## 3 Development of the GPSQC Program  
GPSQC was developed using C#.NET language, which is based on MS Windows GUIs. This program gives the user brief information about GPS observations, time series plots and graphs of quality control parameters. It checks the observation and navigation files from a single station and is available to run on a Windows operating system (Windows 9x, 2000, XP and higher) which is widely used in the multi-tasking environment for PCs. The user interface is relatively easy, as it is based on the Windows series. The fundamental input required by GPSQC is the RINEX Version 2.0 or higher observation and navigation files that contain only GPS data. A RINEX format translator that translates raw data to a RINEX format is publicly available and was developed at the University of Bern. The program forms linear combinations of the GPS range and carrier-phase data to compute the following: (1) the L\({}_{1}\) pseudo-range multi-path for C/A- or P-code observations, (2) the L\({}_{2}\) pseudo-range multi-path for P-code observations, (3) the ionospheric phase effects on the L\({}_{1}\) carrier frequency, and (4) the rate of change of the ionospheric delay. The program also writes data summary files with information about the GPS observations, geometric distribution (_DOP_s, elevation and azimuth) and cycle slips effects, _etc._. The quality control process is completely automated by GPSQC and the resulting report can be used to evaluate the observation data prior to GPS data processing. Figure 1 shows the general flow of the GPSQC program. The GPSQC can be performed to compare the calculated quality control parameters with the specified criteria and give the percentage (%) of data per criterion. This program requires certain criteria before processing. The values of these criteria can influence the results obtained. It is important to detail the three criteria (\(a\sim c\)) as follows:  
_a. Elevation cut-off angle: 15 degrees_  
_b. Minimum gap in data to be detected: 2 min_  
_c. Maximum L\({}_{2}\)-Ionospheric time-rate of change: 800 cm/h (or approximately 0.22 cm/s)_Figure 2 shows the interactive Window of the GPSQC program. By selecting \"Open File\" in the main menu with the mouse button, two RINEX formatted files, observation and navigation files can be chosen from a specified directory. Selecting the QC button or processing icon, GPSQC calculates the eight QC parameters. To following procedure is used to process an individual RINEX file. _Step 1. Load GPSQC._  
_Step 2. Select File/Open using the icon on the toolbar._  
_Step 3. Navigate to the folder containing the files._  
_Step 4. Select \"Observation Files, *.obs\" and \"Navigation Files, *.nav\"._  
_Step 5. Define the criterion (see the red box in Figure 2)._  
_Step 6. Select the 'Processing' icon on the toolbar._  
GPSQC will then begin processing. When the processing is complete, the status bar at the bottom of one window will indicate 'Ready' and the output will be displayed in another window. The progress bar on the bottom right of the window indicates the amount of time it will take to finish data processing. Graphs and time series of quality control indicators, provided so that the user can easily understand the result of quality control will be displayed in the processing window when the processing is complete. Figure 1: General flow of the GPSQC program. The report file can be generated automatically by turning on the selective graphing format option for PDF, MS word and Excel formats. Graphs and time series of quality control indicators that may be plotted for a station include the following: satellite azimuth, elevation, _DOP_, cycle slips on code and phase, number of complete observations, percentage of observations with at least the minimum number of satellites, multipath on L\({}_{1}\) and L\({}_{2}\), and ionospheric delay and its rate on P code. However, we could not define the _exact_ values for quality control parameters because no criteria have been given in previous studies.