system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How can spacecraft determine their speed once out in space?
gpt
There are a number of ways, but one is by measuring the frequency of the radio signal it receives from Earth. The signal is sent at a precise frequency, but because the spacecraft is moving - normally away from earth - the frequency is 'doppler shifted' down - because between each peak in the radio wave and the next, the craft has travelled a little further away, so it takes a little longer for it to arrive. From this the craft can calculate the speed with respect to Earth, and as it knows, from its programming, where and how fast the Earth would be at this time, and the orbit it is in and what direction it is travelling, it can calculate its speed.