• An antenna gimbal system will provide coarse antenna pointing. • Fine pointing and stability of the microwave beam in two axes will be provided by a phase control system. • Reaction control system thrusters will provide the actuation forces for attitude control and station keeping. • A pilot beam (or other means) from the ground rectenna will provide the information necessary to compute the microwave beam pointing errors. 7.1. 6. 2 ATTITUDE CONTROL COORDINATES Figure 7-27 defines the main body and microwave antenna coordinates that are to be used in the pointing and attitude control definitions and discussions. In the reference position the antenna coordinates X , Y , and Z are in alignment A A A with the main body coordinates X , Y , and Z , where: B B B 1. The Z axis is normal to the solar array and aligned to the Sun. B 2. The X minimum inertia axis varies seasonally between X-POP andX-PEP. 3. The Y^ axis in the plane of the solar arrays forms a right hand coordinate frame. 7.1. 6. 3 OPERATIONAL ATTITUDES The reference operational attitude (Sun oriented at all times) has the body axis Z$ aligned with the solar vector. At the solstices (Fig. 7-28), the X^ axis is perpendicular to the ecliptic plane, and at the equinoxes, X^ is perpendicular to the orbit plane. Between these major seasons, the attitude varies to meet the Sun pointing conditions and to maintain structural clearance for the rotating microwave antenna. The antenna, in tracking the ground rectenna, rotates 360° per day about the X axis. In addition, at the solstices the antenna must tilt and roll B ±23. 5° each orbit. A constant tilt angle per orbit is required during the equinoxes. This tilt angle at equinox is a function of the geographical location of the rectenna. Between the major seasons, the total tilt angle varies around the equinox tilt angle.
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