located on the instrument booms will be used, 3 on each boom instrument head. (This will require a very accurate position determination of the boom rotation angles.)
Three 3-axis rate-gyro assemblies will determine the rate of change of any two pointing angles and the spacecraft roll rate. This data will supplement the trackers' information and increase the attitude determination accuracy.
Star tracker parameters:
- Solid state (vice photomultiplier tube)
- 4 arcsecond accuracy (future improvement is expected)
- Magnitude range −1 to +8
- Field of view 6 × 6 degrees
- 6 seconds to search field of view
- 1.2 seconds to search field in 1 × 1 degree search mode
- Has a track mode during which it follows a specific star
- Total weight — 189 kg
- Total power in transit — 324 watts
- Total power after probe separation — 162 watts
A summary of the attitude control systems available to choose from are listed in Fig. 3.6a. The probe's attitude control will be accomplished using two sets of flywheels arranged on 4 axes (described in section 3.3 Power System), and an auxiliary system of hydrazine thrusters. These flywheels will serve as momentum wheels, controlled by the computer using the attitude/rate information, providing torque to maintain spacecraft stability. The 4-axis configuration will enable the reaction wheels to absorb external torques from any direction. The magnitude of this reaction torque is easily modulated by electronic control of the reaction wheel motor current. One disadvantage of this system is the need to control wheel speeds in order to limit vibrational effects. "Unloading" the energy of
