## 1 Introduction  
The quality of GPS positioning is related to a number of error factors [1]. To obtain high-precision positioning results, we need to identify the main error sources impacting the quality of GPS sensing data (_i.e._, GPS observations). In GPS data processing, multi-path, cycle slips, atmospheric delays and quasi-random errors are the main sources that can deteriorate the quality of the observations and subsequently, the quality of positioning results [1, 2]. To obtain consistent high-precision positioning results with the GPS carrier-phase analysis, errors that are not specified in a functional or stochastic model must be detected correctly, removed and controlled in data processing. Reliability, which refers to the ability to detect such errors and to estimate the effects that they may have on a solution, is one of the main issues in GPS data quality control. A comprehensive investigation of quality issues in GPS observations was performed by the Special Study Group (SSG) 1.154 of the International Association of Geodesy (IAG) between 1996 and 1999 [3] and by the University NAVSTAR Consortium (UNAVCO). Two quality control programs, Translation, Editing and Quality Checking (TEQC) and GPS Qualtiy Control (GQC), are now available for GPS observations. The conceptual and theoretical background of TEQC was originally written by [PERSON] [4] at UNAVCO and has been extensively modified by [PERSON], [PERSON] and [PERSON] [5]. Subsequently, a software-toolkit called TEQC was developed by [PERSON] and [PERSON] [5] to treat mixed satellite constellations, such as GPS, GLObal NAvigation Satellite System (GLONASS) and Satellite Based Augmentation System (SBAS). TEQC allows the user to translate the GPS data from a binary format to an ASCII format called the Receiver INdependent Exchange (RINEX) format, which allows for format modifications and quality control of the GPS observations before analysis of the GPS data. TEQC is 100% non-interactive to assist its use with automatically executed scripts, and has a command line interface modeled after common _UNIX_ commands. TEQC has been tested on 32-bit processors with math co-processors from a 486/DX up to a P4 running Windows 95/98/NT/2000/XP with DOS emulation windows [5]. GQC was developed as part of a Ph.D. dissertation [6] in GPS geodesy at the Department of Geomatics, University of Melbourne. GQC was originally called Quimby, but was substantially re-written and re-packaged to have more functionality. GQC provides preliminary quality and data integrity of dual frequency GPS observations in RINEX format. It is able to process individual RINEX files or automatically process files being created by a network of GPS base stations. GQC was written for use with the Microsoft Windows family of operating systems. It will run on all 32 bit Windows operating systems (Win9x, ME, XP, NT and 2000). However, neither of these programs is user-friendly because TEQC and GQC executables are not available on MS Windows GUIs; they are all command line programs. Therefore, the quality control results of GPS observations provided by TEQC and GQC are not easily understood by general GPS users, other than experts, and, in the general, non-academic parts of GPS applications, the data quality check with these programs are often ignored or omitted. Accordingly, the desired precision level is not achieved even though the GPS sensing data observed from field work are post-processed precisely. In this case, GPS observations must be repeated until the desired precision level is achieved. This process is significantly time-consuming and costly. The main purpose of this study is to develop a Windows-based program that calculates the quality control parameter that shows the quality of GPS observations using the GPS sensing data in a RINEX format. This allows general GPS users to easily and intuitively check the quality of GPS observations before post-processing, which will lead to the improvement of GPS positioning precision in diverse areas of GPS applications.