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.
Latest Answers