(2) Truss Configuration The preceding discussion of the column/cable configuration also applies, in general, to the truss. The major exception is that counterweights were not considered. The structural arrangement does not provide a natural attach point for counterweights, although such points could be provided at a modest weight penalty. Propellant required to overcome short-period gravity gradient torque is shown in figure IV-B-4-8 as a function of the length/ width ratio of the solar array. Propellant mass is given as a 30-year total to facilitate comparison with figure IV-B-4-3 which presents similar data for the column/cable configuration. It can be seen that propellant requirements are substantially higher for the truss configuration. High length/width ratios requires less propellant. POP orientation is preferred for the truss as well as for the column/cable configuration. The logic is the same, although numerical values differ. c. Sensing The SPS presents a unique attitude sensing problem in that the axis to be held POP (nominally the Y axis) is not a geometrical axis but the axis of minimum moment of inertia. The relationship between these two axes cannot be precisely determined in advance and varies with time. Therefore, the geometrical axis will be used only as an initial approximation. After start-up, the control computer will monitor the RCS firing command history, infer from this history the actual position of the minimum-inertia axis relative to body axes, and make the necessary corrections to the thruster firing commands to maintain the actual minimum-inertia axis POP. Star trackers together with the known orientation of the orbit plane (from ground tracking) provide a continuous attitude reference. Angular rates will be below the capability of rate gyros and will be computed as time rates of change of attitude. X-axis orientation perpendicular to the solar vector will be detected by sun sensors. The criterion here is orientation of the body axis, not the principal axis of inertia, and the problem is straightforward. Orbit parameters will be determined by ground tracking (optical and/or radar) both for greater accuracy and because orbit corrections must be commanded from the ground to insure coordination with other satellites, since all SPS will be in the same orbit.
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