Planned GPS Civil Signals and Their Benefits to the Civil

Community

Dr. A. J. Van DierendonckAJ Systems

Acknowledgements

• Briefing taken from Chris Hegarty’s Navtech Course to be given at ION-GPS-2001– But then, he is using a lot of my charts

• Briefing includes Tom Stansell’s charts on L2CS signal

• Briefing includes some Navstar JPO charts

Presentation Topics

• GPS Modernization Overview

• New Civil Signals Detail

• Performance Enhancements

GPS Modernization Overview

• Why modernize?

• GPS modernization plans

• New civil signal summary

• Galileo compatibility; plans

Why Modernize?• National policy - GPS is a vital dual-use system• For civil users, new signals/frequencies provide:

– More robustness against interference, compensation for ionospheric delays and wide/tri-laning

• For military users, new signals provide:– Enhanced ability to deny hostile GPS use, greater

military anti-jam capability and greater security

• For both civil/military, system improvements in accuracy, reliability, integrity, and availability

March 1996 Presidential Decision Directive (PDD)

• GPS is free for peaceful use worldwide

• GPS is dual civil/military system

• Selective Availability (SA) to be discontinued by 2006 (occurred in 2000)

• GPS and U.S. augmentations to be managed by Interagency GPS Executive Board (IGEB)

Civil GPS Modernization - National Policy

• February 1997 - DoD and DOT agree to provide a 2nd civil GPS frequency

• March 1998 - IGEB decision to implement two new civil signals

• January 1999 – 3rd civil signal frequency announced - 1176.45 MHz

• February 1999 - IGEB formed 3rd Civil Signal Implementation Steering Group– Established relationship between IGEB and RTCA for

development of L5 signal requirements

Civil Modernization - National Policy (continued)

• November 1999 - IGEB report Implementation of a Third Civil GPS Signal completed– Recommended implementing L5 with:

• 6 dB higher minimum received signal power than L1 C/A code

• 10.23 Mchip/second spreading codes on quadrature channels (no data on one channel)

– Other recommendations regarding coexistence of L5 with existing systems operating at/near 1176.45 MHz

• Link 16

• Distance Measuring Equipment (DME)/Tactical Air Navigation (TACAN)

Modernized Signal Evolution

1227 MHz 1575 MHz1176 MHz

L2 L1L5

P(Y)P(Y)

C/AC/A

P(Y)P(Y)

P(Y)P(Y)

C/AC/A

P(Y)P(Y)

CSCSMM MM

Present Signals

Signals AfterModernization

L5 - New Civil Signal

• Safety-of-life use

• Higher accuracy to users when used with C/A on L1– Similar accuracy as military service today – Much more robust compared with C/A on L1– Greater resistance to interference– Approximately four times more power– Improved data message– Higher chipping rate improves multipath performance

• Located in an Aeronautical Radio Navigation Service (ARNS) band for safety-of-life services use (e.g., civil aviation)

1176.45 MHz

L2 Civil Signal (L2CS)

• More robust civil signal service– Civil users currently only have codeless/ semi-

codeless access to P(Y) on L2

• Increased accuracy– Coded dual-frequency ionospheric corrections at

the receiver in the clear

• Preferred option - advanced signal structure– Better cross-correlation properties than C/A– Data-free component for robust tracking

New GPS Signals - Summary

• Today - 2 navigation frequencies, 3 signals– L1 = 1575.42 MHz (154 × 10.23 MHz)

• Coarse Acquisition (C/A) code• Precision P(Y) code

– L2 = 1227.6 MHz (120 × 10.23 MHz)• P(Y) code

• Near future - 3 navigation frequencies, 7 signals– L1 C/A, P(Y), and M-code– L2 CS, P(Y), and M-code– L5 = 1176.45 MHz (115 × 10.23 MHz)

GPS Spreading CodesSignal Chipping Rate

Carrier frequency

Comments(Mchip/s)(MHz)

