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L2CS Technical Description Tom Stansell

L2CS Technical Description Tom Stansell. Technical Agenda. Signal Development Framework Objectives and Constraints The L2 Civil Signal (L2CS) Description Signal Performance Characteristics Design Decisions and Tradeoffs Eventual Civil Signal Options. Signal Development Framework.

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L2CS Technical Description Tom Stansell

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  1. L2CS Technical Description Tom Stansell

  2. Technical Agenda • Signal Development Framework • Objectives and Constraints • The L2 Civil Signal (L2CS) Description • Signal Performance Characteristics • Design Decisions and Tradeoffs • Eventual Civil Signal Options

  3. Signal Development Framework Objectives and Constraints

  4. Technical Framework (1 of 3) • Civil L2 signal power ~2.3 dB less than L1 C/A • Code chip rate must remain at 1.023 MHz • To separate the M Code and Civil Code spectra • Only one bi-phase signal component available • L5-type quad-phase not possible • L2CS shares L2 with military signals • Definition needed by the first of March • Technical meetings began in mid-January • Definition complete by mid-February • Coordinated with Lockheed-Martin and Boeing • First draft of ICD-GPS-200 PIRN completed

  5. Code Spectra: BOC (10,5) M & C/A C/A code spectrum Effect on GPS noise floor of a strong M code signal

  6. One Civil Component on L1 L1 Phase RelationshipsCivil is 3 dB stronger than P/Y

  7. One Civil Component on L2 L2 Phase RelationshipsCivil is 0.4 dB weaker than P/Y

  8. Technical Framework (2 of 3) • Serve the current large and valuable dual frequency survey, science, and machine control applications • Approximately 50,000 in service • Primary need is for robust carrier phase measurements • Typically use semi-codeless L2 access, but many also are equipped with an L2 C/A capability • Improve cross-correlation for single frequency applications (e.g., wooded areas or indoor navigation) • A strong C/A code signal can interfere with weak signals • Receiver technology has advanced enormously compared with the 1970s when C/A was developed • The outdated C/A should be replaced with a better code

  9. Technology Has Changed 1984 2001 Consumer 12 channel digital with color map Consumer 12 channel digital for under $100 5 Channel Analog

  10. Technical Framework (3 of 3) • New signals on IIR-M and IIF satellites • When will full coverage with the new signals become available? • See estimated launch schedule chart • Will the IIR-M be able to transmit an L5-type message on the L2CS? • Lockheed-Martin implementation study underway • Backup modes will be provided

  11. Signals on IIR, IIR-M, & IIF

  12. Civil Signal Availability

  13. L2 Civil Signal (L2CS) Description

  14. 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 the L5 civil signal

  15. IIF Signal Generation

  16. 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)

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

  18. IIR-M L2CS Preferred Mode • The Preferred mode is the ability to transmit the following signal structure upon command from the Ground Control Segment: • 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, C1, D1)

  19. IIR-M L2CS Backup Mode • One backup mode is the ability to transmit the following signal structure upon command from the Ground Control Segment: • The chip by chip time multiplexed (TDM) combination of the CM and CL codes with the NAV message at 25 bits/sec plus FEC bi-phase modulated on the CM code (A1, C1, D2)

  20. IIR-M L2CS Optional Modes • The ability to transmit any one of the following three signal structures upon command from the 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 NAV message at 50 bits/sec bi-phase modulated on the CM code (A1, C2) • Control Segment implementation is under evaluation for these & the previous options

  21. 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

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

  23. Medium code = CM 10,230 chips 20 msec Long code = CL 767,250 chips 1.5 second Begin and end states Perfectly balanced 37 codes listed in the ICD-GPS-200 PIRN 100 codes defined 37 of the 100 Selected Codes

  24. 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 S/N in a one-code loop • Both codes can be tracked, but CL-only is OK

  25. Narrow Correlator Tracking

  26. Narrow Correlator on L2CS

  27. 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

  28. CNAV Message Content • The CNAV message content is the same as defined for the L5 signal with the following differences and notes: • 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 properly represent the 12-second epoch defined by each sub-frame • The terms provided by the Control Segment representing time bias between the P code and the civil codes for L1, L2, and L5 will be included

  29. Message Sequence Options Type 4 message gives one satellite almanac per sub-frame

  30. CNAV Message Sequencing • Message sequences will be determined by the Control Segment. One possible sequence is three sub-frames grouped into repeating frames of 36 seconds, each containing Ephemeris 1 and Ephemeris 2 messages plus another sub-frame • The third sub-frame of each 36 second frame contains one almanac message or another message when and as needed

