ASCAT Overview

The Advanced SCATterometer (ASCAT) is on board the Metop-A satellite launched in October 2006, as well as the Metop-B satellite launched in September 2012. It is a real aperture radar, operating at 5.255 GHz (C-band) and uses vertically polarized antennas. It transmits a long pulse with Linear Frequency Modulation ('chirp'). Ground echoes are received by the instrument, and after de-chirping, the backscattered signal is spectrally analyzed and detected. In the power spectrum, frequency can be mapped into slant range, provided the chirp rate and the Doppler frequency are known. The above processing is in effect a pulse compression, which provides range resolution.

The prime objective of ASCAT is to measure wind speed and direction over the oceans. In addition, scatterometer data has proved to be very useful in a variety of studies, including polar ice and tropical vegetation. Water plays a unique role at microwave frequencies at which scatterometers are operated. It is the only naturally abundant medium with a high dielectric constant, so increasing the fraction of liquid water contained in soil, snow and vegetation increases the dielectric properties of these media, thereby significantly altering their scattering and absorption behavior. The backscattering coefficient, measured with scatterometers, is dependent on the dielectric properties of the soil surface layer, surface roughness, and vegetation. Thus, ASCAT provides useful data for ice and land applications, such as sea ice extent, permafrost boundary, desertification, etc. Because the scatterometer radar signal can penetrate the surface, ASCAT can also observe subsurface/subcanopy climate-related features (from www.eumetsat.int/Home/Main/Satellites/Metop/Instruments/SP_2010053161611647?l=en).

A more detailed description of the ASCAT instrument is given in Figa-Saldana et al. (2002) and Gelsthorpe et al. (2000). An overview of ASCAT data product can be found at oiswww.eumetsat.org/WEBOPS/eps-pg/ASCAT/ASCAT-PG-index.htm.