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) #1, 37 m #677, 494 m #695, 477 m

Multiple 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) #1, 30 m #734, 488 m

Pseudo-Ranging, all stations within range #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