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Evolution of Differential Loran-C for Navigation Accuracy

Explore the history, challenges, and opportunities of Differential Loran-C from the 1980s to present, its applications, practical challenges, and future direction as a navigation system.

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Evolution of Differential Loran-C for Navigation Accuracy

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  1. Applications of Differential Loran-C32nd Annual ILA MeetingA review ofUSCG R&D Differential Loran-C Study - 1984by LT D.S. Taggart by Mr. Douglas S. Taggart Systems Engineer Overlook Systems Technologies, Inc Vienna, VA

  2. In the pursuit of Accuracy Sandy Hook, New Jersey 9960 Xray/Yankee - 1983 Scatter Plot Agenda • Setting the Stage • Review of the 80’s • Practicality Challenges Then • Opportunities Then • The Early 90’s/National Policy • Where are We Headed? • Challenges Today • The Path-Ahead

  3. Setting the Stage(The Pursuit of Accuracy) • 1974 Loran-C was named as the official Radionavigation system for the Coast Confluence Zone of the US • Emergence of Harbor/Harbor Approach – better than 10 meter accuracy requirement • Receiver technology had not yet experienced the microprocessor boom • Federal Radio Navigation Plan (Inmarsat Act of 1978) • Mix of Federally Provided systems • Signal Stability Studies • USCG R&DC Harbor Monitor Studies • Loran-C Mini-chain, NEUS/SEUS, West Coast, Great Lakes, St Lawrence Seaway • Repeatable vs Absolute Accuracy • In the early 80’s GPS was on the horizon, Selective Availability was in the mix and Differential GPS was being investigated • Downing of Korean Airliner KAL 007 and President Regan’s subsequent offer of GPS Accuracy was a discriminator and “more” was the goal

  4. ts-sam SAM ts-P tm-sam tm-P Review of the 1980’sHarbor Monitor Study(Double Range Difference Model) 1) TDsam=tb + tcd + ts-sam- tm-sam Where tb + tcd = ED (Emission Delay) Secondary 2) ED = TD sam – ts-sam + tm-sam by extension, time difference at P is: TDP= ED + ts-P – t m-P tb Then by substitution TDP = TD sam – ts-sam + tm-sam + t s-P – tm-P And recognizing t = R/v and by further substitution TDP = TD sam (–Rs-sam + Rm-sam + Rs-P – Rm-P)/v Master substituting (– Rs-sam + Rm-sam + R s-P – Rm-P) = DRPP P Result: TDP = TD sam +DRDP/v

  5. P P1 Secondary ts-sam tb SAM ts-P tm-sam P1 Master tm-P P Harbor Monitor Study • Variations that emerged • Seasonal Variations • Weather • Transmitter effects • Control effects • Diurnal effects

  6. New London Differential Demonstration • Applied Harbor Monitor receivers to three sites • Master • Two Secondaries • Averaged delta TD updates relative to an average • Communicated on a VHF voice/data link • Update rate selectable (3 to 15 minutes • Modified a Graphic Display Terminal for direct data display • Modified commercial receiver for delta TD input

  7. Practical Challenges of Differential Then(Provide more accuracy to the Users) • Receiver characteristics/standards • TD to Lat/Long Conversions • Receiver fidelity • Waypoints stored in tenths of microseconds • Precise differential corrections were in the “round off” • Removing time differences on Nautical Charts • Additional Secondary Phase Factor (ASF) surveys/tables • Establishing the references for Differential required surveys • Waypoints on charts • Loran-C outages were problematic • High accuracy goes hand-in-hand with need for higher availability • Dwindling CG resources • Differential was localized (comms link and update validity) • Emergence of other systems (GPS and dGPS) • Global vs Regional Coverage • Comms links, user equipment, infrastructure support, need/mandate It was an uphill battle

  8. Opportunities Then • Complementary relationship between Loran-C and GPS • Using GPS to calibrate Loran-C • Master clock synchronization • “Raw” Loran-C accuracies were superior when compared to GPS with SPS (of 100 meters) • Aviation applications • Dealing with P-static • Momentaries • Aviation Blink • Loran-C had an established customer base and was a proven technology

  9. Early 90’s/National Policy • 1991 Gulf War and the role of GPS • Affordability of microprocessor technology • 1992 Coast Guard announcement to terminate Overseas Loran-C by 31 Dec 1994 • Expansion of Loran-C to mid-continent chains • Coast Guard budget reductions • Foreshadowing of domestic Loran-C termination • Loran Towers fall at Cape Race and Kargaburun, Turkey • Coast Guard's Differential GPS Maritime System • Presidential Decision Directive on GPS • In response to a New York Times article on the September 12,1994 crash of a small plan into the Whitehouse Focus: National Security (accuracy) and Free Service

  10. Where are We Headed? • More Accuracy is no longer the goal • Critical Infrastructure • Reliance on one system is flawed • Balance between security and commercial use/public good • Exclusivity, availability and robustness of the solution is the newdiscriminator

  11. Challenges Today • Role of CG as a navigation service provider in Department of Homeland Security • Constituency is more than maritime • User Equipment • Supply and demand curve is undefined • Seek ways to market Loran-C to fill voids in GPS • Buildings • Urban Canyon’s • Timing/Communication • Critical Infrastructure Is the door on Loran-C half open or half closed ?

  12. The Path-Ahead • National Policy on provision of PNT services is lacking • Advocates of Loran-C must find ways to market benefits as a means of reducing national vulnerabilities • The Department of Homeland Security’s role for leadership • Continuing to reduce costs through modernization and automations will help but there must be more

  13. Contact Information Doug Taggart Overlook Systems Technologies, Inc Suite 400 1950 Old Gallows Road Vienna, VA 22182-3933 703-893-1411ext. 112 703-607-1051 OSD(NII) Wireless U.S. National Grid: 18SUJ 0727 0926 dtaggart@overlooksys.com

  14. LORSTA George Washington Enhancement • It's the Dawn of a New Loran Day! • “I'm pleased to report from the folks currently at the site, that the first Enhanced Loran Station - Loran Station George, Washington - is currently on air and "on the 'nums" with all the newest technology that many of you have been working so hard over the past several years to provide. New buildings, new transmitter, new Timing and Frequency Equipment, new receivers, new cesiums, new ECM, new UPS, new Control Equipment, new....well, you get the point! I think it appropriate that Lorsta George, whose name is synonymous with George Washington, our first President, be the first of the newest breed of stations providing enhanced position, navigation, timing, and data signals for use throughout North America.” • E-mail from Chuck Schue - 10/23/03 W R Systems, Ltd.

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