SAR (Synthetic Aperture Radar) Basics & Applications
**SAR (Synthetic Aperture Radar)** is an active remote sensing technology that sends its own microwave energy toward the Earth and measures the returning signal. Unlike optical sensors, SAR can capture data **day or night, and through clouds** — making it invaluable for reliable, all-weather monitoring. ---
📋 Prerequisites
- Registered Google Earth Engine account.
- Access to GEE Code Editor (code.earthengine.google.com).
Step-by-Step Instructions
Understand How SAR Works
1. The satellite emits microwave pulses toward the ground 2. The pulses interact with surface features and reflect back 3. The sensor records the **strength (backscatter)** and **timing** of the returning signal 4. This data is processed into an image where pixel brightness reflects surface roughness, moisture, and structure **Key advantage:** Microwave wavelengths pass through clouds, smoke, and rain, and don't depend on sunlight — SAR works 24/7 in almost any weather. ---
Learn Key SAR Concepts
- **Backscatter:** The strength of the returned signal — rough surfaces (like forests, urban areas) return strong backscatter; smooth surfaces (like calm water) return weak backscatter - **Polarization:** SAR can send/receive in different orientations (VV, VH, HH, HV) — different combinations reveal different surface properties - **Wavelength Bands:** C-band (Sentinel-1), L-band (ALOS PALSAR), X-band (TerraSAR-X) — longer wavelengths penetrate vegetation/soil more deeply ---
Common SAR Satellites
| Satellite | Operator | Band | Notes | |---|---|---|---| | Sentinel-1 | ESA | C-band | Free, global coverage, 6–12 day revisit | | ALOS-2 PALSAR-2 | JAXA | L-band | Good for forest/biomass studies | | TerraSAR-X | DLR | X-band | High resolution, commercial | | RADARSAT | CSA | C-band | Ice and maritime monitoring | ---
Download Sentinel-1 SAR Data
1. Go to the **Copernicus Data Space Ecosystem** (dataspace.copernicus.eu) 2. Search your area of interest and filter by **Sentinel-1** 3. Choose product type: **GRD (Ground Range Detected)** is easiest for beginners 4. Download the product for your date and area of interest ---
Basic SAR Processing (using SNAP or QGIS)
Using ESA's free **SNAP** software (recommended for SAR-specific preprocessing): 1. **Apply Orbit File** – corrects satellite position data 2. **Radiometric Calibration** – converts raw values to backscatter (sigma0) 3. **Speckle Filtering** – reduces "salt-and-pepper" noise typical of SAR images 4. **Terrain Correction** – geometrically corrects the image using a DEM 5. Export as GeoTIFF and load into QGIS for further analysis ---
Real-World Applications
- **Flood Mapping:** Water appears very dark (low backscatter) — ideal for rapid flood extent mapping, even through clouds during storms - **Deforestation Monitoring:** Sudden backscatter drops indicate forest loss - **Ship/Vessel Detection:** Bright returns against dark ocean background - **Ground Deformation (InSAR):** Comparing SAR images over time detects land subsidence, earthquakes, and landslides at millimeter precision ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- SAR is active — it emits its own energy and works in all weather, day or night
- Backscatter strength reveals surface roughness and moisture
- Sentinel-1 (free, C-band) is the most accessible SAR data source
- Key applications: flood mapping, deforestation, ship detection, ground deformation
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## Practice Questions
1. Why can SAR capture data through clouds when optical sensors cannot?
2. What does low backscatter over water typically indicate?
3. Name one free SAR satellite and its operating agency.
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*Next Tutorial: Working with Landsat & Sentinel Data*