Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory GNSS Bistatic Radar September 14, 2006 Tore Lindgren, Dennis Akos Luleå University of Technology

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Presentation Overview Introduction: - GNSS Introduction - Bistatic Radar Concept - Signal Structure - Measurement Setup Measurements: - Airplane Measurements - Tower Measurements Conclusions and Further Work

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Estimating the Position of a GNSS Receiver GPS receiver r 1 r 2 r 3 r 4 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.

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Estimating the Position of a GNSS Receiver in the presence of a reflection (multipath) GPS receiver r 1 r 2 r 3 r 4 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.

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

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

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

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Measurement Setup Direct Front End Reflect Front End Reflected Front End RHCP Antenna LHCP Antenna PC Processing Results

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Airplane Measurements Lower Left is Northing, Easting, Zone 15 (UTM) Delta East (km) Delta North (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/s Altitude ≈ 582 m

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Correlation Waveform for Direct and Reflected Channels Waveforms are normalized to the maximum of the direct channel

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Identification of Object 45 PRN 6 PRN 10 PRN 18 PRN 21 PRN 29 Image courtesy of the USGS

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Identification of Object Lower Left is Northing, Easting, Zone 15 (UTM) Delta East (km) Delta North (km) Delta East (km) Delta North (km) Blue ellipse indicates possible sources of secondary reflections (right). This intersects with a farm (below). Image courtesy of the USGS More than one farm was found on ellipse Several farms were not detected

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Tower Measurements 300 m 4 element, LHCP, antenna array RHCP patch antenna 4 and 5 April, 2006, Boulder Atmospheric Observatory, Colorado, USA NordNav

Department of Computer Science and Electrical Engineering GNSS Research Group – EIS Laboratory Specular Points and Antenna Illumination 1000 ms coherent averaging Non-coherent averaging over 40 min

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