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 the alignment of the CM inertial Platform. This is done both as checkout of the IMU and its associated equipment and to establish a precise onboard inertial reference as a backup to the inertial reference in the S-IVB Instrument Unit.

The spacecraft attitude at this point is such that the longitudinal axis is in the local horizontal and pointed in the direction of flight. This local horizontal mode is maintained automatically throughout the earth orbit phase by a constant pitch rate equal to the orbital rate of .067 deg/sec. The S-IVB control system provides this mode and maintains it within a one-degree deadband. This orientation in the local horizontal mode assures communication coverage of the spacecraft and launch vehicle antennas when passing over a ground station.

The crew takes over manual control of the vehicle attitude through the S-IVB control system during spacecraft operations requiring specific attitudes. This is the case during the IMU alignment mentioned earlier. Depending on the time of day that launch occurred, the sun location may interfere with the optics line of sight during the IMU alignment. A roll maneuver would then need to be performed by the CM crewman before beginning the alignment. Following the alignment, the crew would return the vehicle to the original roll orientation with the -Z axis of the spacecraft point­ ing down the local vertical.

Because of the relatively high inertias of the vehicle, these maneu­ vers must be performed at relatively slow rates to conserve S-IVB RCS propellant. Present rates are set at .3°/sec. in pitch and yaw, and .6°/sec. in roll. For the IMU alignment operation, then to roll 60°, say, to avoid the sun will require 3 minutes; allowing 10 minutes for alignment, and another 2 minutes to roll back for a total of 14 minutes. This figure, together with the time required to get set up for the operation, means that some 40 minutes have elapsed since liftoff before this operation is completed. The space­ craft's position at this point would be over the Indian Ocean.

Meanwhile, the other systems checkout are being conducted by the other two crew members. Data transmission and voice communication are being maintained over every ground station. In between the stations, data is being recorded onboard for playback when over a station. Tracking periods by ground-based S-band stations (subsequent to the initial insertion ship tracking will have been provided by the Canary Island station for the case of northerly launch azimuths and by Ascension for the southerly azimuths. For the range of around 90° azimuth, tracking by a ground-based station will not be avail­ able until the pass over Australia; however, a ship in the Indian Ocean will be stationed to provide coverage before this time. These