NASA JPL Viking Mars Orbiter Sun Sensor and Canopus Tracker Control Panel

From NASA Facts: Viking Mission to Mars: “The Viking orbiter was stabilized in flight by locking onto the Sun for pitch and yaw references and onto the star Canopus for roll reference. The attitude control subsystem kept this attitude with nitrogen gas jets located at the solar panel tips. The jets would fire to correct any drift. A cruise Sun sensor and the Canopus sensor provided error signals. Before Sun acquisition four acquisition Sun sensors were used and then turned off.

The attitude control subsystem also operated in an all-inertial mode or in roll-inertial with pitch and yaw control, still using the Sun sensors. During correction maneuvers, the attitude control subsystem aligned the vehicle to a specified attitude in response to com- mands from the on-board computer. Attitude control during engine burns was provided in roll by the attitude control subsystem and in pitch and yaw by an autopilot that commanded engine gimballing.

If Sun lock was lost the attitude control subsystem automatically realigned the spacecraft. In loss of Canopus lock, the subsystem switched to roll-inertial and waited for commands from the spacecraft computer. The nitrogen gas supply for the subsystem could be augmented by diverting excess helium gas from the propulsion module, if necessary.

Two on-board general purpose computers in the computer command subsystem decoded commands and either ordered the desired function at once or stored the commands in a 4,096-word plated-wire memory. All orbiter events were controlled by the computer command subsystem, including correction maneuvers, engine burns, science sequences and high-gain antenna pointing.”