1. ILA 4-6 November 2003 Integrated GPS/Loran Sensor for Maritime Operations Wouter J. Pelgrum Reelektronika / Delft University of Technology / Gauss Research Foundation

2. ILA 2003 4-7 Nov 2003 Introduction Two very challenging applications for LC Aviation: The Seven Nines Maritime: The Eight to Twenty Meter This presentation: Maritime - Focus on accuracy - Outline of error budget - Identification & elimination of potential threads

3. ILA 2003 4-7 Nov 2003 Error Budget

4. ILA 2003 4-7 Nov 2003 H-field Antenna Design Issues Why use a H-field antenna and not an E-field - no P-static susceptibility - Less susceptible to near-field phenomena - no grounding needed - Low profile H-field antenna challenges: - More difficult to make low-noise - Parasitic E-field susceptibility - Tuning - Cross-talk - Beam steering algorithm / 180 degrees phase ambiguity

5. ILA 2003 4-7 Nov 2003 Noise Low effective height of an H-field antenna vs. E-field requires special attention to noise design Given a well designed H-field antenna and the Loran-C coverage of the United States, the error due to H-field antenna noise is not (yet) a bottleneck in the total error-budget

6. ILA 2003 4-7 Nov 2003 Rotating a H-field antenna LC 1 LC 2 We need 2 H-field antennas for an omni-directional radiation pattern H-field antenna bias-errors are heading dependant So they are different for all tracked stations And therefore they degrade positioning accuracy

7. ILA 2003 4-7 Nov 2003 E-field Susceptibility In the far field, the E-field has a 90 degrees, 120 pi relation with the H-field Parasitic E-field pickup by a H-field antenna will result in a heading-dependant phase-error (range error) Well designed shielding and/or balancing of the H-field antenna reduces the range error due to E-field susceptibility to the meter level

8. ILA 2003 4-7 Nov 2003 E-field Susceptibility (contd) Single loop used up to 45 degrees. After that, the other loop effectively takes over Maximum range errors of approximately 1 meter achievable with carefully designed shielding and / or balancing Range error due to E-field susceptibility

9. ILA 2003 4-7 Nov 2003 H-field Antenna Tuning Two modes of operation of H-field antennas: Resonance vs Wide-band + Slightly better noise performance @ Q=3 + Some off-band interference rejection + Anti-aliasing OK for Sigma-Delta ADCs - Surrounding metal influences resonance freq - Temperature influence on resonance freq - Multiple LF-Rnav systems reception more difficult Tuning / phase difference between antennas leads to a heading dependant error

10. ILA 2003 4-7 Nov 2003 H-field Antenna Tuning Error Compensation

11. ILA 2003 4-7 Nov 2003 H-field Antenna Tuning Error Compensation (contd)

12. ILA 2003 4-7 Nov 2003 X-talk: Introduction Xtalk causes a heading dependant error Solve Xtalk-problem by: Prevent Xtalk by electrical and mechanical construction of the antenna Measure Xtalk and apply feed-forward correction Auto-calibration by feed-back correction

13. ILA 2003 4-7 Nov 2003 X-talk: some formulas… Ideal dual loop H-field antenna response: Approximation of actual dual loop H-field antenna response: G 1 & G 2 : Gain and tuning mismatch A 21 & A 12 Xtalk Model of X-talk and Tuning

14. ILA 2003 4-7 Nov 2003 X-talk: Measurement of parameters (contd) 130 cm The H-field antenna (yellow box) is rotated inside a measurement loop. The field at the centre of the loop is quite homogeneous due to the large diameter of the loop (1.30m) Computer controlled antenna rotor H-field antenna Cross-section of measurement setup Measurement Setup

15. ILA 2003 4-7 Nov 2003 Capacitive coupling from antenna 1 to antenna 2 Measured Antenna Response Antenna 1 Antenna 2 X-talk: Measurement of parameters (contd)

16. ILA 2003 4-7 Nov 2003 Antenna 1 Antenna 2 Curve-Fitting to find antenna parameters G 1, G 2, A 21 and A 12 X-talk: Measurement of parameters (contd)

17. ILA 2003 4-7 Nov 2003 X-talk: Feed Forward Correction Feed-Forward Correction using G1,A21 G2,A12 Digitized Antenna signals Feed Forward Correction of Antenna

18. ILA 2003 4-7 Nov 2003 Uncorrected Corrected 45 m Comparison of Uncorrected and Feed-Forward Corrected Response after Beam-Steering X-talk: Feed Forward Correction (contd)

19. ILA 2003 4-7 Nov 2003 Corrected (zoomed in) 1.5 m Comparison of Uncorrected and Feed-Forward Corrected Response after Beam-Steering X-talk: Feed Forward Correction (contd)

20. ILA 2003 4-7 Nov 2003 Not Only The Antenna Matters… Attenuators Cables …

21. ILA 2003 4-7 Nov 2003 Xtalk: Work Continues…. Improved measurement loop for better quality and repeatability of factory calibration Separate measurement of X- talk and tuning Extensive testing of calibration-quality Automatic Calibration Investigate influence of cables, attenuators, etc.. 160 cm Computer controlled antenna rotor DUT

22. ILA 2003 4-7 Nov 2003 H-field Antennas Seem Troublesome, Why Again Are We Using Them? Pstatic No grounding needed Low profile Less susceptible to local effects And… True Heading Reradiation Detection

23. ILA 2003 4-7 Nov 2003 Now We Can Rotate the Antenna… … rotate the vessel

24. ILA 2003 4-7 Nov 2003 Error Budget

25. ILA 2003 4-7 Nov 2003 Influence of Vessel on Received Phase

26. ILA 2003 4-7 Nov 2003 Rotating the Vessel…

27. ILA 2003 4-7 Nov 2003 Influence of the Measurement Vehicle Range error due to influence ship might be in the order of the ships size. Effect is most likely larger on E-field than on H-field. Heading dependant error relative constant as long as the antenna orientation with respect to the vessel is fixed. Apply correction method similar to Xtalk correction Develop auto calibration similar as a ship-compass: take a spin and measure the response.

28. ILA 2003 4-7 Nov 2003 Reradiation by Local Objects

29. ILA 2003 4-7 Nov 2003 Reradiation by Local Objects (contd)

30. ILA 2003 4-7 Nov 2003 Reradiation By Local Effects: E-field vs H-field

31. ILA 2003 4-7 Nov 2003 Reradiation By Local Objects (contd) Reradiation is a near-field effect Detect reradiation by looking at the relation between E-field and H-field (ASF survey / reference site) Detect reradiation by looking at the difference between two (ideal) loops (user Rx)

32. ILA 2003 4-7 Nov 2003 Reradiation By Local Objects (contd) Reradiation causes a heading dependant error The effect of reradiation on the range- and position error depends on the beam-steering algorithm and is therefore RX dependant Therefore, ASF mapping is only allowed in a reradiation free environment By detecting reradiation, the problem shifts from accuracy/integrity to availability

33. ILA 2003 4-7 Nov 2003 Conclusions Heading dependant antenna challenges solvable Reelektronika antenna available 2004 Q1 Influence vessel can be (auto) calibrated ASF is a far field phenomenon and has to be measured as such Loran-C: 20 meter of a 3000 meter wavelength = 2.4º GPS: 2.4º of a 20 cm wavelength = 1.3 mm The challenge of getting the accuracy of Loran better than 20 meters is somewhat comparable with GPS better than 1 mm.