ASCAT overview
ASCAT is the Advanced Scatterometer instrument which will fly as part of the MetOp payload. Scatterometers are instruments which are used to infer data on wind speed and direction from radar measurements of the sea surface.
ASCAT relies for its operation on the fact that 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.
From around 800 km altitude, the instrument will transmit well characterised pulses of microwave energy towards the sea surface and will receive and record the resulting echoes for transmission to the ground station. In order to derive both wind speed and direction, the instrument relies on the motion of the satellite and three antennas oriented to broadside and ± 45° of broadside, to make sequential observations of the backscattering coefficient of each point of interest from three directions. The three radar measurements can then be applied to the backscatter/wind velocity model to deduce the wind speed and direction.
In practice, ASCAT will employ two sets of three antennas to make observations in two 500 km wide swaths, on each side of the satellite ground track.
Like its predecessors, the Active Microwave Instruments (AMI) of ERS-1 and ERS-2, ASCAT operates at a frequency in C-band (5.255 GHz) and will provide a day- and night-time measurement capability which is unaffected by cloud cover. The instrument will provide almost complete global coverage of the World's oceans in a period of around 2 days.
Last update: 23 December 2005 | 
