The Apollo Digital Ranging System: More Than Meets The Eye

If you have not observed [Ken Shirriff]’s teardowns and reverse engineering expeditions, then you are in for a treat. His rationalization and demonstration of the Apollo electronic ranging program is a interesting read, even if vintage computing and engineering aren’t section of your usual fare.

The typical Hackaday reader ought to be common with the idea of figuring out the distance of a faraway item by measuring how extended it requires a seem or radio wave to be reflected, these types of as in sonar and radar. Going one more action and measuring Doppler Change – the variance in the returned signal’s frequency – will inform us the velocity of the object relative to our position. It’s so basic that an Arduino can do it. But in the times of Apollo, there was no Arduino. In truth, there were no Built-in Circuits. And Apollo missions went all the way to the moon- considerably way too distant for fairly simple Radar measurements.

How could assortment (length), place, and speed then be measured? The remedy is just one that [Ken] aptly describes as fractal: Every layer of complexity hides beneath it a different layer of complexity. Making use of equations dating from 3rd century China as perfectly as slicing edge weak sign telemetry, Apollo engineers devised a advanced but workable method that used an S-Band transponder to consider details transmitted from a strong floor station and send it again on an additional frequency. A person wonderful hack was to use Stage Modulation to encode the downlink in its place of Frequency Modulation so that Doppler information attained on the uplink wouldn’t be misplaced on the downlink.

By understanding the exact situation of the ground station and the extremely big parabolic antennae, not only could the length and velocity be calculated, but a great estimation of the spacecraft’s situation in 3d room could also be experienced.

From the use of hold off line memory to mixture weak signals to a condition device pc created up of discrete transistor logic, all the way to the slicing edge transponder on the Command Module, the Apollo digital ranging system is an excellent instance of terrific hacks coming out of a application with restricted technological constraints.

We really advocate providing [Ken]’s blog a go through and be confident to verify out the interactive demonstration web pages he’s set up to aid us grasp the genius of the Apollo engineering teams. [Ken]’s been highlighted on Hackaday a quantity of moments reverse engineering this sort of various points as a Yamaha DX7 Synth chip.