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Loran Receiver Technology Cross Rate

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Loran Receiver Technology Cross Rate

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									The following paper was reprinted with permission from The Proceedings of the 27th ILA Convention, 1998.




                                      Loran Receiver Technology
                                     Yesterday, Today and Tomorrow

                            G. Linn Roth, Ph.D. and Paul Schick - Locus, Inc.
                                                   October 1998


Biography
G. Linn Roth, Ph.D., President, Locus, Inc.
Dr. Roth graduated from the University of California at Berkeley and holds a Ph.D. in neurophysiology from the
University of California Medical Center in San Francisco. He performed reearch and taught at the University of
Wisconsin Medical School before joining Nicolet Instrument Corporation (now Thermo Instruments), where he was
the Director of Marketing and a Business Unit Manager. Dr. Roth manages all Locus’ business operations,
including the overall development of Locus’ patented linear averaging digital (LAD) Loran technology and receiver
products.

Paul W. Schick, Project Engineer, Locus, Inc.
Mr. Schick is an honors physics graduate of The Cooper Union, New York, and holds a Master of Science in physics
(electrical engineering minor) from the University of Wisconsin-Madison. He has over 20 years of design
experience. His training has included electromagnetic fields, nonlinear waves, digital circuits, microprogramming,
complex analysis, and symbolic manipulation of differential equations. As a project engineer at Locus for the past
eleven years, he has had primary responsibility for the design and implementation of Locus’ LAD-Loran
technology, as well as digital signal processing software for other RF and digital communication systems. Mr.
Schick holds several patents.

Abstract
The intent of this paper is to generate a public domain document that is an accurate summary of the history, current
status, and future of Loran receiver technology. Such a document is necessary because even in 1998, antiquated
perceptions of Loran performance remain the norm rather than the exception, and Loran’s future is continually
appraised using these antiquated perceptions. However, the future of Loran – and all radionavigation systems –
must be judged on a realistic assessment of today’s and tomorrow’s Loran technology. This paper attempts to
provide that assessment.




                                                                                                                       1
I.   YESTERDAY’S RECEIVERS

     1.   Major commercial R&D Loran development ended circa 1990 for several reasons:

          a.   The growing interest in GPS and expectations for early GPS operational status

          b.   Internal competition with GPS for development dollars

          c.   DOT’s unilateral reversal of established Federal Radionavigation Plan (FRP) policies drastically curtailed
               the Loran market by artificial (i.e. non-market) manipulation of supply and demand.

     2.   During that period, the vast majority of commercial Loran receivers, regardless of application, had the
          following characteristics:

          a.   Hard limited – therefore had no means to measure noise accurately, and there were no uniform industry
               standards to measure signal to noise ratios (SNR).

          b.   Limited processing capabilities – microprocessor and memory capabilities were severely restricted,
               particularly because of size and cost constraints placed on commercial products.

          c.   Many low cost receivers only tracked and analyzed first pulse in group of eight

          d.   Receivers typically averaged Envelope Cycle Delay (ECD) measurements for extremely long time periods
               to determine correct cycle, commonly for hours and sometimes since receiver power on.

          e.   Fixed notch filters or a small number of automatic notches, with accompanying distortion of ECD

          f.   Provided no automatic or only manual Additional Secondary Factor (ASF) adjustments

          g.   Most receivers acquired one chain or triad of stations; a very few acquired a limited number of stations on
               two chains

          h.   No specific means to identify or process skywave interference




                                                                                                                             2
3.   These attributes resulted in the following performance characteristics:

     a.   TDs and ECDs severely degraded due to cross rate interference (CRI) and skywave interference from more
          distant Loran chains

     b.   Poor immunity to continuous wave (CW) interference such as Decca transmitters and burst interference
          such as lightning

     c.   Limited dynamic performance. For example:

          i.    frequent cycle slips

          ii. slow recovery from cycle slips and signal loss

          iii. slow signal acquisition from a cold start

          iv. slow generation of fixes and recognition of changed heading

     d.   Limited range, typically 600-900 miles from transmitter.

     e.   Poorer night time and sunrise/sunset performance due to increased skywave activity

     f.   Loran’s perceived susceptibility to p-static, which remains today despite the following:

          i.    Not a single paper has ever been published quantitatively defining P-static.

          ii. No quantitative P-static data exists or is available in the public domain. In other words, it is impossible
              to investigate digital compensation of P-static interference with computer simulations, because no data
              exist.

          iii. There has never been a paper published with data documenting a Loran receiver’s failure due to P-
               static.

          iv.   In summary, Loran’s infamous P-static susceptibility is completely anecdotal.

          v.    As Mark Twain said, “The truth is not hard to kill, but a lie well told is immortal.” More on P-static
                later.

4.   Even with these “problems”, Loran easily became the most widely used radionavigation system in the
     world because of its:

     a.   exceptional reliability

     b.   repeatable accuracy

     c.   low cost

     d.   number of applications (e.g. aviation, marine, terrestrial, and timing).




