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T. Sakai, K. Matsunaga, K. Hoshinoo, ENRI T. Walter, Stanford University

ION NTM 2006 Monterey, CA Jan. 18-20, 2006. Prototype of SBAS and Evaluation of the Ionospheric Correction Algorithms. T. Sakai, K. Matsunaga, K. Hoshinoo, ENRI T. Walter, Stanford University. Introduction. Implementation of the prototype of SBAS:

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T. Sakai, K. Matsunaga, K. Hoshinoo, ENRI T. Walter, Stanford University

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  1. ION NTM 2006 Monterey, CA Jan. 18-20, 2006 Prototype of SBAS and Evaluation of the Ionospheric Correction Algorithms T. Sakai, K. Matsunaga, K. Hoshinoo, ENRI T. Walter, Stanford University

  2. Introduction • Implementation of the prototype of SBAS: • A prototype of SBAS has been successfully implemented; • Outputs the complete SBAS messages; tested with the SBAS user receiver simulator; • The overall performance is comparable with the MSAS. • Evaluation of ionospheric correction algorithms: • Using the above prototype as an evaluation tool; • Evaluation of the current algorithm: ‘Planar Fit’; the storm detector trips a lot during storm ionospheric condition; • Proposed algorithm with the zeroth order fit reduces the protection levels to the third part of the current algorithm.

  3. Motivation • MSAS is now under operational test procedures for IOC: • Protection levels are hugely conservative with large margins; not reflecting the actual performance; • Needs reducing protection levels to improve availability; • However, MSAS has no ‘Testbed’: • There is only the operational system; cannot be used for testing new algorithms; • The prototype of SBAS will be a powerful tool for evaluation of new algorithms for improvement of MSAS. • QZSS is being developed in Japan; also needs a testbed for investigation of wide-area augmentation technique.

  4. Implemented Prototype • Actually computer software running on PC and UNIX: • ‘RTWAD’ written in C language (not MATLAB, sorry); • Consists of the essential components and algorithms of WADGPS; • Developed for study purpose; does not meet the safety requirements for civil aviation navigation facilities. • Generates the complete SBAS messages: • Outputs one message per second; • 250-bit message without FEC encoding; • Optional output in NovAtel $FRMA format; works as direct input to SBAS user receiver simulator.

  5. Message Sample $FRMA,272,86403.130,138,80811EA4,250,53081FFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFFBBBBBBBBBBBBAC1CD280*7C $FRMA,272,86404.130,138,80811EA4,250,9A0C1FFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFFBBBBBBBBBBBBB7E76F80*0F $FRMA,272,86405.130,138,80811EA4,250,C661FFDFFDFFDFFDFFDFFFBBBBB8800000000000000000000000000036CD8A40*70 $FRMA,272,86406.130,138,80811EA4,250,5306FFBFFFF8000000000000000000000000000000000000000000002B963FC0*0D $FRMA,272,86407.130,138,80811EA4,250,9A091FFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFFBBBBBBBBBBBB806D3340*77 $FRMA,272,86408.130,138,80811EA4,250,C60D1FFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFFBBBBBBBBBBBB924AAE40*08 $FRMA,272,86409.130,138,80811EA4,250,5361FFDFFDFFDFFDFFDFFFBBBBB89000000000000000000000000000021FE640*73 $FRMA,272,86410.130,138,80811EA4,250,9A61FFDFFDFFDFFDFFDFFFBBBBB8A00000000000000000000000000039994D00*05 $FRMA,272,86411.130,138,80811EA4,250,C60A1FFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFFBBBBBBBBBBBBA6BE8CC0*03 $FRMA,272,86412.130,138,80811EA4,250,530E1FFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFDFFFBBBBBBBBBBBBA99E5040*0A Time Message Type ID (6 MSBs) CRC SBAS Satellite PRN Preamble Message Length

  6. Implemented Prototype • Currently running in offline mode: • Used for various evaluation activities; • RINEX observation files are input as monitor station observations; provided from GEONET continuous observation network operated by GSI, Japan; • Thus the distribution of monitor stations is variable and the historical ionospheric storm events can be tested. • Utilizes only code phase pseudoranges on dual frequencies: • Needs no carrier measurements; • Outputs one message per second although RINEX is 30-second sampling.

