G. Beutler

The International GPS Service (IGS): An interdisciplinary service in support of Earth sciences

Abstract 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 for 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.

A new solar radiation pressure model for GPS satellites

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.

Processing Strategies for Regional GPS Networks

At the Center for Orbit Determination in Europe (CODE), one of the IGS Analysis Centers, we are routinely processing the GPS data of a dense European network. In order to improve the height estimates and, very strongly coupled therewith, the modelling of the troposphere for this network, we decided to include low-elevation data into the processing, something that has been done in VLBI analyses for a long time. An appropriate mapping function, the option to weight the observations according to the elevation angle, and the estimation of troposphere gradients were implemented to make best use of the data at low elevations. Problems may arise from phase center variations, multipath, and the troposphere.

Efficient precise orbit determination of LEO satellites using GPS

Abstract 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.

Determination of resonant geopotential terms using optical observations of geostationary satellites

The ellipticity of the Earths equator causes a resonant librational motion of geostationary satellites. Combining astrometric observations of Meteosat satellites from Zimmerwald (Switzerland) and Graz-Lustbuhel (Austria), the resonant geopotential terms C22 and S22 were determined. The accuracy of the parameters provided by the limited campaign is close to that achieved in JGM-3. The comparatively simple observation and reduction technique offers a useful alternative method to monitor the values of the resonant geopotential terms C22 and S22 in order to determine possible time variations or to assess upper limits for variations.

The Effect of Pseudo-Stochastic Orbit Parameters on GRACE Monthly Gravity Fields: Insights from Lumped Coefficients

The official GFZ RL05 monthly GRACE gravity models were processed in a two-step approach. In the first step the orbits were determined. In the second step corrections to the gravity field parameters were estimated, while the orbits were kept fixed. This led to a significant de-noising of the resulting monthly models, but accidentally also to a regularization, i.e., the estimated gravity field coefficients were biased towards the a priori model. We compare the GFZ RL05 models to a revised version RL05a that was determined in a common estimation of orbit and force model parameters. A large number of gravity field coefficients is significantly affected. We relate this effect to the one-hourly stochastic accelerations estimated for orbit determination, and to ignoring the correlations.The official GFZ RL05 monthly GRACE gravity models were processed in a two-step approach. In the first step the orbits were determined. In the second step corrections to the gravity field parameters were estimated, while the orbits were kept fixed. This led to a significant de-noising of the resulting monthly models, but accidentally also to a regularization, i.e., the estimated gravity field coefficients were biased towards the a priori model. We compare the GFZ RL05 models to a revised version RL05a that was determined in a common estimation of orbit and force model parameters. A large number of gravity field coefficients is significantly affected. We relate this effect to the one-hourly stochastic accelerations estimated for orbit determination, and to ignoring the correlations. In the main part of this paper we study the interaction between pseudo-stochastic orbit parameters and gravity field coefficients. To explain this interaction we make use of a time-wise approach to gravity field determination. We apply the linear perturbation theory developed by Hill for circular orbits to compute lumped coefficients of the inter-satellite range-rate observations. We illustrate that the pseudo-stochastic orbit parameters act as a high-pass filter on the lumped coefficients spectra of the range-rates. Because the lumped coefficients are related to the spherical harmonics coefficients via a summation over all degrees, the whole range of gravity field coefficients is affected. This result is of relevance for all approaches to gravity field estimation from orbit observations, where dynamic orbits are introduced a priori and the arc-specific parameters are kept fixed.

GPS/GLONASS orbit determination based on combined microwave and SLR data analysis

The combination of space-geodetic techniques is considered as an important tool for improving the accuracy and consistency of the resulting geodetic products. For GNSS satellites, tracking data is regularly collected by both the microwave and the SLR observation technique. In this study, we investigate the impact of combined analysis of microwave and SLR observations on precise orbit determination of GNSS satellites. Combined orbits are generated for the two GPS satellites equipped with Laser retroreflector arrays and for three GLONASS satellites that are currently observed by the 1LRS network. The combination is done at the observation level, implying that all parameters common to both techniques are derived from both observation types. Several experimental orbits are determined using different observation weights. As the well-known 5 cm-bias between SLR measurements and GPS microwave orbits is unexplained, SLR range biases as well as satellite retroreflector offsets are estimated in addition to the orbital parameters. The different orbit solutions are then compared in order to determine whether and to which extent the SLR measurements influence a microwave orbit primarily derived from microwave observations.