The Test Of GPS TEC Hardware Biases Estimating Methods And WAAS Ionspheric Grid Algorithm

In this paper, two questions of WAAS ionospheric grid corrections were discussed: Firstly, we used 8 GPS observation stations' data over the time span of two months to discuss the stabilization of GPS instrumental biases with a typical GPS TEC hardware biases estimating method. Secondly, the validity of FAA (Federal Aviation Administration) WAAS (Wide Area Augmentation System) ionospheric corrections algorithm was tested. Research achievements concerned had been applied in the task of SZ-Ⅳionspheric refraction corrections successfully.By applying the carrier phase observation to smooth the code observation, an accurate estimate of slant TEC (Total Electron Content) could be obtained. However, TEC contained the GPS differential instrumental delay biases which degrade the accuracy. They must be measured and removed especially for WAAS. Using a typical two-dimensional quadratic model of the vertical TEC, this paper analyzed the variations of GPS combined with satellites and receiver biases, the variations of GPS satellite biases relatively to the mean over two months, and the trend of GPS biases standard deviation with different latitudes. Additionally, this paper discussed the fluctuation of GPS instrumental biases during the storm. It was found that the stability of the results given by the same method differed significantly with the latitude of the stations: the results in mid-high latitude were more stable than those in low latitude. And during the periods of ionospheric storms, great jumps were detected. It was concluded that precautions must be taken when applying this method to solve biases in wide area augmentation system. According to the results, we selected 10-day data from 336 to 345, for ionospheric behavior was smooth during days. Then we computed GPS satellites and receivers delay biases relatively to the certain satellite when undertaking the task of SZ-Ⅳionspheric refraction corrections, which guaranteed the accomplishment of SZ-Ⅳaccurate positioning.The availability and accuracy of the WAAS were evaluated for China. According to the physiognomy of China, IGP (Ionospheric grid points) which were located in a constant 400km-altitude thin shell model above the surface of the Earth were separated by 5°×5°degrees(deg) from 5°to 55°deg latitude north and from 70°to 140°deg longitude east. 10 GPS observation stations were as reference stations. Vertical ionospheric delay of ionospheric pierce points were computed in 8 GPS user stations. Comparison was made between prediction values and observed ones. The results showed that the algorithm prediction error was smaller in mid-high latitude than the one in low latitude, smaller in low solar activity than in high solar activity, and smaller in the morning and at night than in the day time.In conclusion, we had discussed the stability of GPS instrumental biases, which not only was the basis of ionosphere morphology study but also played a significant role in the satellite navigation application with higher accuracy. At the same time, the validity of FAA WAAS algorithm which was aimed at America located a.t mid-high geomagnetism latitude was discussed. The algorithm error was larger in the South of China located at lower geomagnetism. So considering this, we must put forward new WAAS ionospheric grid algorithm.