Why Apollo 11’s telecommunications system made history

The landing of Apollo 11 on the surface of the moon is undoubtedly one of the greatest engineering achievements in human history. Jonathan Wilkins explains why it is important not to forget that the success of the mission is also one of the greatest testaments to advances in the communication technology of the day

This year, the world celebrated the 50th anniversary of man walking on the moon for the first time in history.

Those legendary astronauts, whose names we have all heard since childhood, were carried from Earth, through the darkness of space, to the moon in the equally famous Apollo 11 space shuttle.

When we think of what made the Apollo 11 mission a success, we tend to think of the gruelling training that the astronauts received or the engineering work that went into building Apollo 11 itself.

We don’t tend to think of it as being a success of the communication technology the spacecraft used to communicate with Earth throughout its perilous journey. But we should.

Getting the message across

In any space mission, countless amounts of information must be shared between the spacecraft and the supporting ground crews, such as telemetry data, computer upload information and voice communication. NASA very quickly realised that, just as the Apollo 11 mission was like no other that came before it, the mission would require unique, new communications systems.

Previous missions, operating in Earth’s orbit, used separate radio systems. Ultra-high frequency (UHF) and very high frequency (VHF) standards were used for two-way communications, uplinked data and downlinked telemetry.

Tracking was carried out using a C-band beacon on the spacecraft in combination with ground-based radar systems. These systems may have been enough for missions operating in the Earth’s orbit, but Apollo 11 was going much, much further into space.

The solution that NASA adopted was called Unified S-band (USB) and combined tracking, ranging, command, voice and television data into one antenna.

Voice data was transmitted on a 1.25MHz FM subcarrier, telemetry was done on a 1.024MHz bi-phase modulated subcarrier, and the two spacecraft used modulated S-band downlink frequencies; 2287.5MHz for the command module and 2282.5MHz for the lunar module. The only thing left was to fit in the all-important television broadcast signal.

There was 700kHz of bandwidth available for a television downlink on the USB signal. This was not enough for the standard video cameras of the time, which transmitted 525 scan lines of data at 30 frames per second at 5MHz.