Abstract: Co located geostationary satellites are described as a group of GEO satellites that share the same nominal longitude and a common control box of typical ±0 1° in latitude and longitude.It's convenient to study relative motions in a reference orbital coordinate system using Clohessy Wiltshire equation,provided that the reference orbit is a standard geostationary orbit.However,a set of modified C W equation is presented to avoid a defect of the original C W equation.The satellites follow a combined eccentricity and inclination vector separation strategy,so that the group geometry can be expressed simply by the orbital elements of each member.The group geometry in space (or reference to ground station)can be keeped unchanged with a simple modification of the orbital elements of each member.