RTL-SDR APCO Project 25 (P25) receiver

Balint Seeber has done it again! In this video he shows how to take the RTL-SDR dongle and use it in conjunction with the open source OP25 project to successfully demodulate APCO Project 25 (P25) digital radio voice communications. P25 is a term used to refer to a modulation system used by many public safety agencies for digital radio transmission purposes.

Balint has written an “OP25 Decoder” block shown in the above screenshot of a GNURadio-companion flowgraph which does all the heavy lifting. He promises to release the code soon. Keep an eye on Balint’s website and the OP25 Osmocon project page to catch this outstanding code work!

Of particular interest to any radio hacker living within range of a P25 system is the “key” field in the OP25 Decoder block. This is the user-set variable representing the decryption key field. Watch the video closely for this topic.

Save sample file of short encrypted transmission; set key = x; process file and listen; if audio=garbled, x = x+1. Wash, rinse, repeat until clean. Possibly automate the process to recognize clear speech (a challenge!) We think you understand…

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  1. It would be nice if someone would develop a kit or assembled unit that would take the normal FM
    mobile radio to a modification that would accept with clarity a P25 signal.

    I would enjoy hearing from someone that could point me in the right direction

    Jerry Thacker

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