Viking Orbit Trim Maneuvers

Abstract

THE Viking missions to Mars provided significant navigation challenges. This paper describes the maneuver analyses performed in response to real-time requirements for orbit modification for the first Viking spacecraft prior to landing. Included is a discussion of the propulsive maneuver strategies and operational techniques that were developed to allow for examination and certification of proposed landing sites, to provide for safe entry and descent of the lander, and to cope with hardware failures. Contents Viking was the first United States project to send softlanders to the surface of another planet. Each spacecraft consisted of an orbiter with an attached lander. Following the delivery of this combination to Mars, the spacecraft was inserted into planetary orbit to reconnoiter proposed landing sites. Once a site was certified, the lander separated from the orbiter to descend to the Martian surface. The orbiter's task in this mission phase was to provide the proper orbit geometry and timing to certify the site and prepare for landing. Site reconnaissance specified viewing angle requirements on early spacecraft revolutions about the planet. The dominant angular constraint was considered in terms of an equivalent timing offset which measured how far the spacecraft missed overflying the landing site. Orbit requirements for a safe entry and accurate landing were specified for the spacecraft revolution on which the lander separated from the orbiter (separation orbit). The navigation task was to position the spacecraft so that the downrange and crossrange (DR and XR) capability of the lander could easily acquire the desired site within the entry environment constraints imposed on the lander. The separation orbit also had to guarantee lander-to-orbiter communications at overflight for several orbits after landing. Many trim maneuver strategies were considered1 to obtain a satisfactory trajectory while coping with several error sources, notably the interplanetary navigation delivery, orbit determination,2 and maneuver execution errors. A strategy consisted of a sequence of propulsive maneuvers designed to shape and orient the orbit and to time the spacecraft to allow imaging of proposed landing sites under proper lighting and to achieve a specific separation, orbit which changed often as the site selection process continued. Each maneuver consisted of a sequence of turns, followed by a motor burn to achieve the desired velocity correction AKat the ignition time /ign. The operational timeline required the turns to be specified (with