ILA 4-6 November 2003

Integrated GPS/Loran Sensor for Maritime Operations

Wouter J. Pelgrum

Reelektronika / Delft University of Technology / Gauss Research Foundation

ILA 2003 4-7 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 2003 4-7 Nov 2003

Loran-C TXGroundwave

+Skywave

+ Near Field + Antenna

Rx error

Error Budget

Loran-C TXGroundwave

+Skywave

+Skywave

Near Field + Antenna

Local O

bject

s

CWICRI

Local I

nterfe

rence

Equival

ent R

X nois

e+

CWI

CRI Local Objects

+

Antenna

Local InterferenceEquivalent RX noise

PFSF ASF

Groundwave

PFSF ASF

Tuning

E-fiel

d susc

eptib

ility

X-talk

Boat

Near Field

Ref S

tatio

n Erro

r

Rx errorSpatial decorrelation of Regional ASF

Loran-C TXGroundwave

+Skywave

+ Near Field + Antenna

+CWI

CRI Local Objects

+Local Interference

Equivalent RX noise

Rx error

ILA 2003 4-7 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 2003 4-7 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 2003 4-7 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 2003 4-7 Nov 2003

E-field Susceptibility

.

L

E

E

H

.

.

L

E

E

H

.

.

L

E

E

H

.

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)

H-f

ield

(S

dir

ect

)

E-field (SEfield)

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 2003 4-7 Nov 2003

H-f

ield

(S

dir

ect

)

E-field (SEfield)

E-field Susceptibility (cont’d)

0.10

1.00

10.00

100.00

1000.00

0 10 20 30 40 50 60 70 80 90angle (degrees)

TO

A e

rro

r (m

ete

rs)

-25 dB

-30 dB

-40 dB

-50 dB

-60 dB

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 2003 4-7 Nov 2003

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

ILA 2003 4-7 Nov 2003

Laptop (for monitoring,control & data recording)

‘7499Z’ (7499M + 45ms)

LORADD Loran-C receiver

signalout

Loran-C simulator

Std in

Std in

Sinle loop H-fieldrover antenna

Fixed single loop H-fieldreference antenna

NovAtel OEM4GPS receiver

1 PPS out

GPS Disciplined Rb Oscillator

10 MHz

1 PPS in

Channel 2

Channel 1

H-field Antenna Tuning Error Compensation

ILA 2003 4-7 Nov 2003

0 10 20 30 40 50 60 70 80 90

-12

-10

-8

-6

-4

-2

0

2

4

6

8

Time [minutes]

Dif

fere

nti

al

TD

[n

s]

7499M Double Difference Measurement

Not calibrated

0 10 20 30 40 50 60 70 80 90

-12

-10

-8

-6

-4

-2

0

2

4

6

8

Time [minutes]

Dif

fere

nti

al

TD

[n

s]

7499M Double Difference Measurement

Not calibrated

Calibrated

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

ILA 2003 4-7 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 2003 4-7 Nov 2003

X-talk: some formulas…

sin2

cos1

loop

loop

1sin2

2cos1

122

211

loopAGloop

loopAGloop

Heading Relative

antenna y x toantenna fromXtalk A

xantenna of Tuning & Gain

xy xG

Ideal dual loop H-field antenna response:

Approximation of actual dual loop H-field antenna response:

G1 & G2: Gain and tuning

mismatch

A21 & A12

Xtalk

Model of X-talk and Tuning

ILA 2003 4-7 Nov 2003

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

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 2003 4-7 Nov 2003

0 50 100 150 200 250 300 350

0.5

1

1.5

2

2.5x 10

4 Measured Antenna Gain Response

ga

in

0 50 100 150 200 250 300 350

-2

0

2

4

Measured Antenna Phase Response

Heading (degrees)

Ph

as

e [

us

]

