Fig. 4. Conceptual motion of PF relative to MSS. PF station on the forward side of MSS, it is decelerated with the operating thrusters, and goes down forwards in the descending mode. At the point A (in Fig. 4) properly apart from MSS, PF turns around to be in the ascending mode. Then, the thruster exhaust plasma could be monitored with the instrument on MSS. The behavior of the plasma perpendicular to the geomagnetic field is observed. At the point B, there are two ways to go on to the back of MSS in the same attitude or to return to the original point in front of MSS with the thrust vector changed by 180°. If the thrust vector is pointed perpendicular to the orbit plane, the orbit inclination angle is changed and the plasma flow whose axis is parallel to the geomagnetic field can be realized as shown in Fig. 5. This inclination change may be done with a thruster other than the present electric thrusters. OPERATION CONSTRAINTS There are some constraints on these operations to be considered as listed in Table 3. Direct communication without blocking by the Earth is possible within the orbit phase difference of about 43 deg between PF and MSS. This will be the range limit for the proximity operation. In the vicinity of MSS, the special attention should be paid on the EMI and contamination efflux. These will be measured during the tests. Collision free operation is absolutely important for the flight of the large structures. In the present mission, the acceleration is so gradual that the interception can be refused by computing the orbit with an onboard computer and correcting it with the testing thruster or, in a critical case, hydrazine thruster. In the distant operation, the PF must return to the MSS with the hydrazine thruster with the contingency that the electric thruster system is out of function. The operational area is constrained by the quantity of hydrazine.
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