Image Analysis, Classification and Change Detection in Remote Sensing, 3rd Edition

Radiometric correction of polarimetric SAR imagery 189

Then

h

Gi =

1

m

·mhGi = hGi

and

var(

G) = var

1

m

m

X

i=1

G

i

!

=

1

m

2

· mhGi

2

=

hGi

2

m

=

h

Gi

2

m

. (5.27)

Hence, we see that the variance of the look-averag ed image decreases inversely

as the number of looks.

In real SAR imager y, the neighborhood pixel intensities contributing to

the look averag e will be correlated to s ome extent. This is acco unted for by

defining an equivalent number of looks (ENL) whose definition is motivated

by Equation (5.27), that is, by solving it for m:

ENL = m =

h

Gi

2

var(G)

. (5.28)

The ENL can be estimated in homogeneous regions of an image, where the

contribution of speckle to image statistics