Airprobe-rtlsdr DC spike

HackRF with DC spike

I've been experimenting with the Airprobe-rtlsdr utility which is designed to demodulate non frequency hopping gsm signals and stream the demodulated data to Wireshark. Then in Wireshark you can use the built-in GSM decoding functions to extract cell tower identification etc. You CAN NOT decode the SMS or voice content so stop getting all excited!! That requires a whole lot more effort which would take you from the slightly illegal to the totally busted zone quick smart. Anyway when using the HackRF or it's clone the HackRF-Blue you face the usual large DC spike issue at 0Hz. This stops the demodulator from working so what I have done is add the GNU Radio DC blocker block to Airprobe-RTLSDR and now it (theoretically of course) decodes just fine.

Modifications to airprobe-rtlsdr.py

The modifications to the airprobe-rtlsdr.py file are shown in the above screen shot. I commented out the old code (three lines starting with #) and added the four new lines which insert the dc blocker into the signal chain. You also need to instantiate the dc blocker which is what is happening at the top of the image. Another thing not shown is that you must include the filter code from GNU radio so you need to add an import statement at the start of the code which says:

from gnuradio import filter

HackRF with DC spike removed