Radar Enhanced Frost Filter

The Radar Enhanced Frost (Radar Enh_Frost Filter) resampling option determines the gray level for each pixel by computing the weighted sum of the center pixel value, the mean value, and the variance calculated in a circular kernel surrounding the pixel. To filter pixels located near the edges of the image, edge pixel values are replicated to produce sufficient data.

This filter is used primarily to suppress speckle. It smooths image data without removing edges or sharp features in the images while minimizing the loss of radiometric and textural information. In homogeneous areas, speckles are removed using a low-pass filter. In areas containing isolated point targets, the filter preserves the observed value. In heterogeneous areas, speckles are reduced by convolving the image with a circular kernel. The resulting gray-level value (R) for the smoothed pixel is:

R = Im for Ci <= Cu
R = Rf for Cu < Ci < Cmax
R = Ic for Ci >= Cmax

Where Rf is the result of convolving the image with a circularly symmetric filter whose weighting values (M) for each pixel is:

M = exp(- A * T)

where:

A = Damping Factor * (Ci-Cu)/(Cmax-Ci)
Ci = S / ImCu = SQRT(1/Number of Looks)
Cmax = SQRT(1+2/Number of Looks)
Im = mean value of intensity within the kernel
S = standard deviation of intensity within the kernel
T = the absolute value of the pixel distance from the center pixel to its neighbors in the kernel

The Damping Factor specifies the extent of the damping effect of filtering. OrthoEngine uses a default value of 1 since it is sufficient for most SAR images.

The resulting gray-level value (Rf) for the smoothed pixel is:

Rf = (P1*M1 + P2*M2 + ... + Pn*Mn) / (M1 + M2 + ... + Mn)

where:

P1 .. Pn: gray levels of each pixel in the kernel
M1 .. Mn: weights (as defined above) for each pixel

The Number of Looks and the Image Format of the radar image are usually recorded on the CD jacket or magnetic tape label or in the format specifications provided by the data vendor.

Filter Size:

The kernel is circular with its width and length in odd numbers. You control the size of the kernel with the Filter Size option by typing the number of pixels (width) in the X box and the number of lines (length) in the Y box. Different filter sizes greatly affect the quality of the processed images. If the filter is too small, the noise filtering algorithm is not effective. If the filter is too large, subtle details of the image are lost in the filtering process. The minimum size for the kernel is 3-by-3 pixels. A 7-by-7 kernel usually gives the best results.

Image Format:

The radar images are supplied in one of two Image Formats: Power or Amplitude. Power is the sum of the squares of the real and imaginary values of the complex pixel values in the radar image. Amplitude is the square root of Power. Most radar images are supplied in the Amplitude format to preserve the values. You identify the format used with your images in the Image Format list.

Number of Looks:

You use the Number of Looks to estimate noise variance and to control the amount of smoothing applied to the image. In theory, the correct value for the Number of Looks should be the effective number of looks of the radar image, or close to the actual number, but it may be different if the image was resampled. Using a smaller value for the Number of Looks leads to more smoothing, and a larger value preserves more image features. In the No. of Looks list, select the Number of Looks that you want to apply to the image.