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Naive question: what’s the benefit here to scan downward (from the satellite position) over upward, from the ground?

If the question is about satellite vs ground instrument: the geographic coverage from the satellite is much greater. Geostationary instruments over Europe cover the Atlantic Ocean, Europe, Africa, the Middle East.

If that was not the question, can you provide more detail?

Thanks, this is indeed the question. Thinking out loud: the coverage is probably somewhat conic therefore if you want to scan the ground or lower atmosphere an high altitude is optimal, while scanning the upper atmosphere could be done from the ground.

Perhaps earth's spherical shape gives an advantage to the satellites in both cases ?

Maybe, though a GEO satellite (or really any satellite) will always be much much farther from even the upper atmosphere than the ground will be, so satellites have a pretty dominant coverage advantage.

You can’t drive your sensor bank everywhere on the surface of the earth to point up.

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