  31. Another CNAV Sequence • Another possible sequence is four sub-frames grouped into repeating frames of 48 seconds, each containing Ephemeris 1 and Ephemeris 2 messages plus two other sub-frames • It also will be possible for different satellites to transmit different almanac messages at the same time, as defined or scheduled by the Ground Control Segment

  32. Compact Almanac • A new compact almanac message type is being developed to minimize the time required to collect a complete almanac • Up to 7 satellite almanacs per sub-frame • The new message type will be described in a following presentation

  33. Signal Performance Characteristics

  34. Relative Channel Power Comparing L2CS with C/A on L2

  35. Data & Tracking Thresholds Comparing L2CS with C/A on L2

  36. Signal Acquisition Modern, multiple correlator technology overcomes the L2CS power deficit and permits rapid acquisition of very weak signals C/A code acquisition may be impossible for very weak signals in the presence of a strong C/A signal

  37. Power from IIR-M & IIF Comparing Three Civil Signals

  38. Relative Channel Power Comparing Three Civil Signals

  39. Data & Tracking Thresholds Comparing Three Civil Signals

  40. C/A code acquisition may be impossible for very weak signals in the presence of a strong C/A signal Modern, multiple correlator technology overcomes the L2CS power deficit and permits rapid acquisition of very weak signals Signal Acquisition

  41. Tracking/Data Performance • With 50% power split, 25 bps, and rate-½ FEC • Under moderatedynamic conditions (aviation) • Max acceleration = 29.8 Hz/sec • Maximum jerk = 9.6 Hz/sec2 • BL = 8 Hz • Balanced performance • 300 bit word error rate (WER) is 0.015 with total C/No = 22 dB-Hz • Phase slip probability within 60 seconds is 0.001 with total C/No = 23 dB-Hz

  42. Tracking/Data Performance • With 50% power split, 25 bps, and rate-½ FEC • Under highdynamic conditions • Max acceleration = 300 Hz/sec • Maximum jerk = 100 Hz/sec2 • BL = 15 Hz • Performance • 300 bit word error rate (WER) of 0.015 with total C/No = 24.5 dB-Hz • Phase slip probability in 60 seconds of 0.001 with total C/No = 25.5 dB-Hz

  43. Design Decisions and Tradeoffs Why two codes?Why TDM? Why Chip by Chip? Why L5 type message? Why FEC?

  44. An Old Idea Revived • Transit, the world’s first satellite navigation system, provided a coherent carrier • But GPS used bi-phase data modulation, leaving no carrier • Bi-phase modulation favors data over continuous lock and measurement accuracy • But data is redundant, slowly changing, thus less important • A carrier component makes signal tracking & navigation measurements more robust TransitModulation

  45. Why Two Codes? • Carrier component first accepted for L5 • Two equal power signal components in phase quadrature, each with a separate code • One component with bi-phase data • The other component with carrier & no data • Forward error correction (FEC) raised bit error probability to the level achieved with all the power in one bi-phase signal component • The carrier component improves tracking threshold by 3 dB • Win-win: better tracking, no data degradation

  46. Two L2 Codes • Quad phase was not available for L2 • Two codes provided by time multiplexing one bi-phase signal component • Data with forward error correction on moderate length code, CM • No data on the long CL code, provides a carrier component and a 3 dB better tracking threshold • Longer CL code improves crosscorrelation

  47. Multi-Code Options • Considered 3 ways to provide two codes: • Majority vote of 3 codes • 000=0, 001=0, 010=0, 100=0, 011=1, 101=1, 110=1, 111=1 • One with data, two without data • Tracking only one code loses 6 dB • Knowledge of all three regains 3 dB • Time multiplexed, msec by msec • Time multiplexed, chip by chip

  48. Chip by Chip TDM Chosen • Majority vote eliminated because: • Requires 3 rather than 2 code generators • Requires synch to all 3 codes for best results • No other advantage found • Msec by msec TDM eliminated because: • Requires care to avoid 500 Hz sidetone • No other advantage found • Selected chip by chip TDM • Simple to implement with no disadvantages

  49. Code Length Considerations • The peak cross-correlation between existing C/A codes is ‑23.9 dB • The Gold bound for period 1023 chips • C/A codes are inadequate for indoor navigation • Correlation sidelobe examples for TDM candidates • 20 msec period: 29 dB below full correlation • 200 msec period: 36 dB below full correlation • 1.5 sec period: 47 dB below full correlation

  50. Code Correlation Studies • Fig 1 – Three individual code lengths • Fig 2 – TDM 409,200 • Fig 3 – TDM 1,534,500 (10,230 & 767,250) • This is the selected code pair • CM for faster acquisition • CL for better crosscorrelation • Minimum crosscorrelation protection of 45 dB • Fig 4 – TDM 613,800 (10,230 & 306,900) • Fig 5 – TDM 1,534,500 (1 msec segments)

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