The problem of autonomous orbit determination of the formation flying missions based solely on relative range measurements without any external signals is studied. Observability for all of the participating satellites in this problem is analyzed based on the nonlinear Keplerian model with J 2 term and the nonlinear measurements. It is proven that for most formation flying missions with less than or equal to 13 satellites, the degree of observability for the autonomous orbit determination using only relative range measurements is very weak.