低轨航天器弹道系数估算及热层大气模型误差分析

利用低轨（LEO）航天器在轨期间两行轨道根数（TLEs）数据，结合经验大气密度模型NRLMSISE00，反演计算得到其在轨期间的弹道系数B’，以31年B’的平均值代替弹道系数真值，分别通过标准球形目标卫星对比以及物理参数基本相同的非球形目标卫星对比，对弹道系数真值进行了检验；利用不同外形目标卫星弹道系数在不同太阳活动周内的变化规律，结合太阳和地磁活动变化，估计经验大气密度模型的误差分布. 结果表明，利用反演弹道系数31年的平均值来代替真值，其在理论值的正常误差范围内；大气密度模型误差在210～526km高度范围内存在相同的变化趋势，且模型误差随高度增加而增大；在短周期内B’变化与太阳活动指数F_10.7存在反相关性；密度模型不能有效模拟2008年出现的大气密度异常低. 以上结果表明，经验大气密度模型结果需要修正，尤其是在太阳活动峰年和谷年，此外，磁暴期间模型误差的修正对卫星定轨和轨道预报等也具有重要意义. 

Ballistic coefficient estimation of satellite in low Earth orbit and atmosphere model error analysis

An inversion method is introduced to study the variations of the ballistic coefficient for orbital satellites with different geometrical characteristic. Using satellite tracking data and NRLMSISE00 empirical model of the atmosphere, ballistic coefficients B of the selected satellites are compiled out with this new method. The "true" values of B (B_t) were computed by averaging the 31 years of B values obtained for each satellite. These true B values were validated by comparing the B_t values of two spheres and by comparing the B_t values obtained for a pair of satellites having very similar size, shape and mass. The estimated ballistic coefficients B' variations for a number of selected satellites were then averaged over each year, and compared with solar indices. Some interesting results are found. Firstly, there does not appear to be a bias in the B_t values computed using the 31 years' time period. Secondly, the large similarity between the time series of B ratios is apparent and solar activity related variations in the thermosphere increase in amplitude with increasing height, as do the errors in the NRLMSISE00 model. Thirdly, the anti-correlations of B' variations with the F_10.7 variations were found in short periods. Finally, the NRLMSISE00 model has failed to produce the low thermospheric density during the 2008 solar minimum. The variations in B' point out that the scaling factor is required to correctly predict the true densities from empirical atmosphere models, especially in solar maxima and minima. It is also shown that changes in B ratios during some major geomagnetic storms cannot be explained by atmosphere model.