THE GPS TOOLKIT: WORLD CLASS OPEN SOURCE SOFTWARE TOOLS FOR THE GNSS RESEARCH COMMUNITY Salazar, D., Hernandez-Pajares, M., Juan-Zornoza, M., and Sanz-Subirana, J. Grupo de Astronomia y Geomatica (gAGE) Universitat Politecnica de Catalunya 08034 Barcelona Spain Dagoberto.Jose.Salazar@upc.edu Keywords: GNSS data processing software GPSTk. Abstract The ”GPS Toolkit” (GPSTk) project is an advanced GNSS Open Source Software (GOSS) suite initiated by the Applied Research Laboratories of the University of Texas (ARL:UT), aiming to provide a world class GNSS library computing suite to the satellite navigation community. The objective of this work is to show how the GPSTk suite may be used to rapidly and easily develop, implement and test advanced GNSS data processing applications in a flexible way. The main characteristics and modules of the GPSTk are also presented, as well as the current trends in its development. Results from the GNSS data processing carried out with the examples introduced in this work are shown, as well as a brief comparison with an established GPS software package such as BRUS. 1 Introduction The ”GPS Toolkit” (GPSTk) project is an advanced GNSS Open Source Software (GOSS) suite initiated by the Applied Research Laboratories of the University of Texas (ARL:UT), aiming to provide a world class GNSS library computing suite to the satellite navigation community. In this regard, one of the main goals of the GPSTk is to free the research community from implementing common GNSS algorithms, providing a publicly accessible software repository where those algorithms may be found and freely used. The initial code of the GPSTk was released in summer 2004 and presented at the ION-GNSS-2004 [1], and its functionality has been continuously improving. A very brief list of the tools provided by the GPSTk includes: Handling of observation data and ephemeris in RINEX and SP3 formats, mathematical, statistical, Matrix and Vector algorithms, time handling and conversions, ionospheric and tropospheric models, cycle slip detection and correction, etc. As a typical Open Source project, the GPSTk is released under the GNU LGPL license, allowing freedom to develop both commercial and non-commercial software based on it, and it is actively maintained by a dozen developers around the world using the Internet as communication medium. Development facilities are provided by the popular SourceForge open source application repository, and a website of the project may be found in http://gpstk.sourceforge.net. The GPSTk software suite consists of a core library and extra applications. The library provides several functions that solve processing problems associated with GNSS (for instance, using RINEX files) and is the basis for more advanced applications distributed as part of the GPSTk suite. Apart from the library, the GPSTk suite comes with a wealth of GNSS applications ready to run, explore and include in new developments. Full applications may be found in the ”gpstk/dev/apps” subdirectory, and interesting and easy-to-follow examples of use are located at ”gpstk/dev/examples”. A very important feature of a project of this nature is documentation. In this regard, the GPSTk is