If your neighborhood is something like ours, crossing the street is like taking your life into your own hands. Drivers are increasingly unconcerned with such trifles as speed limits or staying under control and it all happens when they have to connect point A to point B in the shortest possible time. Traffic monitoring with this passive radar it won’t do anything to slow down drivers, but it’s a pretty cool hack that will at least give some idea of traffic patterns.
The principle behind active radar – the type used by police to capture speeds in any neighborhood except yours – is simple: send a microwave signal to a moving object, measure the frequency shift in the reflected signal, and do some math to calculate the relative speed. A passive radar like the one described in the RTL-SDR.com article linked above is quite different. Instead of drawing a target with an RF signal, it relies on signals from other transmitters, such as terrestrial TV or radio outputs in the area. Two different receivers are used, both with directional antennas. One points to the area to be monitored, while the other points directly to the transmitter. By comparing the signals reflected by moving objects received by the first with the reference signal from the second, information on the distance and speed of the objects in the target area can be obtained.
The RTL-SDR test uses a pair of cheap ones Yagi antennas for the nearby DVB-T channel to power their KerberosSDR four-channel coherent SDR device we last looked when it was still in beta. Essentially four SDR keys on a shared board are now available for $ 149. Using it to build a passive radar may not save the neighborhood, but it can be a lot of fun to try.
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