I've seen cubesat projects talk about the altitude of their satellite. Starting orbit is, presumably, known based on deployment conditions. But how does one track the altitude once it's been in orbit for a while?
Do the satellites have some some way to determine altitude and transmit it as part of the telemetry downlink? Presumably from GPS data, as direct measurement sensors seem hard to do - a barometer would need to be incredibly sensitive to be meaningful at that low pressure, and radar altimeters would use too much power. Some other sensor?
Or are there ground tracking systems that offer this info? Is the info public? How good is the resolution on such?