Help with using homography matrix to approximate orbital velocity

I am writing a program that uses images taken aboard the ISS to calculate the speed at which the International Space Station (ISS) is traveling. The framework I have is to take two images (perspective may shift slightly between images) and use SIFT to detect keypoints, which will be matched and filtered with FLANN + Lowe’s ratio test. I then use RANSAC to generate the homography matrix.

What would be the most accurate way to determine the displacement vector? I am unsure which method would be the most accurate. Should I just use the translation components of the homography matrix? Should I average the matched keypoint displacement? Should I transform the matched keypoints with the homography matrix and then average?

Is there anything else I should consider? I have a general idea of what could be done, but I am unsure what will be necessary or useful, or the exact way of implementing it.

Here are some sample images