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Radar Course III
37. Bragg scattering
43. Texture and image analysis
42. Temporal averaging
12. Synthetic Aperture Radar (SAR)
34. Space, time and processing constraints
15. Slant range / ground range
8. Side-looking radars
19. Shadow
10. Real Aperture Radar: Range resolution
11. Real Aperture Radar: Azimuth resolution
9. Real Aperture Radar (RAR)
7. Radar principles
38. Radar image interpretation
35. The radar equation
36. Parameters affecting radar backscatter
16. Optical vs. microwave image geometry
25. Method
18. Layover
32. Landers Earthquake in South California
23. Introduction
27. Interferogramme of Naples (Italy)
29. Interferogramme and DEM of Gennargentu (Italy)
2. Independence of clouds coverage
40. Image interpretation: Speckle
41. Image interpretation: Speckle filters
39. Image interpretation: Tone
20. Geometric effects for image interpretation
22. Geocoding: Geometry
17. Foreshortening
26. First ERS-1/ERS-2 tandem interferogramme
6. Electromagnetic spectrum
30. Differential interferometry
45. Data reduction: 16 to 8 bit, blockaverage vs incrementing
4. Control of imaging geometry
3. Control of emitted electromagnetic radiation
24. Concept
28. Coherence image of Bonn area (Germany)
44. Classification of ERS-1 SAR images with Neural Networks
5. Access to different parameters compared to optical systems
13. SAR processing
33. SAR interferometric products
21. SAR image geocoding
14. ERS SAR geometric configuration
31. The Bonn experiment
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Data reduction: 16 to 8 bit, blockaverage vs incrementing

The figure to the left of the screen was obtained from the full resolution image by means of a block averaging of 16x16 pixels moving window. The average value of each window is written to a new image file which dimension is reduced by 16 times in lines and columns. This is a conventional way to obtain a good quality Quick-Look images starting from a full-resolution image.

On the right part of the screen an 8 bit version of the full resolution image is shown. It has been obtained merely sampling the full image using a step increment of 16 pixels. The process results in a final image that has the same size of the image displayed on the left.

For practical reasons also a 16 to 8 bit conversion is done. Original statistics are conserved best when applying a square root to the original 16 bit data. Especially the information from point targets (strong scattering) are retained.

If mainly distributed targets (e.g. agriculture) are evaluated, a simple division (e.g. by four) is applied. This procedure clips all values greater than 1020, in the presented cases a division by four has been performed

The dimension of the 16 bit full-resolution image which was initially about 128 Mbytes has now been reduced by a factor 256 for the processing and by a factor 2 for the reduction from 16 to 8 bit. This results in a final size for the image of about 250 Kbytes.

The main difference characterizing the two images is in the amount of noise. In the right one it is possible to note that the increase of noise due to the resampling process masks the smaller features of the image and part of the SAR information is lost. In this image, even if the effect of the rain cells is still visible in the central part of the figure, most of the small features such as the presence of ships or the local effects of the wind on the sea surface are lost. Note also that the urban environment (bright part), in the upper part of the left figure, appears, in the right image, defocused because of the noise.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry