Since GPS week 1052, 5 March 2000, the IGS is producing a new combined orbit called the IGS Ultra rapid product, IGU. The combined IGS Ultra rapid products are being made available twice every day, at 3:00 and 15:00 UTC, with a delay of 3 hours after the end of the included data interval, and are based on solutions from up to seven different IGS Analysis Centers. The main reason for the generation of the Ultra rapid products are the requirements, in both timeliness and accuracy, for near-real-time atmospheric monitoring, e.g., weather predictions. Each ultra rapid orbit file covers 48 hours. The first 24 hours of the orbit are based on actual GPS observations (real orbit), the second 24 hours are extrapolated (predicted orbit). Like the IGS Predicted (IGP) orbits, the Ultra rapid orbits are available for real-time usage. However, the quality of the Ultra rapid orbits should be significantly better because the average age of the predictions is reduced from 36 hours (IGP) to 9 hours (IGU). At the 2000 IGS Analysis Center workshop, held at the USNO in Washington, D.C., it was decided that the IGU products were of sufficient quality to replace the IGP products. This change took effect on November 5 with the start of GPS week 1087. We will demonstrate that the accuracy of the IGS Ultra rapid orbits is at the 30 cm level, in a weighted RMS sense, which is significantly better than the 70 cm accuracy of the IGS Predicted orbits, We will also demonstrate that with this orbit quality it is possible to derive tropospheric zenith path delay estimates with a precision of 7 mm, which corresponds to approximately 1 mm precipitable water vapor. This level of precision is only achieved when "bad" satellite predictions are (automatically) detected and handled. (C) 2001 Elsevier Science Ltd. All rights reserved.