C/A 1.023 1575.42 (L1)1023-chip Gold codes repeat every ms

CS 1.023 1227.6 (L2)2 codes per SV each at 511.5 kHz, future

P(Y) 10.23 L1 and L2 Repeats once/week

L5 10.23 1176.45 (L5) 2 codes per SV, future

M 5.115 L1 and L2code modulated by 10.23 MHz square wave, future

Signal Power Spectra

-15 -10 -5 0 5 10 150

0.2

0.4

0.6

0.8

1x 10

-6

Offset from Carrier Frequency (MHz)

Nor

mal

ized

Pow

er S

pect

rum

(W

/Hz) C/A or L2CS

P(Y)M

Notes: (1) C/A codes actually have line spectra - continuous approximation shown.(2) L5 signal spectrum resembles P(Y), except that L5 is also a line spectrum.

GPS Modernization Program• Last 12 Block IIRs

– Add second civil signal (L2CS) and new military signal (M-code) - more signal power

• First 6 Block IIFs (“IIF Lite”) – All of above plus new 3rd civil signal (L5)

– Next (nominally) 6 Block IIFs

– Procured as necessary to sustain the constellation

• GPS III (Full Modernization) – Meet future requirements through 2030 - more M-code

signal power

• Operational Control Segment (OCS)– Evolutionary incremental development

Block IIR- Modified Satellites

L1 L2

L1 Enhancement

• New ME code at -158 dBW

L1 Enhancement

• New ME code at -158 dBW

- Two new military signals - One new civil signal- No changes required to batteries or solar arrays

L2 Enhancements

• New L2CS at -160 dBW

• New ME code at -158 dBW

L2 Enhancements

• New L2CS at -160 dBW

• New ME code at -158 dBW

Block IIF Satellites

L1 L2L1 Enhancements

• New ME code added

L1 Enhancements

• New ME code added

•Two new military signals• One new civilian signal (C/A on L2 already present)

• Could increase power on some of these signals

•Two new military signals• One new civilian signal (C/A on L2 already present)

• Could increase power on some of these signals

L2 Enhancements

• New ME code added

• C/A code on L2

L2 Enhancements

• New ME code added

• C/A code on L2L5

L5 Signal

• New robust Civil Signal

• Power level = -154 dBw

L5 Signal

• New robust Civil Signal

• Power level = -154 dBw

Second Civil Signal

Maintain Space UserService

Third Civil Signal

1 ON 3menu

2

Rockwell

4 5 6

7 WPT

8 POS

9 NAV

CLRMARK

0 OFF

NUM LOCK

FIX FOM 1N 42* 01” 46.12”W 091* 38’ 54.36”EL + 00862 ft

ZEROIZE

The GPS IIISystem

• Relook at entire GPS Architecture to:– Achieve long term GPS performance goals– Limit long-term total ownership costs

• Ensure GPS system properly addresses and is synergized with

– Military and Civil Needs/Systems– Possible augmentation opportunities

• Ensure best GPS system for the nation for the next 30 years

• Relook at entire GPS Architecture to:– Achieve long term GPS performance goals– Limit long-term total ownership costs

• Ensure GPS system properly addresses and is synergized with

– Military and Civil Needs/Systems– Possible augmentation opportunities

• Ensure best GPS system for the nation for the next 30 years

M-Code

GPS III Overview

GPS Modernization Integrated Schedule

Milestones

Space Segment

Control Segment

201720161999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

IIF SV1 Launch

IIF Lite DeliveriesIIF SV7-SV12IIF SV1-SV6

IIF SV1-SV6IIF Lite Launches

IIF SV7-SV12

IIF OCSM-Code/L5

OCS

OCSTraining/Validation

ATP

GPS-IIIFull Capability

IOC

GPS-IIIFull Capability

FOC

DeliverS/W

OT&EComp.

