General ------- The International GPS Service (IGS) continuous to have broad support from the scientific community. During the past year the global network under IGS auspices continued to improve the global distribution of GPS tracking stations. This enhanced the coverage and accuracy of the EOP and ITRF products as generated within the IGS. The IGS network operators and data centers continue to make all this data publicly available in a timely fashion. With the upcoming Ultra rapid products "timely" means that for a subset of stations of the IGS global network the data are made available on a hourly basis rather than a daily basis. More information about the IGS may be found at the IGS web page located at the IGS Central Bureau (http://igscb.jpl.nasa.gov). Current IGS and AC product quality ---------------------------------- Despite the still rapidly increasing processing load due to more stations, additional products (Ultra rapid!), and shortening submission delays, the IGS Analysis Centers managed again to improve and/or maintain their solution precision, timeliness, and reliability. The quality improvement of the IGS products since 1994 is demonstrated in Figure 1 which shows the weighted orbit RMS (WRMS) for the final Analysis Center solutions with respect to the combined IGS Final orbit products. Several Analysis Centers and also the, IGS Rapid orbit products, have reached the 3 to 4 centimeter orbit precision level. Similar levels of accuracy are indicated by the IGS 7-day arc orbit analysis and by comparisons with satellite laser ranging observations of the GPS satellites PRN 5 and 6. The enormous efforts and the resulting improvements of the Analysis Center global solutions are also shown in Table 1 where the yearly averages of weighted orbit RMS values are shown for all Analysis Centers and the IGS Rapid orbit (IGR). Figure 1: Weighted orbit RMS (mm) of the Analysis Center and IGS Rapid (IGR) orbit solutions with respect to the IGS final orbits. The weekly WRMS values from the orbit combination summaries were smoothed for plotting purposes, using a sliding 10 week window.} ------------------------------------- Year COD EMR ESA GFZ JPL NGS SIO IGR ------------------------------------- 1994 11 14 17 12 14 32 21 - 1995 8 10 14 10 9 17 16 - 1996 6 10 9 9 7 15 8 6 1997 4 10 7 6 6 16 7 5 1998 4 10 7 4 5 14 6 5 1999 3 10 7 3 4 9 5 4 ------------------------------------- Table 1: Yearly average weighted orbit RMS (cm) of the Analysis Center and IGS Rapid (IGR) orbit solutions with respect to the IGS final orbits. The primary objective of the IGS is to provide a reference system for a wide variety of GPS applications. To fulfill this role the IGS produces a large number of different combined products which constitute the practical realization of the IGS reference system. Table 2 gives an overview of the estimated quality of these different IGS reference frame products at the start of the year 2000. ------------------------------------------------------- Products Predicted Rapid Final Delay Real-Time 17 hours 13 days Units ------------------------------------------------------- Orbit 50.0 10.0 5.0 cm Clock 150.0 0.5 0.3 ns Pole 0.2 0.1 mas LOD 30.0 20.0 microsec Station Positions 5.0 mm Troposphere Zenith Delay 4.0 mm ZPD ------------------------------------------------------- Geocenter 30.0 mm Terrestrial Scale 2.0 ppb -------------------------------------------------------- Table 2: Quality of the IGS Reference Frame products at the start of the year 2000. Outlook ------- A new, Ultra rapid, IGS combined product (IGU) will become available in the near future. This product will be available for real-time usage, like the IGS Predicted orbits (IGP), but the quality should be significantly better because the average age of the predictions is reduced from 36 to 9 hours. The Ultra rapid products will be generated twice each day at 03:00 and 15:00 UTC. In the months to come, the quality and the reliability of the IGS Ultra rapid (IGU) orbits will be assessed against the IGS Predicted (IGP) and IGS Rapid (IGR) products. When it reaches a satisfactory level the IGU products will replace the IGP and possibly also the IGR products. The plans for the new and improved IGS Reference Frame (RF) realization, as proposed during the 1997 Analysis Center workshop by Jan Kouba, have been finalized in March 2000. Starting with GPS week 1050 the weekly IGS RF realization, as generated at NRCan by Remi Ferland, has become official, see IGSMAIL #2740 for more details. In this new IGS RF realization the combined orbits are made consistent with the combined IGS Reference Frame (SINEX) solution by using both the transformation parameters and the combined ERPs stemming from the SINEX combination. If all goes well the year 2000 should provide us with a few Low Earth Orbiting (LEO) satellites equipped with GPS receivers. A number of missions (CHAMP, SAC-C) are scheduled for launch this year. These LEO mission will add a new and exiting dimension to the IGS activities. However, for the LEO missions it will be mandatory to continue the good cooperation, coordination, and open information exchange as we have practiced in the past IGS years.