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

Integrated GPS/Loran Sensor for Maritime Operations

Wouter J. Pelgrum

Reelektronika / Delft University of Technology / Gauss Research Foundation


Introduction

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

Error Budget


H field antenna design issues

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

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


Rotating a h field antenna

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

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

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

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


H field antenna tuning error compensation cont d

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


X talk introduction

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

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

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 d1

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 d2

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

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

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 d1

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

Not Only The Antenna Matters…

  • Attenuators

  • Cables


Xtalk work continues

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

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

Now We Can Rotate the Antenna…

… rotate the vessel


Error budget1

Error Budget


Influence of vessel on received phase

Influence of Vessel on Received Phase


Rotating the vessel

Rotating the Vessel…


Influence of the measurement vehicle

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


Reradiation by local objects cont d

Reradiation by Local Objects (cont’d)


Reradiation by local effects e field vs h field

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


Reradiation by local objects cont d1

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 d2

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

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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