August 16, 2000IMA1 Mathematical Challenges in GPS: New Opportunities Using New Civil Signals for Multipath Mitigation and Carrier Phase Ambiguity Resolution A. J. Van Dierendonck, AJ Systems

August 16, 2000IMA2 Topics New Civil Signal Requirements Characteristics Summary PN Code Structure and Properties Signal Modulation Data Structure Data Content

August 16, 2000IMA3 Signal Parameters to Consider Frequency & Bandwidth Received Power Code Structure Code Chipping Rate Data Rate Data Structure

August 16, 2000IMA4 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”

August 16, 2000IMA5 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

August 16, 2000IMA6 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

August 16, 2000IMA7 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

August 16, 2000IMA8 GPS C/A Code Deficiencies 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

August 16, 2000IMA9 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

August 16, 2000IMA10 Gold Code Properties Two shift registers of maximum length ever to be used Programmable feedback taps Code length = 2 n - 1, n = no. of stages 2 n + 1 codes, not all good Cross-correlation level inversely proportional to code length

August 16, 2000IMA11 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

August 16, 2000IMA12 Multipath VS Chipping Rate and Bandwidth

August 16, 2000IMA13 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)

August 16, 2000IMA14 Characteristics Summary L5 = MHz Allocated BW = 24 MHz (Requested) Minimum Received Power = -154 dBW PN Code Chipping Rate = 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

August 16, 2000IMA15 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

August 16, 2000IMA16 L5 Codes and Code Properties

August 16, 2000IMA17 L5 Codes Codes with 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 MHz Reset with 1 ms epochs (10,230 chips) Two codes per satellite (4096 available) One for Data channel, one for Data-Free channel

August 16, 2000IMA18 L5 I & Q Code Generators

August 16, 2000IMA19 L5 Code Generator Timing

August 16, 2000IMA20 Cross-Correlation Properties vs. C/A-Code: Zero Doppler Difference

August 16, 2000IMA21 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 < dB Another pair selected as non-standard code

August 16, 2000IMA22 L5 Signal Modulation

August 16, 2000IMA23 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

August 16, 2000IMA24 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

August 16, 2000IMA25 10-ms Neuman-Hoffman Code on I

August 16, 2000IMA26 20-ms Neuman-Hoffman Code on Q

August 16, 2000IMA27 L5 Data Content & Format

August 16, 2000IMA28 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

August 16, 2000IMA29 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 seconds

August 16, 2000IMA30 Message Type 1

August 16, 2000IMA31 Message Type 2

August 16, 2000IMA32 Message Type 3

August 16, 2000IMA33 Message Type 4

August 16, 2000IMA34 Message Type 5

August 16, 2000IMA35 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

August 16, 2000IMA36 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