Also known as: late entry, late-entry, grant update
Late entry is joining a trunked call that is already in progress rather than at its start.1 It works because the control channel does not announce a call only once: while a call is active it periodically re-broadcasts a grant update naming the talkgroup and the voice channel it is using, so a radio (or monitor) that missed the original channel grant can still discover the call and tune to it.
How it works
A call is announced once by an initial channel grant. If a radio was busy, out of range, or scanning another system at that instant, it would otherwise never learn the call exists. To prevent that, the controller re-issues the grant as a grant update (P25 calls this a Group Voice Channel Grant Update) at a regular interval for as long as the call runs. Any radio affiliated to the talkgroup that sees an update while the call is still active retunes to the named voice channel and picks up the audio mid-transmission — hence “late entry.”
The same updates carry the current busy/idle picture of the system: they tell the fleet which channels are occupied by which groups right now. For a listener, late entry is why a call can be followed even if monitoring started after the call began — the necessary information keeps repeating on the control channel.
In practice
The update interval is a system trade-off. Frequent updates make late entry fast and robust but consume control-channel airtime; sparse updates conserve the control channel but lengthen the worst-case delay before a late joiner (or a scanner) locks onto the call. On busy systems updates for many simultaneous calls are interleaved on the single control channel, so a monitor may see a rotating stream of “TG 101 on ch 3, TG 204 on ch 7, TG 101 on ch 3 …” as the controller keeps every active call advertised.
Late entry is also the mechanism that makes trunk-following resilient to plain radio imperfection. Control-channel messages are error-coded but not infallible; a burst of noise, a fade, or a moment of contention can corrupt the one grant that started a call. If that grant were the only announcement, the call would be invisible to anyone who missed it. Because the grant keeps being re-broadcast, a receiver that dropped the first copy simply catches a later one — the repetition is redundancy against loss as much as it is a courtesy to radios that were busy. The same property is what lets a monitor tuning to a system “cold” begin following calls within a second or two rather than waiting for the next call to start from scratch.
Late entry also interacts with encryption and vocoder framing. A voice call carries its own embedded signalling — algorithm and key identifiers, the talkgroup, sometimes the source unit — repeated within the traffic channel itself, so a radio (or monitor) that joins late can recover the call’s identity and cryptographic parameters without having seen the setup on the control channel. That in-band repetition is the traffic-channel analogue of the control channel’s grant updates: both exist so that a receiver arriving mid-call has everything it needs to make sense of what it is hearing.
Relevance to SDR
Late entry is not just a radio feature — it is precisely what makes a trunk-following scanner robust. A monitor that only reacted to the initial grant would miss every call already underway when it tuned in, and would lose a call if it dropped a single message. Because GopherTrunk parses grant updates as well as initial grants on the control channel, it can begin following a call in progress the moment it starts decoding a system, and it can re-acquire a call if it briefly loses a grant. The updates also give it a running map of which voice channels are active, which it uses to schedule receivers efficiently.
Real systems that broadcast these updates include P25 Phase 1/Phase 2 (Group Voice Channel Grant Update messages) and DMR Tier III. As with all trunking signalling, GopherTrunk reads late-entry updates passively from the control channel; the audio it recovers still depends on the traffic being unencrypted.
Sources
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Trunked radio system — Wikipedia, on control-channel signalling that lets radios follow active calls. ↩