SPI ContractDefinitization

M-CodeEarth (18SV)

M-CodeEarth

(24 SV)

CY

L5IOC

L5FOC

GPS III DeliveriesSV1-SV3

SV1-SV3GPS III Launches

SV4 - SVNN

SV4 - SVNN

IIR Mod DeliveriesIIR SV10-SV21

IIR Mod LaunchesIIR SV10-SV21

IIR ModFirst

Launch

GPS Constellation Size

• Through Block IIF modernization, GPS will remain a nominally 24 satellite constellation

• GPS III architecture studies are considering larger constellations as part of system-level trades– Performance benefits of larger constellation– Backward compatibility and costs are two

difficulties

Galileo Compatibility/Plans• U.S. and European Union engaged in high-level

talks on GNSS cooperation– U.S. delegation led by State Department

• Low-level discussions will follow establishment of principles for cooperation

• Lots of less formal discussions in various forums (e.g., International Civil Aviation Organization GNSS Panel, Joint Program Office visits)

• Key issue: should GPS, Galileo share spectrum?

New Civil Signals

New Civil Signals

• L5– Signal structure and pseudorandom noise

(PRN) codes– Navigation message and data format– Spectrum issues

• L2CS– Signal structure and PRN codes– Implementation options

L5 Signal Specification• IGEB Working Group 2 (WG2) chartered to

develop L5 Signal Specification– Formal relationship established with RTCA

Special Committee 159 (SC159) WG1

• December 2000 - RTCA recommendations published (RTCA DO-261)

• Air Force has converted RTCA document into L5 Interface Control Document (ICD-GPS-705)

L5 Characteristics Summary• L5 = 1176.45 MHz• Bandwidth = 24 MHz (filed)• 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 PRN Codes on I and Q

L5 Characteristics Summary (cont’d)

• I and Q Modulation (1 kbps)– Forward Error Correction (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

Data-Free Channel

• No data on Q-channel allows coherent carrier/code tracking– Allows tracking in lower SNR conditions

• Power stolen from data recovered through use of forward error correction (FEC)

Optimum Division of Power Between Data and Dataless Channel

Courtesy of Dr. Tom Morrissey, Zeta Associates

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 and 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

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

Baseline Codes’ Properties

• Same multipath/noise accuracy as P code• Narrowband and CW interference rejection is

much better than the GPS C/A code– Not quite as good as P code– Coupled with encoded data bits

• Wideband noise rejection is same as P code• Direct acquisition capability

– Not practicably available using P code

Autocorrelation Peaks (in dB)

25 50 75 100 125 150 175 200

-45

-40

-35

-30

-25

-20

-15

-10

Typi

cal A

utoc

orre

latio

n Po

wer

- dB

bel

ow F

ull C

orre

latio

n

Delay Offset - chips

Probability of Cross-Correlation Level - 0 to 5 kHz

0

0.02

0.04

0.06

0.08

Pro

bab

ility

Doppler Cross-Correlation - dB-60 -55 -50 -45 -40 -35 -30 -25

0

0.02

0.04

0.06

Pro

bab

ility

Doppler Cross-Correlation - dB-50 -45 -40 -35 -30 -25 -20 -15

0.01

0.03

0.05

0.07

L5 Codes C/A Codes

L5 Power Spectral Density - Reduces the Effect of CW Interference

2000 4000 6000 8000 10000

-50

-40

-30

-20

-10

Typi

cal P

ower

Spe

ctru

m -

dB

Frequency - kHz

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

L5 I and 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

L5 Neuman-Hoffman Codes

• Encoded symbols and carrier– Modulate at PRN Code epoch rate– Spreads PRN 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

