SCA A Spot



Supporting numerical weather prediction, climate monitoring, nowcasting, and ocean state forecasts

SCA A Spot
SCA A Spot

The Scatterometer (SCA) is a real aperture, side-looking radar, emitting microwave radiation towards the Earth's surface and measuring the energy scattered back.

Last Updated

20 June 2022

Published on

19 May 2020

EPS-SG Scatterometer Instrument
Figure 1: Illustration of the SCA instrument (Credit: ESA)

The SCA has some heritage from the ASCAT instrument onboard EPS.

The primary objective of the Scatterometer mission is to support numerical weather prediction (NWP), climate monitoring, nowcasting, and ocean state forecasts at regional and global scales by measuring backscatter cross sections of the sea surface, which carry information about wind vectors.

The secondary objective is to provide information from backscatter signals on land surface soil moisture, leaf area indices, snow-water equivalents and sea-ice types to numerical weather prediction, climate monitoring, and hydrology applications

The basic measurement taken is often called radar backscatter, Normalised Radar Cross Section (NRCS) or σ0. Scatterometers are all weather instruments where the measured σ0 corresponds to different geophysical surface properties over ocean, land or sea ice and at different penetration depths depending on the surface.

The SCA instrument, with a carrier frequency of 5.355GHz (C band), will provide a breakthrough, compared to ASCAT, by delivering information at a doubled horizontal resolution, a widened swath width of 2×660km, and the addition of cross-polarisation measurements. The doubled horizontal resolution will provide oceanic wind vectors closer to the coast lines, further enhancing spatial coverage beyond the increased swath width. The cross-polarisation supports measurements at higher wind speeds without saturation, for the benefit of observing tropical and extra-tropical storms.

Measurement principle

SCA is equipped with six array antennae, each of them consisting of four panels fed by a network of waveguides. The antennae are accommodated on three antenna assemblies. With respect to the spacecraft flight direction, these three antenna assemblies are oriented to broadside (mid beam) and ± 45° from broadside (fore and aft beam, respectively) so that they allow for sequential observations of the backscattering coefficient of each point of interest on the left and the right swath from three directions (see figure).

EPS-SG Scatterometer Overall Geometry
Figure 2: Scatterometer measurement geometry

SCA operates in dual polarisation. The fore and aft antennas transmit and receive vertically polarised waves, however, the two mid antennas can transmit and receive both the vertical (V) and horizontal (H) polarisation. This allows the SCA instrument to provide up to six types of measurement for each swath: fore VV, mid VV, mid VH, mid HV, mid HH and aft VV. Each of these measurements are referred to as a beam or channel.

All the fan antenna beams form a narrow azimuth pattern and a relative wide elevation pattern resulting in a wide swath width (around 660km) parallel to the sub-satellite track. The swaths correspond to incidence angles ranging from approximately 20° to 53.7° for the mid beam and 28.4° to 65° for the side beams. In addition to the full resolution backscatter product, inside each swath the backscatter in each beam is spatially averaged at a set of points on the Earth’s surface in order to produce a resampled product providing collocated measurements suitable for the retrieval of wind vectors and soil moisture index. This generates a regular grid of points, called nodes, with a spacing of around 12.5km in the across track and along track directions.