                                                                                                                         3
   5.   Nevertheless, Loran continues to be judged (and compared to GPS) on the basis of 1980’s technology and
        today’s residual perception of earlier “problems.”
        Loran is viewed as if system performance and technology were optimized years ago, and somehow application
        of new components, techniques, etc. will make no difference or should not be considered. Whatever the
        motivation behind such judgements, they are clearly not objective.

        Loran should be judged on what it is today and what it can be in the future, not what it was yesterday.

II. TODAY’S RECEIVERS
   Using contemporary digital technology and techniques common in GPS receivers, Loran receiver performance has
   been significantly advanced over the last 7-8 years. How was performance improved and what performance is
   available today?
   1.   Recognition that receivers must be multichain and process significant cross rate interference (CRI)
        emanating from distant Loran chains, including skywaves. Why?

        a.   CRI is extremely prevalent throughout the Northern Hemisphere:

             i.   CRI is significant in North America.
                  For example, Locus typically tracks 33-37 stations from 8 chains continuously in Madison, WI, with
                  20-24 stations on groundwave. Figure 1 is from a recent recording.

             ii. CRI is significant in Europe.
                  Figure 2 shows 1998 European recordings demonstrating simultaneous tracking of 19 European,
                  Russian, and North American stations from 7 chains.

             iii. CRI is significant in Asia.
                  Figure 3 shows 1998 recordings in Japan demonstrating simultaneous tracking of 22 Japanese,
                  Korean, Russian, Chinese and US stations from 8 chains.




                                                                                                                       4
Chain / Station               Location             Distance          SNR        Time of Arrival
      5930M                        Caribou, ME          1076 Mi        5 S/N         00000000
      5930X                      Nantucket, MA          1000 Mi        2 S/N         12697998
      5930Y                 Cape Race, Canada           1775 Mi     -13 S/N          32510363
      5930Z               Fox Harbour, Canada           1673 Mi     -11 S/N          44815039
      5990M              Williams Lake, Canada          1644 Mi     -19 S/N          00000000
      5990Y                     George, Canada          1502 Mi     -12 S/N          28164724
      7980M                          Malone, FL          867 Mi      12 S/N          00000000
      7980W                     Grangeville, LA          859 Mi      11 S/N          12763788
      7980X                   Raymondville, TX          1240 Mi        1 S/N         29446192
      7980Y                          Jupiter, FL        1224 Mi      -5 S/N          47121889
      7980Z                  Carolina Beach, NC          878 Mi        8 S/N         61598643
      8290M                          Havre, MT          1064 Mi        3 S/N         00000000
      8290W                       Baudette, MN           455 Mi      20 S/N          11515072
      8290X                         Gillette, WY         819 Mi      14 S/N          27766654
      8290Y                       Williams Lake         1644 Mi     -17 S/N          48304428
      8970M                            Dana, IN          245 Mi      23 S/N          00000000
      8970W                          Malone, FL          867 Mi        9 S/N         17701052
      8970X                         Seneca, NY           635 Mi      17 S/N          33255871
      8970Y                       Baudette, MN           455 Mi      21 S/N          48882409
      8970Z                      Boise City, OK          852 Mi      14 S/N          66932920
      9610M                      Boise City, OK          852 Mi      13 S/N          00000000
      9610V                         Gillette, WY         819 Mi      14 S/N          13703790
      9610W                     Searchlight, NV         1462 Mi     -15 S/N          31890672
      9610X                     Las Cruces, NM          1223 Mi      -3 S/N          44038690
      9610Y                   Raymondville, TX          1240 Mi        1 S/N         58109451
      9610Z                     Grangeville, LA          859 Mi      13 S/N          69340604
      9940M                      Fallon, Canada         1546 Mi     -15 S/N          00000000
      9940W                     George, Canada          1502 Mi     -11 S/N          13563117
      9940X                Middletown, Canada           1749 Mi      -8 S/N          29208822
      9940Y                     Searchlight, NV         1462 Mi     -14 S/N          41517117
      9960M                         Seneca, NY           635 Mi      17 S/N          00000000
      9960W                        Caribou, ME          1076 Mi     -11 S/N          16173428
      9960X                      Nantucket, MA          1000 Mi        4 S/N         28933526
      9960Y                  Carolina Beach, NC          878 Mi        7 S/N         43529247
      9960Z                            Dana, IN          245 Mi      23 S/N          55068549
                             Figure 1: Loran Signal Availability – 1998 US Data
Data illustrating simultaneous tracking of 35 stations from 8 chains as recorded in Madison, WI. Figure
lists chain and station identifiers, station location and distance, Signal to Noise (SNR) and Time Difference
in nanoseconds.




