Depending on where you live, you may or may not hear all police / fire activity on a scanner. Most large cities have 800 MHz radios which can be programmed for hundreds of channels and multiple talk-groups. Most of the police & fire calls of an urgent nature are dispatched by radio, on pre-set frequencies. Car chases, car wrecks, major fires and other "hot" calls, usually go over the air. Non emergency, routine calls such as "cold" vandalism, shoplift cases, etc. are sometimes dispatched by MDT. (Mobil Data Terminal)
Most cities of any size use on-board computers called MDTs to dispatch minor calls, or calls which they don't wish scanner listeners to hear. Not for any super secret conspiracy against the people, but for tactical reasons, or sensitive issues such as child abuse cases.
Most of the dispatch and operational frequencies depend on repeaters, similar to cell phone antenna systems, which re-broadcast the radio messages over a broad area. There are also many simplex channels that are line-of-sight, short range, that do not go through the repeater system. In order to hear those transmissions, such as on tactical SWAT or drug operations, you need to be close (.5 mile +/-) and tuned to the proper channel, and there are many of those. For those who remember the Dragnet & Adam 12 use of "tac-2," well that's what they were using.
I'm sure some smaller, more rural agencies have less sophisticated systems which can still be monitored to a greater degree, but in those cases, where possible, a greater reliance on cell phones is used for tactical or privacy issues.
There's nothing super-secret about any of this, it's just what emergency radios have evolved into. It has been a great tool for multi-agency situations, or major disasters, so finally an officer in a squad car can talk to the fire fighters, or state troopers who are also assigned to the same incident. Here in Portland (Or) the development began following an airliner crash in the 70's and as the area grew, the need for improved communications also grew.
Cogito, ergo armatum sum
NRA Life Member / SAF Member
Retired Police Detective '71-'01 / LEOSA Certified
Naval Aviation Veteran '65-'69
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