* RADAR BACKSCATTER MEASUREMENTS

Radar backscatter (also known as radar cross section or RCS) is measured in units of area (i.e. square meters)

The radar cross section of an object is often related to its actual size, as seen by the radar. For example, the RCS of a sphere is r², where r is the radius of the sphere

Normalized radar cross section, , is given by:

= 10log10 (RCS/A)

where A is the area of the image pixel

Units are decibel (dB)

Example: for an object with an RCS of 10m², and an image pixel size of 10mx10m, = -10dB.

Typical values of for natural surfaces are from + 5dB to - 40 dB

+ 5dB would be a very bright feature in a radar image

- 40 dB would be a very dark feature in a radar image

* DIFFERENT POLARIZATIONS

With different polarizations on transmit and receive, we can measure different polarizations:

- Horizontal transmit, Horizontal receive

- Vertical transmit, Vertical receive

- Horizontal transmit, Vertical receive

Total power (TP) - defined as 0.25( + 2 + )

- Phase difference between HH and VV

measurements

- Correlation between HH and VV measurements

The phase difference is measured in degrees (i.e. not dB) and is therefore number between -180 and +180 degrees.

The correlation coefficient is a measure of how alike the HH and VV scatterers are, and is given as a number between 0 and 1.

= 0 means the HH and VV scatterers are different

= 1 means the HH and VV scatterers are the same

* WHAT CHANGES RADAR BACKSCATTER?

Backscatter can change if we use a different:

- polarization

- frequency

- time of observation

- track angle (angle of radar track compared to True North)

- incidence angle (angle of radar wave at earth's surface, compared with local vertical)

Note that by Pythagoras cos( ) = h / R

* Radar Shadow and Layover

An imaging radar sensor illuminates the ground as it flies over it from a certain direction

In mountainous regions, some parts of the ground may be in shadow

Layover - causes mountains to appear as if they have 'fallen over'

With a little practice, it becomes easy to recognize mountainous areas in radar images from the characteristic bright leading edge of the layover and dark shadows

* LEVELS OF RADAR BACKSCATTER

When looking at a radar image, there are several explanations for the appearance of high and low backscatter:

Possible Explanations for Backscatter ranges:

* Very high backscatter (above -5dB)

- Terrain slopes towards radar

- Radar looking almost straight down

- Very rough surface

- Flooded forest

- Man-made objects

* High backscatter (-10 to 0dB)

- Rough surface

- Dense vegetation, e.g. forest

* Moderate backscatter (-20 to -10dB)

- Moderately rough surface

- Medium level of vegetation, e.g. shrubs, agricultural crops

* Low backscatter (below -20dB)

- Terrain slopes away from radar

- Smooth surface, e.g. water, road surface

- Very dry material, e.g. sand in an arid desert

These are, of course, just rules-of-thumb, to which many exceptions are possible and have been found by scientists analyzing radar image data. There is often considerable overlap between the backscatter ranges, so it might be difficult to differentiate between forest and shrubs, using backscatter alone.

Slides - Table of Contents

Converted to the IBM-PC by Al Wong, sirced03@southport.jpl.nasa.gov

Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109