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Planned GPS Civil Signals and Their Benefits to the Civil Community Dr. A. J. Van Dierendonck AJ Systems.

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Presentation on theme: "Planned GPS Civil Signals and Their Benefits to the Civil Community Dr. A. J. Van Dierendonck AJ Systems."— Presentation transcript:

1 Planned GPS Civil Signals and Their Benefits to the Civil Community Dr. A. J. Van Dierendonck AJ Systems

2 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

3 Presentation Topics GPS Modernization Overview New Civil Signals Detail Performance Enhancements

4 GPS Modernization Overview Why modernize? GPS modernization plans New civil signal summary Galileo compatibility; plans

5 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

6 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)

7 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

8 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)

9 Modernized Signal Evolution 1227 MHz1575 MHz1176 MHz L2L1L5 P(Y) C/A P(Y) P(Y) C/A P(Y) CS MM Present Signals Signals After Modernization

10 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

11 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

12 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)

13 GPS Spreading Codes SignalChipping RateCarrier frequencyComments (Mchip/s)(MHz) C/A1.0231575.42 (L1)1023-chip Gold codes repeat every ms CS1.0231227.6 (L2)2 codes per SV each at 511.5 kHz, future P(Y)10.23L1 and L2Repeats once/week L510.231176.45 (L5)2 codes per SV, future M5.115L1 and L2code modulated by 10.23 MHz square wave, future

14 Signal Power Spectra -15-10-5051015 0 0.2 0.4 0.6 0.8 1 x 10 -6 Offset from Carrier Frequency (MHz) Normalized Power Spectrum (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.

15 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

16 Block IIR- Modified Satellites L1 L2 L1 Enhancement New M E code at -158 dBW L1 Enhancement New M E 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 M E code at -158 dBW L2 Enhancements New L2CS at -160 dBW New M E code at -158 dBW

17 Block IIF Satellites L1 L2 L1 Enhancements New M E code added L1 Enhancements New M E 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 M E code added C/A code on L2 L2 Enhancements New M E code added C/A code on L2 L5 L5 Signal New robust Civil Signal Power level = -154 dBw L5 Signal New robust Civil Signal Power level = -154 dBw

18 Second Civil Signal Maintain Space User Service Third Civil Signal 1 ON 3 menu 2 Rockwell 4 5 6 7 WPT 8 POS 9 NAV CLR MARK 0 OFF NUM LOCK FIX FOM 1 N 42* 01” 46.12” W 091* 38’ 54.36” EL + 00862 ft ZEROIZE The GPS III System 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

19 GPS Modernization Integrated Schedule Milestones Space Segment Control Segment 20172016199920002001200220032004200520062007200820092010201120122013201420152016 IIF SV1 Launch IIF Lite Deliveries IIF SV7-SV12IIF SV1-SV6 IIF Lite Launches IIF SV7-SV12 IIF OCS M-Code/L5 OCS Training/Validation ATP GPS-III Full Capability IOC GPS-III Full Capability FOC Deliver S/W OT&E Comp. SPI Contract Definitization M-Code Earth (18SV) M-Code Earth (24 SV) CY L5 IOC L5 FOC GPS III Deliveries SV1-SV3 GPS III Launches SV4 - SVNN IIR Mod Deliveries IIR SV10-SV21 IIR Mod Launches IIR SV10-SV21 IIR Mod First Launch

20 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

21 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?

22 New Civil Signals

23 L5 –Signal structure and pseudorandom noise (PRN) codes –Navigation message and data format –Spectrum issues L2CS –Signal structure and PRN codes –Implementation options

24 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)

25 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

26 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

27 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)

28 Optimum Division of Power Between Data and Dataless Channel Courtesy of Dr. Tom Morrissey, Zeta Associates

29 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

30 L5 I and Q Code Generators

31 L5 Code Generator Timing

32 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

33 Autocorrelation Peaks (in dB)

34 Probability of Cross-Correlation Level - 0 to 5 kHz L5 Codes C/A Codes

35 L5 Power Spectral Density - Reduces the Effect of CW Interference

36 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

37 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

38 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

39 10-ms Neuman-Hoffman Code on I

40 20-ms Neuman-Hoffman Code on Q

41 Typical Spectral Sidelobes Including 10-Bit Neuman-Hoffman Code 20-Bit Code on Q-Channel reduces spectrum another 3 dB

42 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

43 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

44 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

45 Message Type 1

46 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 m 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 m s pulse width For uncoordinated exercises: – Peak power = 200 W –396,288 pulses/12 s in 200 nmi radius

47 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

48 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.

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

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

51 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

52 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

53 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

54 IIF Signal Generation

55 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)

56 IIR-M Signal Generation B1 is a potential software option to be uploaded by the Control Segment

57 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

58 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

59 L2CS Code Generator Linear shift register generator with 27 stages and 12 taps

60 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

61 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

62 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

63 Performance Benefits Using New Civil Signals

64 Modernization Performance Benefits Dual and triple frequency ionospheric corrections New signal acquisition and tracking Positioning performance after modernization Benefits of increased constellation size Issues

65 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)!

66 Dual-Frequency Ionospheric Correction Accuracy L1 C/A - L2 C/AL1 C/A - L5L2 C/A - L5 0 0.5 1 1.5 2 2.5 3 3.5 4 RMS Error (m) Assumptions: RMS C/A and L2CS code tracking error = 0.3 m, RMS L5 tracking error = 0.1 m

67 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

68 L5 Acquisition Performance 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.

69 L5 Data-Free Channel Enables Phase Tracking at Lower SNR Oscillator effects ignored - use for relative comparison only

70 L5 Multipath Performance

71 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

72 L2CS Low-SNR Performance 23 dB-Hz22 dB-Hz5033.3 & 1/3 21 dB-Hz24 dB-Hz75 25 & ½ 26 dB-Hz24 dB-Hz25 25 & ½ 23 dB-Hz22 dB-Hz50 25 & ½ 23 dB-Hz22.5 dB-Hz50 33.3 & ½ 23 dB-Hz24 dB-Hz50 50 & ½ 23 dB-Hz26.5 dB-Hz5025 & None 23 dB-Hz29 dB-Hz505050 & None 25.5 dB-Hz26 dB-HzCostas50 & None Phase slip = 0.001 with total C/N o = WER = 0.015 with total C/N o = Carrier power percent Data rate (bps) & FEC rate For max acceleration = 29.8 Hz/sec, maximum jerk = 9.6 Hz/sec 2, B L = 8 Hz Selected data rate and forward error correction (FEC) for L2CS

73 GPS Civil Accuracy w/ and w/o New Signals Source: Shaw et. al., GNSS-2000.


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