Radio modem???

[LoLo]

Saw someone driving REALLY slow through the neighborhood today -- up and down the streets -- with a strange antenna on the top of the car. When I passed them the second time, I pulled over (saw a lot of techno gadgets in their car) and I asked them "what is that?" to which I got "what is what...?". They looked surprised and said it was a "radio modem".

OK, I know there are a lot of people here that have MUCH more technical knowledge than I do... so what does a radio modem do, and WHY were they driving the streets???