GROUND-BASED TRACKING

Keeping track of a spacecraft’s motion far away from Earth is a wondrous application of human ingenuity and knowledge. It requires a blend of heavy

engineering and subtle, precise electronics; the first to build and control huge dish antennae, allied to the reception and measurement of vanishingly weak radio signals.

Two techniques were used for Apollo tracking, and both of these were cleverly interwoven into the same radio signal that carried all the communication between the spacecraft and Earth: voice, television and telemetry from the spacecraft’s systems and data uploads from the ground station on Earth to the ship’s computer. Because so many functions were brought together into one S-band radio signal, the system was known as Unified S-band. Its tracking capability could determine range and radial velocity – that is, the distance to the spacecraft and how fast it was approaching or receding along the line of sight of the antenna system on Earth.