One important maintenance task in a tunnel radio system is the coverage test, which is basically a field strength survey. The basic setup consists of a transmitter that is constantly injecting a signal to the tunnel antenna system (TAS).

The Receive Signal (Rx) is constantly measured (sampled) on a RF communications monitor (or test set). Data is downloaded to a portable personal computer and plotted on a X-Y chart “Rx Level versus Distance”.

Most other commercial drive test survey products are designed for outdoor as they use the GPS signal to determine location & distance. Therefore, they do not in underground tunnels (highways or subways).