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The New L2 Civil Signal
LCDR Richard D. Fontana GPS Joint Program Office Wai Cheung and Paul M. Novak Science Applications International Corporation Thomas A. Stansell, Jr. Stansell Consulting


Companion Article GPS World, Sept. 2001


Topics
Acknowledgements Development framework Signal description Acquisition and code tracking Message options Relative signal performance Future choice of signals Signal characteristics summary L2C advantages


Special Acknowledgements
Col. Douglas Loverro ­ why replicate C/A code? Steve Lazar ­ first analysis and R/C code option LCDR Richard Fontana ­ led & coordinated JPO effort Wai Cheung ­ organized, hosted, managed Dr. Charlie Cahn ­ codes, analyses, insight & wisdom Dr. Phil Dafesh ­ lower bit rate & hardware demo Rich Keegan ­ validated receiver feasibility Tom Stansell ­ coherent carrier, guided, presented Dr. A.J. Van Dierendonck ­ alternatives, L5 experience Karl Kovach, Soon Yi, Dr. Rhonda Slattery ­ document


Development Framework
Tight schedule (1.5 months, 3 meetings) Limited chip rate (spectral separation) Bi-phase signal at lower power (shared with P/Y) Application requirements Modern technology (to acquire longer codes) Dramatic increase in new GPS signals


Spectral Separation Limits Civil Chip Rate


Development Framework
Tight schedule (1.5 months, 3 meetings) Limited chip rate (spectral separation) Bi-phase signal at lower power
· L2 civil signal is shared with the military P/Y code · L5 has 2 bi-phase components in phase quadrature · L2 civil power is ~ 2.3 dB less than L1 C/A

Application requirements Modern technology (to acquire longer codes) Dramatic increase in new GPS signals


L1 Signal Component Vector Relationships


L2 Signal Component Vector Relationships

L2 Civil is ~2.3 dB weaker than L1 Civil on IIR-M and IIF Satellites


Development Framework
Tight schedule (1.5 months, 3 meetings) Limited chip rate (spectral separation) Bi-phase signal at lower power (shared with P/Y) Application requirements Modern technology (to acquire longer codes) Dramatic increase in new GPS signals


Two Primary L2C Application Requirements
Dual-frequency civil users
· About 50,000 used for high value applications


Scientific: Cadastral Guidance Land and Marine su

earthquakes, volcanoes, continental drift, weather and construction land survey & control: mining, construction, agriculture offshore land and mineral exploration rvey and construction

· Need a civil code to replace semi-codeless tracking

Single frequency with wide dynamic range
· Avoid crosscorrelation problems of C/A code · E911 inside buildings, forest areas, tree-lined roads


Dual Frequency Transition Issue
Is L2 phase, measured with a code, the same as a semi-codeless phase measurement?
· Semi-codeless L2 phase is L1 C/A phase plus the phase difference between L2 and L1 P/Y phase L2 = L1C/A + (L2P/Y ­ L1P/Y) · Any difference in the P/Y to C/A quadrature phase relationship between L1 and L2 will cause a bias relative to a code-based phase measurement


Are the differences negligible? For sure? Can they be calibrated? Are they stable? How to identify which measurement technique was used? Should both measurements be made during transition?


Development Framework
Tight schedule (1.5 months, 3 meetings) Limited chip rate (spectral separation) Bi-phase signal at lower power (shared with P/Y) Application requirements Modern technology (to acquire longer codes) Dramatic increase in new GPS signals


C/A Code Developed for 1970's Technology

5 Analog Channels


Dramatic Technology Progress since the 1970's


Development Framework
Tight schedule (1.5 months, 3 meetings) Limited chip rate (spectral separation) Bi-phase signal at lower power (shared with P/Y) Application requirements Modern technology (to acquire longer codes) Dramatic increase in new GPS signals


Historic Increase in GPS Navigation Signals

1978 to 2003

2003

2005


Expected Growth in L2C and L5 Signals
Assumes modernization of 12 IIR Satellites


L2C Definitions
L2C ­ the new L2 Civil Signal CM ­ the L2C moderate length code
· 10,230 chips, 20 milliseconds

CL ­ the L2CS long code
· 767,250 chips, 1.5 second

NAV ­ the the L1 C/A CNAV ­ a adopted fo

legacy navigation message provided by signal navigation message structure like that r the L5 civil signal


L2C Signal Options on IIF Satellites


L2C Signal Options on IIR-M Satellites


L2C Code Generation and Definitions


Signal Acquisition and Code Tracking
Normally acquire L2C using CM code (10,230 chips)
· CL code is 75 times longer than CM code · Employ frequency locked or Costas loop during acquisition


CM has data modulation

· Test the 75 possible phases of CL · Acquire CL, track phase with a simple phase locked loop


Improves threshold by 6 dB relative to a Costas loop

After the first, it is possible to acquire CL codes directly
· 19,130 chip search range · Allows longer coherent integration time (e.g., FFT with long sample interval)


Tracking Continuous Code


Tracking Chip by Chip Multiplexed Code


Code Tracking Accuracy
Does a lower code clock rate hurt navigation accuracy?
· Doesn't higher clock improve loop S/N and reduce multipath ? · Two factors eliminate this concern

High S/N in very narrow bandwidth code tracking loop
· · · · · Carrier aided code loops see only ionospheric dynamics Code loop bandwidth of 0.1 Hz entirely adequate Carrier aided code smoothing 0 . 0 0 8 t o 0 . 0 0 3 H z B W Zero baseline tests show centimeter level code noise High accuracy does not require better loop S/N

Multipath mitigation correlator achieves the same multipath performance of a higher clock rate


Multipath Error for Three Correlator Types


P Code Performance from Gated MM Correlator


Two L2C Message Frame Alternatives


Potential Message Improvements
Almanac with 7 orbits in one subframe New ephemeris message
· One rather than two subframes · Better accuracy · Longer validity

Both significantly benefit L2C performance because of its 25 bps message rate


L2C vs. C/A on L2


L1 C/A vs. L2C vs. L5 with IIR-M and IIF Satellites


Relative Data and Carrier Tracking Performance


Balanced Data & Carrier Tracking Thresholds
Data rate (bps) & FEC rate 50 & None 50 & None 25 & None 50 & Ѕ 33.3 & Ѕ 25 & Ѕ 25 & Ѕ 25 & Ѕ 33.3 & 1/3 Carrier power percent C os ta s 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


Civil Signal Characteristics


Civil Signal Choices Functional Differences


Correlation Performance


L2C Advantages
Best crosscorrelation protection (> 45 dB)
· Aids navigation indoors and in forest areas · Provides headroom for increased SV power (GPS III ?) · Reduces impact of narrowband interference

Better tracking and message thresholds than L1 C/A Available years sooner than L5 Lower chip rate than L5
· Saves power, minimizes thermal rise, better miniaturization


Battery powered use, e.g., cell phone and wristwatch products

· More flexible RF/IF filter and signal processing options


L2C Bandwidth and Signal Processing Options
Max Accuracy Lowest Cost

Max Protection