The prime objective of Advanced SCATterometer (ASCAT) is to measure wind speed and direction over the oceans.
Scatterometer data has also 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 behaviour.
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.
With the rapid global coverage, day or night and all-weather operation, ASCAT offers a unique tool for long-term climate studies.
What is ASCAT?
ASCAT is a real aperture radar, operating at 5.255 GHz (C-band) and using vertically polarised 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 analysed and detected. In the power spectrum, frequency can be mapped into slant range, provided the chirp rate and the Doppler frequency are known. The processing is, in effect, a pulse compression, which provides range resolution.
From around 837 km altitude, the instrument transmits well characterised pulses of microwave energy towards the sea surface. Winds over the sea cause small scale (centimetric) disturbances of the sea surface which modify its radar backscattering characteristics in a particular way.
These backscattering properties are well known and are dependent on both the wind speed over the sea and the direction of the wind, with respect to the point from which the sea surface is observed.
On board ASCAT, two sets of three antennas measure the resultant electromagnetic backscatter from the wind-roughened ocean surface, in two 500 km wide swaths, on each side of the satellite ground track. The three antennas on each side are oriented to broadside and ± 45° of broadside, and, so, make sequential observations of the backscattering coefficient of each point of interest from three directions. The three directions are needed to resolve the wind direction ambiguity.