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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?

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There are ground tracking systems that provide this info, usually in the form of TLEs. It is public, except where it is secret. Amateur tracking provides some information on secret satellites.

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  • $\begingroup$ Do you by chance have a link to any such tracking systems, either amateur or not? $\endgroup$ – jgalak Oct 29 '17 at 1:42
  • $\begingroup$ lmgtfy.com/?q=cubesats+tracking+websites $\endgroup$ – JCRM Oct 29 '17 at 7:47
  • $\begingroup$ general info and Info about amateurs $\endgroup$ – JCRM Oct 29 '17 at 8:03
  • $\begingroup$ @JCRM SE comments should be considered temporary; there is always the possibility that they can be cleaned up at any time. It's much better to edit your answer and include them there directly. $\endgroup$ – uhoh Oct 30 '17 at 23:32
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To communicate with the cubesat from a ground station, it is necessary to know before the date and time interval and the direction of the antenna. If the altitude of the satellite is not known before, the orbit and timing of the cubesat is not known and the necessary direction of the ground station antenna is unknown.
A mathematical model for the orbit of the cubesat is necessary and also the variation of the orbit due to drag of the high atmosphere. There is some uncertainity about the actual drag due to atmospheric variations.
If there is communication with ground station, distance to cubesat and its speed may be measured.
It was possible to watch the orbits of satellites in the decades of space science before GPS was available. Before the GPS system was operative, the GPS satellites should be inserted into very precise orbits.

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  • $\begingroup$ What if the cubesat has some sort of active propulsion to change altitude? Especially if the effectiveness of that propulsion is not known ahead of time, and is what's being studied? $\endgroup$ – jgalak Oct 29 '17 at 1:42

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