MobilTrak is a company based out of Birmingham, Alabama (of all places) with a new technology to accurately count radio ratings data for the first time ever. Previously, all advertisers had to go on were diaries, interviews, and other such unreliable surveys.

MobilTrak's system installs small receiver boxes on light polls throughout a city. The amplifier in the receiver of any car driving by creates what I've heard called a "magnetic resonance image", but it was a salesthing talking, so who knows what the actual correct term is.

At any rate, the devices basically pick up the feedback generated by a car's amplifier and speaker cables. From that, they can determine exactly what station people are listening to. It doesn't tell them WHO is listening to WHAT, so your privacy is not directly violated, but it does tell them exactly how many people are listening to any given station.

Even so, the system has raised some eyebrows in privacy circles, and was even mentioned on Howard Stern. Somewhere in the order of 2 dozen cities are currently surveyed by MobilTrak.