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.