  7. User Receiver Simulator • SBAS user receiver simulator: • Also software running on PC and UNIX; • Processes RINEX observation file with L1 pseudorange; carrier smoothing applied; • Decodes SBAS message stream (in NovAtel $FRMA records) and apply them to the observations; • Tested with WAAS and MSAS messages. • The implemented prototype was evaluated by this simulator: • With GEONET observations at some locations; • Output messages work functional; • Evaluates position accuracies and protection levels.

  8. Monitor Stations GEONET for Monitor Stations GEONET for User Stations MSAS Service Area • We used GEONET sites as monitor stations. • Dual frequency observation sampled every 30 seconds. • 6 monitor stations similar to the MSAS. • 5 user locations for evaluation.

  9. Example of User Position Error Standalone GPS Augmented by the Prototype System Horizontal Error Vertical Error Standalone GPS RMS 1.929 m 3.305 m Max 6.993 m 14.48 m Prototype RMS 0.381 m 0.531 m Max 2.867 m 5.451 m • Example of user positioning error at Site 940058 (Takayama; near center of monitor station network). • Period: 22-24 July 2004; active ionosphere condition.

  10. Performance (Nominal) Site 2005/11/14-16 2004/7/22-24 2004/6/22-24 2005/11/14-16 MSAS Hor Ver Hor Ver Hor Ver Hor Ver 940030 RMS Max 0.354 1.695 0.418 2.517 0.432 2.318 0.566 4.455 0.397 2.047 0.602 4.717 0.381 1.659 0.631 2.405 North 940058 RMS Max 0.304 1.487 0.413 2.123 0.381 2.867 0.531 5.451 0.425 2.634 0.603 3.466 0.502 4.873 0.728 3.700 940083 RMS Max 0.353 1.902 0.508 4.452 0.403 2.468 0.592 4.240 0.385 1.757 0.649 3.782 0.637 8.517 0.881 9.396 950491 RMS Max 0.453 3.302 0.647 6.158 0.586 2.143 0.764 5.509 0.491 2.415 0.776 4.574 0.640 3.012 0.730 2.680 South 92003 RMS Max 1.132 6.266 1.102 5.958 0800 4.487 1.317 9.225 0.708 4.507 1.088 6.595 0.982 6.267 1.014 6.614 Unit: [m]

  11. Performance (Severe Storm) Site 2004/11/8-10 2003/10/29-31 Hor Ver Hor Ver 940030 RMS Max 1.546 7.479 1.900 11.44 0.982 5.645 1.057 6.542 North 940058 RMS Max 1.157 7.221 1.560 9.265 0.659 5.194 0.840 6.652 940083 RMS Max 1.057 6.375 1.559 12.80 1.407 14.90 1.863 12.38 950491 RMS Max 1.639 21.90 2.195 23.09 2.164 29.42 2.901 36.31 South 92003 RMS Max 3.302 26.84 3.427 38.86 3.121 15.93 3.356 21.67 • Large errors due to ionosphere. • Users still protected within protection levels. Unit: [m]

  12. Protection Levels (Quiet) Protection level Protection level of MSAS Actual error • Protection levels during quiet ionosphere at site 950491 (second southern user). • Protects users with large margins. • MSAS provided further conservative protection levels.

  13. Protection Levels (Storm) Protection level Actual error • Protection levels during storm ionosphere at site 950491 (second southern user). • Still protects users with large margins. • However protection levels grow large; this means low system availability.

