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Differential Loran

Differential Loran. Ben Peterson, Ken Dykstra & Peter Swaszek Peterson Integrated Geopositioning & Kevin Carroll, USCG Loran Support Unit Funded by Federal Aviation Administration, Mitch Narins, Program Manager International Loran Association, November 4, 2003. Outline.

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Differential Loran

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  1. Differential Loran Ben Peterson, Ken Dykstra & Peter Swaszek Peterson Integrated Geopositioning & Kevin Carroll, USCG Loran Support Unit Funded by Federal Aviation Administration, Mitch Narins, Program Manager International Loran Association, November 4, 2003

  2. Outline • Background & Basic approach • Proposed Modulation • Message Formats • Reed Solomon forward error correction • Integrity from FEC • Synchronization & coset vector • Modulator & receiver status • Data collection example • E field vs H field

  3. Background/Basic Approach • LORIPP determined in 9/02 that some form of LDC would be necessary to enable all-in-view master independent navigation (station ID & cross chain lane ambiguity) • Mitch Narins pushed 9th pulse concept • Absolute time msg is one way to resolve lane ambiguity • In 4/03, Gordon Weeks, Kevin Carroll & I proposed 9th pulse for dLoran • Receiver calculates ASF’s as sum of two terms: • Temporal terms measured at a local base station • A spatial grid based on survey of ASF’s compared to those observed simultaneously at the local base station • Differential corrections will be offsets from published nominal values vice absolute to conserve bits/maintain dynamic range • Effort is to demonstrate that Loran can meet HEA requirements and to determine base station density

  4. Proposed Modulation Scheme • 9th pulse Pulse Position Modulation (PPM) • 32 state PPM, 5 bits/GRI • 3 bits phase, 2 bits envelope & phase • Averages to zero in legacy receivers, CRI increases 0.5dB • Message length is 24 GRI or max of 2.38 seconds • PPM vice IFM means no transmitter modifications, modulation done in software in TFE • Do not have to demodulate more than the strongest signal to get absolute time, positively ID all signals, and to get all corrections for your area • 9th pulse in cross rate would be blanked, other 8 could be cancelled if desired.

  5. Determination of minimum envelope delay between groups of 8 symbols To get same distance as 1.25 usec phase shift need to delay envelope 50 usec (Earlier version had negative phase codes and 45.625 usec between groups, changed to make SSX modulator easier)

  6. Symbol delays in usecd(i) = 1.25 mod(i,8) + 50.625 floor(i/8)

  7. Symbols in the time domainBlue: xxx00, Red: xxx01, Green: xxx10, Magenta: xxx11

  8. Polar plot of symbol space Phase in deg re symbol 0 Delay in usec re symbol 0

  9. Update Rate/Time to 1st Fix/Alarm Limit • Time to alarm: 24 GRI format, max 2.38 sec message length • Time to first fix: By the time dLoran becomes operational, system will be TOT control, 1 vice 2 corrections per LORSTA, 6 vice 12 per monitor site • 3 messages/site @ 2 corrections/message • Assume maximum of 20-40 sites/LORSTA, 60-120 dLoran messages • For dual rated station, update rate/time to 1st fix of 2 to 4 minutes • For single rated station, these times double. • Jupiter and Middletown are only single rated LORSTA’s with significant potential for maritime base stations

  10. Bit Assignments for Time and dLoran messages • (Format for aviation integrity msg TBD)

  11. dLoran for Precise Time Transfer • Format includes base station time base quality term so that timing users can use corrections from high quality sites (NIST, USNO, LORSTA) but would not use maritime sites • Since maritime sites may have GPS for time, if GPS is lost, ASF of nominal strongest signal could be set to zero and all other ASF’s are relative • Performance details in next paper

  12. Aviation Early Skywave Warning • Warning not to use signal • When geomagnetic latitude of midpoint of propagation path exceeds XX degrees, and • Either predicted groundwave signal strength < YY dB, or path > ZZ NM • Message content much less than the available # of bits leaving room for CRC • This enables recovering Reed Solomon error correction performance lost by using RS for integrity • More details on problem in paper tomorrow AM

