I’ve never been especially interested in space or NASA. I find space so intimidating to learn about. And it hurts my brain if I think about it too much – the vastness of it all. It’s a real blind spot in my knowledge. And I must fix that. But watching the film “Apollo 13” has stirred my imagination. I haven’t got to the end of the film yet, but I’m really enjoying it. I like films about people “working the problem”. And one question I’ve always had about Apollo 11’s mission to the moon is: how did NASA get all this near-realtime data in the 1960’s‽ Voice, telemetry, etc. Well, I found out.
The Minor Delay
They got all this data via something very simple: radio.
The Moon is ~384,000 km away. Radio signals travel at the speed of light, giving a one-way delay of ~1.3 seconds (~2.5 seconds round trip). This is why there’s a noticeable pause in all Apollo mission audio.
The Ground Station Network
NASA used a global chain of stations ensuring one was always facing the Moon as Earth rotated (radio needs line of sight). Key stations: California and two in Australia.
The spacecraft used a steerable high-gain dish antenna to beam a focused signal back to Earth.
What Was Transmitted
A single radio link carried multiple streams simultaneously (multiplexed):
- Voice communications
- Biomedical telemetry (heart rate, respiration per astronaut)
- Systems telemetry (hundreds of parameters: pressure, temperatures, voltages, thruster status)
- TV video (slow-scan format, converted before broadcast)
- Ranging data (signal travel time used to calculate precise distance)
How Physical Measurements Become Radio Signals
The full chain:
- Sensor measures something physical (temperature, pressure, voltage)
- Sensor outputs a proportional electrical voltage
- An ADC (Analogue-to-Digital Converter) samples this voltage and converts it to binary numbers
- A multiplexer cycles through hundreds of sensors rapidly, producing a continuous binary data stream
- A modulator encodes that binary stream onto a radio carrier wave by varying one of its properties (Apollo used phase modulation — a phase shift one way =
0, the other way =1) - The wave travels to Earth, where a ground antenna demodulates it, stripping the data back off the carrier
- You’re back to a binary stream — just data
How the Ground Knew Which Reading Belonged to Which Sensor
As I was learning all of the above, my digitally minded brain was confused how it could be done without something like JSON. But no metadata or labels were used. Instead: time-division multiplexing with a fixed frame structure. Before launch, both the spacecraft and ground agreed on a fixed, hardcoded schedule — the multiplexer always cycled through sensors in the same order at the same rate. Position 1 was always sensor X, position 2 always sensor Y, and so on. The ground just counted positions; no labelling needed.
To mark the start of each cycle, a sync word (a distinctive fixed bit pattern) was transmitted at the beginning of each frame. When the ground decoder spotted it, it reset its positional counter.
Readings were organised in two tiers:
- Minor frame — one full cycle through all sensors
- Major frame — multiple minor frames grouped together; slower-changing sensors (e.g. cabin temperature) only appeared once per major frame to conserve bandwidth
This was extremely efficient — zero bandwidth wasted on metadata — but brittle. If sync was lost, the system had to wait for the next sync word to re-lock.
A beautifully simple combination of analogue and digital. Remarkable.