Measuring Robot Velocity in 2D Plane

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I am trying to tune a PID controller for velocities in x and y direction on a wheeled robot. However, in order to achieve this I need to measure the velocity of the robot in xy plane. I cannot use encoders because the robot will be on a hill and be slipping and drifting on purpose. IMU won't cut it either due to its poor accuracy and drift. So far I have tried using the apriltags to measure speed but the resolution/update rate with this approach is too slow (is there a way to speed this up?). I also looked at GPS but even that does not have the required resolution. Does anyone know of alternative ways to measure the speed of robot in 2D plane?