                                                                                                            5
Chain / Station               Location                Distance           SNR         Time of Arrival
    5930M                      Caribou, ME, USA           2564 Mi       -21 S/N          000000000
     5930X                  Nantucket, MA, USA            2822 Mi       -17 S/N          146490014
     5930Y                   Cape Race, Canada            2030 Mi       -20 S/N          253121112
     5930Z                  Fox Harbor, Canada            1991 Mi       -18 S/N          379153161
    7001M                             Bo, Norway          1169 Mi       -23 S/N          000000000
     7001                                                       Mi         0 S/N         000000000
     7001X                   Jan Mayen, Norway            1230 Mi       -16 S/N          144736165
    7499M                          Sylt, Germany           425 Mi          9 S/N         000000000
     7499X                        Lessay, France             97 Mi        26 S/N         120719610
     7499Y                   Vaerlandet, Norway            664 Mi          1 S/N         309790530
     80003                    Simferopol, Russia          1450 Mi       -20 S/N          000000000
    8940M                         Lessay, France             97 Mi        21 S/N         000000000
    8940W                      Soustons, France            422 Mi          4 S/N         160146963
     8940X                         Sylt, Germany           425 Mi       -15 S/N          350808976
    8970M                          Dana, IN, USA          3497 Mi       -14 S/N          000000000
    9007M                           Ejde, Norway           719 Mi         -5 S/N         000000000
    9007W                    Jan Mayen, Norway            1230 Mi       -20 S/N          173570542
     9007X                            Bo, Norway          1169 Mi       -10 S/N          308134380
     9007Y                   Vaerlandet, Norway            664 Mi         -1 S/N         407561593
                           Figure 2: Loran Signal Availability – 1997 European Data
  Data illustrate simultaneous tracking of 19 stations from 7 chains as recorded on the Isle of Wight, UK.
  Figure lists chain and station identifiers, station location and distance, Signal to Noise (SNR) and Time
  Difference in nanoseconds. Data provided by Dr. Nick Ward, Trinity Lighthouse Authority, UK.



Chain / Station               Location               Distance           SNR         Time of Arrival
     5980M                Petropavlovsk, Russia          1883 Mi        -12 S/N          00000000
     6780M                        Hexian, China          1324 Mi         -2 S/N          00000000
     6780X                       Raoping, China          1081 Mi         -7 S/N          13088465
     7430M                   Rongcheng, China              510 Mi        14 S/N          00000000
     7430X                    Xuancheng, China             706 Mi          9 S/N         14656499
     7430Y                        Helong, China            613 Mi        14 S/N          30881029
     7950M               Aleksandrovsk, Russia           1335 Mi        -12 S/N          00000000
     79501                 Petropavlosk, Russia          1883 Mi         -4 S/N          17387621
     79502                     Ussuriisk, Russia           736 Mi        18 S/N          30674875
     8390M                    Xuancheng, China             706 Mi        10 S/N          00000000
     8390X                       Raoping, China          1081 Mi         -7 S/N          15780912
     8390Y                   Rongcheng, China              510 Mi        14 S/N          30263018
     8930M                        Niijima, Japan           504 Mi        23 S/N          00000000
     8930W                       Gesashi, Japan            522 Mi        14 S/N          16669151
     8930X             Minamitorishima, Japan            1562 Mi         -8 S/N          41859666
     8930Y                  Tokachibuto, Japan             940 Mi          6 S/N         55764468
     8930Z                        Pohang, Korea            170 Mi        23 S/N          72146885




                                                                                                              6
   9930M                       Pohang, Korea           170 Mi        24 S/N          00000000
   9930W                     Kwang Ju, Korea           239 Mi        10 S/N          12612778
   9930X                       Gesashi, Japan          522 Mi        13 S/N          27593096
   9930Y                       Niijima, Japan          504 Mi        22 S/N          41023618
   9990M                   Saint Paul, Alaska         3161 Mi       -10 S/N          00000000
                          Figure 3: Loran Signal Availability – 1998 Japanese Data
Data show simultaneous tracking of 22 stations from 8 chains as recorded from Kyushu, Japan. Figure lists
chain and station identifiers, station location and distance, Signal to Noise (SNR) and Time Difference in
nanoseconds.




                                                                                                         7
b. CRI can easily occupy well over 50% of a GRI and dominate a receiver’s background noise.
   For example, a 5 transmitter chain emits 41 pulses per GRI (i.e. 32 from the secondaries plus 9 from the
   master). Using a pulse duration of 200 uS/pulse, then 8 pulses occupy 1.6mS, and transmissions from the
   5 stations take about 8mS or ~10% of a representative 80mS GRI. If there are 6 cross rate GRIs, it is likely
   that over 50% of the primary GRI is occupied by CRI. Of course, these are approximations because the
   interfering GRIs don’t add linearly, but depend on the timing and number of cross rate GRIs.

    Figure 4 is a graphic representation of how much time cross rate will commonly occupy within the nearest
    GRI of interest.




