FM Radar

FM radar using passive transponders

In the classic FM technique, the microwave source sweeps in frequency over time, usually with a linear ramp. By mixing the received signal with the outgoing signal, the difference frequency is proportional to the range at which the reflection occurred. By using this method to illuminate a co-operative target, i.e. a passive transponder, the range to the transponder is uniquely determined in a high-resolution measurement which is essentially free of clutter, since the receiver is locked onto the modulation code of the transponder. 

In the laboratory demonstration illustrated below, range resolutions of a few centimetres have been achieved by illuminating a retro-reflector transponder with a 9.0 - 9.4 GHz swept signal. Multiple channels may be used by allocating unique modulation codes (or frequencies) to each transponder. Thus position may be determined from triangulation on known target positions. 

(click on the image to see a full size diagram)

The hard-wired link between transponders and receiver would be discarded in most practical systems by adopting a carrier-recovery stage at the receiver, or using an incoherent detector in the simplest case. 

Application areas include radio navigation and tracking and control systems such as covert avionic landing aids. In particular, a maritime range finder and tracking system is undergoing development.