Transcript DGPS Overview - edu

Planned GPS Civil Signals and
Their Benefits to the Civil
Community
Dr. A. J. Van Dierendonck
AJ Systems
Acknowledgements
• Briefing taken from Chris Hegarty’s
Navtech Course to be given at ION-GPS2001
– 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
L5
L2
L1
C/A
P(Y)
P(Y)
Present Signals
M
Signals After
Modernization
CS
P(Y)
1176 MHz
1227 MHz
M
C/A
P(Y)
1575 MHz
L5 - New Civil Signal
• Safety-of-life use
1176.45 MHz
• 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)
L2 Civil Signal (L2CS)
• More robust civil signal service
– Civil users currently only have codeless/ semicodeless 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 Codes
Signal
Chipping Rate Carrier frequency
(Mchip/s)
(MHz)
Comments
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
L5
10.23
1176.45 (L5)
2 codes per SV, future
M
5.115
L1 and L2
code modulated by
10.23 MHz square
wave, future
Repeats once/week
Signal Power Spectra
Normalized Power Spectrum (W/Hz)
1
x 10
-6
0.8
C/A or L2CS
0.6
0.4
M
P(Y)
0.2
0
-15
Notes:
-10
-5
0
5
Offset from Carrier Frequency (MHz)
10
15
(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
L1 Enhancement
• New ME code at -158
dBW
L2
L2 Enhancements
• New L2CS at -160
dBW
• New ME code at -158
dBW
- Two new military signals
- One new civil signal
- No changes required to batteries or solar arrays
Block IIF Satellites
L2 Enhancements
• New ME code added
• C/A code on L2
L1 Enhancements
L1 L5 L2
• New ME code added
L5 Signal
• New robust Civil Signal
• Power level = -154 dBw
•Two new military signals
• One new civilian signal (C/A on L2 already present)
• Could increase power on some of these signals
GPS III Overview
The GPS III
System
Maintain Space User
Service
Second Civil Signal
Third Civil Signal
M-Code
FIX
FOM 1
N 42* 01” 46.12”
W 091* 38’ 54.36”
EL + 00862 ft
1 ON
2
3
menu
4
5
6
7
W PT
8
POS
9
NAV
CLR
MARK
0
OFF
NUM
LOCK
ZEROIZE
Rockwell
• 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
GPS Modernization Integrated
Schedule
CY 1999
Milestones
2000
2001
2002
2004
IIR Mod
First
Launch
ATP
IIR Mod Deliveries
Space Segment
2003
IIR Mod Launches
2005
2006
2007
2008
2011
2012
2013
L5
IOC
IIR SV10-SV21
IIR SV10-SV21
IIF SV1-SV6
IIF Lite Deliveries
IIF SV7-SV12
IIF SV1-SV6
GPS III Deliveries
IIF SV7-SV12
SV1-SV3
GPS III Launches
SPI Contract
Definitization
2010
M-Code
Earth
(24 SV)
M-Code
Earth
(18SV)
IIF SV1
Launch
IIF Lite Launches
Control Segment
2009
IIF OCS
Deliver
S/W
M-Code/L5
OCS
OT&E OCS
Comp. Training/Validation
SV4 - SVNN
SV1-SV3
SV4 - SVNN
2014
L5
FOC
2015
2016
2017
GPS-III
GPS-III
Full Capability Full Capability
IOC
FOC
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 (ICDGPS-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
Exclusive OR
Reset to all 1s on next clock
1
2
3
4
5
6
7
8
9
10
11
12
13
Decode 1111111111101
All 1's
XA(t)
XA Coder
XBI State for SV i
XIi(t)
Code Clock
1 ms Epoch
XBI(t+niTc)
Initial XBI State
Reset
XQi(t)
1
2
3
4
5
6
7
8
9
10
11
12
13
Exclusive OR
XBQ(t+niTc)
XBI Coder
XBQ State for SV i
Initial XBQ State
1
2
3
4
5
6
7
8
Exclusive OR
XBQ Coder
9
10
11
12
13
L5 Code Generator Timing
1 ms = 10230
8190
1
XA Code
B
0
8 1
XB Code
1 = 1111111111111
