It is the nav app keeping the display lit up like a Times Square billboard that is sucking the majority of your battery life down, by the way. Your suggestion stands, however.
Why would that use a lot of power though? It's just receiving a signal. I have the impression that it's when sending out a signal that a lot of power is used. And in that sense electromagnetic radiation at GSM, Wifi or visible spectrum-frequencies are all the same.
I'd say it's both keeping the screen on and constantly downloading new map information from the cellular network.