Crazy Danish Hacker – GSM Sniffing 1 – Prerequisites
A radio repeater is usually a radio tower that receives weak signals from handheld, desktop or other radio, and rebroadcasts the same signal at a higher power over a wide area at a different frequency. This allows communications to be extended over a much greater area.
Repeaters are generally made from expensive professional grade radio equipment, however ZR6AIC has been experimenting with creating an ultra low cost repeater out of a RTL-SDR and Raspberry Pi. In his system the RTL-SDR dongle is set up to receive a signal on the 70 cm (420 – 450 MHz) amateur radio band, and then retransmit it using Rpitx on the 2M (144 – 148 MHz) amateur radio band. He also adds a 2M low pass filter to the output of the Raspberry Pi to keep the signal clean.
Rpitx is software for the Raspberry Pi which we have featured on this blog several times in the past. We’ve also seen the qtcsdr software which also uses Rpitx and an RTL-SDR to create a transceiver. Rpitx allows the Raspberry Pi to transmit radio signals without the need for any transmitting radio hardware at all. It works by modulating signals onto a General Purpose I/O (GPIO) pin on the Raspberry Pi. If the GPIO pin is modulated in just the right way, FM/AM/SSB or other signal modulation approximations can be created at a specified frequency. The signal is however not clean, as this type of modulation generates many harmonics which could be dangerous if amplified. If you use Rpitx, always use appropriate filtering hardware.
ZR6AIC’s post goes into detail about how to install and set up the required software onto the Raspberry Pi and how to set up the script to piece all the programs together into a repeater. He’s also uploaded a video demonstrating the system in action on YouTube.