8 = 1111111111101
9 = 1111111111110
2 1
B
0
9 1
2 = State 2040
a) B0 = Initial State at 1 ms (less than State 6152)
1 ms = 10230
8190
1
XA Code
8 1
2 1
8191
B
0
9 1
1 = 1111111111111
XB Code
8 = 1111111111101
9 = 1111111111110
9 1
2 = State 2040
b) B0 = Initial State at 1 ms (greater than State 6151)
B
0
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
Typical Autocorrelation Power - dB below Full Correlation
Autocorrelation Peaks (in dB)
-10
-15
-20
-25
-30
-35
-40
-45
25
50
75
100
Delay Offset - chips
125
150
175
200
Probability of Cross-Correlation
Level - 0 to 5 kHz
0.08
0.07
0.06
Probability
Probability
0.06
0.04
0.05
0.04
0.03
0.02
0.02
0.01
0
-60
0
-55
-50
-45
-40
-35
-30
Doppler Cross-Correlation - dB
L5 Codes
-25
-50
-45
-40
-35
-30
-25
-20
Doppler Cross-Correlation - dB
C/A Codes
-15
Typical Power Spectrum - dB
L5 Power Spectral Density - Reduces the
Effect of CW Interference
-10
-20
-30
-40
-50
2000
4000
6000
Frequency - kHz
8000
10000
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
100 Hz Symbol Clock
GPS L5 Data
Messages
276 bits
Add CRC
300 bits
Encode with
FEC
100 sps
10 - symbol
Neuman-Hoffman
Code
50 Hz Data Clock
1 kbaud
XI(t)
QPSK Modulator
Composite
Signal
1 ms epochs
10.23 MHz
Code Clock
Code Generator
XQ(t)
20 - symbol
Neuman-Hoffman
Code
1 kbaud
Carrier
• (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
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
Neuman-Hoffman Code Value
1
0.5
0
0
1
2
3
4
5
6
-0.5
-1
-1.5
Code Delay - Milliseconds
7
8
9
10
20-ms Neuman-Hoffman Code on Q
1.5
Neuman-Hoffman Code Value
1
0.5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20
-0.5
-1
-1.5
Code Delay - Milliseconds
Typical Spectral Sidelobes Including
10-Bit Neuman-Hoffman Code
Typical Sidelobe Spectrum - dB
-40
-50
-60
-70
-80
0
100
200
300
100 Hz Spectral Lines
400
500
• 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 NeumanHoffman 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
DIRECTION OF DATA FLOW FROM SV
100 BITS
2 SECONDS
1
9
8 BITS
32
33
15
6
BITS
PREAMBLE
MESSAGE TY PE ID
MESSAGE
TOW COUNT*
17 BITS
39
PRN
6
BITS
49
54
55
MSB FIRST
59
73
89
WN
IDOT
n
Crs
10 BITS
14 BITS
16 BITS
12 MSBs
URA INDEX - 4 BITS
ANTI-SPOOF FLAG - 1 BIT
L5 HEALTH - 5 BITS
"ALERT" FLAG - 1 BIT
DIRECTION OF DATA FLOW FROM SV
100 BITS
2 SECONDS
105
121
137
153
MSB FIRST
169
185
193
Crc
Cus
Cuc
Cis
Cic
TGD5
toc
16 BITS
16 BITS
16 BITS
16 BITS
16 BITS
8 BITS
8 MSBs
Crs - 4 LSBs
DIRECTION OF DATA FLOW FROM SV
100 BITS
2 SECONDS
209
219
241
257
265
MSB FIRST
273 277
toc
IODC
af0
af1
af2
TGD
CRC
8 LSBs
10 BITS
22 BITS
16 BITS
8 BITS
8 BITS
24 BITS
RESERVED - 4 BITS
* MESSAGE TOW COUNT = 17 MSBs OF ACTUAL TOW COUNT AT START OF NEXT 6-SECOND MESSAGE
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 ms 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 ms 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 at
40,000 ft
over Harrisburg
Note: Only TACAN/DMEs with frequency assignments from
1157 - 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
L5-Like CNAV
Message
25 bits/sec
Rate 1/2 FEC
Legacy NAV
Message
50 bits/sec
10,230 Chip
Code Generator
511.5 kHz Clock
767,250 Chip
Code Generator
1/2
CM
Code
CL
Code