                    Figure 4: Schematic illustration of cross rate prevalence within a single GRI
    The X axis represents the duration of a single GRI, and the Y axis is amplitude in arbitrary units. Each
    vertical stroke of one color represents a single pulse of cross rate from a single station in another GRI.
    Some vertical strokes are shown in half height for general visual clarity.




                                                                                                                 8
c.   CRI can be much larger than signals from the nearest chain. Navigation normally relies on at least
     three stations, so the limiting factor is generally the third strongest station.

     i.   Signal levels from CRI can be 10-20 times stronger (i.e. 20-30dB) than the signals of primary interest
          for navigation or timing. Figure 5 shows data from recordings made near Fairbanks, Alaska, and the
          highest signal level (dB uV/M) a groundwave or skywave reached during a 24 hour period. Note that
          the maximum level of the Point Clarence, Alaska groundwave signal is 37 dB greater than Shoal Cove,
          Alaska, which is the third largest signal from the local 7960 chain. In addition, note the maximum
          level of skywaves from Williams Lake, Canada (1268 miles) and Ejde, Norway (3441 miles) were
          equal to or 10 dB less, respectively, than the ground wave from Narrow Cape, Alaska (533 miles), the
          second closest station in the chain.


                  GRI/Chain                    Station          Distance (miles)     Signal Level
          7960 / Gulf of Alaska          TOK (M)               185                   78 (G)
                                         Narrow Cape (X)       533                   54 (G)
                                         Shoal Cove (Y)        862                   17 (G)
          8290 / North Central U.S.      Havre (M)             1774                  44 (S)
                                         Baudette (W)          2225                  48 (S)
                                         Gillette (X)          2162                  31 (S)
                                         Williams Lake (Y)     1268                  54 (S)
                                         Port Clarence (Z)     555                   54 (G)
          7970 / Norwegian Sea           Ejde (M)              3441                  44 (S)
                                         Bo (W)                3188                  33 (S)
                                         Sylt (X)              4090                  N/A
                                         Sandur (Y)            3055                  41 (S)
                                         Jan Mayen (Z)         2872                  30 (S)

                 Figure 5. Signals from distant stations can be larger than signals from nearby transmitters
          Groundwave (G) and skywave (S) data taken near Fairbanks, Alaska with distance from recording site
          to transmitter shown. The maximum signal level (in dB uV/M) reached during 24 hour recording is
          indicated.




                                                                                                               9
d. CRI does not average out.

   Since cross rate contains more positive pulses than negative, CRI does not average out as does background
   atmospheric noise. Cross rate “walks” through a GRI in a complex, phase coherent pattern repeating at
   100uS or larger increments, and therefore it can be identified and removed with digital techniques.

   Figure 6 illustrates one important effect CRI processing can have on receiver performance. In this case,
   we show 9960Z TD data from Locus’ original (single chain) prototype receiver taken in 1990 and from our
   current multichain unit. Although the prototype was a linear receiver, and performed better than all other
   commercial units against which it was tested, the TD improvement offered by today’s receiver is dramatic.




                       Figure 6: Representative Comparison of Time Differences Generated
                                  by Single Chain versus Multichain Receivers
   Even though the single chain receiver (left) was perhaps the most advanced model available, the multichain
   receiver (right) showed a dramatic decrease in 9960Z time differences generated from over several days of
   recording.




                                                                                                           10
     2.   Recognition that a receiver must adaptively compensate for skywaves. Why?
          Skywaves are very large, can be extremely dynamic over a short period of time, and travel such long
          distances they can be prevalent even in remote areas.
          a.   In Alaska, for example, skywaves from most of the world’s Loran transmitters (17 GRIs) were present
               within a 24 hour period in 1993 USCG/Locus tests. The most distant skywaves recorded during these
               trials were from Guam, 4700 miles away, and they reached 28 dB uV/M.

          b.   Figure 7 illustrates a sequence of waveforms from 7980X (Raymondville, TX) recorded around
               sunrise in Madison, WI. Raymondville is a 400kW transmitter located 1995kM (1290m) away.




                             Figure 7. Sequence of Skywaves from a Distant Transmitter
          A 37-minute sequence of skywaves recorded around sunrise illustrates the dynamic nature of skywaves.
          Solid lines indicate actual waveforms, and dotted lines indicate idealized Loran pulse. Data recorded in
          Madison, WI from Raymondville, TX (1290m).


     c.   In summary, extensive real world data have conclusively demonstrated skywaves require adaptive
          compensation that cannot be a simple function of distance and transmitter power. Simple ECD averaging
          is not sufficient, and can be so slow as to make correct cycle identification impossible in a reasonable
          timeframe.

3.   A contemporary Loran receiver substantially increases SNR by reducing noise.

     The overall effect is a 12-18 dB SNR improvement beyond that possible on earlier hard limited or linear
     receivers with little or no CRI processing. Consequently, the Loran signals of interest are much cleaner, better
     quantified, and more reliable for position, navigation and timing.

