用GPS观测研究电离层TEC水平梯度

Study of Ionospheric TEC Horizontal Gradient by Means of GPS Observations

双频GPS用户能自动修正电离层总电子含量(TEC)引起的延时误差,但是对于电离层中的不规则体造成的信号闪烁而引起的误差则不能消除.即使是差分GPS系统,电离层误差仍然是其主要的误差源,其中电离层TEC梯度将会影响到系统的定位精度和性能.本文用GPS方法研究了电离层TEC的水平梯度问题,用处于赤道异常区NTUS台站的GPS观测数据作了具体计算.结果表明,在日落以后到子夜前后电离层垂直TEC出现了大的涨落,电离层中的不规则体导致L波段信号强的闪烁,同时还伴随着大而快速变化的电离层TEC水平梯度.对比发现,ROTI指数、电离层TEC水平梯度和电离层垂直TEC三者之间有很好的对应关系,它们的变化特征均由电离层中的不规则体引起.我们认为研究电离层闪烁,特别是在缺乏S_4指数时,电离层TEC梯度也可以作为一个重要的可选参数.

Dual-frequency GPS receiver can automatically correct time delay error caused by ionospheric Total Electron Content （TEC）. But it cannot eliminate signal scintillation error caused by ionospheric irregularities. Even for differential GPS systems, ionospheric error is one of major sources in GPS positioning, for example, error from ionospheric TEC gradient, which will affect accuracy and degrade performance of GPS application system. In this paper, the magnitude of ionospheric TEC horizontal gradient is studied by means of GPS observations. The vertical TEC, and its horizontal gradient and ROTI index over NTUS station, which located in equatorial anomaly region, are derived from GPS observation provided by IGS network. It is found that, between post-sunset and around midnight, the ionospheric TEC has great fluctuation, and intense radio waves scintillation, which is represented by big ROTI index, takes place during this periods. Furthermore, the steep TEC horizontal gradient and its fast variation with respect to time appear. By comparison, the ROTI index, ionospheric TEC horizontal gradient and vertical TEC correlate very well with each other, and their variations characteristics result from density irregularities produced by plasma instability in ionosphere. It is suggested that when scintillation events are investigated, ionospheric TEC gradient is also one of considerable parameters.