Getting Started with Sentinel-1 SAR Imagery: A Beginner's Guide
Quick Answer: Sentinel-1 SAR provides cloud-penetrating radar imagery updated every 6 days. With Off-Nadir Delta, you can access and visualize SAR data directly in your browser — no GIS software needed. This guide walks you through searching, loading, and interpreting your first SAR image.
What is Sentinel-1 SAR?
Sentinel-1 is a radar satellite mission operated by the European Space Agency (ESA) as part of the Copernicus programme. Unlike optical satellites that capture visible light, Sentinel-1 uses Synthetic Aperture Radar (SAR) — an active sensor that transmits microwave pulses and records the backscattered signal.
This gives SAR three major advantages:
- Cloud penetration: Radar waves pass through clouds, enabling observation in any weather
- Day and night operation: SAR provides its own illumination, so it works 24/7
- Surface structure sensitivity: Radar responds to surface roughness, moisture, and geometry
Why Use SAR Data?
SAR imagery is essential for applications where optical data falls short:
| Application | Why SAR? |
|---|---|
| Flood mapping | Water appears dark in SAR, making flood extent clearly visible |
| Ship detection | Metal vessels produce strong radar reflections on dark ocean backgrounds |
| Deforestation monitoring | Works through tropical cloud cover that blocks optical sensors |
| Urban change detection | Buildings produce distinctive radar signatures |
| Soil moisture estimation | Radar backscatter correlates with surface moisture content |
Step-by-Step: Your First SAR Image
1. Navigate to Your Area of Interest
Open the map and zoom to your target area. For this example, let's look at a coastal city — these areas show excellent SAR contrast between water (dark) and urban areas (bright).
2. Open the Satellite Images Panel
Click the satellite icon in the sidebar to open the Satellite Images panel. Select Sentinel-1 GRD as your data source.
3. Set Your Search Parameters
- Date range: Start with the last 30 days for recent imagery
- Polarization: VV is typically best for water/flood applications; VH works better for vegetation
4. Search and Add to Map
Click "Search" to find available imagery. The results will show footprint outlines on the map. Click any result to see metadata (date, orbit direction, etc.), then click "Add to Map" to load the imagery.
5. Interpret the Image
In a SAR image:
- Bright areas: Strong radar return — urban areas, ships, rough surfaces
- Dark areas: Weak radar return — calm water, smooth surfaces, shadows
- Speckle: The grainy texture is inherent to SAR (coherent imaging system noise)
Tips for Better SAR Analysis
- Compare ascending and descending passes — different look angles reveal different features
- Compare multiple dates — load imagery from different dates and toggle between them to spot changes like floods or deforestation
- Adjust visualization parameters — the default min/max may not be optimal for your area
- Consider the polarization — VV emphasizes surface scattering, VH emphasizes volume scattering
Common Questions
Q: Why does my SAR image look grainy? A: This is called "speckle" — it's an inherent property of coherent imaging systems like SAR. It's not noise in the traditional sense, but rather interference between scatterers within each resolution cell.
Q: What's the difference between GRD and RTC? A: GRD (Ground Range Detected) is the basic SAR product. RTC (Radiometrically Terrain Corrected) compensates for terrain effects, providing more accurate backscatter values — especially important in mountainous areas.
Q: How often is Sentinel-1 data updated? A: Sentinel-1 has a 6-day repeat cycle over most land areas, though coverage varies by region.
Next Steps
- Try Change Detection to compare SAR images from different dates
- Explore Flood Mapping with SAR for disaster response applications
- Learn about SAR vs Optical imagery to choose the right data for your analysis
