# Novel 3U Stand-Alone CubeSat Architecture for Autonomous Near Earth Asteroid Fly-By
## 4 Subsystems Analysis
### Attitude Determination and Control System  
The ADCS is a combination of several sensors to determine the state of the spacecraft (e.g., Sun sensors, star trackers, hall-sensors, etc. ), and actuators to control the orientation of the satellite (e.g., reaction wheels, thrusters). Many companies offer integrated ADCS solutions with all necessary sensors and actuators available, including processors capable of doing most of the computations needed for controlling the satellite. Thus these integrated solutions will be the first focus for the ADCS selection. In terms of functionality and performance for the mission discussed in this paper, pointing accuracy (the degree of accuracy with which the ADCS can point the satellite in a specific direction) is the main parameter influencing the selection of the ADCS. The pointing accuracy is mainly limited by three factors: the FOV of the payload, the directional accuracy required for thrusting maneuvers, and the maximum amount of pointing losses allowed for downlink data. For the thrust maneuvers no required accuracy is derived as it is expected to be lower than the payload one; moreover, a proper GNC simulation is left for future works, where the pointing accuracy influence on the propulsive maneuvers is simulated and related to fly-by altitude. Thus, it is assumed the two other factors are the main drivers for the pointing accuracy. It is noted here that factors like jitter and agility are also important for the performance of the payload and communication subsystem. However, as this data is not readily available in most cases, the pointing accuracy will be taken as the major selection criterium. An additional requirements for the ADCS during an asteroid fly-by is related to the relative angular velocity and the payload FOV. The ADCS should allow to dynamically keep track of the target during all the phases of the close approach. The ADCS maximum slew rate should be higher than the maximum relative angular velocity. This assessment depends on the relative velocity and the altitude at the close encounter. Figure 6 reports the relative angular velocity in function of the altitude and relative velocity. Above 450 km of altitude, the required slew rate is below \(1.5^{\circ}/s\) also for high relative velocity (12 km/s). As long as the fly-by altitude decreases, the required slew-rate increases. Then, depending on the ADCS available slew rate and relative velocity, the target fly-by altitude may be tuned. For the actuators it is important that there is sufficient momentum storage in the reaction wheel, or a sufficient of propellant in case of reaction thrusters, and/or that there is a solution to de-saturate the wheels. For most LEO satellites this is done using  
Figure 6: Relative angular velocity at the closest approach in function of the fly-by altitude and relative velocity. magnetorquers, but as the magnetic field of the environment is negligible, this is not an option. For most deep-space applications, this problem is solved using reaction thrusters that provide a counter torque during momentum dumping. There are only a few off-the-shelf options for CubeSats in terms of reaction thrusters. One is the Aurora Resisttojet Module [57], a water-based resistojet. Due to its chemical-free risk and its 12 thrusters, it is able to properly control and de-saturate the CubeSat. However, the minimum volume and mass configuration (0.3U, 0.35 kg of wet mass) increases the complexity of the design. Another option is the VACCO MiPS cold gas propulsion system, which can be used for both translational and attitude maneuvers. It is similar in size, but only contains 5 thrusters which can be used for all 6 degree-of-freedom [38]. GomSpace offers NanoProp CGP3, a flightproven cold gas propulsion system for 3U CubeSats, characterized by 4 individual thrusters, and by 0.5U volume [58].