Applications of Differential Loran-Applications of Differential Loran-CC32nd Annual ILA Meeting32nd Annual ILA Meeting

A review ofA review of USCG R&D Differential Loran-C Study - 1984USCG R&D Differential Loran-C Study - 1984

by LT D.S. Taggartby LT D.S. Taggart

byMr. Douglas S. Taggart

Systems EngineerOverlook Systems Technologies, Inc

Vienna, VA

AgendaAgenda• 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

Sandy Hook, New Jersey9960 Xray/Yankee - 1983 Scatter Plot

In the pursuit of Accuracy

Setting the StageSetting the Stage(The Pursuit of Accuracy)(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

Review of the 1980’sReview of the 1980’sHarbor Monitor StudyHarbor Monitor Study

(Double Range Difference Model)(Double Range Difference Model)

P

1) TDsam=tb + tcd + ts-sam- tm-sam

Where tb + tcd = ED (Emission Delay)

by extension, time difference at P is:TDP= ED + ts-P – t m-P

Then by substitutionTDP = TD sam – ts-sam + tm-sam + t s-P – tm-P

Master

Secondary

SAM

ts-sam

tm-samts-P

tm-P

tb

And recognizing t = R/vand by further substitutionTDP = TD sam (– Rs-sam + Rm-sam + Rs-P – Rm-P)/vsubstituting(– Rs-sam + Rm-sam + R s-P – Rm-P) = DRPP

Result: TDP = TD sam +DRDP/v

2) ED = TD sam – ts-sam + tm-sam

P• Variations that emerged

• Seasonal Variations• Weather • Transmitter effects• Control effects• Diurnal effects

P1

P1

P

Secondary

Master

SAM

ts-sam

tm-samts-P

tm-P

tb

Harbor Monitor StudyHarbor Monitor Study

New London New London Differential DemonstrationDifferential 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

Practical Challenges of Differential ThenPractical Challenges of Differential Then(Provide more accuracy to the Users)(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

Opportunities ThenOpportunities 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

Early 90’s/National PolicyEarly 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

Where are We Headed?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 new discriminator

Challenges TodayChallenges 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 ?

The Path-AheadThe 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

Contact InformationContact InformationDoug TaggartOverlook Systems

Technologies, IncSuite 4001950 Old Gallows RoadVienna, VA 22182-3933703-893-1411ext. 112703-607-1051 OSD(NII)

WirelessU.S. National Grid: 18SUJ 0727 0926 dtaggart@overlooksys.com

LORSTA George Washington LORSTA George Washington EnhancementEnhancement

• 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.