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Comment by comboy | Hacker News Reader

I can't get my head around how this thing works. Apart from the direct GPS signal it's able to also receive the part of it that was reflected by the ocean? How does the reflected signal tell anything about the wind speed?

And btw what are star trackers used for? As a fallback for GPS?

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The earth is bathed in radiation from GPS transmitters and these clever satellites are using doppler radar techniques to see how those signals are being reflected from ocean waves. From the doppler shift you can infer wind fields. Consumer GPS receivers only compute position from these signals, but there is a lot of interesting science you can do from the raw signals with a hacked GPS receiver.

Whoa, doppler shift from this mess from ocean waves, that's amazing. Looks like you registered just to reply, thanks a lot.

IIRC, star trackers are used as both a position and an attitude reference. GPS gives you only position (though more sophisticated systems can be used to determine attitude).

While a GPS outage is a minor inconvenience for most of us, it could be pretty catastrophic for spacecraft if they didn't have another position reference.

EDIT: See below - I was off; they're just attitude, not position

Star trackers are usually just used for attitude, not position. I've heard of research to use them in conjunction with a camera pointed at Earth to resolve coastlines to get position, but it's very theoretical at this point AFAIK.

For those of us just watching, what is attitude? At first I though spellcheck might have corrected it from altitude, but 3 times among 2 differently people I see attitude.

It's orientation, AFAIK, what direction it's facing.

Yep. Think of it like right ascension, declination, and roll from astronomy. Given attitude, your position, and the time, you can know where you are pointing on the Earth (lat/lon).

Altitude is how high your body is.

Attitude is which direction your face is pointing. Imagine a satellite meant to photograph the earth at the right altitude but the wrong attitude could just be taking nice shots of outer space.

What's the difficulty level of writing star tracker software for consumer grade CCD devices? I've always had writing and open sourcing a star tracker on my bucket list, similar to how this person did the same for a from-scratch GPS receiver [1]. Same for determining your position in space using pulsars as beacons [2].

[1] http://www.aholme.co.uk/GPS/Main.htm

[2] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201500...

Interesting, thanks. I wonder at what altitude GPS becomes ineffective? What then? We'll likely need some positioning satellites further out at some point? maybe at L4, L5 Lagrangian points?

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