Integrated gps loran sensor for maritime operations
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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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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

This presentation: Maritime

  • Focus on accuracy

  • Outline of error budget

  • Identification & elimination of potential threads


Error Budget


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


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


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


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


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


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


H-field Antenna Tuning Error Compensation


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


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


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


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


X-talk: Measurement of parameters (cont’d)

  • Measured Antenna Response

Antenna 1

Antenna 2

Capacitive coupling from antenna 1 to antenna 2


X-talk: Measurement of parameters (cont’d)

  • Curve-Fitting to find antenna parametersG1, G2, A21 and A12

Antenna 1

Antenna 2


X-talk: Feed Forward Correction

  • Feed Forward Correction of Antenna

Digitized Antenna signals

Feed-Forward

Correction

using

G1,A21

G2,A12


X-talk: Feed Forward Correction (cont’d)

  • Comparison of Uncorrected and Feed-Forward Corrected Response after Beam-Steering

Uncorrected

Corrected

45 m


X-talk: Feed Forward Correction (cont’d)

  • Comparison of Uncorrected and Feed-Forward Corrected Response after Beam-Steering

Corrected

1.5 m

(zoomed in)


Not Only The Antenna Matters…

  • Attenuators

  • Cables


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


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


Now We Can Rotate the Antenna…

… rotate the vessel


Error Budget


Influence of Vessel on Received Phase


Rotating the Vessel…


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.


Reradiation by Local Objects


Reradiation by Local Objects (cont’d)


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


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)


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


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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