| | | I'll try to be clear. Any signal measured is not the real signal. Ground thermometers are not mercury thermometers, they are thermistors. They measure a signal/blip. Sensors on satellites are tuned to measure certain radiative bands of oxygen in lower, middle, and upper troposphere & stratosphere. Obviously the radiative bands are dependent upon temperature, and so are the blips that are measured.
How you hear an ambulance siren depends on its distance and speed to/away from you. That wave pattern is a doppler shift. On a satellite, how a sensor measures blips depends on speed and distance, which given the # of satellites and GPS capabilities, it's tedious but accurate geometry.
Over time, satellite sensors decay from being exposed to space radiation. That's problem 1. Problem 2 is that their orbits change given that density of extreme portions of the upper atmosphere vary greatly with the sun. That's orbital decay in a vertical sense.
Another problem is drift on a horizontal sense (east/west direction). So for year 1, a satellite is directly over Atlanta, GA, at 0700Z every day. In Year 5, the satellite drifts and actually goes over Atlanta, GA, at 0730Z. The change in geometry can be accounted for, however diurnal impacts cause the measured blips to be different...whether the sun just sets/rises at this time. Anyways, accounting for this drift requires a 'diurnal drift adjustment.' |
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