Making a Silk Purse from a Sow’s Ear, Loran Style

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 entirely those of the author, and do not reflect any position of the Government of Canada.

2. Where was Moses when the lights went out? • Age old Biblical question • Answer: In the Dark • Let’s look at the navigational equivalent

3. Possible GPS Unavailability • Turning off signals • satellites destroyed • jamming of signals

4. Possible GPS Inoperability • Ground stations destroyed • bogus signals • unauthorized control of system

5. 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

6. 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

7. Security • 1) back-up navigation system • 2) provision to friendly forces, yet denial to opponent

8. More Adaptable Loran Receiver • Replace loss of Seneca with: • Carolina Beach - Master • Jupiter, Dana, Nantucket - Secondaries

9. Menu Drop-Downs • 1) Type in GRI • 2) select Master • 3) select Secondary • 4) type in Coding Delay • 5) Repeat 3) & 4)

10. Re-designate / Redesign • With flexible receivers • possibility to: • redesign chains to omit off-air stations • new GRIs to confuse unwanted users

11. 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

12. 2-TD (same chain) & 2-TD (cross-chain) #677, 494 m #695, 477 m #1, 37 m

13. Multiple TD #696, 477 m #1, 25 m

14. Status so far • No change to Loran-C broadcasting • improvement in coverage area with cross-chain • improvement in repeatability with multiple TDs

15. Pseudo-Ranging • All Loran-C stations broadcast in sync. with UTC • receiver computes position & time sync. constant • 3 Time of Arrivals needed

16. Pseudo-Ranging (3 or 4 stations) #734, 488 m #1, 30 m

17. Pseudo-Ranging, all stations within range #740, 499 m #1, 13 m

18. 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

19. Problems with Pseudo Ranging • Timing Control • GPS timing • communication satellites • ground monitors • “hot clocks” • Position calculation • systematic • random

20. Random Errors • e.g., momentary fluctuation in a TD • measure of repeatability • error ellipse computation • natural output of least squares solution

21. Systematic errors • Constants or slowly varying • affects absolute accuracy • e.g. Additional Secondary Factor (ASF) • redundant observations will not solve for ASF

22. Effect of ASF

23. Propagation • Johler et al NBS 573 • Brunavs’ polynomials • Modified Millington • Terrain effects delay Dist from Tx

24. 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

25. 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

26. Thank you • Questions? • David H. Gray • Canadian Hydrographic Service • grayd@dfo-mpo.gc.ca