Vx and Vz are components of reference velocity along the x and z axes, V is the reference velocity magnitude, JJL is the gravitation parameter, and h is the angular momentum. To first order in e, the secular errors due to neglecting e are then given by Eqs. (1-4) with A; = 1.5p/ha. In summary, the circular orbit equations can be used to describe perturbed motion about a moderately elliptic reference orbit if the relative position vector is expressed in a local tangent coordinate system. The accuracy is probably more than adequate for many applications including manual rendezvous in earth orbit, up to an eccentricity of 0.05.