Colorized Sub-aperture Image

Info

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.

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.

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	0.8944, 0.4472, 0.0000	██
5	1.91	0.6000, 0.8000, 0.0000	██
6	1.91	0.2334, 0.9724, 0.0000	██
7	1.91	0.0000, 1.0000, 0.0000	██
8	1.91	0.0000, 0.9724, 0.2334	██
9	1.91	0.0000, 0.8000, 0.6000	██
10	1.91	0.0000, 0.4472, 0.8944	██
11	1.91	0.0000, 0.1644, 0.9864	██
12	1.91	0.0000, 0.0333, 0.9994	██
13	1.91	0.0000, 0.0000, 1.0000	██

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

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.