Colour Coding the Image

Posted on by andrewmeteors

Sentinel-1 transmits radio pulses towards the Earth and measures the echoes to allow a radar image to be formed. The current typical operational mode uses a vertically polarised transmitted pulse with the receivers on the satellite measuring the echo amplitude in both vertical and horizontal polarisations.

Radar echoes with the transmit polarisation are coded as red, with increasing intensity making it brighter. The measured intensity of the cross polarised echo takes the green channel for the colour image. Since there is generally less energy in this polarisation it has been scaled to enhance the sensitivity. Pixels with a mix of both produce the secondary colour yellow.

If you look at your computer screen through polarised dark glasses and rotate your head to maximise the brightness then that is equivalent to the spacecraft measuring the transmitted polarisation. Now rotate your glasses through 90 degrees and the image will go dark. That orientation is the cross polarisation. Take another pair of glasses and put them between the screen and you at 45 degrees, without moving your head, and suddenly you can see the screen brighten again. This is analogous to the polarised radar pulse hitting vegetation, branches and leaves, at an angle with the echo (transmitted light in our desktop scenario) having a cross polarised component.

Large flat areas reflect the radar pulse away from the spacecraft so they appear dark in the image. Aside from Hagley Park several other large areas are also dark, much of it land cleared following the Christchurch earthquakes. The sweep of the Avon River and the large avenues framing the central city are also clearly visible. Bottom right, the south-east corner of the image, exhibits a large area of red, the result of strong vertically polarised echoes. Ferry Road, a main arterial route, leading out from the central city at around 20 degrees south of east is almost perfectly aligned towards the Sentinel-1A spacecraft when it sweeps the city. Buildings aligned with Ferry Road are therefore perfectly placed to reflect the radar signal back towards the spacecraft. Just like our radar reflectors.

Within the analysis package, SNAP, supplied by the European Space Agency to handle Sentinel data, the standard visualisation adds the ratio of transmit polarisation divided by cross polarisation as a blue component of the image. Hard aligned reflectors typically have both red and blue components hence the magenta hue.
Contemplating different colour combinations left me unable to decide on just one colour mix for the Christmas card project. To avoid the dominance of yellow on the card this image places the transmit polarisation on both the red and blue channels with the cross polarisation, frequently an indicator of vegetation, again on green. Red, green and blue coming out as a more neutral, if somewhat cooler white – hints of snow?
With around 6% of the male population being red-green colour blind, some of my students just could not see what I was describing in the images. Here the default Sentinel-1 radar visualisation is reversed: transmitted polarisation (VV) on blue, cross polarisation (VH) on green and the ratio of VV/VH on red.

Ultimately I could not decide on which colour coding to use. Since we had seven reflectors in operation I decided we could justify seven cards. If Newton can decide the music of the spheres justifies seven colours in a rainbow then I figure it is good enough for me. Hope you enjoy the multicoloured variety for visualising the radar data. Ultimately the project is about looking at the city in a different way. Radar view – not Google Earth or Street View.

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Published by andrewmeteors