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Integrated GPS/Loran Sensor for Maritime Operations

Integrated GPS/Loran Sensor for Maritime Operations. Wouter J. Pelgrum Reelektronika / Delft University of Technology / Gauss Research Foundation. Introduction. Two very challenging applications for LC Aviation: The Seven Nines Maritime: The Eight to Twenty Meter.

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Integrated GPS/Loran Sensor for Maritime Operations

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  1. Integrated GPS/Loran Sensor for Maritime Operations Wouter J. Pelgrum Reelektronika / Delft University of Technology / Gauss Research Foundation

  2. 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. Error Budget

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

  7. 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. E-field Susceptibility (cont’d) • Range error due to E-field susceptibility Maximum range errors of approximately 1 meter achievable with carefully designed shielding and / or balancing Single loop used up to 45 degrees. After that, the other loop effectivelytakes over

  9. 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. H-field Antenna Tuning Error Compensation

  11. H-field Antenna Tuning Error Compensation (cont’d)

  12. 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. X-talk: some formulas… • Model of X-talk and Tuning Ideal dual loop H-field antenna response: Approximation of actual dual loop H-field antenna response: A21 & A12 Xtalk G1 & G2: Gain and tuning mismatch

  14. X-talk: Measurement of parameters (cont’d) Measurement Setup H-field antenna 130 cm Cross-section of measurement setup 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

  15. X-talk: Measurement of parameters (cont’d) • Measured Antenna Response Antenna 1 Antenna 2 Capacitive coupling from antenna 1 to antenna 2

  16. X-talk: Measurement of parameters (cont’d) • Curve-Fitting to find antenna parametersG1, G2, A21 and A12 Antenna 1 Antenna 2

  17. X-talk: Feed Forward Correction • Feed Forward Correction of Antenna Digitized Antenna signals Feed-Forward Correction using G1,A21 G2,A12

  18. X-talk: Feed Forward Correction (cont’d) • Comparison of Uncorrected and Feed-Forward Corrected Response after Beam-Steering Uncorrected Corrected 45 m

  19. X-talk: Feed Forward Correction (cont’d) • Comparison of Uncorrected and Feed-Forward Corrected Response after Beam-Steering Corrected 1.5 m (zoomed in)

  20. Not Only The Antenna Matters… • Attenuators • Cables • …

  21. 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 DUT Computer controlled antenna rotor

  22. 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. Now We Can Rotate the Antenna… … rotate the vessel

  24. Error Budget

  25. Influence of Vessel on Received Phase

  26. Rotating the Vessel…

  27. 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. Reradiation by Local Objects

  29. Reradiation by Local Objects (cont’d)

  30. Reradiation By Local Effects: E-field vs H-field

  31. Reradiation By Local Objects (cont’d) • 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. Reradiation By Local Objects (cont’d) • 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. 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.

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