The equations of motion of a small satellite moving along a prescribed trajectory under disturbances are analysed. Problems of this kind have been extensively investigated. The corresponding equations for relative motion errors, caused by the uncertainties in initial conditions and control implementation imperfections, are linearized. The linear equations are reformulated and the evolution equations for optimal ellipsoidal estimates of these errors are derived. It is shown that ellipsoidal bounding of reachable sets is an efficient approach to model uncertain linear dynamical systems. The procedure constructed in this paper allows one to take into account discrete observations and to design control aimed at compensating the disturbances between measurements. These measurements are assumed to be performed with small errors. A numerical example is given which illustrates that the presented control design algorithm is quite efficient and allows one to keep the error between the real and desired motion close to zero.