  14. Protection Level Statistics Site 2005/11/14-16 2004/7/22-24 2004/6/22-24 2005/11/14-16 MSAS Site 2004/11/8-10 2003/10/29-31 Hor Ver Hor Ver Hor Ver Hor Ver Hor Ver Hor Ver 940030 20.02 32.11 22.56 33.69 21.73 34.32 25.82 40.48 940030 101.3 152.7 127.7 181.6 940058 19.41 32.18 22.08 32.58 27.00 37.69 32.83 46.29 940058 91.61 146.0 191.0 231.3 940083 21.62 35.46 23.13 36.99 21.39 37.82 37.67 50.08 940083 89.76 154.0 152.5 249.5 950491 28.37 43.97 26.59 41.24 23.14 39.36 44.34 56.24 950491 100.6 167.7 144.0 229.7 92003 55.34 65.38 35.58 56.11 31.32 53.77 85.79 123.0 92003 109.5 188.6 129.4 216.9 Nominal Ionosphere • RMS of protection levels in meters. • Grows large for storm ionospheric conditions. Severe Storm Ionosphere

  15. Problems • WADGPS Corrections work well: • Positioning accuracy is 0.3-0.6m horizontal and 0.4-0.8m vertical, respectively, over mainland of Japan; • The largest vertical error was less than 40 meters; could support APV-I operation (HAL=40m, VAL=50m). • Protection levels are hugely conservative: • HPL and VPL completely protected users; • However, protection levels were not reflecting the actual performance regardless of ionospheric activities; • Needs reducing protection levels to improve availability. • Investigates this problem using the prototype system.

  16. VPL during Storm Protection level Ionospheric component Actual error • Vertical protection levels during storm ionosphere at site 950491 (second southern user) with the baseline algorithm. • Most of VPL came from ionospheric component. • To reduce protection levels, the primary issue is ionosphere.

  17. UIVE and Actual Residual 5.33 UIVE Actual ionospheric residual • UIVE (user ionospheric vertical error) is interpolated from GIVE (grid ionospheric vertical error). • 5.33 UIVE works as ionospheric component of protection levels. • Large margin even during historical severe storm.

  18. UIVE without Storm Detector 5.33 UIVE Actual ionospheric residual • Without the storm detector algorithm, UIVE was computed like this. • The large UIVE in daytime is resulted in by trip of storm detector. • The actual ionospheric residual exceeded 5.33 UIVE only once even without storm detector.

  19. Storm Detector Problem • The ionospheric storm detector caused a lot of false alert conditions lowering system availability: • When storm detector trips, the associate GIVE value is set to the maximum. • To avoid such a problem there are two possible ways: • Develop an alternative safety mechanism instead of the storm detector; • Develop a method to compute GIVE values instead of setting to the maximum when storm detector trips. • Here we introduce the latter algorithm.

  20. Introducing Zeroth Order Fit 1-st order fit (3 parameters) Rmax Ionospheric delay 0-th order fit (1 parameter) Estimated delay at IGP • We can reduce the order of fit when the storm detector trips; the planar model cannot be applied. • Only one parameter needs to be estimated; equivalent to the weighted average. • Let us see UIVE and the actual residuals induced by the zeroth order fit.

  21. UIVE by Zeroth Order Fit 5.33 UIVE Actual ionospheric residual • UIVE computed with the zeroth order fit without the storm detector algorithm. • UIVE is larger than planar fit. • The largest residual was within 5.33 UIVE even during the historical storm events; the zeroth order fit does not need the storm detector.

  22. Adaptive Algorithm • The zeroth order fit works well and protects residuals within 5.33 UIVE even during storm ionospheric conditions. • Thus the following adaptive algorithm can be employed: • 1. Apply the standard planar fit with the storm detector; • 2. If storm detector does not trip, employ resulted correction and GIVE; • 3. Otherwise, or the number of IPPs is insufficient for the standard planar fit, perform the zeroth order fit. • This algorithm will reduce the number of IGPs with the maximum GIVE due to trip of storm detector.

  23. Protection Levels (Storm) Baseline Algorithm Adaptive Algorithm • Reduced protection levels to the third part; improved availability. • Still protects users with large margins.

