System Components of GRACE-FO
| Instrument System | Function |
|---|---|
| Fluxgate Magnetometer | Provides coarse attitude based on satellite’s position and a model of Earth’s magnetic field. This is helpful as a measure of redundancy and as a system to quickly determine the magnetic field at the expense of accuracy |
| GPS Receiver | Determines the position of the two satellites. It measures the relative change in the positions of the satellites |
| Star Tracker Assembly | Transforms data to inertial reference. It precisely calculates the absolute orientation of each satellite by measuring its relative position with respect to stars. There are 3 redundant star tracker heads |
| High-Performance Gyro Assembly | Measures attitude rates during spacecraft emergency modes. The gyro provides the satellites’ rates of rotation |
| Coarse Earth and Sun sensor | Determines the coarse attitude. This provides faster and rougher attitude updates than the star tracker assembly. This is useful for initial attitude acquisition and redundancy |
| IMU | |
| Magnetic Torquers | |
| Cold Gas Propulsion System | Provides thrust impulses for orbit control |
AOCS Architecture
- Attitude Measurement Assembly
- Digital Earth Sensor (STD) - Roll and Pitch
- Digital Sun Sensor (SSD) - Yaw
- 4 independent 2-axis gyros (GYROS) - For rate of rotation
- Wheels and Magnetotorquer Assembly
- Three 40 Nm reaction wheels (RW) - For torque control
- 2 magnetotorquers (MAC) - For torque generation
- Monitoring and Command Unit (EAIM)
- Propulsion and Solar Array Drive Mechanism (SADM) Assembly
- 4 pressurized tanks in blow-down mode - Each contains 300 kg of Hydrazine
- 2 branches of eight 23.5 N thrusters located on SVM
- Safe Mode Assembly
- Safe mode dedicated electronics unit (T4S)
- Sun Acquisition Sensors (SAS)
- Safe mode software
- OBDH (On-Board Data Handling) Bus1
Info
Blow-down mode - In propulsion systems using Hydrazine (for example), the pressurant gas is pumped into the same bladder tank that contains the propellant. The pressurant “blows down” or forces the propellant into a perforated axial stand-pipe that’s present in the tank, thus causing propulsion. It’s simple and reliable2