10-ms Neuman-Hoffman Code on I

-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

20-ms Neuman-Hoffman Code on Q

-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

Typical Spectral Sidelobes Including 10-Bit Neuman-Hoffman Code

0 100 200 300 400 500

-80

-70

-60

-50

-40

Typi

cal S

idel

obe

Spec

trum

- dB

100 Hz Spectral Lines

• 20-Bit Code on Q-Channel reduces spectrum another 3 dB

L5 Data Content and Format• 5 – Six-Second 300-bit Messages

– Format with 24-bit cyclic redundancy code (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 good performance• Lined up with L1 sub-frame epochs

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 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

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 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

TGD5

8 BITS

185

toc

8 MSBs

193

IODC

10 BITS

209

DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 2 SECONDS

af0

22 BITS

219

af1

16 BITS

241

af2

8 BITS

257

CRC

24 BITS

277

toc

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

RESERVED - 4 BITS

TGD

8 BITS

265

L5 Interference Environment - Primary Concerns

DME/TACAN• Over 1700 U.S. ground beacons • 1 MHz channels across 960-1215 MHz• EIRP = 100 W - 10000 W• 3.5 s pulse width (1/2 voltage)• 2700 - 3600 pulse pairs/s

JTIDS/MIDS• Now 600 terminals (many airborne)• May be 4000 U.S. terminals by 2010• Hops over 51 3 MHz channels from 969-1206 MHz• 6.4 s pulse width • For uncoordinated exercises:

– Peak power = 200 W–396,288 pulses/12 s in 200 nmi radius

L5 Receiver Requirements

• Primary contributors to electromagnetic environment near L5 are pulsed

• More selective front-end (compared to L1 avionics) necessary to limit number of pulses desensitizing receiver

• “Pulse blanking” a low-cost, low-risk method to minimize effects on receiver performance– Performance standards should not specify design, but

will require operation in pulsed environment

Example of Worst-Case DME/TACAN Environment in U.S.

Victim aircraft at40,000 ft

over HarrisburgNote: Only TACAN/DMEs with frequency assignments from1157 - 1209 MHz are shown/analyzed.

SNR Degradation at 40,000 ft - All Known U.S. Emitters

SNR Degradation at 40,000 ft - All Known U.S. Emitters with Reassignment of In-band

DME/TACANs

Summary of L5 Coexistence with Other Systems

• On surface of Earth and at low altitudes, no modifications to existing systems appear necessary

• At high altitudes, many emitters are visible– Some changes to existing environment deemed necessary

in a few regions of the world

– DME/TACAN and JTIDS/MIDS are primary contributors to pulse environment

– U.S. intends to solve high altitude problem (in U.S.) by reassigning, as necessary, in-band DME/TACANs

L2CS Characteristics Summary• L2 = 1227.6 MHz• Bandwidth = 24 MHz (registered)• Minimum Received Power = -160 dBW• PRN Code Chipping Rate = 511.5 kHz for each of

two codes• Time Division Multiplexed (TDM) Signal

– Chip by chip multiplexing of two PRN sequences

– Total chip rate: 1.023 MHz

L2CS Definitions• L2CS – the L2 Civil Signal

• CM – the L2CS moderate length code– 10,230 chips, 20 milliseconds

• CL – the L2CS long code– 767,250 chips, 1.5 second

• NAV – the legacy navigation message provided by the L1 C/A signal

• CNAV – a navigation message structure like that adopted for L5

IIF Signal Generation

C /A C odeG enerator

10,230 C hipC ode G enerator

767,250 C hipC ode G enerator

L5-L ike C N A VM essage

25 b its/sec

C hip by C hipM ultip lexer

1 .023 M H zC lock

T ransm ittedS igna l1 /2

A 1

A 2B 1

B 2

R ate 1 /2 FE C

Legacy N A VM essage

50 b its/sec

511.5 kH z C lock

C MC ode

C LC ode

IIF L2CS Signal Options• The ability to transmit any one of the

following three signal structures upon command from the Ground Control Segment:– The C/A code with no data message (A2, B1)– The C/A code with the NAV message (A2, B2)– The chip by chip time multiplexed (TDM)

combination of the CM and CL codes with the CNAV message at 25 bits/sec plus FEC bi-phase modulated on the CM code (A1)

