One of the very best ways to understand the changes taking place on Planet Earth is to make observations from space.
And to get a true sense of any trends, you really need those measurements to be long-term and unceasing.
Preferably, you use the same type of instrument to make the observations, and when, inevitably, you're required to replace aging equipment, you do so in such a way that the new system can be cross-calibrated with the old.
Few Earth observation programmes get as close to this gold standard as Landsat, the cooperative space mission run by US space agency (Nasa) and the US Geological Survey.
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