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Comment by _moof | Hacker News Reader

> Before this, everybody in the industry thought GPS solved the aerial navigation problem.

Many people in industry believed this but no one with a brain ever did. The vulnerability of GPS has been cause for concern for a long time, and the decimation of the VOR network has always had a lot of people up in arms.

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It seems like we have enough satellites in space that are giving off enough unique signals that we could put together a device that solves the GPS problem without needing GPS itself. I wonder what it would take.

It would take those devices having synchronized atomic clocks, which they do not.

Is it really not possible to converge a location from implicit signals like frequency shift from relative motion? My intuition, maybe wrong, is that with enough compute and a thorough enough ephemeris of what should be broadcasting what, you could derive location.

Its an active research topic called signal of opportunity navigation, and papers has been written saying this is possible, some of the uses the idea you mentioned. I used to be involved in using starlink signals for positioning and there are problems:

1. Public ephemeris from space-track (celestrak) is notoriously inaccurate after a while. Less of a problem for starlink as they give free access to high accuracy ephemeris from their onboard GPS receiver: https://starlink.com/satellite-operators

2. For proprietary signals like starlink they can change signals whenever they want without telling you and break your positioning algorithm. For example, starlink used to transmit some sort of narrowband CW wave at their frequency band, which can be received by cheap ($20 ish) RTL-SDR dongle and satellite TV antenna: https://www.rtl-sdr.com/receiving-starlink-signals-with-an-r... . Everyone speculates that its a beacon signal, research papers were written trying to exploit doppler positioning from those signals. Then the signal disappears, as its possible that they don't intentionally transmit those signal. They you can't use cheap hardware to capture the signal, you need expensive hardware that can capture the full 250 MHz bandwidth. This is not a problem if you use standardized signals where they can even intentionally add positioning signals like 5G or digital TV

Your argument needs to be why it is practical in a smallish device, not that it seems possible.

Like you first have to infer what satellite you are even measuring, before you can decide where it might be.

Disagree. If the objection is just cost/complexiry, that’s really just a statement of how long until it’s practical. GPS itself was computationally impossible in the 1950’s.

If the math works, and it’s just the domain of one-off ridiculously expensive devices today, it’s inevitably feasible in the long run.

Atomic clocks are not precise enough, they need nuclear clocks, which are not developed yet: https://thoriumclock.eu/

GPS uses atomic clocks, not sure why we'd need something more precise for a timing reference on any other satellite system, unless you wanted to get like millimeter-level accuracy using nothing but satellites (and that would require a lot more than just a better clock on the sats).

The clocks aren’t even the biggest source of error in the pseudo ranges.

With nuclear clocks, GPS will be unnecessary.

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