IIR-M Signal GenerationB1 is a potential software option to be uploaded by

the Control Segment

C /A C odeG enerator

10,230 C hipC ode G enerator

767,250 C hipC ode G enerator

L5-L ike C N A VM essage

25 b its/sec

Legacy N A VM essage

25 B its/sec

C hip by C hipM ultip lexer

1 .023 M H zC lock

T ransm ittedS igna l1 /2

A 1

A 2B 1

B 2

R ate 1 /2 FE C

Legacy N A VM essage

50 b its/sec

D 1

D 2 C 1

C 2

511.5 kH z C lock

C MC ode

C LC ode

L2CS Policy Options

• Satellites will have switch for L2 – C/A or L2CS

• Switch control is a policy decision– In the hands of bureaucrats

• Option – When to switch from L2 – C/A to L2CS– Fact 1 – Most current L1/L2 GPS Receivers can use L2 – C/A

code

– Fact 2 – No current L1/L2 GPS Receivers can use L2CS

• Please encourage the bureaucrats to leave C/A on L2 until L2CS is usable (except maybe for occasional tests)

– When most satellites can broadcast L2CS

L2CS Code Characteristics• Codes are disjoint segments of a long-period

maximal code– 27-stage linear shift register generator (LSRG) with

multiple taps is short-cycled to get desired period

– Selected to have perfect balance

• A separate LSRG for each of the two codes• Code selection by initializing the LSRG to a fixed

state specified for the SV ID and resetting (short-cycling) after a specified count for the code period or at a specified final state

• 1 cycle of CL & 75 cycles of CM every 1.5 sec

L2CS Code Generator

DELAY NUMBERS

SHIFT DIRECTION

OUTPUT

INITIAL CONDITIONS ARE A FUNCTION OF PRN AND CODE PERIOD (MODERATE/LONG)

1 3 1 1 3 3 2 3 3 2 2 3

Linear shift register generator with 27 stages and 12 taps

Code Tracking• Early minus late (E-L) code tracking loops try

to center windows, e.g., narrow correlator windows, on code transitions

• For each of the two L2CS codes, there is a transition at every chip– Because the other code is perfectly balanced, the alternate

chips average to zero

– Twice the transitions, half the amplitude, and double the average noise power (time on) yields –3 dB signal-to-noise in a one-code loop

– Both codes can be tracked, but CL-only is OK

The CNAV Message• The CNAV message data rate is 25 bps

• A rate-1/2 forward error correction (FEC), without interleaving, (same as L5) is applied, resulting in 50 symbols per sec

• The data message is synchronized to X1 epochs, meaning that the first symbol containing information about the first bit of a message is synchronized to every 8th X1 epoch

CNAV Message Content• The CNAV message content is the same as

defined for the L5 signal with the following exceptions: – Because of the reduced bit rate, the sub-frame

period will be 12 seconds rather than 6 seconds– The time parameter inserted into each data sub-

frame will provide the 12-second epoch defined by each sub-frame

– Applicable group delay terms for L1, L2, and L5 will be included

Performance BenefitsUsing New Civil Signals

Modernization Performance Benefits

• Dual and triple frequency ionospheric corrections

• New signal acquisition and tracking

• Positioning performance after modernization

• Benefits of increased constellation size

• Issues

Ionospheric Delay Estimation

• Ionospheric delays are inversely proportional to square of frequency

• Having coded access to L2 and L5 will allow civil users to accurately estimate ionospheric delays– This is largest component of stand-alone GPS error

budget now that SA has been discontinued

• Even L2 and L5 can provide a usable correction (in event L1 is lost)!

