Differential drag controllers have been examined in the context of a full system simulation of a target/chaser pair of spacecraft in low Earth orbit. An Extended Kalman Filter has been designed to process measurement sets from GPS receivers on the target and chaser spacecraft. The estimated state from the Kalman Filter is used in a differential drag control algorithm to determine the appropriate control action. Modifications are made to the standard differential drag control algorithms to reduce unnecessary actuations in the presence of errors in the dynamic modeling, control actuation, and incoming measurements. Detailed explanations of the algorithms, dynamic models, and derivations for both the Kalman Filter and the differential drag control laws are presented. Multiple test cases are used to validate the controller performance under a variety of initial conditions. In these simulations, the differential drag control algorithms successfully maneuver the chaser spacecraft from the initial conditions to a final state with instantaneous time-average position (relative to the target spacecraft) of not more than 10 meters in the radial and in-track directions. Modifications to the standard control algorithms ensure that extraneous control actuations are minimized. An optimization algorithm is used determine the time-optimal differential drag control history, and the results are compared to the standard control logic and modified control logic. Based on the optimization results, it is recommended that a system employing differential drag control (especially those with limited computational resources) should use the modified control logic, as it provides a standardized methodology that can be followed in any mission. / text

Benefits of developing missions with multiple formation flying spacecraft as an alternative to a traditional monolithic vehicle are becoming apparent. In some cases, these missions can lower cost and increase flexibility among other situational advantages. However, there are various limitations that are imposed by these missions that are centered on the concept of maintaining the necessary formation. One such limitation is that of the propulsion system required for each spacecraft. To mitigate the complexity and mass of the onboard propulsion, the pairing of electromagnetic actuators and differential drag to replace the functionality of a propulsive system is investigated. By using COTS magnetorquer boards to command satellite orientation, a scenario in which two 3U CubeSats are initially deployed from the ISS NanoRacks at an altitude of 400 km. They are then commanded to achieve a relative separation of 1 km and hold the spacing to demonstrate the capability of formation flight. The scenario was simulated through the MATLAB/Simulink platform and the magnitude of the necessary command torques were determined. By comparison to the ISIS magnetorquer board, the necessary command torques seem relatively high than compared to what the actuator is capable of. The ISIS board may supply ~5e-6 Nm of torque while the mission requires as much as 3e-3 Nm at times. However, by extending the settling time of the control law at the expense of absolute orientation control, the control torques necessary to carry out the simulated mission are well within the bounds of the ISIS magnetorquer boards as well as other COTS boards. With this alteration, mission feasibility is determined. It should be noted that further analysis should be conducted regarding concerns with CubeSat detumble to further confirm feasibility.

Due to mass and volume constraints on planetary missions, the development of control techniques that do not require fuel are of big interest. For those planets that have a dense enough atmosphere, aerodynamic drag can play an important role. The use of atmospheric differential drag for formation keeping was first proposed by Carolina L. Leonard in 1986, and has been proven to work in Earth atmosphere by many missions. Moreover, atmospheric drag has been used in the Mars atmosphere as aerobraking technique to decelerate landing vehicles, and to circularize the orbit of the spacecraft. Still, no literature was available related to formation flying on Mars. To analyze the use of differential drag on the Mars atmosphere, the researcher accessed the two high resolution models available: NASA’s Mars-GRAM and ESA’s Mars Climate Database. These models allowed the simulation of conditions that a spacecraft would experience while in orbit around the planet. To explore the feasibility, the researcher first conducted a study where Mars atmosphere density was compared to Earth atmosphere, determining its applicability. Then, a simulation using MATLAB® was conducted, using a Keplerian two-body problem including the effects of Mars zonal harmonics (i.e. J2) and drag perturbations. Two 6U CubeSat were used in the simulation with deployable drag plates of different sizes, giving the possibility of having five differential drag scenarios as means of formation control. The conclusions showed that, although with some limitations, the use of differential drag as means of autonomous formation flying and proximity operations control is feasible using proven techniques previously validated in Low Earth Orbit. Lyapunov control was selected as the control strategy, where three different methods were evaluated and compared.

No description available.