Werner Gurtner

IGEX: International GLONASS experiment — Scientific objectives and preparation

The GLONASS system is rapidly becoming of great interest for navigation, timing and geodetic applications, usually in combination with the GPS system. The goal of this paper is to describe the first world-wide campaign of observations of GLONASS satellites, organized with a large international participation. This first campaign, called IGEX98 will be organized in fall 1998. In this paper, we will focus on the scientific objectives and present the on-going preparation for this campaign. We will address several important scientific issues that could be solved in the short term by such a campaign. A large number of organizations have already answered positively to the IGEX98 International Call for Participation.

The European Reference System coming of age

More than ten years ago, the advantages of the GPS technology were recognized and a first GPS campaign covering the western part of Europe was organized in order to establish a uniform European Reference Frame (EUREF). Through successive GPS campaigns, the network has been extended towards eastern parts of Europe and various countries have undertaken densification campaigns. The international co-operation within Europe has resulted in the establishment of a high accuracy, threedimensional geodetic network with links to global and national reference systems.

Orbit determination in satellite geodesy

Abstract For centuries orbit determination in Celestial Mechanics was a synonym for the determination of six so-called Keplerian elements of the orbit of a minor planet or a comet based on a short series of (three or more) astrometric places observed from one or more observatories on the Earths surface. With the advent of the space age the problem changed considerably in several respects: (1) orbits have to be determined for a new class of celestial objects, namely for artificial Earth satellites; (2) new observation types, in particular topocentric distances and radial velocities, are available for the establishment of highly accurate satellite orbits; (3) even for comparatively short arcs (up to a few revolutions) the orbit model that has to be used is much more complicated than for comparable problems in the planetary system: in addition to the gravitational perturbations due to Moon and planets higher-order terms in the Earths gravity field have to be taken into account as well as non-gravitational effects like atmospheric drag and/or radiation pressure; (4) the parameter space is often of higher than the sixth dimension, because not only the six osculating elements referring to the initial epoch of an arc, but dynamical parameters defining the (a priori imperfectly known) force field have to be determined, as well. It may even be necessary to account for stochastic velocity changes. Orbit determination is not a well-known task in satellit geodesy. This is mainly due to the fact that orbit determination is often imbedded in a much more general parameter estimation problem, where other parameter types (referred to station positions, Earth rotation, atmosphere, etc.) have to be determined, as well. Three examples of “pure” orbit determination problems will be discussed subsequently: • ⊎ The first problem intends to optimize the observation process of one Satellite Laser Ranging (SLR) observatory. It is a filter problem, where the orbit is improved in real time with the goal to narrow down the so-called range-gate, defining the time interval when the echo of the LASER pulse is expected. • ⊎ Secondly we highlight orbit determination procedures (in particular advanced orbit parametrization techniques) related to the determination of the orbits of GPS satellites and of Low Earth Orbiters (LEOS) equipped with GPS receivers. • ⊎ We conclude by discussing the problem of determining the orbits of passive artificial satellites or of space debris using high-precision astrometric CCD-observations of these object.

Status and development of the European height systems

After a break of ten years, the work on the United European Levelling Network (UELN) resumed in 1994 under the name UELN-95. The objectives of the LJELN-95 project are to establish an unified vertical datum for Europe at the one decimeter level with the simultaneous enlargement of UELN as far as possible to include Central and Eastern European countries. More than 3000 nodal points were adjusted linked to the reference point of LTELN-73 (gauge Amsterdam). The new heights in the system UELN-95/98 are available for more than 20 participating countries.

Processing of GLONASS and combined GLONASS/GPS observations

Abstract The satellite constellation and signal structure of both GLONASS and GPS are comparable. The GLONASS observations can be processed similarly to GPS, provided the differences in the reference frame, the system time and in the frequencies of the satellite signals are taken into consideration. New approaches for cycle-slip detection and ambiguity resolution are presented to account for the satellite-specific frequencies of GLONASS.

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.

The European Vertical GPS Reference Network Campaign 1997 — Concept and Status

The European Vertical GPS Reference Network (EUVN) is designed to contribute to the unification of different height systems in Europe. The most important practical and scientific aspects are contribution to a unique European height datum connection of European tide gauge benchmarks as a contribution to monitoring absolute sea level variations establishment of fiducial points for the European geoid determination preparation of a European Vertical Kinematic Network.

Global GPS Data Flow from Station to User within the IGS

The International GPS Service for Geodynamics (IGS) has been operational since January, 1994. This service was formed to provide GPS data and highly accurate ephemerides in a timely fashion to the global science community to aid in geophysical research. The GPS data flows from the global network of IGS sites through a hierarchy of data centers before they are available to the user at the global and regional data centers. A majority of these data flow from the receiver to global data centers within 24 hours of the end of the observation day. IGS analysis centers retrieve these data daily to produce IGS products (orbits, clocks, Earth rotation parameters, etc.). These products are then forwarded to the global data centers by the analysts for access by the IGS Analysis Coordinator for generation of the rapid and final IGS orbit product and for access by the geodynamics community in general. A discussion of the network data flow, from station to global data center to users, will be presented. Statistics on data quantity, volume, latency, and user access will be given.