As mentioned above by 0xea, @domi007 published 4 blog posts (1,2,3,4) detailing his experience with GSM sniffing and cracking. He also published his recorded presentation about GSM security (slides).
On the same topic, there's also Sniffing GSM with HackRF, Analyzing GSM with Airprobe and Wireshark, three Chaos Computer Club presentations (One discussing ...
@domi007 recently published a series of blog posts detailing his efforts on snooping and decrypting GSM. Insted of USRP, he used rtlsdr. He also used an osmocomBB enabled phone. His efforts build upon the research done by Karsten Nohl on cracking GSM (you can see the video too) and his ~1.5 terabyte of rainbow tables.
If this a commercial device sold in United States, it must be FCC certified and so must have a label with the FCC ID mentioned somewhere. You can use this page to search for any documents FCC has on file for this device. They should mention the frequencies on which the device operates.
As atdre suggested, you might want to check on and learn how usual, known, modulations function. That way you'll usually be able to at least figure out what kind of modulation is in question. In most cases it will turn out to be some known modulation. Read on how AFSK or POCSAG work for example.
Carl Weisman did a series of introductory lessons on RF and ...
How to detect:
obtain Spectrum Analyzer Rigol makes cheap ones. RTLSDR dongle will also work with
get close to transmitting radio (extra points for touching it)
run RF sweep, log RSSI
repeat above until you are sure you found your guy
Frequency hopping might be little annoying, but shouldn't be a problem if ...
Googling for "868 MHZ Arduino" yields several results of transmitters/receivers (transceivers) that can be attached to an arduino, including example programs. You could start with buying 1 of those, put it in receive mode, leave it on for 24h and check what it receives, and check if it really receives data only once in 24h, or maybe more often.
Then, save ...
RF signals are composed of three possible modulations: Amplitude, Frequency, and Phase.
Once you've determined the width and center frequency of the signal, analyze the amplitude changes, if any. Try to determine if the amplitude is periodic. Similarly, look at the frequency and phase.
Most radio systems conform to established standards, so once you've ...
If you can't access the modulation specifics in the documentation or via FCC and patent searches, then you might want to reverse it out of the firmware or chip off.
If you can get an audio-out or oscilloscope visual of the signal, then you may be able to determine the type of modulation -- whether FSK, simple on/off keying, or another data format (...