Transcript No Slide Title

Applications of Differential Loran-C
32nd Annual ILA Meeting
A review of
USCG R&D Differential Loran-C Study - 1984
by LT D.S. Taggart
by
Mr. Douglas S. Taggart
Systems Engineer
Overlook Systems Technologies, Inc
Vienna, VA
Agenda
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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
In the pursuit of
Accuracy
Sandy Hook, New Jersey
9960 Xray/Yankee - 1983
Scatter Plot
Setting the Stage
(The Pursuit of Accuracy)
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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
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Receiver technology had not yet experienced the microprocessor boom
Federal Radio Navigation Plan (Inmarsat Act of 1978)
– Mix of Federally Provided systems
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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
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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’s
Harbor Monitor Study
(Double Range Difference Model)
1) TDsam=tb + tcd + ts-sam- tm-sam
Secondary
ts-sam
SAM
tm-sam
tb
ts-P
Master
tm-P
P
Where tb + tcd = ED (Emission Delay)
2) ED = TD sam – ts-sam + tm-sam
by extension, time difference at P is:
TDP= ED + ts-P – t m-P
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
substituting
(– Rs-sam + Rm-sam + R s-P – Rm-P) = DRPP
Result: TDP = TD sam +DRDP/v
Harbor Monitor Study
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Variations that emerged
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Seasonal Variations
Weather
Transmitter effects
Control effects
Secondary
Diurnal effects
P
P1
ts-sam
SAM
tm-sam
tb
ts-P
P1
Master
tm-P
P
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
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
Opportunities Then
• Complementary relationship between LoranC 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 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?
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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 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
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Buildings
Urban Canyon’s
Timing/Communication
Critical Infrastructure
Is the door on Loran-C half open or half closed ?
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
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
[email protected]
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.