Bolivian Salt Flats Aid Spacecraft Calibration

PCOL writes "Salar de Uyuni is a vast plain of white salt in the mountains of Bolivia, with a total elevation range of less than 80 centimeters - the flattest place on earth. Beginning in 2002, geophysicist Adrian Borsa led a survey that resulted in precise GPS measurements of the salt flat. The flats will be used as a giant calibration device for satellite-based radar and laser altimeters on the CryoSat recovery mission so the spacecraft can more precisely monitor changes in the elevation and thickness of polar ice sheets and floating sea ice. 'Satellites can calibrate their altimeters by bouncing signals off the ocean surface .. because of atmospheric interference, tides and waves, there are uncertainties. Borsa says the salar, now so accurately mapped and with dry, clear skies, is about five times better than the ocean as a reference point.'"

Comments

It's Saturday night

[Dunbal]

And I have probably had a bit too much to drink, so forgive the deeply philosophical question.

      When they mean that it's the "flattest place on Earth", do they mean that it conforms exactly to the curvature of the earth (thus not REALLY flat but earth shaped sort of flat), or is it FLAT flat, as in a chord across the curvature of the earth at that point...

      Sorry, just trying to work out the meaning of "flat" on a round planet... blame the rum.

Re:It's Saturday night

[maxume]

It is somewhat important that the article mentions the 80cm variation in terms of elevation.

In order to be straight line flat, the elevation would have to vary something like 200 meters, based on some rough calculations I did in trying to thing about this. Basically, I assumed that the salt flat is ~100km across, so the each edge is ~50km from the center, and would have an elevation given by the right triangle formed by the radius from the center of the earth and a 50km tangent line(the hypotenuse would be the elevation at the edge). (6300**2+50**2)**0.5=6300.198, subtract the radius, and you are left with 0.198 km, or ~200 meters). This is a simple model of the earth's shape, but it's within a factor of 3 or something.

Re:It's Saturday night

[TimeTraveler1884]

Usually when anything of a GIS nature refers to something as "flat", it is in relation to the WGS84. My understanding is that geodetic systems basically project an perfect, elliptical sphere around the the gravitational center on earth. But I probably have had too much to drink myself so don't hold me to it.

Here's to spending way too much time playing with GPS! Cheers!

Re:It's Saturday night

[goodmanj]

Bear in mind these are small differences: if you could make a perfect scale model of the sea-level surface the size of a billiard ball, it would be rounder and smoother than the ball.

Mind if be pedantic? Not quite true. The difference between pole and equatorial radii at sea level is 22 km. Add in the height of Mt Chimborazu and the depth of the ocean near the South Pole, and we find that Earth deviates from a sphere by about 33 km, and so it's spherical to within +/- 0.26%.

The Billiard Congress of America requires billiard balls to be 2.25" in diameter, to an accuracy of +/- 0.005", or +/- .022%.

So, the Earth doesn't quite pass muster as a billiard ball.

"Give me a pool cue large enough and a place to stand, and I shall sink the Earth in the corner pocket." -- Archimedes Fats

Tearing it up

[BWJones]

Yeah, just don't let Boyd Coddington anywhere near those salt flats. Look what he did to the Bonneville Salt Flats this year when I was out taking photos.

The Great Salt Flats in Utah are flatter...

[JRHelgeson]

The dried Bonneville Salt Flats (open to the public) and its attached military only area called the Dougway Proving Grounds are the flattest place on earth. They have been used by the military for the past 40+ years to calibrate space and weapons systems. Pretty much every land speed record has been made at the Bonneville Salt Flats, including breaking the sound barrier. The variation of altitude is so minimal that it is within the accuracy of the measurement equipment used to calibrate altitude variations, but it has been certified to be less than 1 foot of elevation for every 10 miles.

And every year it gets 'reset'... The springtime runoff from the surrounding mountains will cover the entire salt flats with a perfect 1/2" of water. It is SO COOOL to go out there when there is a *PERFECT* sheet of water covering the salt, it looks like the worlds largest piece of glass. You can actually *SEE* the curvature of the earth. I have a picture of a much younger me 'walking on water' because it is so smooth you cannot tell that the water is only 1/2" (1.5cm) deep.

Working out on the salt flats, doing surveys, the survey crew would drive out 1 mile and hold up a survey marker. At five miles out we could not see them any more, we asked them to raise it up over their heads and we saw the marker rise up over the horizon like it was the sun coming up.

Because it is the worlds largest and flattest spot on earth, my father, an engineer in flight optics systems, has built optical calibration targets used by the military to calibrate autopilot systems, weapons guidance systems, terrain following radar systems, satellite optics systems and all that jazz for the military... which is why I grew up in Utah, am intimately familiar with the flats, and know without a doubt that my dad has worked on black projects that I hope someday he'll be able to tell me about (including flights into and out of the Janet terminal).