4.   A contemporary Loran receiver is “all-in-view,” and its development is analogous to the evolution of GPS
     from sequencing to all-in-view receivers.

     Although not generally recognized, there are similarities in the development of today’s Loran and GPS
     receivers. GPS receivers evolved from single channel sequencing receivers to multichannel, all-in-view
     receivers, as has Loran technology. Excerpts from a GPS receiver flyer of approximately 5 years ago are
     revealing:


                                                                                                                     11
     a.   “___ multi-channel design ensures maximum performance, especially under adverse conditions when the
          satellite signals are attenuated by low elevation angles, multipath interference, or shadowing.”

     b.   “Multi-channel performance means faster acquisition time and faster recovery from signal blockage.”

     c.   “Multi-channel pseudorange measurements ensure a more robust navigation solution and provide an
          important level of redundancy.”


5.   In the context of these statements, it is worthwhile to note todays multichain, all-in-view Loran receivers
     will typically track 2-4 times as many transmitters as an all-in-view GPS receiver - depending on the
     location and conditions – and the performance benefits are quite similar for both.

6.   In summary, multichain tracking, CRI processing, and skywave compensation improve today’s receiver
     performance primarily by:

     a.   Enhancing SNR 12-18 dB over hard limited receivers and reducing short term RMS noise 2-4 times, as
          shown in Figure 8.

     b.   Reducing ECD noise 4 – 6 times (so ECD averaging time is typically reduced 25 times), as shown in
          Figure 9.




               Figure 8. Multichain Tracking Improves SNR 12-18 dB and Typically Reduces Noise 2-4 Times
          Simultaneous TD and SNR recordings from identical receivers programmed to operate in 8 chain (left), 4
          chain (center), and 1chain (right) modes. Note multichain noise (left) is 5.3 nS versus 17.6 nS for single
          chain (right), even excluding cycle slips (a) and (b) on the single chain unit due the short time constant.




                                                                                                                        12
   Figure 9. Multichain ECD Noise Reduction Dramatically Improves Receiver Dynamic Performance
Simultaneous TD and SNR recordings from identical receivers programmed to operate in 8 chain (left), 4
chain (center), and 1 chain (right) modes. Note that multichain ECD noise is about 1/5 single chain ECD
noise (187 nS versus 937 nS, respectively), so the single chain receiver would have to average 25 times
longer to operate at the same level of confidence.




                                                                                                          13
7.   What do these improvements mean to the user regarding a receiver’s real world performance?

     a.   Significantly increases availability and reliability – the 1996 FRP states individual Loran stations have
          99.9% availability/reliability, and 99.7% triad availability/reliability. If a contemporary receiver is
          typically tracking 15-25 stations on groundwave, actual Loran availability is many times better than listed
          in the FRP. Although no formal calculations have been done, we would realistically expect 99.999% or
          better for multistation availability and reliability. Viewed another way, the unavailability of Loran is
          almost non-existent.

     b.   Significantly increases range. Figure 10 shows the Loran coverage from the 1996 FRP based on SNR and
          accuracy determined using antiquated receivers.




          Figure 10. Figure A-4 from 1996 FRP entitled “Loran Coverage Provided by US Operated or Supported
          Loran-C Stations.” Compare coverage with Figures 11-13.




                                                                                                                    14
    c.   In contrast, Figures 11-13 are from a recent MS Thesis at Ohio University and illustrate Loran coverage
         for en-route, oceanic navigation predicted for a modern, multichain Loran receiver.




Figure 11. New Loran-C coverage prediction for remote navigation in the vicinity of the U.S. (2500km radius
coverage from four stations.) Figure and caption from 1997 MS Thesis of W. Huang, Avionics Engineering Center,
Ohio University.




    Figure 12. New Loran Coverage Prediction for En Route and Oceanic Navigation
    Figure from 1997 MS Thesis of W. Huang, Avionics Engineering Center, Ohio University.




                                                                                                                   15
               Figure 13. New Loran Coverage Prediction for En Route and Oceanic Navigation
           Figure from 1997 MS Thesis of W. Huang, Avionics Engineering Center, Ohio University.



d.   Significantly improves coverage – for example, recent USCG/DARPA studies have demonstrated Loran
     penetrates cities much better than previously anticipated, and certainly does so better than GPS or NDGPS
     signals in dense urban canyons.

e.   Significantly improves repeatable accuracy. Figures 14a and 14b show representative 24 hour Loran and
     GPS data from recent recordings. The GPS data are from a 1998 ION paper by Peterson et al, and the
     Loran data are from a September 1998 recording at Locus. The axes on Locus’ data were adjusted to make
     comparison between the data sets easier, but axes are still not identical. Nevertheless, it is very clear the
     Loran repeatable accuracy is considerably better than GPS with S/A. Moreover, Lorans repeatable
     accuracy could be improved considerably if the transmitter controls were upgraded. Assuming the
     transmitters are upgraded in the near future, Lorans repeatable accuracy is predicted by the solid circle.
     Based on literally years of recordings in Madison, repeatable accuracy should then be about 3nS (1m RMS)
     during the day and 10nS (3m RMS) during the night.




