4.9 Microwave Scatterometer

Microwave scatterometers can measure the received power of surface backscattering reflected from the surface of objects. According to a narrow definition, a microwave scatterometer may be a space borne sensor to measure the two dimensional velocity vectors of the sea wind, while according to the wider definition, it also involves air-borne sensors, as well as ground based sensors to measure the surface backscattering as well as volume scattering, such as rain radar.

Microwave scatterometers are classified as two types, pulse type and continuous wave type (CW). The pulse type uses wide band which has restrictions in obtaining a license to operate and in avoid obstructions. CW type has the advantage that the band width can be reduced to 1/100 times that of the pulse type and the price becomes cheaper.

SEASAT-SASS (Seasat-A Satellite Scatterometer) is one of the typical scatterometers. SASS has four fixed antennas to transmit the pulse in a fan beam of 14.5 GHz to four different angles, and to receive the backscattering in subdivided cells through a Doppler filter. Figure 4.9.1 shows the four beam patterns and the incident angles. In accordance with the satellite flight, the same cell of sea area can be observed from both fore beam and aft beam with 90 different angle, which enables the determination of wind direction and wind velocity.

Figure 4.9.2 shows a ground based microwave scatterometer with a rotation system.

ERS-1-AMI Wind Mode (European Remote Sensing Satellite-1-Active Microwave Instrument) and ADEOS-NSCAT (Advanced Earth Observation System-1 NASA Scatterometer) will be available for measurement of velocity vectors of the sea wind with three antennas looking fore, side and aft directions.

Table 4.9.1 compares the basic functions of ERS-AMI Wind Mode and ADEOS-NSCAT. Both have functions to measure wind velocity of the sea wind with an accuracy of 2 m/s or 10 % of the waveheight , and wind direction with an accuracy of 20 .


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