I. Introduction
A variety of sensor technologies are utilized in the automotive industry for both driver assistance functions and autonomous vehicle operation. Millimeter-wave radar is a very important sensor due to its ability to detect targets at relatively long range and under an assortment of weather conditions, which make it superior to other technologies such as lidar or optical imaging in certain roles [1] . Using Doppler processing, radar is also capable of simultaneously detecting both the distance and speed of multiple targets. Additionally, polarimetric radar is able to exploit the polarization response of the target to obtain even more information. This is particularly useful for both discriminating between different types of road surfaces, and assessing road surface conditions, based on the absolute and relative strengths of co- and cross-polarized backscattering exhibited [2] .