Cosmic tape measure

The second ground-based tracking system determined the range or distance to the spacecraft by measuring delay. Most people are familiar with the annoying delay introduced into television interviews carried out over satellite links. It takes light, and therefore radio about one and a quarter seconds to travel from the Moon to Earth, so the return travel time for a signal to a spacecraft at the Moon is about two and a half seconds. Engineers used this delay to measure range by putting a marker onto the radio signal which the spacecraft preserved and returned to Earth. The marker consisted of a digital code called pseudo-random noise, essentially a very large random number carefully chosen not to add undesirable artefacts to the radio spectrum. When the spacecraft synthesised the dowmlink carrier using the 240/221 relationship, it preserved this code, and sent it back to Earth. Engineers recovered the code and compared it with the transmitted code, ‘sliding’ one over
the other until they matched. The amount of ‘slide’ yielded a time for the round trip, and hence, knowing the speed of light, which is fixed, the distance. This technique was powerful enough to measure a spacecraft’s distance to an accuracy of better than 30 metres, and it could do so to a distance of nearly a million kilometres. For all of these ground-based systems, the movement of the ground station due to the rotation of Earth had to be taken into account before deriving measurements of the state vector.