Radar & SAR
Sentinel-1 and radar imagery: backscatter, polarisation, speckle, terrain geometry, floods and change through cloud.
Radar is the one sensor on this site that makes its own light. Sentinel-1 sends 5.5 cm microwaves sideways at the ground and records what comes back, which means it works at night, through cloud, and during exactly the storms that optical satellites miss. That is why flood maps, ship detection and winter crop monitoring are radar work.
The cost is that a radar image is not a photograph of anything. Brightness is roughness, moisture and how the surface faces the sensor — water is black, cities are white, and every pixel carries speckle, a grain that is not noise but interference. Values arrive as linear power with a nodata flag of −32768, averages taken in decibels come out wrong, mountains lean towards the sensor, and two passes from different tracks disagree about a slope that never changed.
The concepts below settle what backscatter is, what VV and VH each see, where speckle comes from and what terrain does to the geometry; the how-to guides find, read, filter, composite and difference real Sentinel-1 scenes; and the fixes are the four ways the numbers come out wrong — noisy, negative, misaligned, or showing water where there is none.
🧭 Start with the concepts
The ideas the how-to guides below assume you already have.
- SAR imagery explained: what radar measures and why it sees through cloud
- SAR polarisation explained: VV, VH and what each responds to
- Speckle explained: why radar images look grainy
- SAR geometry explained: layover, foreshortening, shadow and look direction
- Sentinel-1 products explained: SLC, GRD and radiometrically terrain-corrected
- InSAR explained: measuring millimetres of ground motion from phase
⚙️ Then do the task
Step-by-step walkthroughs with code you can copy and adapt.
- How to find Sentinel-1 scenes in Python
- How to read Sentinel-1 RTC data in Python
- How to convert SAR backscatter to decibels, and average it correctly
- How to filter speckle in Sentinel-1 images
- How to map floods from Sentinel-1 in Python
- How to detect change between two Sentinel-1 images
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🔧 When it breaks
The errors this topic produces, with the cause explained rather than just the command.
Learning paths through this topic
A curated route rather than a list — the guides in the order that builds on itself.
Related topics
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