DOCKS Propagator: An Open-source Adaptive Time-step Trajectory Propagator for CubeSat Missions

DOCKS is an open-source software that computes the major contributors in the system budget for the mission profile of nanosatelites. The deep-space trajectory propagator is a module of DOCKS that can include gravitational forces from point masses as well as complex bodies, solar radiation pressure, and continuous propulsion. Three integrators are provided in the propagator for trajectory generation: a fixed time-step method, Runge-Kutta 4 (RK4), and two adaptive time-step methods, Runge-Kutta-Fehlberg 45 (RKF45) and IAS15. To determine the accuracy of DOCKS, a comparison is carried out with GMAT using RK4. The integrators are then validated using the analytical solution of the two-body problem and compared in high-eccentricity orbits and close encounters. The tests show that RK4 with a small time-step produces accurate results in all scenarios; however, it computes unnecessary points and the accuracy of results is solely dependent on the time-step input by the user. Although RKF45 takes much less time, it is incapable of detecting close encounters because it adapts the time-step only based on the probe's velocity without considering the acceleration. However, IAS15 takes into account the forces acting on the satellite, and thus, generates a well-balanced sampling with fewer points. In addition, IAS15 outperformed all methods by generating highly accurate trajectories in short and long term simulations; however, it can take a much longer time in some scenarios. The addition of IAS15 integrator to the DOCKS Propagator makes it applicable in multiple astrophysical problems and provides a tool that does not require the user to have prior knowledge to determine the accurate time-step needed for the propagation.