Getting a Bearing on ASF Directional Corrections

1. Getting a Bearing on ASF Directional Corrections 32nd Annual Technical Symposium International Loran Association 5 Nov 2003 Boulder, CO

2. Authors • CAPT Richard Hartnett • US Coast Guard Academy • Gregory Johnson • John J. McMullen Assoc. Inc. • Dr. Peter Swaszek • University of Rhode Island • Ken Dykstra • John J. McMullen Assoc. Inc. USCGA-JJMA Presentation for ILA 32

3. Outline • Introduction • Problem identification • Error Sources and Solutions • Summary USCGA-JJMA Presentation for ILA 32

4. Introduction • LORAN-C as “the” backup for GPS • Accuracy, availability, integrity, continuity • Accuracy: • Needs: • Aviation: NPA – RNP 0.3 (309 meters) • Maritime: HEA (8-20 meters) • Limitations: • Spatial and temporal variations in TOA (ASF) observed by the receiver and presented to the position solution algorithm USCGA-JJMA Presentation for ILA 32

5. Loran 2003+ • Multi-station, multi-chain, all-in-view receiver • TOA vs. TD measurements • Separate “errors” into spatial, temporal, and directional components • DSP-based receivers • H-field vs. E-field antennas • Grid approach vs. waypoints USCGA-JJMA Presentation for ILA 32

6. Current Research • USCGA/JJMA activities: • Explore ways to mitigate TOA variation • Current approach: • Model the ASF sources • Spatial effects • Temporal effects • Model the directional effects (H-field) • Estimate parameters/values for the models • “Correct” the TOA observations using the models Of interest today USCGA-JJMA Presentation for ILA 32

7. ASF Variation Model • Assume that the variation in the observed TOAs can be decomposed into 3 independent, additive terms: • Spatial term: • Due to differences in the Loran signal propagation path • Temporal term: • For short term TOA variability on a local region • Directional term: • Based on the relative bearing of the Loran tower to the vessel Key assumptions USCGA-JJMA Presentation for ILA 32

8. Block diagram of approach Current Heading Recent Position Antenna Direction Correction ASF Spatial Correction TOAs From Rcvr TOAs To Position Algorithm ASF Temporal Correction Live Monitor Update USCGA-JJMA Presentation for ILA 32

9. Problem identification • First noticed directional TOA in our data sets over a year ago, started investigating: • Have numerous examples of this: • Airplane rotation data on ground • Convair 580 • Ohio University King Air • Locus antenna/Satmate rcvr • Megapulse antenna/DDC rcvr • Airplane rotation data in air • Convair in CA both rcvrs/antennas • Rockwell data – Locus antenna/satmate • Boat rotations • T-boats • Both Megapulse antenna/DDC and Locus antenna/satmate • Field data, various antennas • DDC, aircraft antenna, small antennas, split loop antennas USCGA-JJMA Presentation for ILA 32

10. Graphs of TOA variation Convair – PC104 - Megapulse Field – DDC – Split Loops Field – SatMate - Locus Tboat – DDC - Megapulse USCGA-JJMA Presentation for ILA 32

11. Expected Position Error • Given that there are TOA variations as the antenna is rotated, what is the effect in the position domain? USCGA-JJMA Presentation for ILA 32

12. Model of TOA Variations USCGA-JJMA Presentation for ILA 32

13. Resulting Position Error USCGA-JJMA Presentation for ILA 32

14. Position Error relative to center USCGA-JJMA Presentation for ILA 32

15. Error Sources • Time delay • Coupling • Antenna phase issues • Reradiation\Local Effects USCGA-JJMA Presentation for ILA 32

16. Time Delay • Unequal phase delay between the two channels leads to directional TOA variation • Sources • Amplifier/Filters • Cables • Antennas USCGA-JJMA Presentation for ILA 32

17. Time Delay – Theoretical Formulation Ideal Actual USCGA-JJMA Presentation for ILA 32

18. Theoretical Effect of Time Difference USCGA-JJMA Presentation for ILA 32

19. Loran Simulator (ICS660 card) RF Front End (Freq Devices gain modules and filters) Ch 1 RF in RF out Ch 2 Ch 1 Ch 2 Loran Receiver (ICS 650 card) Measured Time Delay USCGA-JJMA Presentation for ILA 32

20. USCGA-JJMA Presentation for ILA 32

21. S/w compensation for time delay USCGA-JJMA Presentation for ILA 32

22. Coupling • Some coupling seen with combined loops vs. with split loops • Maximize strong signal on one loop • Should be zero on other loop • This is not the case with some H-field antennas • Maximize strong signal on one loop • Signal is NOT zero on other loop USCGA-JJMA Presentation for ILA 32

23. USCGA-JJMA Presentation for ILA 32

24. Single Loop Phase Issue USCGA-JJMA Presentation for ILA 32

25. Strongest Beam Only USCGA-JJMA Presentation for ILA 32

26. Strongest Beam and Delay Correction USCGA-JJMA Presentation for ILA 32

27. Summary • Split loops (coupling) • Works – when strong signal is max. in one loop is zero on other loop • Time delay correction in software • Works - compensate for unequal delay in antenna and/or amplifier • Loop Phase Shift effect • Use antenna loop with largest magnitude only USCGA-JJMA Presentation for ILA 32

28. Final Remarks • When faced with tremendous coupled challenges we find the best strategy is to do what most engineers on the team do – blame Mitch! • In the future, for harbor mapping we will use both E and H-field antennas – mappings may not be the same - • We are highlighting “solvable challenges” with H-field antennas • New Locus antennas • New Reelelektronica antennas • Presentation by Wouter tomorrow • Calibration is clearly an important issue – Must be done in lab and after installation! USCGA-JJMA Presentation for ILA 32

29. Acknowledgements • Mitch Narins, FAA AND • JJMA Team • Christian Oates • George Sanders • Ruslan Shalaev • Tim Waldie USCGA-JJMA Presentation for ILA 32

30. Questions? rhartnett@exmail.uscga.edu gwjohnson@jjma.com swaszek@ele.uri.edu