An Operational Analysis Ready Radar Backscatter Dataset for the African Continent
2.1. DE Africa’s Sentinel-1 Dataset
2.2. Input and Ancillary Data
2.3. Radiometric Terrain Correction
2.4. Metadata Generation
- A data mask identifying no-data, valid and radar shadow (invalid) pixels. No-data pixels are those outside the scene boundary. The radar shadow mask maps pixels inside and near radar shadow where the backscatter coefficient normalization is not reliable. The radar shadow is first defined in the GRD pixel grid as where normalized scattering area is smaller than 0.05. This binary shadow mask is dilated by 1 pixel using a maximum filter and then nearest neighbor resampled into the range Doppler projected output grid coordinate. This ensures all output pixels potentially affected by radar shadow are masked.
- A local incidence angle image. This is the angle between the radar antenna and the normal of the local ground surface calculated for each pixel. For each output pixel, an angle is computed against the ground topography defined by the DEM.
- A scattering area image. The output area values are bi-linear interpolated into output pixel grid from the unitless normalization factors () that have been used to convert to in the GRD pixel grid. This area image provides information on the terrain relative to the radar illumination geometry. On the fore-slope (slope facing the sensor), scattering area is large and foreshortening occurs hence effective ground resolution is lower than the nominal resolution. On the back-slope, scattering area is small (or close to zero in the shadow) and a larger number of pixels in radar geometry are mapped to a smaller number of pixels in the ground geometry resulting in a higher effective resolution on the ground. The scattering areas are therefore inversely correlated with effective resolutions and can be used as weight when combining images from different viewing geometries . This scattering area can also be used to derive equivalent incidence angle. As shown in Equation (3) in , for an ellipsoidal Earth model, the ratio of reference area to reference area is equal to tangent of the incidence angle. For RTC, each illuminated area does not correspond to a single incidence angle, hence may serve as a proxy when an angle estimate is needed.
3.1. Radiometric Terrain Correction
- Minimal artifacts near radar shadow. Inside and near the radar shadow, the scattering area is either zero or a small value, so a small uncertainty in the scattering area leads to large uncertainty in . Pixels that are potentially affected by this uncertainty are masked out and we do not see obvious artifacts on the edge of the shadow masks (Figure 2c,f).
- Minimal dependence on look angle. For this mountainous area, local incidence angle changes significantly within a scene and between descending and ascending orbits (Figure 2a,d). The normalized backscatter measurements (Figure 2c,f) and the difference between measurements from descending and ascending orbits (Figure 2h) do not show variation correlated with the terrain facets.
3.2. Geometric Accuracy
3.3. Data Access through DE Africa Platform
4. Discussion and Example Applications
4.1. Water, Coast, and Wetland
4.2. Crop Phenology
4.3. Forest Monitoring
Data Availability Statement
Conflicts of Interest
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|Centre frequency||5.405 GHz (C-band)|
|Acquisition mode||Interferometric Wide swath (IW)|
|Polarization||VV + VH|
|Coordinate reference system||WGS 84 (EPSG:4326)|
|Pixel spacing||(≈22.2 m at the Equator)|
|Pixel grid origin|
|Dimension||ALE (m)||RMSE (m)|
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Yuan, F.; Repse, M.; Leith, A.; Rosenqvist, A.; Milcinski, G.; Moghaddam, N.F.; Dhar, T.; Burton, C.; Hall, L.; Jorand, C.; Lewis, A. An Operational Analysis Ready Radar Backscatter Dataset for the African Continent. Remote Sens. 2022, 14, 351. https://doi.org/10.3390/rs14020351
Yuan F, Repse M, Leith A, Rosenqvist A, Milcinski G, Moghaddam NF, Dhar T, Burton C, Hall L, Jorand C, Lewis A. An Operational Analysis Ready Radar Backscatter Dataset for the African Continent. Remote Sensing. 2022; 14(2):351. https://doi.org/10.3390/rs14020351Chicago/Turabian Style
Yuan, Fang, Marko Repse, Alex Leith, Ake Rosenqvist, Grega Milcinski, Negin F. Moghaddam, Tishampati Dhar, Chad Burton, Lisa Hall, Cedric Jorand, and Adam Lewis. 2022. "An Operational Analysis Ready Radar Backscatter Dataset for the African Continent" Remote Sensing 14, no. 2: 351. https://doi.org/10.3390/rs14020351