Polarisation

When the radar transmits microwaves, they are polarised into a vertical or horizontal plane. The antenna also detects the scattered signal in a particular polarisation -- not necessarily the same as that transmitted. This gives four possible images of the same scene. They are labelled HH, HV, VH and VV for Horizontal/Vertical <transmit>-<receive>. However, if the SAR is correctly calibrated, the backscatter reciprocity principal implies that the HV and VH images are the same.

Instruments such as the JPL AirSAR can image a scene in two orthogonal polarisations simultaneously. This is known as quadrature polarisation. It enables the complete scattering matrix to be computed. Other instruments, such as the Envisat ASAR system, can produce data for both VV and HH polarisations, though not the complete scattering matrix. Fully polarimetric data will also be available from future planned satellite SAR instruments.

Whenever more than one polarisation is available, it is likely that improved results can be achieved by processing both/all polarisations jointly.