# How is the velocity of a spaceship measured?

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I get that with air crafts you can use a pitot tube to measure air speed, but what do you do if there is no air

In: Physics

If you’ve got line of sight to Earth, you can theoretically use the GPS constellation.

An Earth-based observation telescope or other monitoring station could watch you go and radio the data to you.

The movements of planets and moons are known. By tracking one or more of those objects relative to the spacecraft, it would be trivial for a computer to do the math.

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The velocity of a spacecraft is measured by taking advantage of something called the *Doppler effect.*

If you’ve ever watched an ambulance go past you, you’ve heard the pitch of the siren change as it approaches and goes past you, right?

That frequency shift is directly related to the object’s velocity relative to the measurer’s frame of reference, so if a satellite (edit: or spaceship!) is constantly broadcasting a signal on a known frequency, ground-based computers can calculate the velocity from the perceived redshift of the signal.

Usually some combination of inertial guidance with star tracking and ground-based tracking via Doppler. the Deep Space Network antennas can measure spacecraft speeds accurately to within hundredths of a millimeter per second anywhere in the solar system.