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 9 th pulse concept –Absolute time msg is one way to resolve lane ambiguity –In 4/03, Gordon Weeks, Kevin Carroll & I proposed 9 th 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 9 th 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 9 th 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 usec d(i) = 1.25 mod(i,8) + 50.625 floor(i/8)

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

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

9. Update Rate/Time to 1 st 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 1 st 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 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. Encoding & Decoding 45 data bits modulo 32 adder RS (24,9) encoder RS (24,9,X) bounded distance decoder c * + modulator demodulator channel modulo 32 subtraction c * + 45 data bits -

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

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

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

19. Integrity performance of the (24,9) RS code when > 6 errors results in rejection P UE undetected error P F decoder failure Error count uP UE (u)P F (u) 0,1,2,3,4,5,600 7,8,901 101.1 × 10 -10 1 – P UE (10) 115.5 × 10 -10 1 – P UE (11) 121.4 × 10 -9 1 – P UE (12) 132.3 × 10 -9 1 – P UE (13) 143.0 × 10 -9 1 – P UE (14) 153.2 × 10 -9 1 – P UE (15) 163.2 × 10 -9 1 – P UE (16) 173.2 × 10 -9 1 – P UE (17) 183.2 × 10 -9 1 – P UE (18) 193.2 × 10 -9 1 – P UE (19) 203.2 × 10 -9 1 – P UE (20) 213.2 × 10 -9 1 – P UE (21) 223.2 × 10 -9 1 – P UE (22) 233.2 × 10 -9 1 – P UE (23) 243.2 × 10 -9 1 – P UE (24)

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 SEPT LSU TTX to Waterford, E field, no CRI canceling, errors only decoding Note: 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 Locus TM LRSIIID with E field antenna Locus TM 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

28. Comparison of E & H field accuracy during turns

29. 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

30. 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.

31. Very good Question! 9 th 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.