Starting with LIMESDR using GNURADIO under Linux

In 2016 I have participated in crowdfunding of the LIMESDR project which developed a powerful SDR based on a chip from LIME Microsystems, and recently received my unit (version 1.4s). I have put the unit into a metallic enclosure and used 10 SMA/u.fl pigtails to get all RX and TX ports on the front panel. I have also attached heatsinks on the 3 big chips and on the voltage regulator to prevent overheating, during RX only operation I can now comfortably hold finger on the heatsinks.


Concerning software, I am using LIMESDR under Gentoo Linux, where some of the required software is provided as a package (e.g. Gnuradio), some had to be installed separately (SoapySDR, LimeSuite., gr-osmosdr with soapysdr support (gentoo package is out of date and does not support soapy yet) while lms-suite was not necessary.
In GRC schematics, LIMESDR is employed using osmocomm source/sink, with device argument string "soapy=0,driver=lime" and antenna name chosen from the list displayed by 'SoapySDRUtil --probe="driver=lime"'.

I have first tried the software radio "hello world" application, WFM receiver using gnuradio companion lime_wfm_rx.grc .

Then I tried the TETRA receiver using LIMESDR - worked out of the box, great! As of spring 2017, Prague municipal police still does not encrypt their radio traffic.

Spoofing GPS signal using GNURADIO and LIMESDR under Linux

Next I tried generating a fake GPS signal as described here. I have used a unix fifo file to pass the simulated GPS baseband data between gps-sdr-sim and GRC to transmit it in real time and I did only transmission, using my Garmin GPS to receive. I used 16-bit samples, have put the right GPS frequuency, and tweaked a bit the gains; this is my GRC file. I used the following options for gps-sdr-sim:
./gps-sdr-sim -e brdc3540.14n -l 35,-10,0 -d 6000 -T 2017/05/03,11:11:11 -s 10000000 -b 16
I performed the experiment in a basement to avoid interference with any "real" GPS signal, although the generated signal was certainly much stronger than the 'real' one is, and also not to leak the signal outside. I used a log-periodic broadband antenna (purchased long time ago from Ettus research together with USRP) to transmit the signal and put the GPS device close to it.

My Garmin was showing super strong satellite signals, and I got a position fix quickly. Great! The technology advanced so much that you do not need to be a superevil from a James Bond film to transmit fake GPS signals ;-).



I have designed a simple RF amplifier with RF2126 to enhance the output power of LIMESDR. I have written a code to facilitate such design (RF impedance matching and S-parameter calculations), see here.

Note that spoofed GPS signals might be recognized, see here and Schneier blog and that GPS signals at military frequencies are cryptographically protected anyway.

More experiments with LIMESDR on the way...

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