Preprocessing is an essential aspect for zero- and double-difference GPS software packages. In the first case we have to produce “clean” code and phase observations on the single receiver and single satellite level, in the second case solely double-difference observations have to be checked. The checks usually are performed on the “minimum constellation” level, i.e., the single receiver level for zero-, the single baseline level for double-difference packages. When analyzing the code observations stemming from a permanent array, this step may be performed in a much more efficient and robust way because the known geometry and, if available, the known atmospheric delays may be removed from the original observations. In an array of n receivers observing m satellites this leaves us with n · m observations and n+m − 1 unknowns (the clock parameters relative to a reference clock). The degree of freedom of f=n · m − ( n +m − 1 ) (for, e.g., n= 10 and m= 10, we have f= 81) allows for a very robust detection of outliers and enables generating a satellite clock file based on code measurements (with very much reduced multipath and noise characteristics). A similar step may be performed with the differences of phase observations between subsequent epochs. Using an analogous procedure as in the case of code observations we may generate phase files with all cycle slips flagged or, in the case of “small-area” arrays, even with cycle slips repaired. Both steps, phase and code cleaning, are performed in the same program unit. We discuss this new development and present first results and applications using data from the AGNES (Automated GPS Network Switzerland) and the IGS (International GPS Service) Networks.

Efficient precise orbit determination of LEO satellites plays an important role for near real-time studies of GPS satellite occultations for meteorological purposes. Precise point positioning for each epoch is one approach to achieve this goal. Using IGS orbits and precise clocks for the GPS satellites the positions are generated by the combination of code derived positions and phase derived position differences. Fitting an orbit based on a physical model to the positions promises to complement a procedure that meets the requirements regarding precision and processing speed. This efficient procedure is tested with data of TOPEX/POSEIDON.

In October 1998 the IGEX field campaign, the first coordinated international effort to monitor GLONASS satellites on global basis, was started. Currently about 40 institutions worldwide support this effort either by providing GLONASS tracking data or in operating related data and analysis centers. The increasing quality and consistency of the calculated GLONASS orbits (about 25 cm early in 2000), even after the end of the official IGEX field campaign, are shown. Particular attention is drawn to the combination of precise ephemerides in order to generate a robust, reliable and complete IGEX orbits product. Some problems in modeling the effect of solar radiation pressure on GLONASS satellites are demonstrated. Finally, the expected benefits and prospects of the upcoming International GLOnass Service-Pilot Project (IGLOS-PP) of the International GPS Service (IGS) are discussed in more detail.

The largest error in currently used GPS orbit models is due to the effect of solar radiation pressure. Over the last few years many improvements were made in modeling the orbits of GPS satellites within the International GPS Service (IGS). Howeer, most improvements were achieved by increasing the number of estimated orbit and/or solar radiation pressure parameters. This increase in the number of estimated satellite parameters weakens the solutions of all estimated parameters (not only orbit parameters). Because of correlations the additional orbit parameters may introduce biases in other estimated quantities, for example the length of day. We present a recently developed solar radiation pressure model for the GPS satellites. This model is based on experiences and results gained at the Center for Orbit Determination in Europe (CODE) in the context of its IGS activities since June 1992. The performance of the new model is almost an order of magnitude better than that of the existing ROCK models. It also allows a reduction of the number of orbit parameters that have to be estimated. © 1999 John Wiley & Sons, Inc.

Since 21 June 1992 the International GPS Service (IGS) produces and makes available uninterrupted time series of its products, in particular GPS observations from the IGS Global Network, GPS orbits, Earth orientation parameters (components x and y of polar motion, length of day), satellite and receiver clock information, and station coordinates and velocities.At a later stage the IGS started exploiting its network for atmosphere monitoring, in particular far ionosphere mapping and for troposphere monitoring. This is why new IGS products encompass ionosphere maps and tropospheric zenith delays, both with a very high temporal resolution. This development will be even more pronounced through the advent of many space-missions carrying GPS, or combined GPS/GLONASS receivers for Various purposes. The achievements of the IGS are only possible through a unique voluntary cooperation of a great number of active organizations.This article gives an informative overview for the broader scientific community of the spectrum of problems that is addressed today using IGS/GPS techniques. (C) 1999 COSPAR. Published by Elsevier Science Ltd.

