The GNU Radio conference (GRCon15) is a yearly conference discussing all matters related to GNU Radio, an open source graphical block based DSP programming application that is compatible with most SDR’s, including the RTL-SDR. The conference started on August 24 and is due to close this Friday August 28, however many of the presentation slides are now available for viewing on their website.
This year there are many interesting talks, including a speech by Balint about radio direction finding, RF sniffing and digital FPV on drones. There are also several tutorial presentations that show how to install GNU Radio, how DSP sampling works, an intro to analog RF concepts and how to build a software radio from scratch.
Georg used his TI Chronos watch to transmit a signal copied from remote controls which are used to open his house gate, garage door, light switches and set his house alarm. When he discovered that the watch signal was not transmitting properly, he used his RTL-SDR to compare the signal coming from the watch to the original signals from the remote controls to help him with debugging. In order to detect the bit stream from the RF signal, he used a GNURadio program for decoding wireless temperature sensors, which he modified slightly to work with his watch.
After checking for local causes of interference and finding nothing, they decided that the interferer must be coming from further away. To find the location of the jamming signal they did some radio direction finding. This involved driving around with Yagi and magnetic loop antennas and RTL-SDR and USRP N200 SDRs and then measuring the signal strength at various points.
For the software they used a custom GNURadio block which calculated the power spectra using the FFTW C library, and averaged the results to disk. They then post processed the data to calculated the RFI power, and correlated the data with GPS coordinates recorded on his phone.
After all the data was processed, they discovered that the interference originated from an FM radio tower which had a faulty FSK telemetry link. They notified the engineer responsible who then replaced the link and the interference disappeared.
Redhawk is a graphical signal processing application that is similar to GNURadio. YouTube user REDHAWKWaveforms has posted a video tutorial showing how to build a simple FM receiver using the RTL-SDR in Redhawk.