Exercise 1: Spiraling Hurricane Katrina from cloud tops to ocean waves - continued
b) Ocean waves - ASAR
Radio Detection And Ranging (Radar) systems are active sensors which provide their own source of electromagnetic energy. Active radar sensors emit radio wave radiation in a series of pulses from an antenna. When the energy reaches the target, some of the energy is reflected back towards the sensor. This backscattered microwave radiation is detected, measured and timed. The time required for the energy to travel to the target and return back to the sensor determines the distance or range to the target. By recording the range and magnitude of the energy reflected from all targets as the system passes by, a two-dimensional image of the surface can be produced.
Because radar provides its own energy source, images can be acquired day or night. Microwave energy is also able to penetrate through clouds and most rain, making it an all-weather sensor. This makes it capable of piercing through Hurricane Katrina’s cloud tops.
Open the ASAR image of Hurricane Katrina. Inspect its histogram first and see if you can understand why LEOWorks is not capable of displaying the image correctly.
Historgram of ASAR image
Because the backscattered radiation is measured with a very high accuracy, the pixel values cover a much wider range of values than the pixel values of usual 8-bit images. 8-bit images store pixel values in the range of (0 to 255). The accuracy of the ASAR instrument requires a higher range of pixel values. To allow a much wider range of pixel values, ASAR image pixels are stored in a 32-bit format (float32) rather than 8-bit. The range of pixel values displayed in the LEOWorks viewer is 0-255 (8-bit), therefore it is not capable of displaying 32-bit images accurately on your computer screen.
Your next task consists in projecting this radar image onto the 8-bit scale by applying interactive stretching.
Inspect the radar image now.
1. Compare the image with a map in an atlas. Can you locate the coastline at the top of the image?
2. Describe how the different features of Katrina are visualised in the radar image.
3. Using the Measure tool, measure the distance from the eye of Katrina to the coast. How large is the diameter of the eye of Katrina?
Unlike in the MERIS image, you will not identify clouds because, as mentioned above, the microwave radiation used by radar sensors can pierce through clouds.
Now it is time to examine the ocean surface below the hurricane.
Once more, try to draw conclusions from the direction of the spiraling ocean to the direction of the spiraling hurricane itself.
As radar imagery (e.g. acquired by ASAR) is created in a very different way to optical imagery (e.g. acquired by MERIS), it also needs to be interpreted differently.
Instead of measuring intensity of reflected and emitted radiation within a spectral band, the backscattered portion of the microwave pulse emitted by the active sensor is measured. High values, meaning bright pixels, indicate that a comparatively big portion is scattered back, and vice versa.
4. What does ocean brightness in the radar image reveal about the roughness of the sea surface?
The following picture may help to answer this question.
Relationship between greyscale of radar images and backscattering of different targets
As you should have understood from the previous question, rough surface areas of the ocean appear bright in a radar image, whereas smooth ocean surfaces appear dark.
Once more, inspect the greyscale values of the hurricane’s eye and the edges of the eye with the help of this information.
5. Can you derive any information on the wind speed in the eye of Katrina?
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