The Extended Center for Orbit Determination in Europe (CODE) Orbit Model, an empirical orbit model proposed by Beutler and colleagues in 1994, has been tested extensively since January 1996. Apart from six osculating Keplerian elements, this orbit model consists of nine (instead of the conventional two) parameters to take into account the deterministic part of the force field acting on the satellites. Based on the test results an improved orbit parameterization is proposed. The new orbit parameterization consists of the conventional two parameters plus three additional parameters, a constant and two periodic terms (a cosine and a sine term), in the X-direction to model the effects of the solar radiation pressure. Results based on one full year of routine orbit estimation, using the original and the new orbit parameterization, are presented to demonstrate the superiority of the new approach. An improvement of the orbit estimates with at least a factor of two is observed!

Space geodesy provides an efficient technique to determine contemporary crustal deformations. This paper discusses the computation of tectonic motions in the Mediterranean area from observations acquired by Satellite Laser Ranging (SLR) and Global Positioning System (GPS) instruments.The SLR contribution is based on measurements on LAGEOS-1, taken in the period September 1983 to December 1992. For this analysis, the period was divided into 38 consecutive intervals. An independent station coordinates solution was computed for each subinterval. During this 9.3 year interval, the WEGENER project organized four mobile SLR observation campaigns in the Mediterranean region, resulting in three or four independent position solutions for the majority of the sites in this area. The GPS contribution is based on observations taken during the Euref'89 campaign, which took place from May 16 to 28, 1989, and during the WEGENER/GPS-92 campaign, which took place from July 29 to August 3, 1992, as a sub-campaign for the IGS Epoch'92 campaign. Most of the space-geodetic reference points in the Mediterranean area were occupied with GPS receivers during both campaigns, providing two valuable additional position solutions.The time-series of position solutions for each station are converted into individual station motion vectors. The horizontal components of the latter reveal a clear and consistent picture of the deformations taking place in the area: the northward motion of Arabia, the lateral escape to the west of Anatolia, the NE-SW expansion in the Aegean Basin and the northward motion of Africa being transduced into the central part of the Mediterranean. The results are in agreement with the qualitative kinematical models for this region, derived from geophysical evidence. The vertical station motion solutions are generally very small and insignificant from a statistical point of view. The internal and mutual consistency of the independent SLR and GPS network solutions is also investigated. The agreement is shown to be at the level of 5-20 mm.

During the International GPS Service for Geodynamics (IGS) Epoch '92 campaign a special sub-campaign was Organized to occupy all WEGENER/MEDLAS SLR sites with Trimble SST GPS receivers. This campaign, WEGENER/GPS-92, was initiated by the Institute for Applied Geodesy in cooperation with Delft University of Technology. The purpose was to observe the whole SLR network, for the first time, with GPS to establish a connection between the SLR and GPS reference frames. The dataset was analyzed at the Section Space Research and Technology, using the GIPSY software developed at the Jet Propulsion Laboratory and the Bernese GPS software version 3.4. In the analysis a weighted combination of the orbits provided by the IGS was used to model the motion of the GPS satellites. The results show a baseline length repeatability, which is a measure for the precision, in the order of 5 mm. More importantly the GPS coordinate solution compares very well with the most recent SLR coordinate solution computed at Delft University of Technology. The rms differences are 3.8, 5.2 and 12.5 mm in North, East and Up, respectively, using 14 sites. It is therefore obvious that GPS will play an increasingly important role in the monitoring of the deformation and a further densification of this network. Especially since starting January 1, 1994 the IGS has become operational and combined orbits are being produced and made available with only a few week delay.