Orbital Rendezvous Systems

Abstract

The sybstem requirements for orbital rendezvous systems vary greatly depending on the mission in question. A few of the mission parameters which significantly affect subsystem requirements are: 1. Launch Window. The permitted launch window may be very short in an emergency rescue mission or could be relaxed on a supply mission. The size of the launch window, in turn, affects the variation in terminal geometry, and the ascent and terminal phase performance requirements. 2. Ground Tracking and Control On-board Tracking and Control. For many earth-orbiting missions the ground track will be well covered by tracking stations which can determine a precise ephemeris for the target and/or the chaser, which, in turn, permits ground control of orbital maneuvers up to some point in the terminal phase. This procedure would not be possible, of course, for lunar orbital rendezvous. A related question is then the point in the terminal phase at which relative position and velocity data are adequate for terminal guidance. 3. Cooperative: vs. Non-cooperative Target. Rendezvous with a non-cooperative target usually requires a high powered skin tracking radar capability of acquisition at fairly extended ranges whereas a cooperative target permits use of a transponder, greatly reducing power requirements, and also pernuts cooperative attitude reorientation and corrective maneuvers of the target. 4. Manned vs. Unmanned. It, has been demonstrated by ground simulation that a pilot can visually monitor and control portions of the terminal phase of a rendezvous and docking mission, whereby the tracking and computation requirements for such missions can be greatly altered. Even within the framework of a given mission there are a number of major system parameters to be determined. For example, the question of whether tracking sensors can be fixed to the body or require isolation from vehicle motions in the acquisition and terminal phases, in turn depends on the nature and accuracy of the vehicle stabilization and control systein, and the number and location of corrective rockets which will affect the reorientation requirements to achieve velocity corrections. Furthermore, the number and size of these corrections, which depend on tracking accuracy, will affect whether or not tracking is required during such corrections. The factors which lead to a choice among radar, optical or infrared targeting during the various phases will he discussed. Finally, the discussion and comparison of various guidance laws for controlling the closing rate and lateral corrections will be discussed, including the effects on propulsion requirements and tracking accuracy requirements. In particular, proportional navigation, bang-bang correction, and orbital maneuver laws will be compared.