Density Model Corrections Derived from Orbit Data to Characterize Upper Atmospheric Density Variations

One goal of this research is to estimate density model corrections using readily available Satellite Laser Ranging (SLR) data, and to demonstrate this approach's validity for additional satellites with similar data sets in the future. The research also aims to utilize previously unused or little used sources of orbit state data to generate corrections to existing density models. These corrections yield estimated density corrections which lead to better drag estimates, improved orbit determination and prediction, as well as an enhanced understanding of density variations in the thermosphere and exosphere. This research primarily focuses on using SLR data. This examination will give a better idea of obtainable improvements in atmospheric density. Consideration will also be given to the effects of varying levels of geomagnetic and solar activity. This work established the validity of using SLR data to estimate atmospheric densities by comparing results for the ANDE Castor satellite to results for the CHAMP and GRACE satellites for the same time periods. The density correction factors and standard deviations comparing the baseline model densities to the derived atmospheric densities are also examined for the ANDE Castor satellite. For the entire family of ANDE satellites, the uncertainty in atmospheric density is established for each arc. The uncertainties are significantly higher at the beginning of the arc for each of the satellites, and the uncertainties also increase as the satellites drop in altitude. Preliminary density values for the Special Purpose Inexpensive Satellite (SPINSat) are also derived

DERIVATION AND OBSERVABILITY OF UPPER ATMOSPHERIC DENSITY VARIATIONS UTILIZING PRECISION ORBIT EPHEMERIDES

Several models of atmospheric density exist in today's world, yet most possess significant errors when compared to data determined from actual satellite measurements. This research utilizes precision orbit ephemerides (POE) in an optimal orbit determination scheme to generate corrections to existing density models to better characterize observations of satellites in low earth orbit (LEO). These corrections are compared against accelerometer derived densities that are available for a few select satellites, notably, the CHAMP and GRACE satellites. These corrections are analyzed by determining the cross correlation coefficients and root-mean-squared values of the estimated corrected densities as compared to the accelerometer derived densities for these satellites. The POE derived densities showed marked improvement using these methods of comparison over the existing empirical density models for all examined time periods and solar and geomagnetic activity levels. The cross correlation values for the POE derived densities also consistently out-performed the High Accuracy Satellite Drag Model (HASDM). This research examines the ability of POE derived densities to characterize short term variations in atmospheric density that occur on short time scales. The specific phenomena examined were travelling atmospheric disturbances (TAD) and geomagnetic cusps, which had temporal spans of less than half the period of the satellite's orbit, more specifically spans of between four and ten minutes, and less than three minutes respectively. Density variations of shorter duration are more difficult to observe even in accelerometer data due to diurnal variations that arise from cyclical increases due to the satellite passing from the darkened side of the earth to the lit side. This research also examines the effects of a vertically propagating atmospheric densities by looking at periods of time during which both the GRACE and CHAMP satellites have coplanar orbits, during which perturbations can be examined for their capability to extend vertically through the atmosphere, as well as their observability in POE derived densities. Additionally, this research extends the application of optimal orbit determination techniques to an additional satellite, the TerraSAR-X, which lacks an accelerometer. For LEO, one of the greatest uncertainties in orbit determination is drag, which is largely influenced by atmospheric density. There are many factors which affect the variability of atmospheric densities, and some of these factors are well modeled, such as atmospheric heating and to some degree, the solar and geomagnetic activity levels, though some variations are not modeled at all. The orbit determination scheme parameters found to perform best for most cases were a baseline model of one of the three Jacchia based baseline models, a density correlation half-life of 18 or 180 minutes, and a ballistic coefficient correlation half life of 1.8 minutes. All three Jacchia based models performed very similarly, with the CIRA-1972 model edging out the other two overall. The density correlation half-life's optimal value was usually 180 minutes, though for specific levels of geomagnetic activity, a half-life of 18 minutes was preferable. During the coplanar periods for both the GRACE and CHAMP satellites, both satellites showed minor density increases that occur on the unlit side of the earth near the equator. These increases were mostly unseen in the precision orbit ephemeris (POE) derived densities, though the POE derived densities did show a slight response to these perturbations. The secondary density increases were seen in both GRACE and CHAMP accelerometer data, and likely existed both above and below the orbits of these two satellites. The TerraSAR-X densities found for the time period examined in this study using POE data showed deviations from empirical density models of up to 10% for peak atmospheric density values. The CHAMP and GRACE POE derived densities showed a greater relative deviation from the empirical density models during peak density periods, and the deviations for the CHAMP and GRACE satellites' empirically predicted densities much better approximated the density values found using the accelerometers aboard both satellites. As the TerraSAR-X satellite lacks its own accelerometer, the POE derived densities are assumed to be a more accurate representation of the atmospheric densities

Thermospheric density variations: Observability using precision satellite orbits and effects on orbit propagation

This is the published version. Copyright 2013 American Geophysical Union.This paper examines atmospheric density estimated using precision orbit ephemerides (POE) from the CHAMP and GRACE satellites during short periods of greater atmospheric density variability. The results of the calibration of CHAMP densities derived using POEs with those derived using accelerometers are examined for three different types of density perturbations, [traveling atmospheric disturbances (TADs), geomagnetic cusp phenomena, and midnight density maxima] in order to determine the temporal resolution of POE solutions. In addition, the densities are compared to High-Accuracy Satellite Drag Model (HASDM) densities to compare temporal resolution for both types of corrections. The resolution for these models of thermospheric density was found to be inadequate to sufficiently characterize the short-term density variations examined here. Also examined in this paper is the effect of differing density estimation schemes by propagating an initial orbit state forward in time and examining induced errors. The propagated POE-derived densities incurred errors of a smaller magnitude than the empirical models and errors on the same scale or better than those incurred using the HASDM model