This paper reviews the history and the current status of the relationship between the backscattering coefficient σ° and the wind-speed u at microwave frequencies. When σ° is assumed to be proportional to uγ, measurements have indicated that γ lies in the vicinity of 1.4 to 2.0. Under similar conditions with incidence angle between 30° and 80°, γ for HH polarization is usually larger than that of VV polarization and γ for upwind or downwind is larger than that of the crosswind. Better surface truth and controlled experiments are still needed to obtain a more specific value for γ under a given condition. Theoretical modeling of radar sea scatter indicates that near vertical incidence the physical optics method is applicable while for incidence angles approximately in the range 30° to 80° the return can be explained by Bragg scattering due to capillary waves when the tilting effect of the large-scale waves is also included. The methods used in extracting wind vectors from SEASAT scatterometer measurements are outlined. Areas where further experimental and theoretical studies are needed are indicated.

Radar determination of winds at sea

The capability of ERS-1 wind scatterometer (WSC) data for land surface studies at regional and global scales is investigated. Calibrated data, available since April 1992, consist of backscattering coefficient /spl sigma//spl deg/ values estimated over a 50 km ground resolution cell within a 18-59/spl deg/ incidence range. The WSC covers latitudes between 80/spl deg/S and 85/spl deg/N. Data quality is investigated. Results show a good radiometric stability throughout a two-year period (April 1992-March 1994), a good measurement accuracy and a very good intercalibration between antennas. A global backscattering coefficient image of the world is presented and statistics of /spl sigma//spl deg/ are extracted for the main land surfaces. A strong dependence between this image and global vegetation and elevation map is observed. Finally, WSC and Seasat-A Scatterometer System (SASS) data of typical regions are compared. Results indicate the high capability of ERS-1 WSC data for land surfaces monitoring at global and regional scales.

Use of ERS-1 wind scatterometer data over land surfaces

ERS-1 wind scatterometer (WSC) data is analysed over a wide range of terrain types for the period May 1992-April 1994. Comparison is made with Global Vegetation Index (GVI) data for the monitoring of vegetation dynamics. Results show that WSC data display a well-pronounced seasonality over most vegetated surfaces. The highest sensibility to vegetation dynamics is found over semi-arid regions and boreal zones. In these two cases, there is a marked seasonality in environmental parameters which is well depicted by σ0 temporal profiles. However, the sensibility of the ERS-1 response is much less pronounced over densely vegetated surfaces. In yspite of its low spatial resolution, the usefulness of a C-band scatterometer for monitoring vegetation dynamics is shown.

Monitoring global vegetation dynamics with ERS-1 wind scatterometer data

This paper describes the methods used to develop performance requirements and design characteristics for the microwave scatterometer (SASS) ocean-surface wind sensor on the NASA SeaSat-A satellite. Wind vector measurement requirements from the SeaSat user community such as wind speed and direction accuracy, resolution cell size, grid spacing, and swath width formed the basis for defining instrument characteristics. The resulting scatterometer is designed for 14.6 GHz using four fan beam antennas to measure wind speed and direction over a 1000-km swath width with a resolution cell size 50 \times 50 km. Results presented show scatterometer accuracy satisfies user requirements for wind speed from 4 m/s to greater than 24 m/s for the nominal SeaSat-A orbit of 790 km altitude, 108\deg inclination, and 0.001 eccentricity.

The SeaSat-A satellite scatterometer

Seasat—A 25-year legacy of success

The Skylab microwave radiometer-scatterometer experiment S-193 was designed to collect simultaneous information on the radiometric emission and the backscatter from the earth from orbital altitude for the first time. A primary driving force was ascertaining the response from the oceans of both active and passive systems to variations in wind, and the ability of the passive system to aid in calibrating the active system. Numerous overland applications were also explored. Preliminary results of backscatter and emission from sea and atmosphere are shown for hurricane Ava and for two flights across the Gulf of Mexico. The results tend to confirm the most recent theoretical prediction of an approximately square-law relation between scattering coefficient for horizontal polarization and windspeed, with a somewhat smaller dependence for vertical polarization. Overland measurements show that scatterometer signals at 30 deg increase and radiometer signals decrease for terrain that has recently experienced rainfall.

N. M. Hatcher

Papers

Simultaneous active and passive microwave response of the earth - The Skylab radscat experiment