The amazing archaeology on one of my Apollo artifacts — Ken Shirriff did a teardown and deep dive into how the Apollo Command Module transponder, pictured here, could figure out the 3D location and speed of the spacecraft, using the primitive electronics of the day, and using a single radio link (that was also carrying voice, data and TV!).
HACKADAY summarizes: “Each layer of complexity hides beneath it another layer of complexity. Using equations dating from 3rd century China as well as cutting edge weak signal telemetry, Apollo engineers devised a complex but workable system that used an S-Band transponder to take data transmitted from a powerful ground station and send it back on another frequency. One great hack was to use Phase Modulation to encode the downlink instead of Frequency Modulation so that Doppler data gained on the uplink wouldn’t be lost on the downlink. From the use of delay line memory to aggregate weak signals to a state machine computer made up of discrete transistor logic, all the way to the cutting edge transponder on the Command Module, the Apollo digital ranging system is an excellent example of great hacks coming out of a program with tight technical constraints. We highly recommend giving [Ken]’s blog a read and be sure to check out the interactive demonstration web pages he’s put up to help us grasp the genius of the Apollo engineering teams.”
I agree! See: http://www.righto.com/2022/04/the-digital-ranging-system-that.html
And make sure to see this interactive page, showing the Chinese Remainder Theory with a spacecraft slider control at the top of the page: https://righto.com/apollo/ranging.html
And the pseudo-random code generator made from a shift register and XOR gate: https://righto.com/apollo/ranging-xor.html
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