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

2

Topics

• L5 Signal Design Status

• Characteristics Summary

• PN Code Structure and Properties

• Signal Modulation

• Data Structure

• Data Content

3

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

4

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)

5

L5 Codes and Code Properties

6

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

7

L5 I & Q Code Generators

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

Exclusive OR

Initial XBI State

Exclusive OR

All 1's

1 ms Epoch

Code Clock

XA(t)

XBI(t+ n iT c)

XI i(t)

XA Coder

XBI Coder

XBI State for SV i

ResetXQ i(t)

XBQ(t+ n iT c)

1 2 3 4 5 6 7 8 9 10 11 12 13

Initial XBQ State

Exclusive OR

XBQ Coder

XBQ State for SV i

Decode 1111111111101

Reset to all 1s on next clock

8

L5 Code Generator Timing

XB CodeB0

XA Code1 1

1

8 1

B0

1 ms = 10230

8190

1 = 1111111111111 8 = 1111111111101

a) B0 = Initial State at 1 ms (less than State 6152)

9 = 1111111111110

9

2 = State 2040

2

B0

XB Code

XA Code1 1

119 9

1

B0

1 ms = 10230

8190

8191

1 = 1111111111111 8 = 1111111111101

b) B0 = Initial State at 1 ms (greater than State 6151)

8 2

9 = 1111111111110 2 = State 2040

9

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

10

L5 Signal Modulation

11

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

GPS L5 DataMessages

Add CRC10 - symbol

Neuman-HoffmanCode

Code Generator10.23 MHzCode Clock

1 ms epochs

XI(t)

1 kbaud

XQ(t)

Encode withFEC

100 sps

20 - symbolNeuman-Hoffman

Code

1 kbaud

276 bits 300 bits

50 Hz Data Clock

QPSK ModulatorComposite

Signal

Carrier

100 Hz Symbol Clock

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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

13

L5 Neuman-Hoffman Codes

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10

Code Delay - Milliseconds

Neu

man

-Ho

ffm

an C

od

e V

alu

e

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Code Delay - Milliseconds

Ne

um

an

-Ho

ffm

an

Co

de

Va

lue

10 ms Code on I 20 ms Code on I

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L5 Data Content & Format

15

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

16

Message Content

• Mostly, content is same as on L1• Exceptions:

– Group delay terms added– L5 Health added– Different Text Message– PRN number added

17

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

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Message Type 1

8 BITS

MESSAGE TYPE ID

6BITS

PREAMBLE

PRN

6BITS

MESSAGETOW COUNT*

17 BITS

"ALERT" FLAG - 1 BIT

WN

10 BITS

9 3315 39 49

L5 HEALTH - 5 BITS

55

URA 5 INDEX - 4 BITS

IDOT

14 BITS

59

n

16 BITS

73

Crc

16 BITS

Cus

16 BITS

89

105 121

Crs - 4 LSBs

DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 2 SECONDS

DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 2 SECONDS

Cuc

16 BITS

137

Cis

16 BITS

153

Cic

16 BITS

169

1

TGD15

8 BITS

185

toc5

8 MSBs

193

IODC

10 BITS

209

DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 2 SECONDS

af05

22 BITS

219

af15

16 BITS

241

af25

8 BITS

257

CRC

24 BITS

277

toc5

8 LSBs

273

32 54

ANTI-SPOOF FLAG - 1 BIT

* MESSAGE TOW COUNT = 17 MSBs OF ACTUAL TOW COUNT AT START OF NEXT 6-SECOND MESSAGE

Crs

12 MSBs

TIQ5 - 4 BITS

TGD25

8 BITS

265

19

Message Type 2

8 BITS

MESSAGE TYPE ID

6BITS

PREAMBLE

MESSAGETOW COUNT*

17 BITS

"ALERT" FLAG - 1 BIT

32 BITS

9 15 33

M0

32 BITS

65

e

28 LSBs

i0

32 BITS

97

129

DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 2 SECONDS

DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 2 SECONDS

0

32 BITS

161

1

8 MSBs

193

225

DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 2 SECONDS

toe

16 BITS

24 BITS

241

IODE

10 BITS

265

CRC

24 BITS

24 LSBs

276

32

275

FIT INTERVAL FLAG - 1 BIT

A

e - 4 MSBs

RESERVED - 1 BIT

277

* MESSAGE TOW COUNT = 17 MSBs OF ACTUAL TOW COUNT AT START OF NEXT 6-SECOND MESSAGE

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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?

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Satellite Group Delays

I5

Q5

TIQ5

GD2

2L1L T]f/f1[

L1 C/A-P(Y)C/A

P(Y)L1

P(Y)

C/AL2

L5

L2 C/A-P(Y)

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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 TIQ5 message parameter

• “TGD” terms provided for single-frequency L5 users and L2-L5 users

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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

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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