Like rattler said they use GPS positioning , they will usually hit a key on their mobile come that lets the DPS base know they are out on traffic and GPS gives the dispatcher the exact location of that unit the trooper will enter licenses plates , DL and other info via computer , if that trooper should have an emergency or a suspect that becomes combative they can hit the emergency button on their radio to notify dispatch , I've heard situations on my scanner when monitoring Houston DPS where the dispatcher will say units be advised 2Dxxx is out at ( location ) and has a suspect non compliant , and I don't hear anything on any DPS channels prior to this saying anything about a traffic stop or needing back up , which tells me it's not broadcasted over the air , it's done by GPS and computer and an emergency button