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

ILA 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

ILA Nov 2003 Error Budget

ILA 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

ILA 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

ILA 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

ILA 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

ILA 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

ILA Nov 2003 H-field Antenna Tuning Two modes of operation of H-field antennas: Resonance vs Wide-band + Slightly better noise 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

ILA Nov 2003 H-field Antenna Tuning Error Compensation

ILA Nov 2003 H-field Antenna Tuning Error Compensation (contd)

ILA 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

ILA 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

ILA 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

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

ILA 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)

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

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

ILA 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)

ILA Nov 2003 Not Only The Antenna Matters… Attenuators Cables …

ILA 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 cm Computer controlled antenna rotor DUT

ILA 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

ILA Nov 2003 Now We Can Rotate the Antenna… … rotate the vessel

ILA Nov 2003 Error Budget

ILA Nov 2003 Influence of Vessel on Received Phase

ILA Nov 2003 Rotating the Vessel…

ILA 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.

ILA Nov 2003 Reradiation by Local Objects

ILA Nov 2003 Reradiation by Local Objects (contd)

ILA Nov 2003 Reradiation By Local Effects: E-field vs H-field

ILA 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)

ILA 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

ILA 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.