ELF probably would not work for video and audio though. It has an extremely low throughput, still images would work but would be very compressed or poor quality or time consuming. ELF was used for brief text messages to the subs.
"the low data transmission rate of a few characters per minute and, to a lesser extent, the one-way nature due to the impracticality of installing a transmitter of the required size on a submarine"
I figured Destro had tapped into or bought air time on satellites that cover the US like SATMIX 6 hosted by Mexico. The transmitter must be extremely powerful to penetrate layers of concrete and earth. It also need to be powerful enough to penetrate whatever GPS shielding they have in place.
Commercial GPS receivers are not very powerful; look at how the portable one in your car acts up if it does not have a clear "view" to the sky. Reality is a GPS receiver in operation that far underground would not be able to get a fix on 3-4 satellites. Maybe they did not know the robots were that deep below ground.
The robots must have an extremely sensitive receiver since the operators are sending commands the the robots as they fight in the PIT.
Anyone have any more ideas?