                                                                                                               16
                                                                  GPS position over 24 hours




                   Figure 14a, 14b. GPS versus Loran – Repeatable Accuracy
GPS data taken over a 24 hour period in a stationary position are from Peterson et al, 1998. Loran data
taken over 24 hour period on Locus LRSIII in a stationary position, and are from Locus, Inc. Circles
surrounding individual data points show true Loran repeatability during times between transmitter timer
jumps. Solid circle with arrow represents predicted Loran repeatable accuracy if transmitter timing jumps
were eliminated by transmitter upgrade program and is artificially offset for visual clarity.




                                                                                                        17
f.   Significantly improves dynamic performance – for example, cycle slips, slow recognition of cycle slip, and
     slow recovery after cycle slips were prevalent problems with single chain receivers. Figure 15 shows
     ECD data from 3 identical receivers operated simultaneously, but each was configured to function in 8
     chain, 4 chain, or single chain mode respectively. As can be readily appreciated, single chain ECD is over
     5 times noisier than 8 chain acquisition, and older receivers would have to average 25 times longer to
     achieve the same level of confidence in cycle selection. With contemporary technology, the reality is cycle
     slips do not occur.




     Figure 15 (same data as Figure 9). Contemporary multichain receivers do not cycle slip and offer
     significantly enhanced dynamic performance.
     Simultaneous ECD recordings from identical receivers programmed to operate in 8 chain (left), 4 chain
     (center), and 1chain (right) modes.




                                                                                                              18
8.   Today’s Loran works much better than yesterday’s, but Loran’s performance is still far from optimized.
     Why?

     a.   Today’s receivers suffer from some of the same constraints as encountered by yesterday’s receivers, i.e.
          lack of processing power and over-reliance on hardware, primarily analog components used for filtering.
          For example, Locus current receiver may discard up to 75% of the data acquired because of processor
          overload.

     b.   The same processor limitations typically restrict a receiver to single chain navigation solutions instead of
          multichain and master independent navigation.

     c.   However, the primary limitation is adequate control of the transmitters. What is required?

          i. New Cs Clocks
          The existing clocks are aging, drift considerably, and seem to be susceptible to temperature variations at the
          sites. They need to be replaced with new Cs clocks with adequate temperature stability. For example,
          Figure 16 shows the phase of a Locus receiver clock relative to a Cs standard when the receiver is locked to
          8970M (Dana, IN, about 245 miles from Madison, WI). The pattern of transmitter clock drift over this 3
          day period and the approximate 100 nS adjustment are evident.




                               Figure 16. Continuous Drift of Cs Clock at 8970MTransmitter
                          Note even with the daily 100nS step adjustments, the Cs continues drifting.




                                                                                                                         19
   ii. New Timing Control
       There are instabilities in the timing controllers and new ones are needed. For example, Figure 17 is
       an overnight recording of 8970Y TDs derived from 8970Y TOA (Baudette, MN) minus 8970M TOA
       (Dana, IN). These and much other data illustrate the basic background noise seen by the receiver is
       about 2.5nS RMS, but that background is dominated by 20nS oscillations or jumps. With these
       oscillations removed, the loran signal supplied to modern receivers would be very steady, repeatable,
       and appropriate for use as a pseudolite in hybridized GPS/Loran receivers, if the system is
       synchronized to UTC.




                      Figure 17. Representative 20nS Transmitter Jumps or Oscillations
8970Y TD data derived from 8970Y TOA minus 8970M TOA and taken from an overnight data run. Note
numerous TD shifts/oscillations of approximately 20 nS, and background noise of approximately 2.5nS RMS.




                                                                                                           20
iii. Synchronization to Universal Time Coordinated (UTC)
     If the Loran system is synchronized to UTC, it will allow use of Loran stations as GPS pseudolites.
     Synchronization is a straightforward, relatively inexpensive process that was ordered by the US
     Congress several years ago. Other international Loran systems are now synchronized to UTC
     standards, such as the NELS synchronization to UTC Brest. The Cs clock at the Pohang, Korea
     transmitter is very tightly synchronized to GPS time using the USNO’s “melting pot” method for
     robust, automated time transfer.
iv. Implement Time of Emission (TOE) Control
    TOE would eliminate many problems associated with system area monitoring (SAM) stations and save
    significant upgrade and ongoing operations/maintenance expenses for those stations. SAM control
    evolved when Loran was primarily a marine system in order to maintain stable TDs over a fixed
    geographic area. Given Lorans widespread use in a variety of markets, the ability of new receivers to
    track signals much more accurately, and the cost savings of eliminating some monitoring sites, TOE
    control should be implemented.




