The Apollo Digital Ranging System: More Than Meets The Eye

Jean J. Sanders

If you have not seen [Ken Shirriff]’s teardowns and reverse engineering expeditions, then you’re in for a treat. His clarification and demonstration of the Apollo electronic ranging method is a interesting examine, even if vintage computing and engineering aren’t aspect of your standard fare.

The normal Hackaday reader must be common with the principle of identifying the distance of a faraway item by measuring how extensive it normally takes a sound or radio wave to be mirrored, this sort of as in sonar and radar. Going a different step and measuring Doppler Change – the distinction in the returned signal’s frequency – will explain to us the velocity of the object relative to our position. It is so uncomplicated that an Arduino can do it. But in the days of Apollo, there was no Arduino. In truth, there have been no Integrated Circuits. And Apollo missions went all the way to the moon- considerably way too distant for rather simple Radar measurements.

The TPAC contained transistor logic for the ranging computer system

How could assortment (length), placement, and velocity then be calculated? The answer is just one that [Ken] aptly describes as fractal: Each layer of complexity hides beneath it yet another layer of complexity. Using equations dating from 3rd century China as perfectly as cutting edge weak signal telemetry, Apollo engineers devised a advanced but workable process that utilized an S-Band transponder to choose facts transmitted from a potent ground station and send it again on an additional frequency. A single terrific hack was to use Period Modulation to encode the downlink in its place of Frequency Modulation so that Doppler information received on the uplink would not be lost on the downlink.

By figuring out the precise position of the floor station and the quite substantial parabolic antennae, not only could the distance and pace be calculated, but a excellent estimation of the spacecraft’s placement in 3d room could also be had.

From the use of delay line memory to combination weak signals to a state equipment pc produced up of discrete transistor logic, all the way to the slicing edge transponder on the Command Module, the Apollo electronic ranging program is an great instance of excellent hacks coming out of a system with restricted technological constraints.

We really advise offering [Ken]’s website a study and be absolutely sure to check out the interactive demonstration net internet pages he’s place up to assist us grasp the genius of the Apollo engineering groups. [Ken]’s been highlighted on Hackaday a quantity of instances reverse engineering this kind of numerous things as a Yamaha DX7 Synth chip.

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