How do the increasing number of LEO satellites keep track of their position? I know they use ground stations, but do they employ private companies to keep track for them? It seems like there are way too many satellites for the ground stations. Or are there lots of ground stations too?
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1$\begingroup$ possibly helpful but not duplicates: What would be a “big picture” understanding of how the orbits of Earth satellites are monitored? and also Are commercial communications satellites in GEO being constantly monitored by telescopes? plus perhaps How exactly does TDRSS provide tracking? (Tracking and Data Relay Satellite System) I think this question needs its own answer. $\endgroup$– uhohAug 19, 2020 at 17:19
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2 Answers
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Initially, when a new satellite is launched in space orbital dynamics models using launch and deployment parameters to provide an estimation of the orbit of the target satellite.
As @pearsonartphoto said on another comment, many LEO satellites do have GPS (or other GNSS systems) on-board to retrieve information on the exact position of the spacecraf,t which could then be transmitted back to Earth via a ground-station or via a ground-station network like SatNOGS.
Most satellite operations teams get their info on the position of their satellite from Space Situational Awareness Services such us SpaceTrack; I recommend checking out this overview of Space Situational Awareness Services to get some understanding of the techniques used and the information they provide to satellite operator teams.
Since nowadays many LEO satellites are routinely deployed in batches, identifying which orbital object tracked by Space Situational Awareness services corresponds to which satellite can be challenging. Satellite ground-station networks can assist identification by using doppler shift measurements across several of their ground-stations to facilitate timely identification.
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1$\begingroup$ How does GPS work in satellites though? I thought all GPS devices were required to disable themselves if they detect themselves being above a certain altitude or ground-speed due to SALT/START/etc? $\endgroup$– DaiAug 20, 2020 at 8:23
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2$\begingroup$ While all consumer grade GPS devices must abide to certain limits, this is not necessarily true for military or space grade devices @Dai $\endgroup$ Aug 20, 2020 at 8:32
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$\begingroup$ Wouldn't the military be the first sector that would be required to abide by those GPS limits imposed by SALT or START? It's not like I'm going to build my own ICBM (yet). $\endgroup$ Aug 20, 2020 at 14:34
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2$\begingroup$ It's ITAR rather than SALT and START which is the ploblem for GPS units. Military/space grade GPS devices have paperwork trails that are supposed to stop them getting into the wrong hands. $\endgroup$ Aug 20, 2020 at 16:26
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$\begingroup$ @llama rephrased the sentence, thanks for noticing... apparently I'm not a native English speaker $\endgroup$– elkosAug 21, 2020 at 6:30
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There are a lot of different way the positions are tracked. Typically, some combination of the following 3 things is done to track satellites (And then using orbital dynamics to propagate things forward)
- GPS signals- Can be received, especially in LEO. A bit harder to do above the constellation (MEO or HEO).
- Space Track- Mostly US, but also other governments that are allied track the satellites and provide periodic updates to the public. This is using radars primarily, and possibly other sensors, to detect the satellites.
- Ranging- Sending a "ping" to a satellite and seeing how long it takes to respond to the signal will give you a pretty exact position. Also related is detecting the first moment a satellite can be heard, Doppler shift, and several other similar things.
answered Aug 19, 2020 at 17:45
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2$\begingroup$ Sending a "ping" to a satellite and seeing how long it takes will give a pretty exact distance and a not so precise direction. For a pretty exact position a better direction measurement is neccessary. $\endgroup$– UweAug 19, 2020 at 18:14
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1$\begingroup$ One potential challenge with using ping data on many of the modern, low cost satellites, is that much of the communications are implemented with the help of software and do not offer "deterministic" or fully predictable time delays. Older systems that relied on coherency between uplink and downlink are more predictable and have less error with the ping method. $\endgroup$ Aug 20, 2020 at 2:39
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2$\begingroup$ @Uwe: but if you do it a number of times during the orbit you can then do a fit to the orbital parameters. You don't have to have direction at all. If you have it accurately enough, it can help. $\endgroup$ Aug 20, 2020 at 3:07
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2$\begingroup$ You are right that pinging will only give a distance, but several pings, combined with orbital dynamics, can allow one to determine pretty accurately the orbit of a satellite. $\endgroup$– PearsonArtPhoto ♦Aug 20, 2020 at 11:31