The new concept proposed during the development of the first Russian orbit-borne scatterometer SCAT-3 requires an additional study for estimating its efficiency and comparison with the current scatterometer concepts. Using the fan antenna pattern (with angular dimensions 1◦ × 6◦), we have reduced the antenna rotation speed by about a factor of three compared with the prototype (the “SeaWinds” scatterometer) and measured the backscattering cross section for each wind cell at the horizontal and vertical polarizations. The numerical model of the scatterometer was developed with allowance for the technical characteristics of the radar, orbital parameters, and observation scheme. The scatterometer operation is simulated with the subsequent swath formation and partitioning into the wind cells. It is shown that using the fan pattern in the scatterometer, one can improve the accuracy of the wind-direction r5etrieval in a wind cell due to employing the radiometric resolution in the processing algorithm. The main error in determining the wind direction is related to the ambiguity ±180◦, which is caused by the type of the azimuthal dependence of the backscattering cross section. With the help of the two-dimensional median filtering, we can significantly reduce the wind-direction retrieval error. This error can probably be smaller than that for the current scatterometers.

We have chosen a “SeaWinds” scatterometer with an orbital altitude of about 800 km as a prototype of the first Russian orbital scatterometer. An involuntary decrease in the orbit altitude to 650 km made us choose between conservation of the initial swath width 1800 km or the incidence angles with the swath-width decrease to 1500 km. A wider swath width has the advantage of a better coverage of the world-ocean surface. However, it leads to an increase in the local incidence angles and, hence, a decrease in the reflected-signal power. As a result, the signalto-noise ratio decreases and an error in the wind velocity and direction reconstruction because of the equipment noise increases. The error of the wind-velocity vector reconstruction for the same drive and antenna is the choice criterion. During the study, the mathematical model of the scatterometer is developed, the numerical simulation for both swath widths is performed, the data are processed, and the reconstruction accuracies of the wind velocity and direction are compared. It is shown that the reconstruction accuracy can significantly be improved if the measurement for two polarizations is used. The results obtained also show that the wind velocity is sufficiently well reconstructed for both swaths, while the wind-direction reconstruction accuracy in the case of a wider swath is worse than that required by the technical specifications for the scatterometer. Therefore, the swath width of the new scatterometer should be 1500 km.

Orbital scatterometry is briefly overviewed and its trends are indicated. Two scatterometer concepts are currently considered for trade-offs: with fixed and rotating antenna systems. The concept with a rotating antenna system was selected and SeaWinds was chosen as the prototype for the first Russian scatterometer. The scatterometer concept was then further developed and instead of two pencil beams, a fan-beam antenna was proposed about 1° × 6°. The fan-beam antenna allows successive measurements for horizontal and vertical polarization in each wind vector cell WVC. This increases the number of observations of the WVC at different incidence and azimuth angles during flight. The scatterometer parameters required to implement the proposed measurement geometry for an orbit altitude of 650 km and a swath width of 1525 km are discussed. A numerical scatterometer model that accounts for both the specifications and the observation geometry is developed. The scatterometer performance, with subsequent formation of a swath and splitting into WVCs, is simulated. The procedure of wind vector retrieval includes two stages: 1 determining wind speed and wind direction in a single WVC; and 2 using the information from adjacent WVCs to correct wind direction. It is shown that the accuracy of wind direction retrieval by a WVC can be increased by simultaneous radar cross-section RCS measurements at vertical and horizontal polarization. The basic error in determining wind direction is due to a 180° wind direction ambiguity caused by the form of RCS azimuth dependence. Two-dimensional median filtering is commonly employed in scatterometry to increase the accuracy of wind direction retrieval. In this study, two-dimensional angular median filtering was employed and it is shown that the error in wind direction retrieval significantly decreased. The results of the research indicate that wind field can be retrieved by the new scatterometer with the level of precision required.

This study continues our previous explorations of the new space-borne scatterometer concept. Using a fan-beam antenna and simultaneous RCS measurements at the horizontal and vertical polarizations, one can improve the accuracy of wind speed and wind direction retrieval.