How to communicate with the 36-year-old space probe using GNU Radio

In May 2014, a group of enthusiasts successfully established communication with the ISEE-3 spacecraft, which was launched in 1978, successfully completed its mission in 1999, and has since been mothballed.

Now the participants of this project have published instructions (and are ready to send the source code) so that anyone can repeat the experiment and send commands to the probe. Unless, of course, you have a large enough satellite dish.

GNU Radio is a software for digital radio processing. Programs on GNU Radio can be used with external antennas. The visual environment of the GNU Radio Companion or libraries in C ++ and Python is suitable for programming.
')
The device communication system with ISEE-3 is quite simple. The main detail is the software-defined radio system USRP N200 , which is connected to the laptop via Gigabit Ethernet.



The outgoing signal from the USRP radio system at a frequency of about 2.05 GHz (S-band) is transmitted to the room where the control equipment is installed with a satellite antenna. After processing a 400-watt amplifier, the signal is fed to the antenna.

Enthusiasts used a satellite dish at the Arecibo Observatory to transmit.


Satellite Dish at Arecibo Observatory

The authors say that the most difficult part was to understand hundreds and hundreds of pages of NASA documentation. They described everything: from antenna design to details of signal modulation and the layout of the earth communication system.

Signal transmission is carried out using PCM / FSK / AM / PM modulation. Team size is 60 bits. First, the command is sent to an FSK modulator with a center frequency of 8.25 KHz and a deviation of 750 Hz.

In ancient times, engineers tried to make a radio receiver on a spacecraft as simple as possible. Therefore, instead of installing a clock recovery circuit on the probe, they transmitted a clock signal along with a command from the ground. It is modulated in AM with the FSK subcarrier. Thus, the subcarrier carried the clock signal (amplitude, green graph in the diagram), and transmission information (frequency, blue graph in the diagram). The FSK / AM signal (red graph in the diagram) was then modulated for a frequency of 2.05 GHz. The circuit was not designed for maximum energy efficiency or bandwidth.



See more about modulating the signal here . The authors will send the program code upon request.