# Presentation: 482061

## 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**

## 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

- 1176.45 MHz

**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)**

## L5 Codes and Code Properties

## 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 I & Q Code Generators

## L5 Code Generator Timing

## 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 Signal 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

**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

**10 ms Code on I**

**20 ms Code on I**

## L5 Data Content & 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

**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)

**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**

## Message Type 1

## Message Type 2

## 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, 2********, 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

- I5

- Q5

- TIQ5

- L1 C/A-P(Y)

- C/A

- P(Y)

- L1

- P(Y)

- C/A

- L2

- L5

- L2 C/A-P(Y)

## L5 Message Group Delay Parameters

**Clock correction parameters (a****f05****, a****f15****, a****f25****) 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 ********T****IQ5**** message parameter**
**“****T****GD****” terms provided for single-frequency L5 users and L2-L5 users**

## 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

**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**