Scan, and find voice channels that are using MPT. Run TrunkView, feed it with audio from a voice channel and look at the 'Call clear-down' messages.

Hopefully you will see something like:

09:18 - CLEAR| Call clear-down                | Ch: 787  Freq: 000.0000 MHz
             |   Return to Control Ch: 895    |                            

or

09:18 - MAINT| Call maintenance message       | (021)  Org: 1425  Oper: 6
             |   Ch: 787   Freq: 000.0000 MHz |                          

From that and your scanner readout you will know:

1. the frequency  (voice frequency 180.6375 MHz assumed here)
2. the channel number (787)
3. the control channel number (895)

The 'MAINT' message is not always transmitted.


Base frequency = Voice frequency - (channel x step)

Almost all European systems use 12.5 kHz channel steps, so the above example works out:

Base frequency (channel 0)

= 180.6375 - (787 x 0.0125) MHz = 180.6375 - (9.8375) = 170.8000

So now you know the base frequency: 170.8000 MHz.


From the more useful 'Call clear-down' message you would also know that:

Control Channel frequency

= Base frequency + (channel x step) = 170.8000 + (895 x 0.0125) = 170.8000 + (11.1875) = 181.9875

As you can see, the above (real) example is a Band 3 system with standard numbering (as used by Fleetcomm in the UK).