There are used in drug interdiction and other coast guard functions. My brother was an Air Force test pilot that tested these for that purpose (he was originally a surveillance pilot flying kc-135s). They are basically a kc-135 in a small jet (e.g., leer jet 35) form factor, and not carrying a team of technicians in the back but instead just equipment, or maybe a technician or two. Officially he's not supposed to know.
Officially he's not supposed to know and that was the unofficial excuse they gave him before he started asking too many questions. And then he gave you the information he wasn't even supposed to have, facing unemployment and disciplinary actions. How naive can you be?
