OnAirTwo-way radio · Experimental
Decode P25 with an SDR
OnAir receives P25. APCO Project 25 Phase 1 (TIA-102) digital two-way radio, C4FM, 12.5 kHz: conventional channels and trunked control channels.
What P25 is
APCO Project 25 is the digital radio standard of public safety in North America and other regions, conventional or trunked.
Frequencies and where it is used
APCO Project 25 Phase 1 (TIA-102), C4FM, 12.5 kHz, conventional and trunked.
In OnAir the tuning shortcut is Digital voice, 12.5 kHz; you can enter any frequency your radio reaches.
What OnAir shows for P25
Calls with talkgroups, unit IDs and emergency flags; trunking: WACN, system, NAC, RFSS / site, adjacent sites, channel identifiers, voice grants, talkgroup activity; voice through your mbelib. Receive only: encrypted calls are only reported as encrypted, nothing is decrypted. Follow the rules on receiving radio where you live.
This receiver is built from the published specification and checked end to end on simulated signals. It has had little or no testing on real transmissions yet, so reports and recordings on Discord are welcome.
Which radio you need
This is a shortwave mode, so the radio has to tune HF. An SDRplay, Airspy HF+ or an RTL-SDR in direct-sampling mode do that directly; with a HackRF an HF upconverter or a direct-sampling receiver is the usual route. A long wire or loop antenna matters more than the radio.
Questions about P25
Can I decode P25 with an RTL-SDR?
An RTL-SDR can receive HF only in direct-sampling mode, which OnAir offers in its radio settings; results depend on the antenna. SDRplay and Airspy HF+ radios are better suited.
Is it legal to receive P25?
In many countries it is illegal to intercept or disclose private radio communications. OnAir is a receiver: encrypted calls show up only as encrypted and are never decrypted, and voice needs a library you install yourself. Follow the law where you live.
Can I try P25 in OnAir without a radio?
Yes. Most modes include a built-in synthetic test signal, so you can open the screen and try the controls before you own a radio.
Try it without a radio
OnAir includes built-in synthetic test signals for most modes, so you can open this screen, see the layout and try the controls before you own a radio. The Tour button walks you through the app.