New method for on-orbit-determination of parameters for guidance, navigation and control

For most scientific satellite missions it is required that the attitude control system works with the greatest possible accuracy to achieve the missions objectives. Therefore it is necessary to know the non-linear system "satellite" with its different parameters e.g. the moments of inertia. the thruster parameters, mass, etc. as exactly as possible. But in most cases these parameters cannot be determined on earth so it has to be done on-orbit. For that reason an applicable estimation method must be found which is robust against all kinds of disturbances acting on the satellite: disturbing torques, noise and offset of the satellites sensors and actuators. Another condition is that only the measurements of the satellite sensors (attitude and angular velocity) can be used for determination. Because of these constraints and the complex non-linear behaviour of the satellite linear and conventional non-linear methods cannot be used for the estimation. A new method must be used. The idea is to compare the sensor measurements which are influenced by the just mentioned disturbances and the results of a computer-aided simulation of the non-disturbed satellite. With the difference between the measured and simulated sensor informations a cost function is determined. In an iteration loop the parameter values are estimated by varying them until this function reaches its minimum. This is performed with the simplex-method by NELDER and MEAD. Here the applicability and accuracy of this method is proved exemplarily by estimating the parameters of a satellites thruster system. This method delivers the parameters sufficiently exact independent from initialized errors in the simulation and disturbances on the real satellite. This method is also applicable to other parameters Re for example the moments of inertia or the solar pressure torque, etc.. (C) 2002 International Astronautical Federation. Published by Elsevier Science Ltd. All rights reserved.