Aren't these kind of rigs used to survey, triangulate and record large numbers of ground-based transceivers? If so, they are similar to rigs which do geological surveys with packs of machines you can see traversing likely oil reserves. Odd looking, six or so trucks, bumper to bumper, running 50 yards or so, pausing, an almost inaudible hammering on the ground, then rushing ahead to the next stop. Logs of these soundings are reportedly transmitted directly to home base, encrypted, so the trucks cannot be hijacked or intercepted by competitors. So the wishful thinking goes. In the case of aerial surveys of radio frequencies, it is possible to spoof signals by tucking the genuine in amongst "static." But successful evasion depends on the snoopers not have quick-witted algorithms to snatch the static, even if it's burst transceivals, and make sense of it. If you encrypt the sneaky signal you're dead meat. Again, learning from the oil patch pirates, you can lay off over the horizon and grab the sounding data by reading the ground signal in the clear not the encrypted air data. But beware of chaff sent out by the trucks to thwart that such angle snooping. Some packs of trucks are reconnaissance by fire. As are some of the aerial surveys, which invite tracking of the overt apparatus to snatch the pirates' interceptions by way of reverse engineering the snoop with an array so large you wouldn't know it was there unless you had an antenna capable of reading it. Some locate components of the secondary receiver city-wide, or maybe wider. Some radio engineers think the mobile phone system transceivers are used for that very, very wide array purpose tucked dual-use below the consumer usage. Would mobile phones also serve as unwitting contributors to a large area transceiver? Sophicticated geological sensors can certainly get pretty good readings from heavy traffic on the interstate, and it gets better as the highways deteriorate. Concrete expansion joints are wondrous impactors.