Reversing a vehicle while towing a trailer can be challenging for many drivers, particularly for those who only tow on an occasionally basis. Systems used to assist a driver with backing a trailer typically estimate the heading angle of the trailer relative to that of the vehicle, i.e., the hitch angle. In the current Ford Trailer Backup Assist (TBA) system, the hitch angle is determined utilizing the existing reverse camera with added software in the image processing module. One potential issue for the vision-based hitch angle estimation approach is that environment factors may limit the system usage, since either the camera lenses or the target may be blocked or partially blocked. Furthermore, it is very difficult to apply the vision-based approach to gooseneck or fifth wheel trailers. In this paper, a yaw rate based hitch angle observer is proposed as an alternative sensing solution for TBA. In addition to the existing vehicle yaw rate sensor which is primarily used by the vehicle stability control system, a second yaw rate sensor is placed on the trailer to measure its yaw motion. Based on the kinematics model of the vehicle-trailer, an instantaneous hitch angle is first derived by utilizing vehicle yaw rate, trailer yaw rate, vehicle velocity and vehicle/trailer parameters provided by the TBA system. Due to signal errors and parameter uncertainties, this instantaneous hitch angle may be noisy, especially at lower vehicle speed. To improve the estimation accuracy, a Kalman filter (or an extended Kalman filter) which is based on the vehicle-trailer kinematics model is then formulated to obtain the optimal estimate, with the instantaneous hitch angle treated as a pseudo-measurement. Experimental results verify the accuracy and robustness of the proposed approach, and that the same algorithm can be applied to both conventional trailers and gooseneck/fifth wheel trailers.