Decoding Satellite-based Text Messages with RTL-SDR and Hacked GPS Antenna

[Carl] just found a yet another use for the RTL-SDR. He’s been decoding Inmarsat STD-C EGC messages with it. Inmarsat is a British satellite telecommunications company. They provide communications all over the world to places that do not have a reliable terrestrial communications network. STD-C is a text message communications channel used mostly by maritime operators. This channel contains Enhanced Group Call (EGC) messages which include information such as search and rescue, coast guard, weather, and more.

Not much equipment is required for this, just the RTL-SDR dongle, an antenna, a computer, and the cables to hook them all up together. Once all of the gear was collected, [Carl] used an Android app called Satellite AR to locate his nearest Inmarsat satellite. Since these satellites are geostationary, he won’t have to move his antenna once it’s pointed in the right direction.

Hacked GPS antenna
Hacked GPS antenna

As far as antennas go, [Carl] recommends a dish or helix antenna. If you don’t want to fork over the money for something that fancy, he also explains how you can modify a $10 GPS antenna to work for this purpose. He admits that it’s not the best antenna for this, but it will get the job done. A typical GPS antenna will be tuned for 1575 MHz and will contain a band pass filter that prevents the antenna from picking up signals 1-2MHz away from that frequency.

To remove the filter, the plastic case must first be removed. Then a metal reflector needs to be removed from the bottom of the antenna using a soldering iron. The actual antenna circuit is hiding under the reflector. The filter is typically the largest component on the board. After desoldering, the IN and OUT pads are bridged together. The whole thing can then be put back together for use with this project.

Once everything was hooked up and the antenna was pointed in the right place, the audio output from the dongle was piped into the SDR# tuner software. After tuning to the correct frequency and setting all of the audio parameters, the audio was then decoded with another program called tdma-demo.exe. If everything is tuned just right, the software will be able to decode the audio signal and it will start to display messages. [Carl] posted some interesting examples including a couple of pirate warnings.

If you can’t get enough RTL-SDR hacks, be sure to check out some of the others we’ve featured in the past. And don’t forget to send in links to your own hacking!

Announcing: International Hack Day, August 11th.

There is no single and definitive definition of what hacking is. We all have different versions of similar ideas in our head, but depending on your background and area of enthusiasm, hacking means something different. While has many definitions of the word itself, none seem to cover what we see on a daily basis.

We set out to define “hacking” ourselves. We tossed around words like “modify”, “kludge”, “explore”, and “create”. Each time we committed an increasingly vague definition onto the page, we decided it was too narrow and tossed it in the proverbial trash. The variations were just too many.

What we do know is that “hacking” seems to breed advancement and innovation. Much like mutations in an evolutionary chain, each hack pushes the topic in a slightly new direction, inspiring others and thereby perpretuating the evolutary event. In a very short time we’ve witnessed hacking bring forth the evolution of wagons to cars, kites to airplanes, and the creation of the computer.

We at Hackaday would like to declaire August 11th to be “International Hack Day”. A day to celebrate hacking in all of its diverse forms. From soldering to sewing, coding to carbonating, knitting to knurling, we want you to keep on hacking. Take August 11th as a day to show pride in your hacking. Waive your hacker flag high and educate those around you.

We have asked many of our friends to contribute their personal definition of hacking. Here they are, in the order they were received.

Continue reading “Announcing: International Hack Day, August 11th.”