Navigating by radio signal

Doppler: how fast it moves

When a spacecraft moves away from or towards an antenna, the frequency of its signal shifts slightly. By tracking that shift, the Deep Space Network measures the spacecraft’s speed along the line of sight. For navigation the most useful form is two-way: a station sends a signal up, the spacecraft returns it, and the ground compares the two.

Ranging: how far away it is

For ranging, the station sends a coded signal that the spacecraft sends straight back. The delay of the round trip gives the light time between station and spacecraft, and so the distance.

Delta-DOR: where it is in the sky

The third method uses two stations on different continents at once. Both listen to the spacecraft, and then to a quasar, a very distant natural radio source whose position is known. Comparing the two gives the spacecraft’s angle in the sky very precisely. The technique, called delta-DOR, uses the Goldstone–Madrid or Goldstone–Canberra pairs, and one measurement takes about an hour.

Why spacecraft must think for themselves

Radio signals travel at the speed of light, but the distances are so large that the delay is measured in minutes. During Perseverance’s landing on Mars, its signals took 11 minutes to reach JPL, while the whole descent lasted about seven. By the time engineers heard that the landing had begun, it was already over. Every step had to be programmed in advance. The story of the landings is on the Mars page.