A Preliminary Study of Loran-C Additional Secondary Factor (ASF) Variations

1. A Preliminary Study of Loran-C Additional Secondary Factor (ASF) Variations International Loran Association 31st Annual Convention And Technical Symposium October 27-30, 2002

2. Authors • CAPT Richard Hartnett • US Coast Guard Academy • Gregory Johnson • John J. McMullen Assoc. Inc. • Peter Swaszek • University of Rhode Island • Mitchell J. Narins • US Federal Aviation Administration USCGA-JJMA Presentation for ILA-31

3. Outline • Introduction • Methodology of Data Collection • Error Sources and Bounds • August Flight Tests • Highlights of ASF Corrected Loran data • Conclusions • Future work USCGA-JJMA Presentation for ILA-31

4. Introduction • Concept: • LORAN-C as “the” backup for GPS • NPA – RNP 0.3 • Obvious issues: accuracy, availability, integrity, and continuity • USCGA Goal: Improve our understanding of those variations in TOA that could be mitigated (ASFs) USCGA-JJMA Presentation for ILA-31

5. This report: • Report on flight tests (August 2002) to collect concurrent LORAN and WAAS GPS position data (equipment, methodology, variation characteristics) • From ground data estimate TOA ASF in the vicinity of an intended approach • (post-)Process some approach flight data to determine feasibility of 1/6 NM accuracy using local correction USCGA-JJMA Presentation for ILA-31

6. Data Collection Methodology • Use USCGA PC-104 TOA ASF Measurement System • Run receiver in reference mode at a known location (surveyed or DGPS/WAAS position) • Save TIC and RAW TOA, SNR, ECD measurements for each station at 2 sec. intervals • Collect data over 1-2 hr period • Post-process to calculate ASF’s at 2 sec. intervals • ASF, SNR, and ECD are used to determine if ASF is valid • Average ASF’s over the 2 hr period USCGA-JJMA Presentation for ILA-31

7. 1 PPS Novatel OEM-4 WAAS GPS 10 MHz PC-104 Rcvr DS-345 Signal Generator 12.8 MHz 2 Channels RF Save to disk: TIC, RAW TOA, ECD, SNR, etc. TOA ASF Measurement System H-field antenna USCGA-JJMA Presentation for ILA-31

8. PC-104 Based TOA ASF Measurement System • CPU board, Gatefield DSP board, oscillator board (opt), Power and I/O boards • Provide stabilized clock frequency • 12.8 MHz from Cesium clock stabilized signal source • GPS 1PPS signal • H-field antenna • Two crossed loops USCGA-JJMA Presentation for ILA-31

9. ASF Calculation (TIC + Raw TOA - Rcvr Delay - Emission Delay - Predicted TOA ) mod 200 μsec = EDC + ASF USCGA-JJMA Presentation for ILA-31

10. ASF Definition • Define TOA ASF as the additional propagation delay due to a non-seawater path • Need to separate out system timing errors from ASF value • Need to know exactly when stations transmit, relative to UTC (use GPS 1 PPS as time reference). • Master: existing TTM equipment • Secondary: LSU/PIG installed TOTM equipment • Difference between measured and calculated TOA is ASF + time offsets USCGA-JJMA Presentation for ILA-31

11. Nantucket TOTM • Secondary TOTM data for time period of flight tests (Julian days 233-241, 2002) • X-axis = hours starting from midnight of first day • Y-axis = offset from UTC in ns 9960 5930 USCGA-JJMA Presentation for ILA-31

12. Carolina Beach TOTM 7980 9960 USCGA-JJMA Presentation for ILA-31

13. ASF Measurement Error Sources • Timing Errors • Master and Secondary • Receiver Errors • 1 PPS stability • Sampling resolution • Receiver tracking accuracy • Receiver calibration • Cross-coupling in H-field antenna USCGA-JJMA Presentation for ILA-31

14. Error Bounds USCGA-JJMA Presentation for ILA-31

15. Loop cross-coupling Variation in TOAs as aircraft antenna is rotated 720 degrees. Dana Caribou Seneca Carolina Beach Nantucket USCGA-JJMA Presentation for ILA-31

16. Loop cross-coupling Close-up of Nantucket data from previous slide USCGA-JJMA Presentation for ILA-31

17. Ohio University King Air USCGA-JJMA Presentation for ILA-31

18. CG Equipment on King Air USCGA-JJMA Presentation for ILA-31

19. August 2002 Flight Test USCGA-JJMA Presentation for ILA-31

20. Flight routes Seneca Dana Carolina Beach Malone USCGA-JJMA Presentation for ILA-31

21. 22 August, 9960 Data USCGA-JJMA Presentation for ILA-31

22. 22 August, 8970 Data USCGA-JJMA Presentation for ILA-31

23. 22 August, 5930 Data USCGA-JJMA Presentation for ILA-31

24. 8/22 BLM TOA Distributions USCGA-JJMA Presentation for ILA-31

25. 8/27 W29 TOA Distributions USCGA-JJMA Presentation for ILA-31

26. Portland ME, 26 Aug 02 Loran (red), Corrected Loran (green) and DGPS (blue) positions during approach to Portland, PWM. Corrected Loran is the raw TOA’s corrected with measured ASF values, ED offsets, and Master TOT offsets. USCGA-JJMA Presentation for ILA-31

27. Portland ME, 26 Aug 02 Radial error measured from WAAS GPS position USCGA-JJMA Presentation for ILA-31

28. Conclusions • ASF collection procedure: • Methodology works • Further resolution of error sources • Sample flight tests: • TOA All-In-View receiver (with timing corrected and static, local ASF correction) provided <1/6 NM accuracy on approaches (to 20 miles out) • Potential for LORAN to achieve RNP 0.3 USCGA-JJMA Presentation for ILA-31

29. Future Work • Examine spatial and temporal ASF variations • Daily data collection at USCGA (multi-week) • Collect additional spatial data in December and January • Receiver improvements • Increase sampling rate to 25.6 MHz to increase resolution • Resolve receiver calibration • Apply ASF corrections to Locus receiver TOAs USCGA-JJMA Presentation for ILA-31

30. Acknowledgments • Christian Oates, George Sanders, Ken Dykstra • JJMA, Inc. • Dave Diggle, Brian Branham, Jamie Edwards • Ohio University • Ben Peterson • P.I.G. • USCG Loran Support Unit • USCG NAVCEN USCGA-JJMA Presentation for ILA-31