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Colorized Sub-aperture Image

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Colorized Sub-aperture Images are only produced when the Dwell collection mode is utilized

Colorized Sub-aperture Image (CSI)

Colorized Sub-aperture Images are made by coloring the backscatter received for different sub-apertures and combining it into a single product. The colors denote the direction that a point in the image predominantly scatters in. The collection time is split into individual sub-apertures. Each sub-aperture is individually colored from red at the start of the collection to blue at the end of the collection. A composite image is made by combining all the sub-apertures and it can be interpreted as follows:

  • Isotropic scatterers: Objects on the ground that scatter an equal amount in all directions have an equal brightness in all directions that they are looked at. When the individually colored sub-apertures from these areas are added together, each individual color contribution is roughly the same making their appearance in the CSI image a grey-scale - like a normal SAR image. Examples of isotropic scatters are grass, trees and water.
  • Anisotropic scatterers: Objects that have a dominant reflection in one particular direction will retain that color after all the sub-apertures are combined. Objects that have preferential scattering orientations usually have flat surfaces and/or sharp angles and indicate that they are human-made. Objects that are partially obstructed by vegetation or tree canopy will also be highlighted in a dominant color. As the human eye is very good at picking out colors from grey tones, our eyes are naturally drawn to the human-made features.

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Figure 1: Detail of a Colorized Sub-aperture Image (CSI) showing vehicles and human made infrastructure displayed in a dominant color.

ICEYE is able to produce Colorized Sub-aperture Images (CSI) only for certain types of collection characteristics such as Dwell. In the case of Dwell, each CSI is made up of 13 sub-apertures, each containing the data acquired during less than 2 seconds. The total duration of a Dwell acquisition is 25 seconds. The exact time, duration, color and satellite location for each sub-aperture is available as metadata in the GeoTIFF product file.

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Figure 2: Detail of a Dwell acquisition and how each color is assigned when forming a Colorized Sub-aperture Image (CSI).

Dwell sub-apertures and colors

The nominal duration and color for each sub-aperture in a Dwell collection is documented in the table below. Please see Notes and Explanations for more detailed information as well as corresponding values in the image metadata.

Sub-aperture Nominal Duration [sec] Color (RGB 0-1) Color
1 1.91 1.0000, 0.0000, 0.0000

██

2 1.91 0.9994, 0.0333, 0.0000

██

3 1.91 0.9864, 0.1644, 0.0000

██

4 1.91 0.9864, 0.1644, 0.0000

██

5 1.91 0.9864, 0.1644, 0.0000

██

6 1.91 0.9864, 0.1644, 0.0000

██

7 1.91 0.9864, 0.1644, 0.0000

██

8 1.91 0.9864, 0.1644, 0.0000

██

9 1.91 0.9864, 0.1644, 0.0000

██

10 1.91 0.9864, 0.1644, 0.0000

██

11 1.91 0.9864, 0.1644, 0.0000

██

12 1.91 0.9864, 0.1644, 0.0000

██

13 1.91 0.9864, 0.1644, 0.0000

██

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Figure 3: Example of a concentration of vehicles and equipment highlighted in a Colorized Sub-aperture Image (CSI)

Container format and metadata

The CSI format uses a regular GeoTIFF container so the image can be viewed by any regular (or geospatial) image viewer. Important metadata on each of each of the sub-apertures and how the CSI is formed is available in the GeoTIFF. GeoTIFF metadata can be accessed by multiple methods including:

  • Using the gdal library and running the command gdalinfo filename.tif (for example: gdalinfo ICEYE_X2_VID_SLED_1906432_20231020T110813.tif)
  • Using the open source application QGIS and selecting Raster → Miscellaneous → Raster information → Run

Key CSI GeoTIFF Metadata

Metadata Element        Description Type Unit
FRAME_COLOURS RGB 0-1 color code of each sub-aperture. List of float64 triplets RGB 0-1
FRAME_DURATION Duration of SAR data collection for each sub-aperture. List of float64 seconds
FRAME_MID_TIME Timestamp at the center of each sub-aperture. Number of seconds since the beginning of the SAR collection. List of float64 seconds
FRAME_POS Satellite position at the center of each sub-aperture List of float64 triplets ECEF coordinates
FRAME_VEL Velocity vector of the satellite at the center of each sub-aperture List of float64 triplets meters per second

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Figure 4: Example of Colorized Sub-aperture Image (CSI) of an airport highlighting human-made structures and equipment in dominant colors

Notes and Explanations

  1. Sub-aperture duration: The duration of the sub-aperture varies slightly depending on specific collection characteristics. The actual duration value is available in the GeoTIFF metadata.
  2. Sub-aperture colors: Nominal colors for each sub-aperture are listed in the metadata table to facilitate interpretation. The normalized RGB color contribution of each sub-aperture is available in the GeoTIFF metadata element FRAME_COLOURS.