Decoding FSK

Get help identifying an unknown signal.
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HobbyMan
Posts: 38
Joined: Sat Apr 07, 2018 1:55 am

Decoding FSK

Post by HobbyMan » Fri Apr 27, 2018 11:21 pm

Hey,

I have been trying to decode FSK with GNU radio to no luck. the problem is, i dont know very much. i just started using SDR a couple weeks ago and installed GNU radio yesterday. I tried following this but i got confused halfway through. Anybody got a simpler option?

rtlsdrblog
Site Admin
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Joined: Mon Nov 19, 2012 11:54 pm

Re: Decoding FSK

Post by rtlsdrblog » Fri Apr 27, 2018 11:33 pm

What type of FSK? There are many FSK decoders out there e.g. Qtmm AFSK1200 for APRS, various FSK modes available in MultiPSK too http://f1ult.free.fr/DIGIMODES/MULTIPSK ... esF6CTE_en.

HobbyMan
Posts: 38
Joined: Sat Apr 07, 2018 1:55 am

Re: Decoding FSK

Post by HobbyMan » Sat Apr 28, 2018 2:39 am

There are alot of thing going on in multipsk, how exactly do i get it to do what i want it to do? the signal, i believe, is BFSK at 409.8 MHZ. Here is an IQ recording.

rtlsdrblog
Site Admin
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Re: Decoding FSK

Post by rtlsdrblog » Sun Apr 29, 2018 4:05 am

Do you know anything about the signal other than its modulation? Whose sending it, and what is it used for? If it's unknown you'll probably need to do some manual reverse engineering work on it which is not that easy.

multiPSK is mostly for FSK signals found in the HF band.

HobbyMan
Posts: 38
Joined: Sat Apr 07, 2018 1:55 am

Re: Decoding FSK

Post by HobbyMan » Sun Apr 29, 2018 4:33 am

Good thing is, i know exactly what its used for :D . Its weather data being sent between the AWOS (Automated Weather Observation System) tower and the terminal building at my airport. The first pulse in the recording is data being sent by the observation tower to the terminal for processing, the second pulse is data being sent from the terminal back to the tower. I have no idea as to what is being sent back to the tower, as the only purpose of the tower is to collect weather data and send it to the terminal, which broadcasts the data using a speech synthesizer on the VOR freq. i could get some pictures of the equipment if you need them.

HobbyMan
Posts: 38
Joined: Sat Apr 07, 2018 1:55 am

Re: Decoding FSK

Post by HobbyMan » Wed May 02, 2018 1:31 am

I’m a little Impatient, so I’m bumping this. Sorry if it’s against something.

rtlsdrblog
Site Admin
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Re: Decoding FSK

Post by rtlsdrblog » Thu May 03, 2018 8:24 am

Hmm sorry i'm not sure. If you can find documentation for whatever protocol AWOS uses it would help you create a decoder.

I don't think you can just plug it into a generic FSK decoder and expect data. You have to figure out how the data is sent and encoded first.

HobbyMan
Posts: 38
Joined: Sat Apr 07, 2018 1:55 am

Re: Decoding FSK

Post by HobbyMan » Thu May 03, 2018 8:55 pm

Okay, i did some research. according to the manual for DataRadio Integra (which is the type of transceiver that is used for this), the modulation type is DRCMSK (Differential Raise-Cosine Minimum Shift Keying). now i gotta look for something that can demodulate that :shock:

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