Anyone want to know how GPS works?

[Eriku]

Tbh I'm more interested in knowing what the deal with the girl with the fish is...

with.

[Santa_Claus]

mine uses batteries.

[Eriku]

What, your fish?

Is it one of those which sing a shitty tune when you clap?

[leo-rcc]

How can GPS be be used for altimeters?

[piscator]

How can GPS be be used for altimeters?

Sorry this took so long....


"Pseudoranges" (ie the trilateration distance measurements observed earlier) from at least 4 SVs are required for a receiver to compute a 3D fix on a reference ellipsoid, and from there to pretty much any other defined surface, like "Sea level".
The short (and very belated) answer to your question is that your receiver uses a minimum of 2 (3D) triangles.

It's important to remember that the sats are in a dynamic 3D array, and that some geometries are better than others, but they are all transient. GDOP (geometric dilution of precision) is a rough inverse calculation of how well your receiver can resolve its position on the Z-axis at a given time and place. The less dilution, the stronger the geometry. Drink it down.

[charlou]

*interrupts*

Nice to see you again, piscator.

*carry on ... *

[piscator]

[Pensioner]

Do the clocks run slower on the satellites to take account of special relativity.

[Feck]

Do the clocks run slower on the satellites to take account of special relativity.

And who goes up to put them forward an hour in the spring and one back in autumn ?

[Pappa]

Do the clocks run slower on the satellites to take account of special relativity.

And who goes up to put them forward an hour in the spring and one back in autumn ?

Midgets.

[Pensioner]

Do the clocks run slower on the satellites to take account of special relativity.

And who goes up to put them forward an hour in the spring and one back in autumn ?

Midgets.

Ginger welsh guys do it, do they?

[piscator]

Do the clocks run slower on the satellites to take account of special relativity.

Oh yes... SV clocks need to time sync to within a few nanoseconds to broadcast valid position data to themselves and you, yet they are in a different spacetime curvature zone and have very high velocities relative to your handheld receiver.

Relativistic corrections in all 3 segments of the GPS system (space, control, user) make GPS usable. The clock corrections alone enhance the accuracy of your cell phone GPS by a factor of over a thousand.


The pixies are there to help us find meaning in it all

[Orwellian]

i have a degree in geodesy, so chances are i can answer your questions or bullshit my way through them.

Satellites send signals to the GPS units on the ground Measuring the distances to the known points (the satellites) the relative location of the GPS unit on the ground can be calculated using geometry. Basically, think of an imaginary sphere around the satellite with the satellite at the center. Based on the time it takes the signal to get to the GPS unit, its position on the imaginary sphere can be located. To get the GPS unit's location in 3 dimensional space, you need 4 satellites, I believe. And the location is the position on each of the imaginary circles intersecting at the GPS unit. Or, something like that.

[piscator]

^^ Yeah. What he said!

'Cept I like to think of a point in space surrounded by ellipses representing the orbits of SVs. This point, by something more than happy coincidence, happens to orbit the sun in the approximate center of Earth's mass and lets clever geomancers originate what they like to call an "Earth-Centered, Earth Fixed" coordinate system shaped something like an elliptic sphere or symmetrical egg.
From a reference surface centered in this coordinate system,, other coordinate systems more closely resembling actual or equipotential surfaces (like sea level) can be derived or translated for a given point in time.