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Comment by 0n34n7 | Hacker News Reader

There is no GPS in orbit. You need a point of reference and stars are pretty (99.99x10^zillion)% reliable, and yes, the SRS used the same idea.

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who gave you that impression - ofc there is GPS in orbit. it is just that the usual commercial GPS software refuses to deliver coordinates above a certain height or a certain speed. this is ostensible to prevent GPS from being used in non American ICBM and cruise missile weapons.

the GPS birds are in a 20k km orbit and you should be able to use them just fine at lower orbits (starlink sats are planned to be in several fairly low orbit shells 320/550km).

I suspect you can even use GPS all the way out to say the moon if you are a bit creative. (catching GPS sats coming aroud the earth for positioning, or using a special dish antenna to get reflected GPS signals)

> 20k km

A double metric "kilo" prefix like that feels awkward. I know it's common, but I've always felt that we should just say "megameter" instead. (Also, a metric ton should just be called a "megagram".)

Agreed with megameter, but disagree with tonne: the kilogram is the base unit that physical constants etc. are measured against (e.g. Newton's constant is measured in Newton meter squareds per kilogram squared). It can be less confusing to talk about microkilograms than grams when doing physics.

I wonder if I will live long enough to see GPS coverage extended to the full surface of Luna, perhaps with some transmitters in lunar orbit.

How directional are the GPS antennas? They look like the point earthwards, but I suppose you could pick up some signal above them as well.

Wouldn't reflected signals be inaccurate due to additional time of flight?

Well yeah, you can't just duct tape a TomTom to it. But you can buy GPS receivers designed for space.

https://gdmissionsystems.com/en/communications/spaceborne-co...

There is still GPS in leo. It can use more power than some smaller satellites can budget for is a reason it's not always used.

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