akar.earthSAR: Definition
What is SAR (synthetic aperture radar)? How radar satellites see through cloud to monitor tropical forests, peat and mangroves in Indonesia.
SAR (synthetic aperture radar) is an active remote sensing technology that sends microwave pulses from a satellite or aircraft and records the energy scattered back. Because it supplies its own signal and microwaves pass through cloud, it can image the ground day or night in any weather.
Why it matters
Optical satellites like Sentinel-2 are blind under cloud. In the humid tropics that can mean weeks or months without a clear view. SAR closes that gap. It is widely used for near real-time deforestation alerts, flood and water level mapping, and tracking changes in forest structure.
How it works
SAR sensors operate in different wavelength bands:
- C-band (around 5–6 cm), used by Sentinel-1, responds mainly to the top of the canopy and is good for change detection.
- L-band (around 24 cm), used by missions such as ALOS PALSAR and NISAR, penetrates deeper into the canopy and relates better to woody biomass.
Analysts look at backscatter strength, polarisation and changes between dates. Several public alert systems use Sentinel-1 radar to flag clearing in the tropics; see our guide to deforestation alerts.
Indonesia and Southeast Asia angle
SAR is a core tool for monitoring peatland and mangroves in Indonesia. It can detect flooding and drainage on peat, burn scars after fires and clearing in mangrove belts. For REDD+ and EUDR monitoring, radar alerts help catch clearing in the wet season when optical images are cloudy.
Radar data needs specialist processing, such as speckle filtering and terrain correction, before analysis. Free Sentinel-1 data and analysis-ready products on cloud platforms have made that easier for project teams without in-house radar experts.
Common mistakes
- Treating radar images like photos. Backscatter is affected by moisture, terrain and look angle.
- Using C-band to estimate biomass in dense forest, where it saturates quickly.
- Ignoring slope effects in mountainous areas such as the Sumatran highlands or Papua.
Combine SAR with optical data and field plots, as explained under remote sensing and in remote sensing for biomass.
Frequently asked questions
Why is SAR useful in Indonesia?
Indonesia is cloudy for much of the year. SAR uses microwaves that pass through cloud and work at night, so it gives regular images when optical satellites cannot.
Can SAR estimate biomass?
Partly. Longer wavelengths such as L-band respond to woody biomass, but the signal saturates in high-biomass forest. SAR is best combined with lidar and field plots.