Field Guide · term

Also known as: radio ID, unit ID

A radio ID (unit ID) is the unique identifier of an individual radio on a system, distinct from the talkgroup it is using.1 It appears in control-channel signalling and in analog in-band schemes like MDC1200. Where a talkgroup answers which group is talking, the radio ID answers which specific unit — a particular officer’s portable, a fire engine, a dispatch console.

voice transmission radio ID 4567 TG 101 every transmission identifies the individual radio
A radio ID uniquely identifies the transmitting unit, so you see who is talking, not just which group.

How it works

Each transmission and affiliation carries the source radio ID, so a monitor can see which unit is talking, not just which group. On a group call the voice channel’s embedded signalling includes the source ID alongside the destination talkgroup; on a private call both the source and destination radio IDs appear. Registration and affiliation messages on the control channel expose radio IDs even when no call is in progress.

Radio IDs are numeric and scoped to the system. On P25 a Source Unit ID is a 24-bit value that is unique within the network defined by the WACN and system ID; DMR uses its own 24-bit radio IDs. Fleets often assign ID ranges by department, so the numeric ID alone can hint at a unit’s role even without an alias table.

Variants

  • Trunked source ID — carried in P25/DMR control and voice signalling for every keyup.
  • Analog ANI — MDC1200 and similar in-band schemes send a radio’s ID as a short data burst at the start (and optionally end) of an analog FM transmission.
  • Special / system IDs — some values are reserved for consoles, the system itself, or all-call addresses rather than a physical subscriber.

In practice

Radio IDs let a listener follow a specific unit rather than a whole talkgroup, and they are central to fleet mapping: correlating IDs with talkgroups and timing reveals who works with whom. Hobbyist databases publish alias tables mapping numeric IDs to unit names for known systems, though many agencies rotate or withhold them.

Relevance to SDR

GopherTrunk’s radio-ID view merges live radio IDs with any alias catalogue, letting you track individual units across a system. Because the source ID is present in both control signalling and the voice channel’s embedded data, GopherTrunk can tag each logged call with the transmitting unit, not merely the group — the basis for per-unit history and filtering.

Sources

  1. Trunked radio system — Wikipedia, on subscriber/unit identifiers in trunking signalling. ↩

See also

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