Page 8 - Spec Tech Vol 1 Issue 11
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SPACE SENSORS
Scatterometers are unique among satellite backscatter signal. The wind direction is found
remote sensors in their ability to determine by determining the angle that is most likely to
the wind direction over water. be consistent the backscatter observed from
Scatterometers can provide a wealth of wind multiple angles. In roughly 5 minutes, a
velocity observations over the earth's bodies satellite in a low polar orbit will move far
of water. These wind observations have a enough to view a point on water surface from
wide variety of applications including angles spanning 90 degrees. The
weather forecasting, marine safety, mathematical function describing the fit of the
commercial fishing, El Nino prediction and observed backscatter (as a function of the
monitoring, and long term climate studies. wind direction) usually has multiple minima
The exceptional accuracy of the recent (ambiguities). Ideally, the best fit corresponds
NASA Scatterometer (NSCAT) is leading to to the true direction of the wind. Typically, the
development of new applications. At next best fit is in approximately the opposite
COAPS, we have found that examining direction, and the next two minima are in
animations of scatterometer winds can directions roughly perpendicular to the wind
inspire new scatterometry applications. We direction. The process of selecting the
have generated wind animations (NSCAT, direction from among the multiple minima is
QSCAT) for 37 overlapping regions called ambiguity selection. Noise in the
spanning the global oceans. To date, all observations can change the quality of fit and
scatterometers have been active microwave thereby cause incorrect directions (also known
sensors: they send out a signal and as aliases) to be chosen. NSCAT ambiguity
measure how much of that signal returns selection has proven to be much better than
after interacting with the target. Microwaves previous scatterometers, with roughly 90%
are Bragg scattered by short water waves; successful selection of the correct ambiguity.
the fraction of energy returned to the Most of the problems with ambiguity removal
satellite (backscatter) is a function of wind occur for low wind speeds, where the signal is
speed and wind direction. The wind speed weak and easily confounded by noise.
can be determined from the strength of the
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