Each county has a different set of circumstances, and the state deals with each on the basis of the particular comm system in place. In counties with 800 mHz trunked systems where the state has interoperability, there are shared talkgroups and tactical freqs that allow the troopers with 800 mHz radios to communicate with local units. In counties where there is no 800 mHz system, the nearest state site with the interoperability infrastructure is equipped with a radio on the designated interops freq for that county and which can be patched to the designated state channel for interops. In a large number of the counties with no 800 mHz system, the troopers have a radio on the county frequencies, and which is either issued to the trooper by the entity in charge or is supplied by the trooper himself.
You can tell the VIPER sites from the state's other 800 mHz sites by the large number of directional antennas mounted on the tower. These antennas are attached to the various local department radios that are rack mounted in the tower house and connected to the ACU1000 or ACU 2000 interoperability unit. The ACU is controlled from the comm center in the troop zone where the VIPER site is located.
As a rule, there would be at least one site per troop comm center, but in some cases, coverage areas and interstate comms require more than one site. The state's digital microwave system makes possible the patching together of systems from and to anywhere in the state via the microwave linking at the VIPER sites, and particular VIPER sites along the state borders enable the patching of any VA, TN, GA, and SC systems linked to the respective states' systems.
It should be noted that not all VIPER comms require the ACU. Some VIPER comms between NC entities with compatible 800 mHz systems are done through 800 mHz infrastructures shared by local and state systems. Those comms are accomplished by shared talkgroups and/or patching certain talkgroups within each system through the internal routing of the state and local trunking control systems themselves.
Since the frequencies are licensed to the local and state agencies and are listed in the RR database already, there aren't any separate listings that will give you a clue as to what method the trooper in a given location will use to talk to local units. If you're listening to local traffic and hear an SHP unit number used, there's your clue that the trooper has a radio on the local frequencies.
Counties with compatible radio systems use mutual aid frequencies to communicate, and those with incompatible systems use cross monitoring or some form of multiple radio systems.
Did all that help? My home county is Mecklenburg, VA, by the way.
