Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory...

Department of Computer Science and Electrical EngineeringGNSS Research Group – EIS Laboratory

GNSS Bistatic Radar

September 14, 2006

Tore Lindgren, Dennis Akos

Luleå University of Technology


Presentation Overview

Introduction: - GNSS Introduction

- Bistatic Radar Concept

- Signal Structure

- Measurement Setup

Measurements: - Airplane Measurements

- Tower Measurements

Conclusions and Further Work


Estimating the Position of a GNSS Receiver

GPS receiver

r1

r2 r3r4

• Satellite positions are known.

• The distance is determined by time-of-arrival

• Distance and position of at least 4 satellites is required to determine 3D-position and receiver clock error.


Estimating the Position of a GNSS Receiver in the presence of a reflection (multipath)

GPS receiver

r1

r2 r3r4

Reflection

• Multipath is caused by reflections that corrupt the time-of-arrival measurement.

• Most of the reflected signal change polarization.

• Multipath minimized with a good antenna.


GNSS Bistatic Radar Concept

• The delay of the reflected signal can be used to determine the height above the ground of the receiver.

• The shape of the reflected signal can be used to determine properties of the ground (roughness and soil moisture).

GPS receiver with

zenith RHCP&

nadir LHCP antennas

Specular reflection

height above ground


GNSS Bistatic Radar Concept

• An object can cause a reflection with longer delay than the specular reflection.

Reflected GNSS signals can be used as a bistatic radar system.

Specular reflection

Reflecting object

GPS receiver with

zenith RHCP&

nadir LHCP antennas


Signal Structure

• GPS signal buried ~18 dB under noise floor

• 24 Satellites transmitting on same frequency – CDMA

• Pseudo Random Noise code (PRN code), 1023 bit long

• Correlate with locally generated C/A code to remove CDMA coding (i.e. make signal 1023 times stronger)


Measurement Setup

Direct Front End

Reflect Front EndReflected Front End

RHCPAntenna

LHCPAntenna

PC

Processing Results


Airplane Measurements

0 10 20 30 40 50 600

5

10

15

20

25

30

35

40

45

Lower Left is 4604808 Northing, 408000 Easting, Zone 15 (UTM)

Delta East (km)

De

lta N

ort

h (

km)

Image courtesy of the USGS

• Airborne (Cessna Aircraft) dynamic bistatic GPS data collection

• 1-July, 2005, Iowa / Des Moines, USA

Aircraft speed ≈ 75 m/sAltitude ≈ 582 m


Correlation Waveform for Direct and Reflected Channels

Waveforms are normalized to the maximum of the direct channel

m 293.05chip 1


Identification of Object


Delta East (km)

De

lta N

ort

h (

km)

0 10 20 30 40 50 600

5

10

15

20

25

30

35

40

45

PRN 6

PRN 10

PRN 18PRN 21

PRN 29



Identification of Object


Delta East (km)

De

lta N

ort

h (

km)

0 0.5 1 1.5 2 2.5 3 3.50

0.5

1

1.5

2

2.5

3

3.5

Delta East (km)

De

lta N

ort

h (

km)

0.4 0.45 0.5 0.55 0.6 0.65

2.3

2.35

2.4

2.45

2.5

2.55

Blue ellipse indicates possible sources of secondary reflections (right).

This intersects with a farm (below).


• More than one farm was found on ellipse • Several farms were not detected


Tower Measurements

300 m

4 element, LHCP, antenna array

RHCP patch antenna

• 4 and 5 April, 2006, Boulder Atmospheric Observatory, Colorado, USA

• NordNav


-400 -200 0 200 400 600 800

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

100

East (m)

Nor

th (

m)

Area illuminated by antennaPRN 6

PRN 29

PRN 30

Center of BeamTower

Specular Points and Antenna Illumination

• 1000 ms coherent averaging• Non-coherent averaging over 40 min


Conclusions and Further Work

Secondary reflections can be used for object detection.

Advantages:- Passive system- Complete earth coverage

Disadvantages:- GPS signals are weak- Dependent on geometry and radar cross section of reflecting object

Use phase information of reflected signal to increase accuracy.

-300

-200

-100

0

-400

-200

02468

x 10-6

north (m)East (m)

Nor

mal

ized

Pow

er

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory...

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