  Frequency-Modulated Continuous-Wave (FMCW) lidar is a recently emerging
technology that additionally enables per-return instantaneous relative radial
velocity measurements via the Doppler effect. In this letter, we present the
first continuous-time lidar-only odometry algorithm using these Doppler
velocity measurements from an FMCW lidar to aid odometry in geometrically
degenerate environments. We apply an existing continuous-time framework that
efficiently estimates the vehicle trajectory using Gaussian process regression
to compensate for motion distortion due to the scanning-while-moving nature of
any mechanically actuated lidar (FMCW and non-FMCW). We evaluate our proposed
algorithm on several real-world datasets, including publicly available ones and
datasets we collected. Our algorithm outperforms the only existing method that
also uses Doppler velocity measurements, and we study difficult conditions
where including this extra information greatly improves performance. We
additionally demonstrate state-of-the-art performance of lidar-only odometry
with and without using Doppler velocity measurements in nominal conditions.
Code for this project can be found at: https://github.com/utiasASRL/steam_icp.
