# A Method to Reduce Non-Nominal Troposphere Error
## 2 Analysis of troposphere Meteorological Data in China  
This paper analyzes troposphere meteorological data of eight iGMAS stations and 10 IGS stations in China during 2015 and 2016. The meteorological data were obtained from the IGS website and iGMAS stations.  
Figure 3 shows the locations of the eight iGMAS stations and 10 IGS stations in China. The red asterisks represent the iGMAS stations, while the blue triangles represent the IGS stations.  
Figure 2: Representation of a skylplot of the selected optimal satellites under abnormal troposphere.  
Figures 4-6 indicate the abnormal troposphere conditions on 24 July 2016 at the Beijing fangshan station.  
According to the data at points A, B and C for pressure, relative humidity and temperature in Figures 4-6, respectively, the troposphere meteorological parameters at the Beijing fangshan station on 24 July 2016 are observed to fluctuate remarkably, indicating abnormal troposphere conditions. Clearly, the maximum change in pressure is 15 hPa/h, the maximum change in temperature is 11.1 \({}^{\circ}\)C/h, and the maximum change in relative humidity is 27.9%/h.  
Figure 4: Abnormal pressure condition.  
Figure 3: Locations of eight International GNSS Monitoring Assessment System (iGMAS) stations and 10 International GNSS Service (IGS) stations in China.  
The troposphere delay is related to the temperature, pressure and relative humidity conditions at ground stations. In monitoring the GBAS integrity, it is assumed that the ground station and the aircraft experience approximately the same meteorological conditions, and, thus, the troposphere delay can be eliminated using the differential correction method. However, under abnormal troposphere conditions, the ground station cannot eliminate the troposphere delay by differential correction, and as a result, the differential correction residual troposphere error increases.  
According to troposphere meteorological data from 18 stations collected during 2015 and 2016, the temperature change rate is less than 3 \({}^{\circ}\)C/h, the pressure is almost constant and the relative humidity change rate is less than 4% in the absence of abnormal troposphere conditions.  
Based on the troposphere meteorological data during 2015 and 2016, the most serious troposphere anomalies at eight iGMAS stations in China are shown in Table 1.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Time**} & \multirow{2}{*}{**Station**} & \multicolumn{2}{c}{**Maximum Change in**} & \multicolumn{2}{c}{**Maximum Change in**} \\  & & **Temperature (\({}^{\circ}\)C/h)** & **Pressure (hPa/h)** & **Relative Humidity (\%/h)** \\ \hline
16 July 2015 & bjf1 & 11.3 & 13.9 & 27.7 \\
27 June 2015 & canb & 10.7 & 13.1 & 29.1 \\
29 June 2015 & chu1 & 12.5 & 15.9 & 31.7 \\
28 July 2015 & cptn & 12.6 & 15.9 & 33.7 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The most serious troposphere anomalies at eight iGMAS stations in China.  
Figure 5: Abnormal relative humidity condition.  
Figure 6: Abnormal temperature condition.  
The most serious troposphere anomalies at 10 IGS stations in China during 2015 and 2016 are shown in Table 2.  
Tables 1 and 2 indicate that the eight iGMAS stations and 10 IGS stations in China all experienced troposphere anomalies during 2015 and 2016.