  13. Synchronization/HMI • An unsynchronized transmitter and receiver pair will not yield accurate data • In early tests w/CRC, erroneous messages were “corrected” by RS, and then accepted by 24 bit CRC • Could use decoder failure as a way to test synchronization: • Issue of cyclic-like nature of RS code • Issue of the effect of error correction

  14. Coset vector • Solution: use a constant coset vector c* • Add to codeword before transmission • Subtract from channel observation before decoding • Effects of c*: • Cancelled if synchronized • Usually cause decoding failure if unsynchronized (high probability)

  15. c * 45 RS (24,9) + data modulator encoder bits modulo 32 adder channel c * RS (24,9,X) 45 bounded + data demodulator distance bits decoder modulo 32 subtraction Encoding & Decoding -

  16. Bounded Distance Decoding • Release codeword if HD(r,c)  threshold for some codeword c; otherwise, decoder failure Hamming distance HD = Yellow = correct Orange = undetected error Gray = failure

  17. Probability of HMI • Random observations: • 24 random symbols • Error if they fall in an incorrect decodable region

  18. Aviation Threshold w/16 bit CRC Maritime Threshold Aviation Threshold

  19. Integrity performance of the (24,9) RS code when > 6 errors results in rejection PUE undetected error PF decoder failure

  20. dLoran Status • TTX – Modulator & receiver finished (too many times due to numerous changes in msg format) • Two versions of prototype user equipment • Comms only receiver that gets Loran data from Locus receiver, calculates dLoran fix & send NMEA message • Combined comms/navigation receiver (both E and H field) • Base stations write messages to hard drive of modulator PC • SSX – Prototype • TSC under contract as of early September • Will modify TFE to send msg via serial port requesting next msg, & modulate signal. • PIG will modify software to generate msg & send via RS232 • Expect prototype by 1 DEC • Starting data collection effort to evalute dLoran accuracy

  21. Transmission test: 30 SEPTLSU TTX to Waterford, E field, no CRI canceling, errors only decodingNote: Smallest cross rate pulse that can cause error is -7dB Transmitter off (Fraction of last 10 msgs)

  22. Data Collection Example • 65’ US Army Corps of Engineers Survey Vessel Shuman in Cheaspeake Bay 29-30 October • LocusTM LRSIIID with E field antenna • LocusTM SatMate with H field antenna • DGPS for ground truth • Receivers in TOA mode; TOA’s relative to common, but free running oscillator • Software automatically starts at 0700 & quits at 1600 every weekday

  23. Path of Shuman during depth survey

  24. Scatter plot of fix errors

  25. LRSIIID Accuracy vs Estimated Delay

  26. Test of directional dependence of H field antenna

  27. Comparison of E & H field accuracy during turns

  28. Summary • Modulation & message format is hopefully frozen at least for the proof of concept phase • TTX modulator & LDC receiver succesfuuly demo’ed, SSX modulator under development • Very early in the ASF data collection and accuracy analysis effort, early data promising • E field better than H field at this point, calibration and/or antennas with less bias may solve problem

  29. Acknowledgements, etc. • Funded by Federal Aviation Administration • Mitch Narins – Program Manager For additional info: kmcarroll@lsu.uscg.mil Or benjaminpeterson@ieee.org -Note- The views expressed herein are those of the authors and are not to be construed as official or reflecting the views of the U.S. Coast Guard, the U. S. Federal Aviation Administration, or the U.S. Departments of Transportation and Homeland Security.

  30. Very good Question! 9th pulse PPM vs EUROFIX • Both have comparable data rates & message lengths, EUROFIX could easily transmit the same data we are proposing. Why a new format?? • Main reason is ability to cancel 8/9 of cross rate pulses • For maritime & timing users can merely blank cross rate: Only issue for aviation where short time constants preclude cross rate blanking • It is possible to cancel cross rate with Eurofix • After demodulation & data wipeoff – if demodulation errors, canceling not effective • After demodulation and decoding (& data wipeoff) – need delays of up to 3 seconds for completion of message • Formats are completely compatible, same transmitter can transmit both, same receiver can receive both.

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