Global Navigation Satellite System Global Navigation Satellite System is the Generic term given to Satellite navigation systems using passive ranging as a means for navigation. The most well-known is the U.S. GPS system. This consists of 24 satellites in a Medium Earth Orbit (MEO) with a semi-major axis of 26,560 km and an inclination of 55°. Each satellite transmits navigation data (satellite position, time, etc.) at a data rate of 50 bits per second. This navigation data is overlaid on a sequence of 1023 bits (chips) that is unique for each satellite. This sequence, called PRN code, is repeated each millisecond, thus the chip width is approximately 293.05 m [2]. The distance to the satellite is measured by correlating the incoming signal with a PRN code generated in the receiver. In GNSS bistatic radar the path delay of the reflected signal measured by comparing the PRN code of the reflected channel with the direct channel. Flight Test During data collection campaigns over the summers of 2004 and 2005 researchers at University of Colorado, Boulder, USA, digitally sampled direct and reflected GPS spectrum from an airborne platform using zenith- and nadir-pointing antennas [3]. This section describes data collected over Iowa, USA, on July 1, 2005, from a small Cessna airplane. The direct channel data was used to gather information about the aircraft’s position and velocity. The average speed of the aircraft was 75 m/s and the AGL was approximately 232 m. This was approximated using a simple relationship given in Fig. 1, which assumes a flat ground and also that the elevation and azimuth of the satellite is the same at both the receiver and the specular point. This assumption will be used throughout this paper. Fig. 2. shows a correlation waveform of the reflected channel normalized to the strength of the direct signal. In addition to the expected specular reflection, direct leakage and a secondary reflection can be seen. The direct leakage is likely due to signals propagating around the aircraft and/or cross-talk between the direct and reflected channels. This does, however, not affect the results. Of more interest here is the secondary reflection with a path delay of barely 1500 m. Several of these secondary reflections were seen in the collected data, the duration was typically less than 100 ms which implies that they are due to reflections of large structures only when the geometry between the GPS satellite, aircraft, and reflecting object [4].  Fig. 2. Correlation waveform showing direct leakage, specular reflection, and secondary reflection strengths as a function of path delay [4]. |