Radar polarisation is the orientation of the electric (E) vector in an electromagnetic wave, frequently horizontal or vertical, in conventional imaging radar systems. Polarisation refers to the orientation of the plane of the electric field (E), as opposed to the magnetic field (M)
image

Remote sensing radars are usually designed to transmit either vertically polarised or horizontally polarised radiation. This means that the electric field of the wave is in a vertical plane or a horizontal plane. Likewise, the radar can receive either vertically or horizontally polarised radiation, and sometimes both. The planes of transmitted and received polarisation are designated by the letters H for Horizontal and V for Vertical. Thus the polarisation of a radar image can be HH, for horizontal transmit, horizontal receive, VV for vertical transmit, vertical receive, HV for horizontal transmit vertical receive, and vice versa (VH).
When the polarisation of received radiation is the same as the transmitted radiation, the image is said to be like-polarised When the polarisation of received radiation is the opposite of the transmitted radiation, the image is said to be cross-polarised cross polarisation requires multiple-scattering by the target and therefore results in weaker backscatter than like-polarisation Satellite radars generally use like-polarisation because the cross-polarised signals are too weak to produce a good image.
Polarisation is established by the antenna, which may be adjusted to be different on transmit and on receive. Reflectivity of microwaves from an object depends on the relationship between the polarisation state and the geometric structure of the object. Possible states of polarisation, in addition to vertical and horizontal, include all angular orientations of the E vector, and time varying orientations leading to elliptical and circular polarisations.

https://earth.esa.int/handbooks/asar/CNTR5-5.html