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III. TOMORROW’S LORAN - A COMPONENT OF INTEGRATED GPS/LORAN RECEIVERS

      1.   Initially based on commercial digital signal processing chips (DSPs) to enable processing of all available
           Loran data. Subsequent generations will use application specific integrated circuits (ASICs) to reduce
           size, cost, and power consumption. In other words, Loran receivers can become as small and
           inexpensive as GPS receivers; only establishment of the market and manufacturing volume stand in
           the way.

      2.   Virtually all processing will be done in software, so adaptation to different markets and product
           changes will be faster, less expensive. In addition, such problems as ECD distortion by analog filters
           will be eliminated, thereby improving dynamic receiver performance and reliability regardless of the
           environment.

      3.   Small, inexpensive Loran H-field antennas and combined GPS/H-field antennas will become
           commonplace. E-field antennas, regardless of the application, will be 20” or less in length. There
           are small OEM Loran H-field antennas now on the market, as well as combined marine radiobeacon
           (300kHz) H-field and GPS antennas from several companies. Depending on the application, some
           users will prefer the true redundancy of physically separate GPS and Loran antennas.

      4.   Through use of H-field antennas, digital noise blanking techniques, and additional processing using
           DSP techniques, outstanding Loran performance will be conclusively demonstrated and documented
           under P-static and thunderstorm conditions.

           a.   As illustrated above, the application of contemporary digital technology to Loran already provides
                major performance advances with regard to elimination of cycle slips, immunity to noise and burst
                interference, rapid signal acquisition and quantification, etc.

           b.   Some specific advantages of H-field antennas in aviation applications include:

                i.      inherent 3dB SNR advantage over E-field

                ii.     microprocessor capabilities enable receiver to steer antenna internally with no need for input
                        from external device

                iii.    greater immunity to high-voltage, low-current interference characteristics of P-static

                iv.     small, and can be integrated with GPS antenna into a single device

                v.      Size and robustness makes Loran more practical regardless of application.

      5.   Integrated Eurofix capabilities will provide differential GPS (DGPS) corrections and the following
           benefits:

           a.   Improved GPS availability and continuity through calibrated Loran, plus external integrity, for the
                primary GPS applications (e.g. GA aircraft, marine, terrestrial, and timing). Synchronization of Loran
                to UTC and improved Loran transmitter control enables Loran transmitters to be used as GPS
                pseudolites. Given multichain acquisition capabilities of new Loran receivers, combined GPS/Loran
                availability should easily exceed .99999.

           b.   Many times more terrestrial and oceanic DGPS coverage than the NDGPS system, even if NDGPS
                were to be fully deployed. Note Eurofix will also provide much better DGPS penetration into cities
                and remote areas for vehicle location, recreation and other applications.

           c.   WAAS-level accuracy and integrity performance for aviation receivers with the additional major
                benefit of triple or more redundancy (depending on how many Loran stations the receiver can track).


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          Note the cost of implementing Eurofix is approximately $10M versus about $3B for WAAS, so even if
          only considered a national insurance policy, Eurofix is phenomenally cost effective.

     d.   True redundancy for all radionavigation applications, i.e. Eurofix eliminates the national sole-means
          vulnerabilities and limitations of WAAS, LAAS and NDGPS systems.

     e.   Enhance international acceptance of GPS and sales of GPS receivers by eliminating sole-means
          dependence and liability issues, and by providing individual countries a cost effective, autonomous
          radionavigation system ideally complementary to GPS.

6.   Real-time Eurofix DGPS corrections will eliminate or significantly reduce Loran’s ASF bias and
     improve Loran’s absolute accuracy to better than GPS with S/A.

     a.   Real time DGPS corrections provided through Eurofix will enable the receiver to generate ASF
          corrections dynamically. Depending the requirements of the application, individual ASF corrections
          can be good for a few minutes to several days? For example, an integrated GPS/Loran receiver used
          by a hiker in the northern Rockies would only require DGPS calibration infrequently, while an airplane
          receiver during approach should be calibrated every 5 seconds to provide WAAS-like performance in
          the approach phase. Both performance standards can be met by Eurofix.

     b.   Because of reduced memory costs and microprocessor performance improvements, ASF maps can also
          be stored in memory in perhaps 20 minute squares and applied by the receiver as a function of the
          dynamics, position, and season. In other words, stored ASF maps would enable a hybrid GPS/Loran
          receiver to provide GPS level performance, even if GPS were blocked, jammed, or lost for any reason.

     c.   Moreover, Loran’s repeatable accuracy today is better than GPS repeatable accuracy with S/A.
          Improvements in the timing control of the Loran system will improve Loran’s repeatable accuracy to
          approximately 1-2 meters short term, 5 meters daily, and 5-30 meters seasonally, using a 60? second
          time constant.

