In the Thunderbirds 11th episode of season 1, Sun Probe, the ship fails to fire its boosters because of "radiation" but somehow Thunderbird was able to fire them remotely...
How, if the solarnoughts on the ship itself couldn't fire the boosters themselves, could they be fired by remote control?
Warning: Speculation alert!
Considering how hot it was getting, it may be that the heat was interfering with the electronic componentry aboard Sun Probe. Consequently, any attempts by the crew to fire the boosters would have been futile. Hence, by firing the safety beam which somehow interferes with the navigation systems, the retros could be fired. This theory does explain why a beam would be able to fire the retros when the crew couldn't, but it is just speculation on my part I'm afraid!
The episode is pretty specific that the safety beam is an independently controlled safety system, presumably akin to the remote detonation sequence on the Space Shuttle. In the event that the crew are incapacitated or their primary controls fail (in this case, due to radiation), the crew on the ground can trigger this system externally by means of a "safety beam".
If I had to guess, I'd assume that the retro backup system was situated at the back of the ship (close to the main engines and facing the Earth and where the radiation would be at the least), whereas the primary controls are located at the front of ship, where the radiation would be at its strongest. This would explain why the safety system still works but the primary controls don't.
