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L5 Signal Characteristics. Dr. A.J. Van Dierendonck, AJ Systems Dr. Chris Hegarty, MITRE Co-chairs RTCA SC159 WG1 GPS L2/L5 Industry Day May 2, 2001. Topics. L5 Signal Design Status Characteristics Summary PN Code Structure and Properties Signal Modulation Data Structure Data Content.

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l5 signal characteristics

L5 Signal Characteristics

Dr. A.J. Van Dierendonck, AJ Systems

Dr. Chris Hegarty, MITRE

Co-chairs RTCA SC159 WG1

GPS L2/L5 Industry Day

May 2, 2001

topics
Topics
  • L5 Signal Design Status
  • Characteristics Summary
  • PN Code Structure and Properties
  • Signal Modulation
  • Data Structure
  • Data Content
l5 signal design status
L5 Signal Design Status
  • Signal design is complete
  • RTCA SC159 published an L5 Signal Specification
    • Some specification details affected by SV design included comments
      • Phase Noise specification
      • Correlation Loss specification
      • Detailed Received Power specification
      • Inter-frequency and inter-code delays
    • To be resolved in ICWG process as SV design evolves
l5 characteristics summary
L5 Characteristics - Summary
  • L5 = 1176.45 MHz
  • Bandwidth = 24 MHz (20 MHz null-to-null)
  • 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 each)
  • 50 bps data (rate 1/2 FEC encoded)

1176.45 MHz

l5 codes
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
l5 code performance summary
L5 Code Performance Summary
  • 74 Codes have been selected
    • 37 I, Q pairs
  • Maximum non-peak autocorrelation  -30 dB
  • Maximum cross-correlation with other selected codes  -27 dB
  • Average autocorrelation and cross-correlation  -42 dB
  • Maximum cross-correlation between I, Q pairs < -74.2 dB
  • Another pair selected as non-standard code
l5 i q code and symbol modulation
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
neuman hoffman codes
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
      • Reduces SV cross-correlation most of the time
      • Provides more robust symbol/bit synchronization
l5 neuman hoffman codes
L5 Neuman-Hoffman Codes

10 ms Code on I

20 ms Code on I

l5 data content and format
L5 Data Content and 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
message content
Message Content
  • Mostly, content is same as on L1
  • Exceptions:
    • Group delay terms added
    • L5 Health added
    • Different Text Message
    • PRN number added
l5 message types of 64 possible
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
group delay considerations
Group Delay Considerations
  • GPS time defined based on L1/L2 P(Y) iono-free measurement combination
    • Clock correction terms in NAV data convert SV time to GPS time
  • C/A-to-P(Y) timing variations specified to be < 10 ns, 2s, but no corrections provided
    • One reason why L1/L2 C/A code will never be as accurate as L1/L2 P(Y)
  • L5 introduces new group delay offsets
    • Q: How to tie L5 into GPS time?
satellite group delays
Satellite Group Delays

C/A

L1 C/A-P(Y)

L1

P(Y)

P(Y)

L2

L2 C/A-P(Y)

C/A

I5

L5

Q5

DTIQ5

l5 message group delay parameters
L5 Message Group Delay Parameters
  • Clock correction parameters (af05, af15, af25) provided to correct SV time to GPS L5 time
    • Simplest way to provide accurate time for L1 C/A-L5 users
    • Presumes Control Segment monitors L1 C/A and L5
  • I5-to-Q5 transition offset corrected by DTIQ5 message parameter
  • “TGD” terms provided for single-frequency L5 users and L2-L5 users
summary
Summary
  • L5 Signal Specification complete
    • RTCA publication in December 2000
  • L5 signal and message structure provide many advanced features relative to C/A
    • Improved ranging precision and accuracy
    • Robustness in interference/low SNR conditions
    • Flexible message format for future growth
special acknowledgements
Special Acknowledgements
  • Swen Ericson, MITRE
  • 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