Vishal Ray, University of Colorado Boulder; Daniel Scheeres, University of Colorado Boulder
Keywords: Prediction, Estimation, Drag Coefficient, Atmospheric Drag, Gravitational Field, Solar Radiation Pressure
Abstract:
In most satellite tracking operations, the gravity field is truncated at an order and degree based on their average effects on the motion of the satellite. Even though the contribution of the higher-order gravitational harmonics towards the propagation of the satellite states may be negligibly small as seen in isolation from other forces, their aliasing with the non-gravitational force coefficients can significantly affect the prediction capabilities in the tracking process. The correlation of the gravity field with the non-gravitational forces of atmospheric drag and solar radiation pressure (SRP) renders the estimates of their force coefficients non-physical that in turn degrades the accuracy of the predicted satellite states (NRC, https://doi.org/10.17226/13456). This occurs since the magnitude of higher-order gravity field coefficients can be as large as the non-gravitational accelerations up to degree and order 130 at around 400 km altitude, even though their net effect is averaged out for long-term motion. Therefore, an arbitrary order of truncation of the gravitational field model is detrimental to orbit determination. It is imperative to select the order depending on the altitude as well as orbit orientation with respect to the Sun-Earth line. An additional dependence on the satellite attitude profile stems from the correlations between the SRP and drag forces. In this work, we carry out a study to map the correlations between the different forces across varying orbital regimes and satellite attitude profiles. The order and degree to which the gravitational field should be modeled in order to mitigate the effects of the correlation with the non-gravitational force coefficients is derived for different orbital regimes. For example, for a satellite at 400 km, the gravitational field should be modeled to an order and degree 60 to get an accurate estimate of the SRP coefficient. At a given altitude, the effects of the higher order gravitational harmonics on the SRP coefficient and in turn, the drag coefficient, are less significant if the orbital plane is nearly perpendicular to the Sun-Earth line. With increasing altitude, the correlations between the gravitational field and non-conservative forces become smaller. Introduction of higher-fidelity models for the non-conservative forces adds an additional layer of complexity to the problem because of the different sensitivities of the parameters in these models to the various forces. At AMOS, we plan to bring to attention the corrupting effects of the correlation of the gravity field with the non-gravitational forces on the estimated force coefficients that ultimately reduces the prediction accuracy. Therefore, simply selecting the order of truncation of the gravity field based on their effects on satellite motion is not enough to ensure accuracy. In order to expect improved performance with the high-fidelity models of the non-gravitational forces, the gravity field has to be selected accordingly.
Date of Conference: September 17-20, 2019
Track: Astrodynamics