GPS无线电掩星资料特点
GPS RO Data Characteristics
中文摘要：
GPS（Global Positioning System）指全球（卫星）定位系统，最初的设计是为了美国军方的精确定位，于1980年代与1990年代早期开始应用。GPS共由距地球表面约20200km高度的6个轨道上运行的24颗卫星组成，这些卫星以2个L波段频率（f1=1.57542GHz，f2=1.22760GHz）持续发射电磁波。GPS为将无线电掩星（Radio Occultation）技术应用到地球大气领域提供了基础。
GPS发射的无线电信号传播在真空中是一条直线。但实际射线从GPS卫星发射，途经地球电离层和中性大气层时，受电离层电子密度分布和大气折射率的影响， 路径有不同程度的弯曲，从而延迟这些信号到达低轨（Low Earth Orbiting，LEO）接受卫星的时间。基于卫星的精确位置与运行速度，可以导出总的弯角（bending angle）。由于GPS发射卫星和低轨接受卫星的相对运动，从大气顶部到地球表面的整个大气层都有射线穿过，因此可以获得弯角的垂直廓线。GPS无线电信号的波长较长，大约为20cm，故这些信号传播途经大气层时不受气溶胶和云雨的影响。通过把两个波段的信号传播延迟量进行一种线性组合，可以消除电离层对信号传播路径的影响。剩余的无线电信号传播延迟量便仅仅包含中性大气中大气折射率的影响。
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Abstract：
GPS stands for Global Positioning System. It came into use in 1980s and early 1990s. Initially, it was designed for accurate positioning in US army. This system is composed of 24 satellites circling on six orbits at about 20200 km high above the earth. These satellites continuously emit radio electromagnetic waves through two L-band frequencies ( f1=1.57542 GHz and f2=1.22760 GHz). GPS provides a practical foundation for applying radio occultation (RO) technique to the observation of the Earth's atmosphere.
The radio signal emitted from GPS transmits linearly in vacuum. However, when the signal passes through the ionosphere and neutral atmosphere, its transmitting path bends at varying degrees according to the electron density distribution in the ionosphere and the influence of atmospheric refractivity. As a result, the time when the signal reaches a receiver onboard a low-earth-orbiting (LEO) satellite is delayed. The total bending angle can be derived from the precise positions and moving speeds of both satellites emitting and receiving this signal. Because of the relative movement of GPS transmitting satellite and the receiving satellite at low earth orbit, the radio signal penetrates through the whole atmospheric layer from the earth surface to the top of the atmosphere. Therefore, a vertical profile of the bending angle can be obtained. The GPS signals have long wavelength of about 20cm and could thus pass through aerosols, cloud and precipitation in the atmosphere. A linear combination of the signal delay at two wavelengths could eliminate the impact of ionosphere on signal transmitting path. The residual delay accounts for the delay caused by the atmospheric refractivity in the neutral atmosphere.