Low thrust relative orbital transfer uses orbital motion equations (such as the Clohessy-Wiltshire equations published in 1961) to explain the differences in orbits and displacements of target and chaser spacecraft.
In 1960, W. H. Clohessy and R. S. Wiltshire published the Clohessy-Wiltshire equations, which presents a rather simplified model of orbital relative motion, in which the target is in a circular orbit, and the chaser spacecraft is in an elliptical or circular orbit. 