  24. GIVE Statistics Baseline algorithm Adaptive algorithm GIVEI = 14 Maximum GIVE • Current baseline algorithm produced the maximum GIVE (GIVEI=14) for 50% of IGPs. • The adaptive algorithm reduced the maximum GIVE conditions and replaced to GIVEI=13. GIVEI = 15 Not Monitored

  25. Reduction of Protection Levels Site 2004/11/8-10 2003/10/29-31 Hor Ver Hor Ver 940030 Baseline Adaptive 101.3 27.31 152.7 41.83 127.7 32.73 181.6 48.88 940058 Baseline Adaptive 91.61 22.93 146.0 37.36 191.0 31.32 231.3 47.87 940083 Baseline Adaptive 89.76 24.69 154.0 41.05 152.5 40.14 249.5 61.95 950491 Baseline Adaptive 100.6 29.73 167.7 48.65 144.0 41.13 229.7 64.01 92003 Baseline Adaptive 109.5 38.26 188.6 65.21 129.4 44.01 216.9 73.00 • RMS protection levels in meters during storm ionospheric conditions. • The adaptive algorithm reduced protection levels to the level of third part of the baseline algorithm.

  26. System Availability Site LNAV/VNAV Availability APV-I (LPV) Availability 2004/11/8-10 2003/10/29-31 2004/11/8-10 2003/10/29-31 940030 Baseline Adaptive 38.2 % 81.7 % 29.2 % 62.8 % 37.1 % 77.6 % 28.4 % 59.0 % 940058 Baseline Adaptive 37.4 % 88.2 % 26.1 % 65.9 % 36.9 % 86.8 % 25.9 % 62.1 % 940083 Baseline Adaptive 39.4 % 83.3 % 15.8 % 36.8 % 39.1 % 81.6 % 15.8 % 34.9 % 950491 Baseline Adaptive 38.6 % 69.6 % 20.0 % 34.3 % 38.3 % 68.7 % 19.9 % 33.9 % 92003 Baseline Adaptive 26.3 % 34.6 % 14.0 % 18.8 % 25.7 % 33.8 % 14.0 % 18.7 %

  27. Upcoming Plans • Realtime operation: • For implementation and tests integrity functions (TTA); • RTWAD runs with causality to input observations; little modification for realtime operation; • Signal biases will be computed day by day; • ENRI is installing realtime monitor stations; 6 stations up to now and one more shortly; • Evaluation activities in offline mode: • Effects of additional monitor stations (IGS stations); • Testbed for dual frequency SBAS.

  28. Realtime Stations Realtime Statins MSAS Stations MSAS Service Area • We have already installed 6 stations with realtime datalink to ENRI, Tokyo. • Additional station in Toyama is to be installed shortly.

  29. Experimental Equipment Trimble 4000SSE in Sapporo NovAtel MiLLennium and IP converters at ENRI, Tokyo

  30. Conclusion • A prototype of SBAS has been successfully implemented: • Overall accuracy: 0.3-0.6m horizontal and 0.4-0.8m vertical; • Protection levels completely protects users. • Evaluation of ionospheric correction algorithms using prototype: • The current algorithm caused a lot of the maximum GIVE; • Adaptive algorithm will reduce the protection levels to the third part of the current algorithm and improve availability. • Future works will include: • Realtime operation of prototype system; • Simulation of dual frequency SBAS. • Contact: sakai@enri.go.jp

  31. Ionospheric Delay: Quiet

  32. Ionospheric Delay: Storm

  33. MSAS Architecture GPS Constellation MTSAT • 2 GEO • 2 MCS • 2 MRS • 4 GMS Sapporo GMS NTT 64Kbps User Kobe MCS L-band 1Mbps Hitachiota MCS Fukuoka GMS Tokyo GMS K-band Ground Link KDD 64Kbps MCS Master Control Station Hawaii MRS Monitor and Ranging Station MRS Naha GMS Australia MRS GMS Ground Monitor Station

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