making a silk purse from a sow’s ear, loran style david h. gray canadian hydrographic service...
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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.
Where was Moses when the lights went out?
• Age old Biblical question
• Answer: In the Dark
• Let’s look at the navigational equivalent
Possible GPS Unavailability
• Turning off signals• satellites destroyed• jamming of signals
Possible GPS Inoperability
• Ground stations destroyed
• bogus signals• unauthorized control
of system
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
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
Security
• 1) back-up navigation system
• 2) provision to friendly forces, yet denial to opponent
More Adaptable Loran Receiver
• Replace loss of Seneca with:
• Carolina Beach - Master
• Jupiter, Dana, Nantucket - Secondaries
Menu Drop-Downs
• 1) Type in GRI• 2) select Master• 3) select Secondary• 4) type in Coding
Delay• 5) Repeat 3) & 4)
Re-designate / Redesign
• With flexible receivers • possibility to:
– redesign chains to omit off-air stations
– new GRIs to confuse unwanted users
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
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
Multiple TD
0
100
200
300
400
500
600
2-TDX-ChainMultiple TD
#1, 25 m
#696, 477 m
Status so far
• No change to Loran-C broadcasting
• improvement in coverage area with cross-chain
• improvement in repeatability with multiple TDs
Pseudo-Ranging
• All Loran-C stations broadcast in sync. with UTC
• receiver computes position & time sync. constant
• 3 Time of Arrivals needed
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
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
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
Problems with Pseudo Ranging
• Timing Control– GPS timing
– communication satellites
– ground monitors
– “hot clocks”
• Position calculation– systematic
– random
Random Errors
• e.g., momentary fluctuation in a TD
• measure of repeatability
• error ellipse computation
• natural output of least squares solution
Systematic errors• Constants or slowly
varying• affects absolute
accuracy• e.g. Additional
Secondary Factor (ASF)
• redundant observations will not solve for ASF
Effect of ASF
Propagation
• Johler et al NBS 573• Brunavs’ polynomials• Modified Millington• Terrain effects
Dist from Tx
delay
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
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
