THE PROBLEM
It is hard enough to fix a new piece of scientific equipment when it goes out of order in the laboratory. And when the equipment is sailing around the earth a couple of thousand miles up in the sky a repair job ought to be impossible. But during the last two months of 1962 we found this not to be true at all. That was when the Telstar I satellite began to misbehave and eventually would not obey the commands we sent it from the ground. This presented us with a nice little problem: We had to find out exactly what was wrong with the satellite and then—the really tough job—devise a way to cure the trouble. We were able, finally, to do both these things, after a combination of logical deduction, trial-and-error experimentation, and laboratory testing—plus a certain amount of plain good luck. Our “cure,” unfortunately, turned out to be only a temporary one, for our patient had a relapse some weeks later. However, the story of how we went about doing our never-before-attempted task should give you an idea of the things you have to improvise in the laboratory when an experiment doesn’t work out exactly as you had planned.
The Telstar Command Circuit
As shown on pages [32] and [33], the operation of the Telstar I satellite was controlled by orders sent from the ground on a frequency of 123 megacycles. Fifteen different commands could be given to the satellite, each a coded signal made up of a series of ones and zeros. The signals, as you can see in the [table below], turn on or off the radiation experiments, the telemetry, the communications equipment, and the orientation coil. These are important functions, and we wanted them to be going on when they were needed. But we did not want them to be operating continuously.
Command was the only Telstar I function that we felt had to be “redundant,” so two duplicate chains of components were provided. As you can see in [the block diagram on the next page], the satellite has two radio receivers in parallel, so that one can operate if the other fails. There are also two command decoders, which take the pulse-coded signals from the receivers and translate their zeros and ones into usable instructions. In the command switching control, these instructions operate nine relays that turn on or off the power to all the electronic circuits except the command receiving chain, which operates continuously.
Telstar I’s telemetry unit reported back 112 measurements every minute over the 136-megacycle frequency. These told both what the satellite encountered in space and the condition of the satellite’s own components—as indicated by a variety of different sensors. The telemetry also gave a check on whether the commands sent to the satellite were actually obeyed.
The Fifteen Telstar Commands
| COMMAND | FUNCTION |
|---|---|
| A | Turns on traveling-wave tube filament voltage |
| B | Turns on traveling-wave tube helix and collector voltages; energizes all transistor circuits associated with communications experiments |
| AA | Turns off traveling-wave tube helix, collector, and filament voltages; de-energizes transistor circuits |
| C | Turns on traveling-wave tube by applying anode voltage |
| CC | Turns off traveling-wave tube anode voltage |
| D | Turns on telemetry and energizes radiation experiment circuits |
| DD | Turns off telemetry and de-energizes radiation experiment circuits |
| E | Turns on current in orientation torque coil |
| EE | Turns off current in orientation torque coil |
| F | Connects telemetry encoder No. 1 to circuit |
| FF | Connects telemetry encoder No. 2 to circuit |
| SS | Performs duties of AA, CC, DD, EE, and FF; de-energizes 136-mc beacon transmitter and removes all load from storage battery |
| S | Connects storage battery back into the circuit and energizes the 136-mc beacon transmitter |
| T1 | Turns off command receiver and decoder No. 2 for 15 seconds, so that command receiver and decoder No. 1 can be tested |
| T2 | Turns off command receiver and decoder No. 1 so that No. 2 can be tested |
