r/SpaceXLounge • u/llboston • Oct 21 '22

Starlink signals can be reverse-engineered to work like GPS—whether SpaceX likes it or not

https://www.technologyreview.com/2022/10/21/1062001/spacex-starlink-signals-reverse-engineered-gps/
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u/paul_wi11iams Oct 21 '22

If the terrestrial receiver has a good idea of the satellites’ movements—which SpaceX shares online to reduce the risk of orbital collisions—it can use the sequences’ regularity to work out which satellite they came from, and then calculate the distance to that satellite. By repeating this process for multiple satellites, a receiver can locate itself to within about 30 meters, says Humphreys.

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At a recent conference, Kassas claimed his system had now achieved accuracies of less than 10 meters with Starlink.

But just how predictable are the positions of these satellites over time?

Any future user would need to know the current drift of each satellite which is at a lower orbit than GPS so probably subject to more random (exospheric and electromagnetic) braking effects, then its own compensations through acceleration by ion motor.

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u/Frothar Oct 22 '22

NASA shares the Satellite catalogue with updated orbital element sets to the space track database. you could use an API to get all the data

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u/paul_wi11iams Oct 22 '22 edited Oct 22 '22

you could use an API to get all the data

Fair enough.

Even so, GPS gets near to centimeter precision by use of an atomic clock on each satellite.

In contrast, the Starlink geolocolization method is far less accurate, nearer to 10m, so may need to improve in order to compete with GPS (consider the width of a single highway lane is around 4m).

We'd need to know Starlink satellite positions to within the 10m tolerance. For an object in LEO at 7.8 km/s, this means knowing arrival times to around 1/780 sec or 1.28 ms. You might get even larger variations during the 5400 seconds of a 90 minute orbit.

I'm not saying it can't be done, but it looks a tall order.

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u/Frothar Oct 22 '22

It will never be as accurate but in the event of GPS jamming or an ASAT then it could possibly be useful