GNSS Bistatic Radar

1
GNSS Bistatic Radar

• September 14, 2006
• Tore Lindgren, Dennis Akos
• Luleå University of Technology

2
Presentation Overview

• Introduction - GNSS Introduction
• - Bistatic Radar Concept
• - Signal Structure
• - Measurement Setup
• Measurements - Airplane Measurements
• - Tower Measurements
• Conclusions and Further Work

3
Estimating the Position of a GNSS Receiver

• 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.

4
Estimating the Position of a GNSS Receiver in the
presence of a reflection (multipath)

• 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.

5
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).

6
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.

7
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)

8
Measurement Setup
Processing Results
9
Airplane Measurements

• Airborne (Cessna Aircraft) dynamic bistatic GPS
  data collection
• 1-July, 2005, Iowa / Des Moines, USA

Aircraft speed 75 m/s Altitude 582 m
Image courtesy of the USGS
10
Correlation Waveform for Direct and Reflected
Channels
Waveforms are normalized to the maximum of the
direct channel
11
Identification of Object
45
Image courtesy of the USGS
12
Identification of Object
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

Image courtesy of the USGS
13
Tower Measurements

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

4 element, LHCP, antenna array
RHCP patch antenna
14
Specular Points and Antenna Illumination

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

15
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.