Autonomous Control System for Precise Orbit Maintenance

In this paper, we describe a closed-loop autonomous control system that enables orbit operations to be performed without the need of any ground segment. The growing availability of GPS receivers on satellites provides an excellent means for autonomous orbit determination and our work builds upon previous work on orbit determination algorithms developed here at Surrey. The orbit is described using a set of epicycle parameters which provide an analytic model of LEO orbits. The parameters in this model are estimated onboard the satellite using a Kalman filter. We describe an enhancement to this software which provides both control as well as estimation of the orbit parameters and a discussion of how atmospheric drag has been included in the model. The goal of the control part of the software is to ensure that the orbital altitude of the satellite never falls outside of a prescribed window due to drag. We present results of the orbit maintenance software which has been successfully running on Surrey's minisatellite UoSat-12. This satellite is in a 650 km altitude orbit at inclination 64.57o . The satellite has been manoeuvred into a repeat ground track orbit so that the satellite repeats its ground track every 7 days. The orbit maintenance software then attempts to maintain the satellite in its resonant orbit, and also to slowly manoeuvre the satellite into a frozen orbit so that the altitude at each pass does not vary.