Orbit Trim Strategies for the 1975 Mars Viking Mission

Abstract

Propulsive maneuver strategies have been developed for the Mars orbital phases of the Viking mission, including the activities before and after lander deployment. These strategies have been formulated as fixed sequences of orbit parameter-correction maneuvers which will be performed on specified spacecraft revolutions. Certain adaptive features are also available to ensure efficient use of propellants. Numerical simulations demonstrate that it is possible (to within a very high probability) to satisfy mission requirements on orbital navigation, using these strategies and Viking technology. HE United States will send two unmanned spacecraft to Mars during the 1975 launch opportunity. Each of these spacecraft will orbit Mars and deploy a soft-lander to the surface of the planet as part of the Viking Project. These objectives place a number of interesting and stringent requirements on the control of the satellite orbit to obtain near periapsis reconnaissance of the landing site and to prepare for lander release. As a result, the preflight navigation analyses and maneuver strategy development have been considerably increased in scope and complexity over previous missions. Reference 1 describes the Viking mission objectives and overall navigation profile from trans-Mars injection through the postlanding station-keeping phase. During the Mars orbit trim (MOT) phase of each flight, a sequence of orbit trim spacecraft propulsive maneuvers is performed to remove the effects of orbit determination and maneuver execution errors, plus any intentional biases, remaining after earlier maneuvers in the flight. The sequence of trim maneuvers is performed according to a predetermined strategy designed to achieve the mission objectives. This paper describes the maneuver objectives and the resulting strategies for the Mars orbital phases of the Viking mission, including the activities before and after lander deployment. The prelanding activities are treated in Sec. II-IV, with the consideration of postlanding activities being reserved for Sec. V. Numerical results are included in Sec. IV and V to demonstrate that the strategies satisfy all mission requirements which have been identified The preflight navigation analyses for Viking have included the analyses of the two typical missions defined in Table 1. The . typical candidate for the first spacecraft is referred to in this paper as Mission 1, while the candidate for the second spacecraft is called Mission 2. These example missions are considered in this paper to clarify techniques and to provide realistic numerical results. They are not intended to contribute a parametric analysis of all possible Viking missions. Table 1 Selected parameters for Missions 1 and 2 Parameter Mission 1 Mission 2