When NASA gave permission for a group of enthusiasts to attempt to reconnect the ISEE-3 probe, one of the big problems they needed to solve was communicating with something that was launched in the late 1970s. The solution involves software defined radio (SDR) via GNU Radio, a USRP radio module, and a very big dish antenna.
While theoretically possible using nothing but hardware (as it was originally done), the use of SDR allows for rapid updating and prototyping at a fraction of the cost of hardware. The team was able figure out the protocol from NASA’s original specifications, define it in software and implement a solution that fell well within their budget and schedule. Permission to use the enormous dish antenna at Arecibo in Puerto Rico was the final piece of the puzzle.
Now that they can communicate with the probe, they continue to evaluate the status of its various components with the hopes of being able to gain navigational control.