It is done with the help of radio waves. Scientists use a network of ground-based antennae that support interplanetary spacecraft missions, e.g. NASA’s Deep Space Network (DSN), Indian Deep Space Network (IDSN), and European Space Tracking (ESTRACK) Network etc. These networks enable scientists to explore the solar system and the universe using radio and radar astronomy observations.
A radio telescope sends up a radio signal from Earth with a time code on it towards the spacecraft. After the signal bounces back, scientists calculate the amount of time it takes for the signal to travel from Earth to the spacecraft and back to find out how far it is from Earth. The angle at which the radio telescope is pointing when it receives the signal, reveals the direction in which the ship is travelling.
A more precise method of zeroing in on the spacecraft location is by using a technique called triangulation. Two different radio telescopes receive radio signals from the ship and scientists measure its distance from each telescope. Then the scientists measure the distance between a known object in space that doesn’t change its location (a star, for example) and the telescopes. Comparing the distance to the spaceship and distance to the known object helps to pinpoint the spaceship’s locations.
Picture Credit : Google