Radar Backscatter Satellite Data
Radar backscatter (σ⁰) is the intensity of microwave energy reflected from the Earth's surface back towards the radar sensor. Unlike optical sensors, which depend on sunlight and cloud-free conditions, Synthetic Aperture Radar (SAR) emits its own signal and works in all weather, day and night.
Piksel provides one radar product from the Sentinel-1 mission operated by the European Space Agency (ESA).
Available Product
| Product | Sensor | Polarisation | Resolution | Coverage | Revisit | Best for |
|---|---|---|---|---|---|---|
| Sentinel-1 RTC | C-band SAR | VV+VH or HH+HV | 20 m | Oct 2014 – present | 6–12 days | All-weather monitoring, floods, deforestation |
When to Use Radar
- Is the area often covered by cloud during the tropical wet season? — Radar penetrates cloud. Optical data does not.
- Need rapid flood mapping? — Radar detects standing water with high contrast against land.
- Need to detect landslides and ground movement? — SAR is sensitive to changes in surface geometry.
- Analysing mangroves or wetlands? — Backscatter is sensitive to surface moisture content.
- Need colour reflectance or vegetation indices (NDVI, EVI)? — Use Surface Reflectance.
- Need land surface temperature? — Use Surface Temperature.
Combining radar and optical data (sensor fusion) brings their complementary information together: optical data supports vegetation classification, while radar captures surface structure and moisture.
About Radar Backscatter
SAR sensors transmit microwave pulses towards the Earth's surface and measure the intensity and polarisation of the returning signal. Backscatter values (σ⁰) are affected by:
- Surface roughness — smooth surfaces (calm water) reflect the signal away from the sensor (low values), while rough surfaces (forest, urban areas) scatter more of it back (high values)
- Dielectric constant — influenced by surface moisture content (wet soil scatters more strongly than dry soil)
- Acquisition geometry — the radar wave's incidence angle affects the response
Polarisation
Sentinel-1 acquires data in two polarisation channels at once. Ratios between polarisations (such as VV/VH) are often used to distinguish surface types:
- VV+VH — the default mode over land; VH is sensitive to vegetation structure
- HH+HV — used in some cross-polarised modes and for ice monitoring
Radiometrically Terrain Corrected (RTC)
RTC products apply topographic and radiometric corrections so that backscatter values can be compared between scenes and over time, regardless of slope or acquisition geometry. This matters for time-series analysis — without RTC correction, backscatter values from slopes facing the sensor will be falsely high, while values from slopes facing away from the sensor will be falsely low.
Backscatter values in Sentinel-1 RTC are stored on a linear scale (float32). For analysis, they are often converted to a decibel (dB) scale: σ⁰(dB) = 10 · log₁₀(σ⁰_linear). dB values range from around −25 dB (water) to −5 dB (very rough surfaces / buildings).