摘要
GPS无线电掩星反演地球大气参数方法是上世纪八十年代末提出的一种具有挑战性的理论。在目前的地球大气监测手段中,它具有诸多优势,如全天候观测、高垂直分辨率、全球覆盖性、高性能价格比等等,因而有着独特的地位。目前国际上已经有多个国家为实施该方法发射了多颗小卫星。目前该理论在其应用过程中不断地得到发展和完善。我们试图通过该项研究初步掌握该技术的特点及其实施技巧,以便为将来国内实施GPS无线电掩星技术反演地球大气参数项目时提供某种程度上的理论研究支持。
对GPS无线电掩星技术反演地球大气参数方法中的若干问题进行了较为全面的分析和研究。
1)给出GPS无线电掩星反演地球大气参数过程中计算大气折射角的解析表达式。以圆轨道假设下的大气折射角计算值为先验约束,采用迭代法对不引入圆轨道假设情况的大气折射角进行归算,在此基础,利用反演方法得到了引入和不引入圆轨道假定两种情况下大气参数(气压和温度)的差分序列。结果表明: 卫星圆轨道假设对GPS 无线电掩星反演大气参数的影响,在气压方面为1mbar左右,而在气温方面为10K左右。这一结果支持了目前无线电掩星定性误差估计研究中通常引入卫星圆轨道假设这个近似处理方法的合理性;同时也表明:若在高精度反演地球大气参数时,摒弃圆轨道假定是必要的.
2)探讨了空基无线电掩星技术应用于反演地球大气参数方法,并将该方 法编制成资料处理软件;对实测资料进行处理和分析后,给出了地球大气参数反演结果。发现:指数函数外推法引入数据处理后大大改进了大气温度的归算结果,而对气压归算的影响却很小。
3)对MSISE90模型进行了简要的介绍,并指出Hedin本人在该模型问世后
对它的一些修正、改进之处。基于修正后的MSISE90大气密度模型,生成了一个可供GPS无线电掩星反演地球大气参数时使用的先验温度序列。
4)GPS掩星监测大气方法中需要载有GPS接收机的卫星的精密定轨信息,而该卫星的轨道误差对GPS掩星监测大气的影响进行分析是十分必要的。基于地球大气模式CIRA86,我们采用了三维射线跟踪方法模拟了5次完整的GPS掩星事件,估计了LEO卫星的轨道误差对GPS掩星测量中关键参数的影响。
5)利用GPS/MET实验中5组GPS和LEO卫星的实际轨道,分别模拟了无LEO卫星轨道误差和有轨道误差情况下的GPS信号在电离层中的传播,并生成了对应的电离层延迟量。就掩星观测中电离层延迟对LEO卫星轨道误差的响应程度进行了估计和分析。初步结果显示:在掩星观测中,相对于同样大小的横向T和法向N的轨道误差而言,径向轨道误差对掩星监测大气的影响是最为显著的。
6)对无线电信号在电离层中的传播路径进行了数值模拟;针对掩星观测中的五组实际卫星轨道数据(包括GPS和LEO卫星),给出了在太阳射电辐射流量(FLUX)分别为0,70,160和240等几种情况下的电离层延迟量对10.7cm波长的太阳辐射流量的响应结果;这分别对应着无太阳辐射、太阳射电辐射处于活动低谷、平静期和高峰期等几种情形。从中可以发现,掩星观测中电离层延迟量对10.7cm波长的太阳射电辐射流量的响应表现为如下特性:电离层延迟同参数FLUX的大小有明显的对应关系,即FLUX越大,则掩星观测中的电离层延迟越大;对于上升掩星情况而言,掩星观测中电离层延迟量起先逐渐增加,然后达到某一峰值,其后逐渐下降;在掩星观测的初期和中期,太阳射电辐射处于活动低谷、平静期和高峰期等几种情形之间的电离层延迟量差异都较为显著,而在掩星观测的后期,几种情形相互间电离层延迟量的差异都比较小。
7) 对应用于局部地区大气监测的LEO卫星轨道设计中的力学模型进行了精化,并给出了重新设计结果。
Some problems are solved or thorough explored in sensing Earth's atmosphere via GPS radio occultation methodology. Firstly, an analytic expression for the angle of refraction in GPS radio occultation in the general case is presented. The angle of refraction without assuming a circular orbit is calculated iteratively starting with the value from the circular assumption. The atmospheric parameters are then calculated by inversion with and without the circular assumption. It is shown that the bias caused by the circular assumption is up to about 1 mbar in the pressure and up to about one degree Celsius in the temperature. On one hand, this result supports the practice of using the circular assumption in qualitative error analyses; on the other hand,
it shows the necessity of foregoing the circular assumption when the highest accuracy is required of the atmospheric parameters. Secondly, the way of remote sensing of terrestrial atmospheric parameters using spaceborne GPS radio occultation technique is discussed. Based on this method, data analysis software has been coded and is used to reduce real radio occultation data. It is confirmed that exponential-function extrapolation contributes much to the improvement of inverted temperature; however it does much less for pressure inversion. Compared with relevant published results from abroad, our results are, at large, agreeable with foreign investigators' relevant works. Thirdly, MSISE90 atmospheric density model is briefly introduced and the amendments in this model by Hedin are pointed out. Based on MSISE90, a priori temperature series are produced for input parameter in GPS radio occultation data reduction. Fourthly, orbit information of the Low Earth Orbiter carrying a GPS receiver is needed in the conception of atmosphere monitor by GPS occultation. It is unavoidable to analyze the effects of orbit uncertainty of the low Earth Orbiter on the observable in GPS occultation for atmosphere monitor. Based on the existent Earth atmosphere model, three dimensional ray-tracing techniques are employed to simulate five representative complete GPS occultation events. The effect of the orbit error is quantified on the key observable in occultation. Fifthly, based on Haselgrove's and Budden's Equations, the propagation process of GPS signal in the ionosphere is simulated. Five groups of orbit data for GPS and LEO satellites are employed to simulate and generate the ionospheric phase delay respectively in two cases, one is LEO orbit without forced perturbation
and the other is LEO orbit with forced perturbation. The response of ionospheric phase delay to the LEO satellite orbit biases is assessed and analyzed. The preliminary results have shown that the response of ionospheric phase delay to the LEO orbit biases is much greater in the radial direction than those in the transverse or the normal directions. Sixthly, the propagation of radio signal in the ionosphere is simulated. Based on five data sets of real satellite state vectors from radio occultation measurements, the phase delays are respectively calculated with four different FLUX, i.e., FLUX=0, 70, 160 and 240. It corresponds to four representative conditions, that is to say, no solar activity, low solar activity, average solar activity and peak solar activity. The simulation has shown such features as follows: ionospheric phase delay is obviously correlated with solar radio flux, and the greater solar radio flux, the greater ionospheric phase delay; as for a rising occultation event, at the beginning, the phase delay increases, then it reaches a peak, finally it decreases till the end of occultation event. The phase delay differences among three different solar radio flux conditions tend to enlarge in the first most part of occultation event, however, they tend to narrow in the rest of the occultation event. Finally, some force models are adapted for LEO orbital design dedicated to the regional atmospheric monitor by radio occultation. The orbit design are renewed.
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