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 to provide continuous coverage of the ground station with the high-gain antenna during the translunar coast phase (except for sites west of about 20° W longitude). This will not be the case for the transearth coast phase, but the situation is not considered to be a problem since the ground station (because of its higher radiative power) will always be able to contact the spacecraft over the omni antennas, and request that the roll maneuver be discontinued for a period, if continuous communications were required for some reason.

A second midcourse correction will be made about one hour after entering the moon's sphere of influence, several hours before reaching lunar orbit. It is executed with the SPS in the same manner described previously.

Following another rest period, the crew begins a period of considerable activity which will continue for the next several hours through lunar landing and the first exploration period. These activities will be discussed in detail in a later paper by Mr. Loftus, but the highlights will be mentioned here.

About three hours before lunar orbit insertion, the LM will be checked out. After pressurizing the LM, which has leaked down during the long coast period, one of the three crew members will transfer to the LM through the tunnel and begin an activation and checkout of the LM systems (figure 33). This is considered desirable for two reasons: First, it provides the ground and the crew with the first knowledge since leaving the launch pad that the LM systems will indeed be able to function properly for a lunar landing. The discovery of some system malfunction which would preclude lunar landing may be sufficient reason not to commit the mission to lunar orbit.

The second reason for LM checkout prior to lunar orbit is to assure that the descent propulsion system is available as a ready means of abort in the event of SPS failure during lunar orbit insertion. Since the abort mode using the descent propulsion system also requires the use of the LM guidance and control system, it will be necessary to activate, checkout and align this system also. Similarly, the Environmental Control and Electrical Power Systems will need to be activated. A second crewman will join the first when freedom from his duties in the CM permits.

Continuous ground tracking confirms that the spacecraft is indeed on the proper course and the decision is made to proceed to lunar orbit insertion. The LM crew has returned to the CM by this time and the next operation is to orient for lunar orbit