Making a Silk Purse from a Sow’s Ear, Loran Style David H. Gray Canadian Hydrographic Service...
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![Page 1: Making a Silk Purse from a Sow’s Ear, Loran Style David H. Gray Canadian Hydrographic Service Ottawa Disclaimer: The opinions expressed in this paper are.](https://reader036.fdocuments.us/reader036/viewer/2022062518/56649eb15503460f94bb7558/html5/thumbnails/1.jpg)
Making a Silk Purse from a Sow’s Ear, Loran Style
David H. Gray
Canadian Hydrographic Service
Ottawa
Disclaimer:
The opinions expressed in this paper are entirely those of the author, and do not reflect any position
of the Government of Canada.
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Where was Moses when the lights went out?
• Age old Biblical question
• Answer: In the Dark
• Let’s look at the navigational equivalent
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Possible GPS Unavailability
• Turning off signals• satellites destroyed• jamming of signals
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Possible GPS Inoperability
• Ground stations destroyed
• bogus signals• unauthorized control
of system
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Loran-C good back-up to GPS
• Different parts of radio spectrum
• terrestrial vs. space• signal strength• most of northern
hemisphere• fully de-bugged• rugged, robust
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Present Loran-C receivers fragile to loss of coverage
• If Seneca goes off air
• then:• loss of coverage in NE
USA, Great Lakes, St. Lawrence River
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Security
• 1) back-up navigation system
• 2) provision to friendly forces, yet denial to opponent
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More Adaptable Loran Receiver
• Replace loss of Seneca with:
• Carolina Beach - Master
• Jupiter, Dana, Nantucket - Secondaries
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Menu Drop-Downs
• 1) Type in GRI• 2) select Master• 3) select Secondary• 4) type in Coding
Delay• 5) Repeat 3) & 4)
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Re-designate / Redesign
• With flexible receivers • possibility to:
– redesign chains to omit off-air stations
– new GRIs to confuse unwanted users
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Test Cases
• Every 2° interval of Lat. & Long. in Western Hemisphere
• Calculated semi-major axis of error ellipse (Repeatability)
• Max. Range = 700 n.m.
• More than 1200 points calculated
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2-TD (same chain) & 2-TD (cross-chain)
0
100
200
300
400
500
600
2-TDCross-Chain
#1, 37 m
#677, 494 m #695, 477 m
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Multiple TD
0
100
200
300
400
500
600
2-TDX-ChainMultiple TD
#1, 25 m
#696, 477 m
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Status so far
• No change to Loran-C broadcasting
• improvement in coverage area with cross-chain
• improvement in repeatability with multiple TDs
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Pseudo-Ranging
• All Loran-C stations broadcast in sync. with UTC
• receiver computes position & time sync. constant
• 3 Time of Arrivals needed
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Pseudo-Ranging (3 or 4 stations)
0
100
200
300
400
500
600
2-TD
X-Chain
Multiple TD
Pseudo Ranging, 4stns
#1, 30 m
#734, 488 m
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Pseudo-Ranging, all stations within range
0
100
200
300
400
500
600
2-TD
X-Chain
Multiple TD
Pseudo Ranging, 4stnsPseudo Ranging, allstns
#1, 13 m
#740, 499 m
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Requirements for Pseudo-Ranging
• Each station identifiable
• Stations either a Master or Secondary
• Can be double-rated• Pairings based on
most frequently used in 4-Station solutions
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Problems with Pseudo Ranging
• Timing Control– GPS timing
– communication satellites
– ground monitors
– “hot clocks”
• Position calculation– systematic
– random
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Random Errors
• e.g., momentary fluctuation in a TD
• measure of repeatability
• error ellipse computation
• natural output of least squares solution
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Systematic errors• Constants or slowly
varying• affects absolute
accuracy• e.g. Additional
Secondary Factor (ASF)
• redundant observations will not solve for ASF
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Effect of ASF
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Propagation
• Johler et al NBS 573• Brunavs’ polynomials• Modified Millington• Terrain effects
Dist from Tx
delay
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Conclusions
• North American chains do not have to be “carved in stone”
• can be available to friendly forces but denial to opponents
• Rx’s could track 2 or more chains
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Conclusions (2)
• Increase in service area and accuracy
• With ASF knowledge, position accuracy can match repeatability– 37 m present, 2 TD
– 37 m 2 TD X-chain
– 25 m multi-TD
– 30 m 4 stn, pseudo range
– 12 m multi- pseudo range
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