# InnCube--A Wireless Satellite Platform to Demonstrate Innovative Technologies
## 3 Spacecraft Overview
### Attitude Determination and Control System--ADCS  
Given a 600 km sun-synchronous LEO, InnoCube is expected to be subject to external disturbance forces lower than \(10^{-7}\) Nm, not considering the satellites residual magnetic moment. It is hard to estimate the residual magnetic moment, but literature analysis expects it to be less than 0.1 Am\({}^{2}\) for a typical CubeSat [20; 21]. Considering 0.2 Am\({}^{2}\) torque rods allows for an active magnetic actuation with a maximum torque of \(1\times 10^{-5}\) Nm depending on the orbital position. System requirements imposed on the ADCS derive mainly from the EPISODE payload. The communication subsystem does not require specific attitude control because of the antenna's omnidirectional characteristics. The EPISODE payload requires two main scientific attitude control modes: GNSS Antenna Pointing and Laser Ranging Ground Station Tracking. GNSS Antenna Pointing resembles an anti-nadir tracking, which ensures an unobstructed field-of-view into medium Earth orbit (MEO) of the GNSS antenna. Assuming a \(\pm\)50 deg antenna beam width leads to a \(\pm\) 130 deg anti-nadir attitude requirement. Assuming a 600 km circular orbit leads to maximum attitude of 3.75 deg/min for anti-nadir tracking. Laser ranging ground station tracking orientates the LRR in the direction of the laser ranging ground station and tracks this specific attitude. Figure 10 associates different contact elevation angles with required torque for optimum tracking performance. The LRR tolerates \(\pm\)17 deg deviation from the optimum tracking orientation which is 20 deg off-axis pointing of optimum line-of-sight. With final simulation still pending, 2.3 mNm has been chosen to be the minimum torque requirement for the attitude actuation. In order to accommodate these requirements, a typical 3-axis ADCS will be implemented [18; 19]:Figure 11 gives a basic system overview: attitude determination will be carried out using redundant magnetic and gyroscope sensors combined with six custom two axis sun-sensors, developed at the University of Wuerzburg. An extended Kalman filter will be used for sensor-data fusion and final attitude estimation. This allows an absolute attitude reference necessary for the attitude controlling as well as the estimation of the gyroscope biases. A yet to be determined quaternion feedback controller will serve for the nadir and tracking control laws. A basic B-dot bang bang detumbling controller can be used for the initial damping of the rotational rates as well as for possible desaturation of the wheels.  
The low dynamic GNSS antenna pointing mode can be accommodated by the use of three magnet-torquer actuators only. The magnet-torquers will be based on a self-developed ferrite core torque rod. Because of the aforementioned minimum torque requirements, laser ranging ground station tracking requires the use of a three-axis reaction wheel actuation system. The magnet-torquers are utilized to desaturate the wheels if necessary. The ADCS software includes the telecommand and telemetry management as well as fault detection and isolation (FDIR) procedures.  
Figure 11: Block diagram of attitude determination and control system (ADCS) subsystem.  
Figure 10: Example required torque for optimum pointing of different contact elevation angles.  
The ADCS hardware (except for the external sun-sensors) will be combined in a dedicated mechanical structure, which will subsequently be mounted inside the satellite. This structure also includes the ADCS mainboard along with the microcontroller running the ADCS software. The advantage of this closed-box design is the improved stand-alone testing capabilities without the need for a completely assembled satellite structure. Figure 12 shows the CAD-model (computer-aided design model) of the ADCS box which can be integrated into the satellite using the mounting wedgelocks on the upper and lower sides. The only electrical interface is the backplane-connector mounted on the mainboard.