Development of real-time orbital propagator software for a Cubesat's on-board computer

Abstract

A precise orbit propagator was developed for implementing on a CubeSat's on-board computer for real-time orbital position and velocity determination and prediction. Knowledge of the accurate orbital position and velocity of a Low Earth Orbit (LEO) Cubesat is required for various applications such as antenna and imager pointing. Satellite motion is governed by a number of forces other than Earth's gravity alone. The inclusion of perturbation forces such as Earth's aspheric gravity, third body attraction (e.g. Moon and Sun), atmospheric drag and solar radiation pressure, is subsequently required to improve the accuracy of an orbit propagator. Precise orbit propagation is achieved by numerically integrating a set of coupled second order differential equations derived from satellite's perturbed equations of motion. For the purpose of this study two numerical integrators were selected: RK4 - Fourth order Runge Kutta method and RKF78 - results from embedding RK7 into RK8. The former is a single-step integrator while the latter is a multi-step integrator. These integrators were selected for their stability, high accuracy and computational efficiency. An orbit propagation software tool is presented in this study. Considering the processing power of Central Processing Unit (CPU) of CubeSat's on-board computer and a trade-off between precision and computational cost, the 10 x 10 and 20 x 20 gravity field models, the Exponential atmospheric model and Jacchia 70 static atmospheric model, were implemented. A 60 x 60 gravity field model is also investigated for reference. For validation purpose the developed software tool results were compared with results from Systems Tool Kit (STK) and Satellite Laser Ranging (SLR) using SUNSAT satellite reference orbit.