Dual-Frequency Ionospheric Correction Accuracy

L1 C/A - L2 C/A L1 C/A - L5 L2 C/A - L50

0.5

1

1.5

2

2.5

3

3.5

4R

MS

Err

or (

m)

Assumptions: RMS C/A and L2CS code tracking error = 0.3 m,RMS L5 tracking error = 0.1 m

L5 Performance Features

• Coherent carrier increases code/carrier tracking loop robustness– No advantage for initial acquisition– Can be an advantage for reacquisition

• Higher chipping rate provides superior multipath performance to C/A code

• High power and signal design provide robustness against interference

L5 Acquisition Performance

0.100

1.000

28.5 29 29.5 30 30.5 31 31.5 32 32.5 33 33.5 34

C/N0 - dB-Hz

PR

OB

AB

ILIT

Y O

F D

ET

EC

TIO

N

L1 - 60 ms

L5 - 15 ms

VS NOISE - 15 ms

L5 has 10 times as many “chips” to be searched for acquisition as C/A code, butsuperior L5 cross-correlation properties allows faster searches per dwell.

L5 Data-Free Channel Enables Phase Tracking at Lower SNR

20 21 22 23 24 25 26 27 2810

-20

10-15

10-10

10-5

100

Pro

babi

lity

of C

ycle

Slip

in 1

s In

terv

al

S/N0 (dB-Hz)

PLL on datalesschannel

L1 C/A withCostas loop

• Oscillator effects ignored - use for relative comparison only

L5 Multipath Performance

-0.0005

-0.0004

-0.0003

-0.0002

-0.0001

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0 0.2 0.4 0.6 0.8 1

MULTIPATH DELAY - C/A CHIPS

MU

LTIP

AT

H E

RR

OR

EN

VE

LO

PE

S -

C/A

CH

IPS

20 MHz BW, 10.23 MHz CODE

20 MHz BW, 10.23 MHz CODE20 MHz BW, C/A

20 MHz BW, C/A

24 MHz BW, 10.23 MHz CODE24 MHz BW, 10.23 MHz CODE

ALPHA = 0.01

0.1 C/A Chip Spacing

L2CS Performance Features

• Same multipath performance as C/A-code

• Data-Free channel (CL code) and low data rate enable low signal-to-noise tracking– Indoor or under-foliage applications– Excellent cross-correlation properties facilitate

tracking with large signal level variations from satellite-to-satellite

L2CS Low-SNR Performance

23 dB-Hz 22 dB-Hz 5033.3 & 1/3 21 dB-Hz 24 dB-Hz 7525 & ½26 dB-Hz 24 dB-Hz 2525 & ½23 dB-Hz 22 dB-Hz 5025 & ½23 dB-Hz 22.5 dB-Hz 5033.3 & ½ 23 dB-Hz 24 dB-Hz 5050 & ½23 dB-Hz 26.5 dB-Hz 5025 & None23 dB-Hz 29 dB-Hz 5050 & None

25.5 dB-Hz 26 dB-Hz Costas 50 & None

Phase slip = 0.001 with

total C/No =

WER = 0.015 with total

C/No =

Carrier power percent

Data rate(bps) &

FEC rate

For max acceleration = 29.8 Hz/sec, maximum jerk = 9.6 Hz/sec2, BL = 8 Hz

Sel

ecte

d da

ta r

ate

and

forw

ard

erro

r co

rrec

tion

(F

EC

) fo

r L

2CS

GPS Civil Accuracy w/ and w/o New Signals

Error Source Typical Range ErrorMagnitude (meters, 1)

Without SA Without SAplus 2 or morecoded signals

Selective Availability 0.0 0.0

Atmospheric Error Ionospheric Tropospheric

7.00.2

0.010.2

Clock and Ephemeris Error 2.3 2.3

Receiver Noise 0.6 0.6

Multipath 1.5 1.5

Total User Equivalent Range Error (UERE) 7.5 2.8

Typical Horizontal DOP (HDOP) 1.5 1.5Total Stand-Alone Horizontal Accuracy, 95% 22.5 8.5

Source: Shaw et. al., GNSS-2000.