7.   Integrated or Hybridized GPS/Loran receivers. Important user benefits include:

     a.   GPS/Loran receivers will be integrated within one box, and there will be a single user interface to
          simplify receiver operation. Integrated devices will reduce size, cost, and power consumption in all
          applications, and competition will continually drive prices down/performance up.

     b.   Individual users who may not have a sophisticated understanding of the vulnerabilities and limitations
          of radionavigation systems will enjoy the safety and reliability of true radionavigation redundancy,
          regardless of the application. A similar, stronger argument in support of Loran could be made for
          other countries who do not want to risk dependence of their marine, aviation, terrestrial, and
          telecommunication infrastructure on a sole-means, US controlled GPS system.

8.   Integrated or Hybridized GPS/Loran receivers. Important national benefits include:

     a.   A proven, reliable, and cost effective means to provide true radionavigation redundancy/safety for
          individual users and groups and elimination of national sole-means vulnerabilities, as identified in the
          Presidential Commission on Critical Infrastructure Protection (PCCIP) and other national reports.

     b.   Market stabilization of Loran will increase sales volumes and decrease end user prices by focusing
          industry R&D dollars on developing core technologies applicable to all radionavigation applications.
          For example, neither VORs nor NDGPS can be used for timing applications; VORs are inappropriate
          for marine or terrestrial use; and NDGPS is inappropriate for aviation or widespread terrestrial use.
          Loran is the only system that can complement GPS in all these growing markets, and therefore offers
          the greatest opportunity drive down future product costs.




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c.   Hybridized receivers will promote international acceptance by assuaging global concerns with GPS
     control, availability, liability, etc. Individual governments will have autonomous control of a single,
     reliable radionavigation system – Loran - that can service their various aviation, marine, terrestrial,
     and telecommunication needs even if GPS is unavailable, regardless of the reason. Remember,
     constructing a national Loran system is very inexpensive, fast, and does not require sophisticated
     technology. Some examples are China, Kingdom of Saudi Arabia, and India.

d.   Indeed, Loran is our best example of the cooperative sharing of international radionavigation
     resources in 1998. For example, Japanese telecommunication companies can use Korean and Chinese
     Loran signals for precise time and frequency applications. Canadian pilots use US Loran signals on a
     daily basis, and German vessels can track Norwegian, French and Russian Loran signals to navigate.
     As a working model, Loran offers current lessons and more immediate promise than the uncertainty of
     future international GPS cooperation, as illustrated by concerns raised during the ICAO conference last
     June.

e.   Hybridized receivers will increase international sales of GPS receivers in substantial markets such as
     China and Europe, where control and liability concerns limit market acceptance.




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CONCLUSIONS

        1.   Today’s Loran receivers have considerably better performance than yesterdays, and the next
             generation receivers will offer another quantum leap in performance. All performance, economic,
             and policy evaluations of Loran must be based on objective contemporary standards, not outdated
             perceptions.

        2.   The overall performance of today’s Loran system is limited by aging transmitter controls that are in
             dire need of upgrades, not by receiver technology.

        3.   Transmitter upgrades, receiver improvements, and enhancements such as Eurofix make
             contemporary Loran an exceptionally high performance radionavigation system ideally
             complementary to GPS because:

             a.   the two systems have no single thread failures;

             b.   each system can synergistically enhance the other’s performance; and

             c.   the two systems together are substantially better than either alone.

        4.   Loran is the only other radionavigation system so uniquely complementary to GPS, and can be used
             in virtually all the same applications as GPS, i.e. marine, aviation, terrestrial, and timing. As such,
             Loran remains a remarkably cost effective national resource that should be sustained and strongly
             supported by the DOT in a comprehensive radionavigation policy that is nationally, rather than
             agency directed.

        5.   Finally, a national policy endorsing Loran as a complement to GPS will accelerate the international
             acceptance of GPS as the world’s primary radionavigation system. Loran could be the best thing to
             ever happen to GPS.



ACKNOWLEDGEMENTS
I would like to acknowledge my co-author, Paul Schick. Paul is one of those unique individuals who has a strong
theoretical background as well as extensive knowledge of what can be done with contemporary hardware and software.
Paul has made many important contributions to Loran, and I expect him to make many more in the future.

REFERENCES
1 Roth, G. Linn, Blandino, Thomas, and Schick, Paul “New Loran Receiver Technology Significantly Improves
  Overall system Performance and Substantiates Loran Viability as GPS Backup”. Proceedings of the Institute of
  Navigation National Technical Meeting, January 1996.

2 1996 Federal Radionavigation Plan, Department of Defense and Department of Transportation.

3 Huang, Wen-Jye “Investigation of The Benefits of Multi-Chain Loran-Chain Loran-C and Hybrid GPS/Loran-C
  Positioning. Ohio University Thesis, March 1997.

4 Peterson, B et al, “Integrated GPS/Loran, Structures and Issues”. Report 05-98, USCG Academy, Center for
  Advanced Studies, June 1998.




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