Capacitive coupling from antenna 1 to antenna 2

Measured Antenna Response

Antenna 1

Antenna 2

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

ILA 2003 4-7 Nov 2003

0 50 100 150 200 250 300 350

0.5

1

1.5

2

2.5x 10

4 Curve-Fitting to Antenna Gain Response

ga

in

0 50 100 150 200 250 300 350

-2

0

2

4

Curve-Fitting to Antenna Phase Response

Heading (degrees)

Ph

as

e [

us

]

Antenna 1

Antenna 2

1sin2

2cos1

122

211

loopAGloop

loopAGloop

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

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

ILA 2003 4-7 Nov 2003

X-talk: Feed Forward Correction

0 50 100 150 200 250 300 3500

0.5

1

1.5

2

x 104 Corrected antenna gain

ga

in

0 50 100 150 200 250 300 350

-1

0

1

2

3

Corrected antenna Phase

Ph

as

e [

us

]

Heading (degrees)

signal

Feed-ForwardCorrection

usingG1,A21G2,A12

Digitized Antenna signals

Feed Forward Correction of Antenna

ILA 2003 4-7 Nov 2003

0 50 100 150 200 250 300 350-25

-20

-15

-10

-5

0

5

10

15

20

Phase response of Beam-Steered Antenna

Heading (degrees)

rang

e er

ror

[m]

Uncorrected

Corrected45 m

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

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

ILA 2003 4-7 Nov 2003

50 100 150 200 250 300 350

-1

-0.5

0

0.5

1

Phase response of Beam-Steered Antenna

Heading (degrees)

ran

ge

err

or

[m]

Corrected

(zoomed in)

1.5 m

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

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

ILA 2003 4-7 Nov 2003

Not Only The Antenna Matters…

Attenuators Cables …

ILA 2003 4-7 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..

160 cm

Computer controlled antenna rotor

DUT

ILA 2003 4-7 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 2003 4-7 Nov 2003

Now We Can Rotate the Antenna…

… rotate the vessel

ILA 2003 4-7 Nov 2003

Loran-C TXGroundwave

+Skywave

+Skywave

Near Field + Antenna

Local O

bject

s

CWICRI

Local I

nterfe

rence

Equival

ent R

X nois

e+

CWI

CRI Local Objects

+

Antenna

Local InterferenceEquivalent RX noise

PFSF ASF

Groundwave

PFSF ASF

Tuning

E-fiel

d susc

eptib

ility

X-talk

Boat

Near Field

Ref S

tatio

n Erro

r

Rx errorSpatial decorrelation of Regional ASF

Error Budget

ILA 2003 4-7 Nov 2003

Influence of Vessel on Received Phase

+

-

+-

ILA 2003 4-7 Nov 2003

Rotating the Vessel…

+

-

+-

ILA 2003 4-7 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 2003 4-7 Nov 2003

Loran-C TXGroundwave

+Skywave

+Skywave

Near Field + Antenna

Local O

bject

s

CWICRI

Local I

nterfe

rence

Equival

ent R

X nois

e+

CWI

CRI Local Objects

+

Antenna

Local InterferenceEquivalent RX noise

PFSF ASF

Groundwave

PFSF ASF

Tuning

E-fiel

d susc

eptib

ility

X-talk

Boat

Near Field

Ref S

tatio

n Erro

r

Rx errorSpatial decorrelation of Regional ASF

Reradiation by Local Objects

ILA 2003 4-7 Nov 2003

Reradiation by Local Objects (cont’d)

ILA 2003 4-7 Nov 2003

12 14 16 18 20 22 24-20

0

20

40

60

80

TOA Lessay (6731M)[n

s]

time [h]

H-field

E-field

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

ILA 2003 4-7 Nov 2003

+

-

+-Direct (primary) signal

Reradiated (secondary) signal

EP+HP

ES+HS

, ,

0 0,

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)

ILA 2003 4-7 Nov 2003

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

ILA 2003 4-7 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 = 2.4ºGPS: 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.