Use Phase Image to generate a phase image for AIRSAR or SIR-C data as a means to measure the phase difference between the horizontal and vertical polarizations. The phase difference is measured in either radians or degrees and ranges from -p to p or -180 degrees to 180 degrees.

A piecewise linear stretch that sets -p and p to 0 and 0 radians to 255 will map the positive and negative angles to the same level.

Additionally, use Band Math to calculate the absolute value of the result, using the expression abs(b1).

1. From the Toolbox, select one of the following options:
   
• Radar > AIRSAR > AIRSAR Phase Image: The Input Stokes Matrix Files dialog appears.
• Radar > SIR-C > SIR-C Phase Image: The Input Data Products dialog appears.
2. Click Open File and select the input compressed data file.
3. Click OK. The Phase Image Parameters dialog appears.
4. In the Phase Images field, select the check boxes next to the bands to use for the phase image calculation.
5. Select Radians or Degrees for output angle units.
6. Perform optional spatial subsetting by clicking Spatial Subset.
7. Select output to File or Memory.
8. Click OK. ENVI adds the resulting output to the Layer Manager.