大气掩星反演误差特性初步分析

Preliminary analysis of error characteristics in atmospheric inversion of GPS radio occultation

GPS大气掩星探测技术可以获得全球大气折射率、气压、密度、温度和水汽压等气象参数,该技术基本原理是基于几何光学近似的Abel积分反演.地球扁率、电离层传播时间延迟、大气大尺度水平梯度、多路径传播现象等因素在某些高度范围影响大气反演的精度.本文采用模拟的方法,分析其中地球扁率及电离层对反演结果的影响,并讨论局部圆弧修正及电离层修正的效果.利用CHAMP掩星实测轨道数据和有关电离层和大气经验模式、采用三维射线追踪方法模拟计算几种情形下的GPS掩星观测附加相位数据,对模拟数据进行反演,将反演气象参量剖面与模拟时给定模式剖面进行比较,得到了0～60km高度范围内的反演误差.误差统计分析结果表明,局部圆弧中心的修正以及电离层修正,对于高精度的GPS掩星反演是非常重要的;电离层修正残差仍是制约30～60km高度范围内反演精度的重要因素,进一步完善和优化大气掩星反演需要发展新的电离层修正算法.

GPS radio atmospheric occultation technique can measure the profiles of the global meteorological fields of the neutral atmosphere, from which we can obtain the atmosphere refractivity, pressure, density, temperature, and water vapor. The basic theory of the technique is the inversion method of Abel integration. Some factors such as the Earth＇s oblateness, the ionospheric dalay, the large-scale horizontal refractive index gradients in the atmosphere, and atmospheric multipath can influence the precision of retrieval. In this paper, we discuss the effect of the Earth＇s oblateness and ionosphere on the retrieval, and analyze the result of the local curvature correction and the ionosphere calibration. The excess phase delay is simulated by using the observed CHAMP orbit data, the ionospheric model and experiential atmospheric model, and the method of 3D-tracing technology. Inversion errors of the height from 0 to 60 km are obtained by comparison of the retrieval parameter profile and model profile, and studied by using statistical methods for several occuhations. The result shows that local curvature calibration and ionosphere calibration are very important for GPS occultation inversion of high precision. Ionospheric residual influence is the main reason of restricting the retrieval precision from 30 km to 60 km, and it is necessary to develop new ionosphere calibration methods for atmosphere inversion of higher precision.