Field Guide · term

Also known as: priority scan, priority monitor, priority scanning

Priority scan is a monitoring strategy in which a receiver follows a chosen set of talkgroups and, when more than one is active, prefers the higher-priority group — so an important call can preempt a lower-priority one that is already being listened to.1 On a trunked system this is driven entirely by the control channel: the scanner reads every channel grant, compares the granted talkgroup against its priority list, and points its limited receiver capacity at the most important active call.

control channel grants TG 402 (P3) active TG 101 (P1) active receiver followsTG 101 (higher)
When a higher-priority talkgroup becomes active, the receiver leaves the lower-priority call to follow it.

How it works

The listener assigns each talkgroup on the scan list a priority level. As the scanner decodes the control channel, each grant tells it that some talkgroup has gone active on a voice channel. The scanner then applies its rule:

  • If nothing is currently being followed, tune to the newly granted call (if it is on the scan list).
  • If a call is already being followed and a higher-priority talkgroup is granted, preempt the current call and switch to the higher one.
  • If a lower-or-equal-priority talkgroup is granted, note it but keep the current call.

On conventional (non-trunked) radios, priority scan means periodically interrupting a lower-priority channel to sample the priority channel for activity. On a trunked system the logic is cleaner because the control channel announces activity — the scanner never has to leave a call to check whether a priority group is busy; it simply watches the grants. This makes trunked priority scanning both faster and more reliable.

In practice

Priority scan is what lets a monitor with only one or two receivers cover a system that may have dozens of active talkgroups. Priorities are usually arranged so that dispatch/primary channels outrank tactical or administrative ones, and emergency calls are commonly given an implicit top priority so they always break through. A well-tuned priority list is the difference between hearing the important traffic and being stuck on a long, low-value transmission while the call that matters plays out unheard on another channel.

A second decision is what to do when a higher-priority call ends. A common policy is resume: if the preempting call finishes while the earlier, lower-priority call is still up, the receiver falls back to whatever is still active — so the listener does not lose the lower call entirely, only the portion that overlapped. Systems and scanners also provide a priority hold or hold option that pins the receiver to one talkgroup regardless of what else is granted, which is the opposite behaviour: it is useful when a listener wants to stay with a developing incident and deliberately ignore everything else. The right choice depends on whether the user is monitoring broadly or focused on one event, and a good implementation makes both easy to express.

Priority scan is also where the number of available receivers shows through. With a single tuner, priority is strictly one-at-a-time and preemption really does mean abandoning one call for another. With several tuners, “priority” becomes an allocation problem — assign the scarce receivers to the highest-value active calls and let lower ones go unheard only when there are more calls than tuners — which is a richer and more forgiving version of the same idea.

Relevance to SDR

Priority scan is a scanner behaviour rather than a system feature, and it maps directly onto how GopherTrunk allocates its receivers. GopherTrunk parses every channel grant on the control channel and can weight talkgroups so that a limited pool of tuners is always assigned to the highest-value active calls, preempting a lower-priority call when a more important one begins — including surfacing emergency-flagged traffic first. Because the decision is driven by control-channel metadata, GopherTrunk can make it before committing a receiver to demodulate any audio, which is what keeps a software-defined scanner responsive across a large channel pool.

Real systems where this applies include P25 Phase 1/Phase 2, DMR Tier III, and Motorola trunking. The priorities are the user’s policy; GopherTrunk supplies the control-channel awareness that makes the policy actionable.

Sources

  1. Trunked radio system — Wikipedia, on following talkgroups by monitoring control-channel grants. ↩

See also