In this contribution, the experimental results of testing a monocular visual odometry algorithm in a real rover platform over flat terrain for localization in outdoor sunlit conditions are presented. The algorithm computes the three-dimensional (3D) position of the rover by integrating its motion over time. The motion is directly estimated by maximizing a likelihood function that is the natural logarithm of the conditional probability of intensity differences measured at different observation points between consecutive images. It does not requiere as an intermediate step to determine the optical flow or establish correspondences. The images are captured by a monocular video camera that has been mounted on the rover looking to one side tilted downwards to the planet's surface. Most of the experiments were conducted under severe global illumination changes. Comparisons with ground truth data have shown an average absolute position error of 0.9% of distance traveled with an average processing time per image of 0.06 seconds.