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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
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
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)

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
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 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.