In this paper we discuss the use of a monitoring camera on board a satellite to detect unknown satellites coming so close as to cause collision risk. We assume that a camera on board our satellite tracks direction angles of an unknown target satellite and try simulations of determining the target's trajectory relative to our satellite. Simulations show that we cannot determine uniquely the target's trajectory, while showing that we can decide whether we need a collision avoidance maneuver and can determine its strategy when we need it. The on-board camera does not need precise alignment, as the bias in direction angles can be estimated as an unknown parameter. On-board monitoring can thus be practical for orbital risk avoidance.