ASCAT (Advanced Scatterometer)
Mar 7,2026

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Introduction

ASCAT (Advanced Scatterometer) is a active microwave radar instrument carried aboard the MetOp series of polar-orbiting satellites operated by EUMETSAT . As a real aperture radar scatterometer, ASCAT transmits C-band (5.255 GHz) microwave pulses and measures the backscatter from the Earth's surface to derive key geophysical parameters . The primary operational application is ocean surface wind vector retrieval, providing measurements of wind speed and direction over global oceans . Additionally, ASCAT data is used for soil moisture monitoring over land surfaces and sea ice detection . The instrument features two independent swaths of backscatter measurements, enabling wide spatial coverage and frequent revisit times.

File Structure

ASCAT data products are distributed in multiple formats and processing levels depending on the specific geophysical parameter and application. The main components are as follows:

Level 2 Ocean Surface Wind Vector Products: Distributed in netCDF version 3 or 4 format . Each file typically corresponds to one complete orbit (approximately 100 minutes) or 3-minute orbital granules, with the orbit beginning defined near the South Pole . Files contain wind vector retrievals processed using geophysical model functions such as CMOD5.n or CMOD7 .
Backscatter Data (Sigma-0) : The fundamental measurements include normalized radar cross-section (σ0) values at multiple incidence and azimuth angles, stored within the netCDF structure. A Hamming filter is applied to spatially average the Level 1 Sigma-0 data over swath grid cells (25 km or 12.5 km resolution) .
Grid Structure: Data is organized along-track and across-track within the instrument swath. Standard products include 25 km sampling resolution (effective resolution ~50 km) and 12.5 km sampling products . Global coverage extends from -89.6° to 89.6° latitude and -180° to 180° longitude .
Metadata: Includes satellite platform identifier (MetOp-A, B, or C), orbit number, processing time, geophysical model function version, quality flags, and spatial coverage information .
Soil Moisture Products: Distributed as time series at discrete grid points defined on a sinusoidal projection, aggregated into cells that are rectangular in latitude-longitude projection. Data for each grid cell is bundled as zip files containing all grid point time series within that cell .
Enhanced Resolution Image Products: Available in BYU .SIR file format (gzipped for storage efficiency), created using the Scatterometer Image Reconstruction (SIR) algorithm with filtering . These products achieve higher spatial resolution (4.45 km pixel spacing) by combining multiple satellite passes, at the cost of reduced temporal resolution .
Coordinate Reference System: Level 2 wind products typically use the EPSG:4326 geographic coordinate system (WGS 84) . Soil moisture products use sinusoidal projection for grid definition .

Pros

All-weather capability: Active microwave sensing enables data acquisition through clouds and independent of solar illumination.
Dual-swath design: Provides wide spatial coverage (two 550 km swaths) with gap-free measurement between swaths.
Climate data record stability: Long-term reprocessed datasets (since 2007) provide consistent wind vector climate data records suitable for climate studies .
Near-real-time availability: Products available approximately 2 hours after measurement for operational applications .
Multiple resolution options: 25 km and 12.5 km products allow flexibility between noise levels and spatial detail .
Free and open access: All intellectual property rights belong to EUMETSAT, but products are granted to every interested user free of charge (with copyright attribution required) .

Cons

Coarse spatial resolution: Native resolution (25-50 km) limits applications in coastal zones and small-scale features .
Land contamination: Near-coastal wind retrievals contain less geophysical information due to land contamination in the measurement footprint .
Data volume: Full orbit files in netCDF format can be large for extensive time series analysis.
Complex processing required: Geophysical model functions (CMOD algorithms) require specialized knowledge for optimal use.
Limited temporal sampling: Polar orbiting platform provides ~2 passes per day at mid-latitudes (ascending and descending), limiting diurnal cycle characterization .
Standardization variations: Different processing centers (EUMETSAT OSI SAF, NOAA, KNMI) may use slightly different algorithms or format variations .

Application Scenario

ASCAT (Advanced Scatterometer) data is primarily utilized in marine meteorology and oceanography for operational ocean surface wind monitoring, supporting weather forecasting, storm tracking, and marine safety services . In climate research, long-term ASCAT wind climate data records enable studies of global wind patterns, trends, and air-sea interactions . Over land, surface soil moisture products derived from ASCAT backscatter measurements support hydrological modeling, drought monitoring, and agricultural applications . The data is also employed in sea ice monitoring and polar research, where the enhanced resolution image products (4.45 km pixel spacing) enable detailed observation of ice edge dynamics and polar processes . Numerical weather prediction centers assimilate ASCAT wind data to improve forecast accuracy, while the research community utilizes the dataset for validating climate models and studying ocean-atmosphere coupling mechanisms.