The power of cross-correlating: from GPS reception to passive RADAR using SDR

CDMA systems rely on encoding data streams radiated by multiple emitters on the same carrier frequency with (ideally) orthogonal codes. Recovering the signal from each emitter requires identifying the code assiociated with each source, which hence also requires recovering the carrier to account for relative emitter/receiver motion (Doppler shift), thermal drift and oscillator bias. We demonstrate this concept with the reception of GPS signal — a constellation of satellites orbiting 20000 km over the surface of the earth — with 20 euro worth of equipment centered on a DVB-T receiver designed for receiving neighbouring television signals. We extend the concept to passive radar, in which a radiofrequency emitter (television, broadcast radio) signal reflected on a mobile target is used for identifying the velocity and position of the target. In this approach, no active source is needed: RADAR measurement is only a matter of correlating the direct and reflected signal, after identifying the Doppler induced frequency shift.

Slides + Video available at https://archive.fosdem.org/2015/schedule/event/xcorr/