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A. J. Van Dierendonck, AJ Systems

Mathematical Challenges in GPS: New Opportunities Using New Civil Signals for Multipath Mitigation and Carrier Phase Ambiguity Resolution. A. J. Van Dierendonck, AJ Systems. Topics. New Civil Signal Requirements Characteristics Summary PN Code Structure and Properties Signal Modulation

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A. J. Van Dierendonck, AJ Systems

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  1. Mathematical Challenges in GPS: New Opportunities Using New Civil Signals for Multipath Mitigation and Carrier Phase Ambiguity Resolution A. J. Van Dierendonck, AJ Systems IMA

  2. Topics • New Civil Signal Requirements • Characteristics Summary • PN Code Structure and Properties • Signal Modulation • Data Structure • Data Content IMA

  3. Signal Parameters to Consider • Frequency & Bandwidth • Received Power • Code Structure • Code Chipping Rate • Data Rate • Data Structure IMA

  4. Frequency and Bandwidth Considerations • Wide bandwidth required for good multipath mitigation • Original allocation = 24 MHz • Frequency separation from L1 and L2 required for large baseline carrier phase ambiguity resolution • Called “Tri-Lane Ambiguity Resolution” IMA

  5. New Signal Frequency Selection Requirements for Aviation • Allocated and protected as ARNS • As seen by the user, the signal must be independent of L1 and L2 signals • Completely redundant signal • User need not rely on other signals for acquisition and tracking IMA

  6. Other Requirements for Good Aviation Signal • Increased power relative to L1 (-154 dBW) • To overcome higher interference levels • Due to radar, DME/TACAN and JTIDS • 20 MHz broadcast bandwidth (minimum) • To provide required accuracy in presence of noise and multipath • Registration will be for 24 MHz IMA

  7. More Requirements • Improved code cross-correlation properties • To overcome inadequacies of L1 C/A code • To provide better signal integrity • To allow for more satellites in view • Improved bandwidth efficiency • Better accuracy and alarm limit margin • Reduce the effects of “Evil Waveforms” • Improved parity/CRC and data encoding • To provide better signal and data integrity IMA

  8. GPS C/A Code Deficiencies • 1.023 MHz chipping rate does not provide bandwidth efficiency • Very susceptible to waveform distortion • Cross-correlation properties are marginal • Acquisition problems • False alarms and wrong signal acquisition • Susceptible to CW and narrow-band interference IMA

  9. Characteristics of Longer, Higher Chipping-Rate Gold Codes • Longer codes reduce cross-correlation to more acceptable levels • Although, acquisition time is longer • Higher chipping-rates provide better accuracy • Easy to implement IMA

  10. Gold Code Properties • Two shift registers of maximum length ever to be used • Programmable feedback taps • Code length = 2n - 1, n = no. of stages • 2n + 1 codes, not all good • Cross-correlation level inversely proportional to code length IMA

  11. Candidate Code Properties • Same multipath/noise accuracy as P code • Narrowband and CW interference rejection is 10 dB better than the GPS C/A code • Not quite as good as P code, but OK • Wideband noise rejection is same as P code • Direct acquisition capability • Not practicably available using P code IMA

  12. Multipath VS Chipping Rate and Bandwidth IMA

  13. Cross-Correlation Properties • Code length reduces size of cross-correlation peaks (see Spilker, ION red book, volume I) • 3 dB for each factor of 2 (different Doppler, worst case) • 6 dB for each factor of 4 (same Doppler) • 12 dB for a factor of 8 (same Doppler) IMA

  14. Characteristics Summary • L5 = 1176.45 MHz • Allocated BW = 24 MHz (Requested) • Minimum Received Power = -154 dBW • PN Code Chipping Rate = 10.23 MHz • QPSK Signal • In-Phase (I) = Data Channel • Quadraphase (Q) = Data-Free Channel • Equal Power in I and Q (-157 dBW) • Independent PN Codes on I and Q IMA

  15. Characteristics Summary (Cont.) • I and Q Modulation (1 kbps) • FEC encoded 50 bps data on I (100 sps) • Further encoded with 10-bit Neuman-Hoffman Code • Q encoded with 20-bit Neuman-Hoffman Code • More details to follow IMA

  16. L5 Codes and Code Properties IMA

  17. L5 Codes • Codes with 2 - 13 stage shift registers • Length of one (XA code) = 8190 chips • Length of second (XB code) = 8191 chips • Exclusive-Or’d together to generate longer code • Chipping rate of 10.23 MHz • Reset with 1 ms epochs (10,230 chips) • Two codes per satellite (4096 available) • One for Data channel, one for Data-Free channel IMA

  18. L5 I & Q Code Generators IMA

  19. L5 Code Generator Timing IMA

  20. Cross-Correlation Properties vs. C/A-Code: Zero Doppler Difference IMA

  21. L5 Code Performance Summary • 74 Codes have been selected • 37 I, Q pairs • Max non-peak autocorrelation  -30 dB • Maximum cross-correlation with other selected codes  -27 dB • Maximum cross-correlation between I, Q pairs < -74.2 dB • Another pair selected as non-standard code IMA

  22. L5 Signal Modulation IMA

  23. L5 I & Q Code and Symbol Modulation • (Coded) coherent carrier in-quadrature with data • Allows for robust code & carrier tracking with narrow pre-detection bandwidth • Independent codes to remove QPSK tracking bias IMA

  24. Neuman-Hoffman Codes • Encoded symbols and carrier • Modulate at PN Code epoch rate • Spreads PN Code 1 kHz spectral lines to 50 Hz spectral lines (including FEC) • Reduces effect of narrowband interference by 13 dB • Primary purpose of NH Codes • Also allows detection of narrowband interference • Reduces SV cross-correlation most of the time • Provides more robust symbol/bit synchronization IMA

  25. 10-ms Neuman-Hoffman Code on I IMA

  26. 20-ms Neuman-Hoffman Code on Q IMA

  27. L5 Data Content & Format IMA

  28. L5 Data Content & Format • 5 – Six-Second 300-bit Messages • Format with 24-bit CRC (same as WAAS) • Encoded with Rate 1/2 FEC • To make up for 3-dB QPSK reduction • Symbols modulated with 10-bit Neuman-Hoffman Code • Messages scheduled for optimum receiver performance • Lined up with L1 sub-frame epochs IMA

  29. L5 Message Types (of 64 possible) • Message Type 1 - Ephemeris/Clock I • Message Type 2 - Ephemeris/Clock II • Message Type 3 - Ionosphere/UTC • Message Type 4 - Almanac • Message Type 5 - Text Message • Anticipated that Ephemeris/Clock Messages would be repeated every 18-24 seconds IMA

  30. Message Type 1 IMA

  31. Message Type 2 IMA

  32. Message Type 3 IMA

  33. Message Type 4 IMA

  34. Message Type 5 IMA

  35. Message Content • Mostly, content is same as on L1 • Clock parameters describe L1-C/A/L5 combined offset rather than L1-P/L2-P combined offset • L1/L5 Group Delay variable for single frequency users • Add L5 Health • Different Text Message • Add PRN number • Peculiar L5 information can be provided by civil community IMA

  36. Special Acknowledgements • Gary McGraw, Rockwell Collins • Peter Fyfe, Boeing • Karl Kovach, ARINC • Keith Van Dierendonck • Tom Morrissey, Zeta Associates • Tom Stansell & Charlie Cahn • Rich Keegan (Leica) • Jim Spilker IMA

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