Trajectory Control Programs in Support of Apollo Missions

the trajectory of a spacecraft in flight has been predicted, control measures are required to ensure that the trajectory data is applied in a consistent manner when calculating the many trajectory-related parameters required by the flight controllers of the space fligh t. This paper describes the queue control techniques used in generating the predicted trajectory and the subsequent use of the trajectory data. The queue control logic that is discussed requires a minimum amount of main storage while a task is waiting to be performed. The real-time computer programs' used in support of the Apollo missions are composed of highly complex mathematical programs, various control programs, and a sophisticated operating system. This paper describes a set of control programs, referred to as the trajectory control programs, that make use of features provided by the operating system to enhance the integrity and consistency of the output from some of the mathematical programs. A large percentage of the data viewed by the flight controllers of a space flight is calculated from a table of position and velocity vectors that describe the predicted spacecraft trajectory for the next several hours.' The predicted trajectory, or ephemeris, is used to calculate such items as the time to fire retro-rockets to land in a given place, the direction in which a given radar antenna should be pointed to make contact with the spacecraft as soon as it crosses the horizon, which tracking stations will sight the spacecraft in a given revolution, and how a planned maneuver will change the current trajectory. Since many critical decisions are based on ephemeris data, great care must be taken to ensure that the ephemeris is updated in a timely manner and that other programs that might be affected by a changing ephemeris are locked out while the ephemeris is being generated. For example, data needed for determining the positions of contact stations might be rather disjointed if part of it were 12 QUARLES IBM SYST J