A GUIDE TO USING INTERNATIONAL GNSS SERVICE (IGS) PRODUCTS (2003)

Since 1994, the International GNSS Service (IGS) ha s provided precise GPS orbit products to the scient ific community with increased precision and timeliness. Many national geodetic agencies and GPS users interested in geodetic positioning have adopted the IGS precise orbits to achieve centimeter level acc uracy and ensure long-term reference frame stability. Rel ative positioning approaches that require the combination of observations from a minimum of two GPS receivers, with at least one occupying a station with known coordinates are commonly used. The user position can then be estimated relative to one or multiple reference stations, using differenced carr ier phase observations and a baseline or network estimation approach. Differencing observations is a popular way to eliminate common GPS satellite an d receiver clock errors. Baseline or network process ing is effective in connecting the user position to the coordinates of the reference stations while the pre cise orbit virtually eliminates the errors introduc ed by the GPS space segment. One drawback is the practical constraint imposed by the requirement that simultaneo us observations be made at reference stations. An alte rnative post-processing approach uses un-difference d dual-frequency pseudorange and carrier phase observations along with IGS precise orbit products, for stand-alone precise geodetic point positioning (sta tic or kinematic) with centimeter precision. This is possible if one takes advantage of the satellite cl ock estimates available with the satellite coordina tes in the IGS precise orbit/clock products and models systema tic effects that cause centimeter variations in the satellite to user range. Furthermore, station tropo spheric zenith path delays with mm precision and GPS receiver clock estimates precise to 0.03 nanosecond are also obtained. To achieve the highest accuracy and consistency, users must also implement the GNSS-specific conventions and models adopted by the IGS. This paper describes both post-processing approaches, summarizes the adjustment procedure and specifies the Earth and space based models and conventions th at must be implemented to achieve mm-cm level positioning, tropospheric zenith path delay and clo ck solutions. The International GNSS Service (IGS), formerly the International GPS Service, is a voluntary collabora tion of more than 200 contributing organizations in more than 80 countries. The IGS global tracking network of more than 300 permanent, continuously-operating GPS stations provides a rich data set to the IGS Analy sis Centers, which formulate precise products such as s atellite ephemerides and clock solutions. IGS Data Centers freely provide all IGS data and products fo r the benefit of any investigator. This paper focus es on the advantages and usage of the IGS precise orbits and clocks. Currently, up to eight IGS Analysis Centers (AC) contribute daily Ultra-rapid, Rapid and Final GPS orb it and clock solutions to the IGS combinations. The da ily computation of global precise GPS orbits and clocks by IGS, with centimeter precision, facilitat es a direct link within a globally integrated, refe rence frame which is consistent with the current Internat ional Terrestrial Reference Frame (ITRF). Since 200 0 the ultra-rapid product originally designed to serve me teorological applications and support Low Earth Orbiter (LEO) missions, has been made available. The ultra-rapid product has since become useful to many other real-time and near real-time users, as well. For mo re information on the IGS combined solution product s and their availability see the IGS Central Bureau ( see http://www.igs.org/components/prods.html).