Various architectures of Software Defined Radio (SDR) based RAdio-frequency Detection And Ranging (RADAR) systems are investigated in the context of the impact of oscillator stability.
The most intuitive need for a high stability local oscillator is for velocity measurement measured as a Doppler shift. The local oscillator fluctuation would be detected as a target velocity variation: this concept is addressed with a passive RADAR systems using GRAVES as an illuminator of opportunity. Since the Continuous Wave (CW) RADAR GRAVES does not provide any range information, the location of the targets are identified with angle of arrival measurements using a distributed system of SDR synchronized using White Rabbit.
White Rabbit synchronized X310 SDR receivers recording the GRAVES signal backscattered by planes and satellites recorded on the second Nyquist zone, allowing for Direction of Arrival (DoA) analysis and identifying the target location despite the lack of ranging capability of the non-cooperative CW source.
Various architectures of Synthetic Aperture RADAR (SDR-SAR) systems have been investigated, using either a mobile non-cooperative source (spaceborne Sentinel1 RADAR) for bringing the spatial diversity with a fixed receiver for mapping target azimuths, or moving the emitter/receiver pair on a rail in a system similar to Ground-Based SAR (SDR-GB-SAR). In the latter, a covert emission source is using a WiFi emitter streaming a pseudo-random signal for detecting range and azimuth of the targets. Since the azimuth compression is a Fourier transform along the antenna position relying on the phase of the signal to compress the signal in the direction of arrival, local oscillator stability is again involved and time durations much longer than the two-way trip for ranging measurement (frequency offset a kHz to tens of kHz for time of flight in the 100 us to ms range).
Map of targets obtained with the covert SDR-GB-SAR operating with a 5.8 GHz WiFi emitter as non-cooperative source
All development files are available at https://github.com/jmfriedt/SDR-GB-SAR/ or https://github.com/jmfriedt/sentinel1_pbr
Reference:
J.-M. Friedt, Covert Ground Based Synthetic Aperture RADAR using a Wi-Fi emitter and SDR receiver, FOSDEM (2024)