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The purpose of the Wind Scatterometer is to obtain information on wind speed and direction at the sea surface for incorporation into models, global statistics and climatological datasets. It operates by recording the change in radar reflectivity of the sea due to the perturbation of small ripples by the wind close to the surface. This is possible because the radar backscatter returned to the satellite is modified by wind-driven ripples on the ocean surface and, since the energy in these ripples increases with wind velocity, backscatter increases with wind velocity.


The three antennae generate radar beams looking 45deg. forward, sideways, and 45deg. backwards with respect to the satellite's flight direction. These beams continuously illuminate a 500 km wide swath (see the figure) as the satellite moves along its orbit. Thus three backscatter measurements of each grid point are obtained at different viewing angles and separated by a short time delay. These "triplets" are fed to a mathematical model which calculates surface wind speed and direction. The main technical characteristics of the Wind Scatterometer are listed below:

Spatial resolution >=45 km (along and across track)
Radiometric resolution (4 m/sec) <=8.5% (mid beam)
<=9.7% (fore/aft beam)
Radiometric resolution (24 m/sec) <=6.5% (mid beam)
<=7.0% (fore/aft beam)
Radiometric stability CMIS: <=0.57 dB
IIS: <=0.46 dB
Cross polarisation >=15 dB
Swath width >=500 km
Swath stand-off 200 km to right of sub-satellite track
Localisation accuracy +-5 km (along and across track)
Wind direction range/accuracy 0 - 360deg. / +-20deg.
Wind speed range/accuracy 4 m/s - 24 m/s / 2 m/s or 10 %

A transmit pulse is produced by the Scatterometer Electronics and amplified by the IF Radar unit, converted to an RF signal in the transmitter/converter unit and amplified by the High-Power Amplifier. The transmit signal is routed to the correct antenna by the Circulator Assembly which in this mode is under the control of the Scatterometer Electronics.


The received signal is down-converted, amplified by the IF Radar and routed to the Scatterometer Electronics. A measurement sequence of 3.763 seconds (see the figure) corresponds to 25 km along the sub-satellite track at a satellite altitude of 785 km and is continuously repeated in the wind mode without any gap. This sequence involves four sets of measurements, regularly spaced, for each antenna beam (fore, mid and aft). Each series corresponds to 32 measurement pulses on each beam. Noise measurements and internal calibration are regularly performed in the interval between the transmitted pulse and the reception of the return echo.

For the mid-beam, the return echo is filtered and sampled in complex form I and Q, while for the fore and aft-beams, as the doppler variation is significant over the swath width (20 KHz near swath to 140 KHz far swath), a programmable doppler compensation law is applied to the received signal before filtering and complex sampling.

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