NASA and other space agencies use various methods to calculate the distance of a spacecraft relative to Earth when it is not at a fixed position. Here are a few commonly used techniques:
Radar Tracking: NASA employs radar systems to track spacecraft and measure their distance accurately. By bouncing radio waves off the spacecraft, the time it takes for the signal to travel back to Earth can be measured. Since the speed of light is known, the distance can be calculated using the equation distance = speed of light × time taken.
Deep Space Network (DSN): NASA operates the Deep Space Network, a system of large antennas located at different locations around the globe. These antennas communicate with spacecraft and receive signals from them. By precisely measuring the time it takes for the signals to travel between the spacecraft and the DSN antennas, the distance to the spacecraft can be determined.
Triangulation: When a spacecraft is in view of multiple ground-based antennas or spacecraft, triangulation can be used to calculate its distance. By measuring the angles between the spacecraft and the different observing points, along with known baseline distances between the points, trigonometric calculations can determine the spacecraft's position and distance.
Onboard Navigation Systems: Spacecraft often have onboard navigation systems that can determine their position and velocity relative to Earth. These systems utilize sensors such as star trackers, gyroscopes, and accelerometers to calculate the spacecraft's trajectory and distance traveled.
Range Rate Doppler Shift: The Doppler effect can be used to estimate the spacecraft's relative velocity and, in turn, the distance. By analyzing the frequency shift in the radio signals received from the spacecraft, caused by its motion toward or away from Earth, scientists can determine its velocity and distance.
These techniques are often used in combination, depending on the mission requirements and the capabilities of the spacecraft and ground-based tracking systems. By employing precise measurements and sophisticated calculations, NASA can accurately determine the distance of a spacecraft relative to Earth, even when it is not at a fixed position.