Chip by Chip
Multiplexer
B2
A1
B1
A2
C/A Code
Generator
1.023 MHz
Clock
Transmitted
Signal
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 Generation
L5-Like CNAV
Message
25 bits/sec
D1
10,230 Chip
Code Generator
511.5 kHz Clock
767,250 Chip
Code Generator
1/2
Rate 1/2 FEC
D2
Legacy NAV
Message
25 Bits/sec
C1
Legacy NAV
Message
50 bits/sec
CM
Code
CL
Code
B1 is a potential software
option to be uploaded by
the Control Segment
C2
Chip by Chip
Multiplexer
B2
A1
B1
A2
C/A Code
Generator
1.023 MHz
Clock
Transmitted
Signal
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 (shortcycling) 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
DELAY
NUMBERS
3
3
L2CS Code Generator
2
3
3
2
2
3
1
1
1
3
OUTPUT
INITIAL CONDITIONS ARE A FUNCTION OF PRN AND CODE PERIOD (MODERATE/LONG)
SHIFT DIRECTION
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-tonoise 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 subframe will provide the 12-second epoch defined
by each sub-frame
– Applicable group delay terms for L1, L2, and
L5 will be included
Performance Benefits
Using 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
4
3.5
RMS Error (m)
3
2.5
2
1.5
1
0.5
0
L1 C/A - L2 C/A
L1 C/A - L5
L2 C/A - L5
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
1.000
PROBABILITY OF DETECTION
L1 - 60 ms
L5 - 15 ms
VS NOISE - 15 ms
0.100
28.5
29
29.5
30
30.5
31
31.5
32
32.5
33
33.5
34
C/N0 - dB-Hz
L5 has 10 times as many “chips” to be searched for acquisition as C/A code, but
superior L5 cross-correlation properties allows faster searches per dwell.
L5 Data-Free Channel Enables Phase
Tracking at Lower SNR
0
Probability of Cycle Slip in 1 s Interval
10
L1 C/A with
Costas loop
-5
10
PLL on dataless
channel
-10
10
-15
10
-20
10
20
21
22
23
24
25
S/N0 (dB-Hz)
26
27
28
• Oscillator effects ignored - use for relative comparison only
L5 Multipath Performance
MULTIPATH ERROR ENVELOPES - C/A CHIPS
0.0005
ALPHA = 0.01
0.0004
0.0003
0.1 C/A Chip Spacing
0.0002
0.0001
0.0000
20 MHz BW,
20 MHz BW,
20 MHz BW,
20 MHz BW,
24 MHz BW,
24 MHz BW,
-0.0001
-0.0002
-0.0003
10.23 MHz CODE
10.23 MHz CODE
C/A
C/A
10.23 MHz CODE
10.23 MHz CODE
-0.0004
-0.0005
0
0.2
0.4
0.6
MULTIPATH DELAY - C/A CHIPS
0.8
1
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
Selected data rate and forward error
correction (FEC) for L2CS
L2CS Low-SNR Performance
Data rate
(bps) &
FEC rate
50 & None
50 & None
25 & None
50 & ½
33.3 & ½
25 & ½
25 & ½
25 & ½
33.3 & 1/3
Carrier
power
percent
Costas
50
50
50
50
50
25
75
50
WER = 0.015
with total
C/No =
26 dB-Hz
29 dB-Hz
26.5 dB-Hz
24 dB-Hz
22.5 dB-Hz
22 dB-Hz
24 dB-Hz
24 dB-Hz
22 dB-Hz
Phase slip =
0.001 with
total C/No =
25.5 dB-Hz
23 dB-Hz
23 dB-Hz
23 dB-Hz
23 dB-Hz
23 dB-Hz
26 dB-Hz
21 dB-Hz
23 dB-Hz
For max acceleration = 29.8 Hz/sec, maximum jerk = 9.6 Hz/sec2, BL = 8 Hz
GPS Civil Accuracy w/ and w/o New Signals
Error Source
Typical Range Error
Magnitude (meters, 1)
Without SA
Without SA
plus 2 or more
coded signals
Selective Availability
0.0
0.0
Atmospheric Error
Ionospheric
Tropospheric
7.0
0.2
0.01
0.2
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)
Total Stand-Alone Horizontal Accuracy, 95%
1.5
1.5
22.5
8.5
Source: Shaw et. al., GNSS-2000.