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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 l.jpg

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 l.jpg
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


Background basic approach l.jpg
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


Proposed modulation scheme l.jpg
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.


Determination of minimum envelope delay between groups of 8 symbols l.jpg
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)


Symbol delays in usec d i 1 25 mod i 8 50 625 floor i 8 l.jpg
Symbol delays in usec symbolsd(i) = 1.25 mod(i,8) + 50.625 floor(i/8)


Symbols in the time domain blue xxx00 red xxx01 green xxx10 magenta xxx11 l.jpg
Symbols in the time domain symbolsBlue: xxx00, Red: xxx01, Green: xxx10, Magenta: xxx11


Polar plot of symbol space l.jpg
Polar plot of symbol space symbols

Phase in deg re symbol 0

Delay in usec re symbol 0


Update rate time to 1 st fix alarm limit l.jpg
Update Rate/Time to 1 symbolsst 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


Slide10 l.jpg


Dloran for precise time transfer l.jpg
dLoran for Precise Time Transfer symbols

  • 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


Aviation early skywave warning l.jpg
Aviation Early Skywave Warning symbols

  • 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


Synchronization hmi l.jpg
Synchronization/HMI symbols

  • 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


Coset vector l.jpg
Coset vector symbols

  • 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)


Encoding decoding l.jpg

c symbols

*

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

-


Bounded distance decoding l.jpg
Bounded Distance Decoding symbols

  • Release codeword if HD(r,c)  threshold for some codeword c; otherwise, decoder failure

Hamming distance

HD =

Yellow = correct

Orange = undetected error

Gray = failure


Probability of hmi l.jpg
Probability of HMI symbols

  • Random observations:

    • 24 random symbols

    • Error if they fall in an incorrect decodable region


Slide18 l.jpg

Aviation Threshold w/16 bit CRC symbols

Maritime Threshold

Aviation Threshold


Slide19 l.jpg

Integrity performance of the (24,9) RS code when > 6 errors results in rejection

PUE undetected error

PF decoder failure


Dloran status l.jpg
dLoran Status results in rejection

  • 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


Slide21 l.jpg

Transmission test: 30 SEPT results in rejectionLSU 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)


Data collection example l.jpg
Data Collection Example results in rejection

  • 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



Scatter plot of fix errors l.jpg
Scatter plot of fix errors results in rejection





Summary l.jpg
Summary results in rejection

  • 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


Slide30 l.jpg

Acknowledgements, etc. results in rejection

  • Funded by Federal Aviation Administration

    • Mitch Narins – Program Manager

For additional info:

[email protected]

Or

[email protected]

-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.


Very good question 9 th pulse ppm vs eurofix l.jpg
Very good Question results in rejection! 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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