making a silk purse from a sow’s ear, loran style david h. gray canadian hydrographic service...

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

Thank you

• Questions?

• David H. Gray• Canadian

Hydrographic Service• grayd@dfo-mpo.gc.ca