of March in 44 B.C. mark a turning point in Roman history, the transition from Republic to Empire, so might this week mark complete world domination. GPS is now ¾ down the last section of road that leads to the fully modernized Block III generation. Galileo will reach, numerically, 1/3 of the total number of satellites it needs for full operational capability, although there is some doubt about whether all satellites now in orbit can be counted as full integers. BeiDou will mark its 15th operational satellite, out of a planned total of 35, with the new philatelically commemorated rising. And, as mentioned, IRNSS will pass its halfway point this weekend.
Ironically, just as GNSS begins to show signs of approaching its apogee (similar to the dawning of Empire in the Augustan Era that followed Caesar’s assassination on the Ides of March), the world is starting to turn away from, or turn beyond, GNSS.
GNSS will remain at the core of our navigation and positioning technologies — as Roman values remain at the core of Western civilization. But we need to go now to multi-sensor approaches for several reasons:
some requisite positioning data, such as precise attitude, is not optimally derived solely from GNSS measurements;
despite their increasing numbers, GNSS satellites will never be ubiquitous enough to be visible in sufficient numbers everywhere;
threats such as jamming and interference will likely surmount all efforts at single-solution resilience to overcome GNSS vulnerability.
‘Twas ever thus. With rise come decline, with ripeness, decay. Sic transit Gloria.
The Russian company Credo-Dialogue has released Credo GNSS 1.0, a GNSS processing software.
Credo GNSS 1.0 is designed for processing of satellite geodetic measurements in differential mode. In this mode, the simultaneous operation of two or more receivers forms the baseline.
The input can use the following types of data:
satellite geodetic measurements and ephemeris format RINEX (2.0-3.2);
satellite geodetic measurements and ephemeris formats satellite geodetic receivers (in accordance with the import module);
import point coordinates from text files in any format, user-configurable;
precise ephemeris (can be downloaded automatically to the time span of the project); and
Also in the program, users can view images from web services such as Google Maps, Bing and Express Kosmosnimki.
Credo GNSS supports a variety of coordinate systems, including Transverse Mercator, Mercator, PseudoMercator, Lambert Conformal Conic and Orthographic.
To learn more about the software, click here or view the video below.
