[nuccy]

If you see satellites then likely you see even more stars. Unlike satellites the stars barelly move (actually they do, see "proper motion" [1]) relatively to each other, so a catalogue of stars (two coordinates values and two proper motion values) along with the time of observation is sufficient to be used over decades, unlike NORAD orbit elements requiring regular updates. With stars you need just one image at a known time to find your location, with satellites it is much much more complicated: you need to know where the sun is, you need few images of a satellite or even a video (likely on top of image of stars anyway) to distinguish it from the stars and to solve the trajectory.

1. https://en.m.wikipedia.org/wiki/Proper_motion

[anovikov]

How do you find your location from one image of stars? It is possible if you have a precise vertical but you don't have a precise vertical on a moving UAV. That is, you need an inertial system on top that will provide you with a vertical.

With satellite images, you don't need anything apart from time. And no, you don't need to "make a video to see satellites move", you start with your approximate location, make an image and find satellites within a circle where each of them might be, starting with the slowest moving - furthest away from you - ones (they provide poorest precision of coordinates because parallax is small, but you need to start with something, but their search circle will also be smaller), locating those, you get better coordinates of yours and the search circle for each satellite becomes smaller, then you can find faster moving satellites too to get precise coordinates of yourself.

[nuccy]

You are right: to find a location from a star image you need a true horizon, but unless UAV is pulling some Gs even a basic accelerometer would give you the horizon, accuracy of that estimation will limit the accuracy of your location.

Regarding satellites: so "starting with the slowest moving" requires a series of images, doesn't it? Then how do you know "your approximate location"? From stars? In theory I understand what you say but practically it would be much more complicated and the obtained accuracy would not be better than with the stars, since in either case you also need a horizon to know your location.

[anovikov]

No you just "start looking" on a single image.

Know your approximate location: by dead reckoning. You will need coordinate fixes once every few minutes anyway and you know your direction precisely enough from the same stars, error only comes from wind direction not being precisely known. So we are speaking of correcting for at most tens of kilometers of error. 10km at a typical distance of 1000km to a low orbit sat is <1 degree and only about 10 arcmin to a typical medium earth orbit satellite.

Astrometry allows for locating objects down to about 0.2 pixel reliably and to 0.1 pixels in optimal conditions, so a typical wide-angle camera that might have about 40 arcsecond pixels will easily give 8 arcsecond precision, for a satellite 4000km away (about 2000km orbit at 30 degrees elevation), that's 170 meters of location error, which is more than good enough for navigation (final targeting is done by optical pattern recognition on the ground anyway).

>since in either case you also need a horizon to know your location.

No you don't. Benefit of using satellites is that the source of coordinate data is the parallax of satellites vs stars. It works without having a vertical/horizon.

Simply put, we calculate that in a predicted location the satellite will be at a certain pixel distance from a few of the closest stars on the photo. And it will be a few pixels off that predicted point. Distance and direction of that error allows for calculation of discrepancy of predicted vs real location (and repeating this process on several satellites visible on same photo, allows to decrease the error by removing outliers - which might be noise/space rays on images or errors in star catalogs or orbital elements data, or satellites changing their orbits - and averaging the results).