Browsing by Author "Rossouw, Nico Chris"
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- ItemA GPS-based on-board orbit propagator for low earth-orbiting CubeSats(Stellenbosch : Stellenbosch University, 2015-12) Rossouw, Nico Chris; Steyn, W. H.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: On-board knowledge of satellite position is vital to all space missions. Due to the stringent power budgets of the CubeSat form factor, permanently active on-board GPS receivers for accurate navigation solutions are infeasible. Analytical techniques such as SGP4, which have been used since the 1970s, are often identified as the natural replacements. Unfortunately, due to inherent position errors of more than 1 km RMS, such techniques are unfit for some missions that prioritise precision. This emphasises the need for a new strategy for accurate on-board orbit determination and propagation. In an attempt to conserve power, it is proposed that the GPS receiver is activated intermittently such that a duty cycle of less than 15% (including the time-to-first-fix) is required. The orbit will be estimated during the activated period, after which navigation solutions will be obtained by means of propagation algorithms during the deactivated parts. Two approaches, one based on an analytical method and the other on numerical integration, are designed and implemented in the C programming language. These systems are simulated using actual on-board GPS datasets from SumbandilaSat and NigeriaSat missions. The analytical approach is based on SGP4. TLE parameter offsets are estimated by filtering the difference between the GPS and SGP4 output instantaneous Kepler elements, and an additional time offset is calculated for an in-track correction. Simulations revealed RMS and maximum 3D position errors of 200 m and 1 km, respectively, for a 12.5% duty cycle. The numerical approach employs an Extended Kalman Filter with a 4th order Runge-Kutta integrator for propagation. Reducing the propagator’s orbit dynamics model to only a 9th order JGM-3 geopotential and a Jacchia atmospheric density model for aerodynamic drag still produced fairly accurate results. Simulations revealed RMS and maximum 3D position errors of 60 m and 300 m, respectively, for a 10.7% duty cycle. The proposed system therefore delivers a remarkable accuracy improvement over standard analytical propagators at only a fraction of the power required by permanently active on-board GPS receivers.