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Development of web-based automated GPS processing engine
 S. Q. Salman n8004016@ccs.iitb.ac.in  Prof. M. N. Kulkarni kulklarni@civil.iitb.ac.in Department of Civil Engineering, IIT Bombay Mumbai
Introduction
The Global Positioning System (GPS), based on carrier phase processing, is now popularly accepted as the most effective technique for surveying and many other applications involving positioning. In the conventional scenario for GPS surveying based on the post-processing mode, the users need to purchase a minimum of two sets of GPS receiver hardware as well as the associated processing software package. This adds significantly to the system cost, and in addition, GPS survey operation incurs additional logistical costs due to the need to operate both base (or reference) receiver, as well as the user receiver. In many instances the data post-processing steps may be more labour-intensive than the field data capture task, involving the operation of complex software, and subsequent transformation of the resulting coordinates into a form appropriate for the application These are the factors, which make it a costly technique. World Wide Web (WWW), which involves communication and data exchange, has revolutionized many fields. It has mollified the difficulties involved in GPS surveying, which involves a lot of data handling. The information on Internet includes GPS raw data (which is provided by International GPS Service (IGS) and many other organizations), as well as some GPS processing software packages (Satirapod C, 2000). A GPS data processing software package developed using Matlab code is already available on the Internet for free download (Strang, Borre, 1997). With the availability of precise GPS ephemeris and satellite clock information, the Jet Propulsion Laboratory (JPL) has initiated some work on an automated GPS data analysis service for single-point, static positioning known as the ‘Auto Gipsy (AG) service’ (Zumberge et al., 1997; Zumberge, 1999). This service allows users to submit data via the Internet, but users are required to have their own FTP server and only data obtained from a dual-frequency receiver can be processed. (Satirapod C, 2000). Most recently, JPL has presented a paper on an Internet-Based Global Differential GPS System which can produce single-point positioning solutions in real-time (Muellerschoen et al., 2000). Alternatively a Web based GPS processing engine can be developed which enables the users to submit their rover station file via web page. The web page can be accessed from anywhere around the world. This data will be processed automatically by the web server application and the users will be notified of their results via email. The user need not have a FTP server for using this facility. In this implementation, the user needs to operate one GPS receiver and the complementary reference data file can be obtained from a database. The system can be improved further if the agency provides its own base station data. In this case the accuracy of the results will be improved by the use of multiple reference station concept. The development of this facility leads to some data processing schemes not possible with commercial off-the-shelf GPS software. For example, true GPS network-based processing, where multiple reference receiver data is used (Rizos et al., 1999) is possible, permitting higher performance (in the form of higher accuracy, less observation time or longer baselines) than is currently the case. This paper first describes the design and configuration of the web based application followed by the future developments and implementation issues.  Fig.1: The HTML page of the web-based automated GPS processing system System Design and Configuration Types of Web Server Applications A web server application receives Hyper Text Transfer Protocol (HTTP) request messages from a web server. Web server applications consist of either a Common Gateway Interface (CGI) or use a web server Application Programming Interfaces (APIs) which is usually a Dynamic Link Library (DLL) or a Server side script like Preprocessor Hyper text (PHP). CGI could be implemented in any programming language but have lacked performance. When a request for a CGI script is made it would spawn another process. In the case of the web application referred to in this paper, PHP is used which is a server-side, cross-platform, HTML embedded scripting language. It interacts seamlessly with the Oracle Data base and is executed efficiently. Oracle 8i Database is used for storing the reference station data. When user sends a rinex file a pro*c (SQL embedded in C++) program scans the file to get the start and end of the session and procures data for that time interval from the Oracle database.  Fig.2: Multi-reference receiver Network, Hongkong
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