John Dow

Ocean tides and tectonic plate motions in high precision orbit determination

Abstract A study of the influence of solid earth, ocean and atmospheric tides on the orbits of earth satellites has been carried out, allowing the computation of the spectrum of tidal perturbations on the orbital elements. Some 170,000 laser ranging normal points from over 3 years of tracking were combined in a simultaneous solution for the positions and tectonic motions of the stations of the laser network, earth rotation parameters, and a set of tidal and gravity coefficients. Tectonic plate motion estimates show a reasonable agreement with a recent geophysical model (Minster and Jordan). The principal objective of the work was the development of an improved tidal model from Lageos for use in satellite orbit determination.

Satellite determinations of Earth orientation with emphasis on GPS

Abstract The “classical” satellite-based technique for determination of earth orientation (polar motion and changes in UT1/length of day) is laser ranging to the Lageos spacecraft. With the deployment of more GPS satellites and many high quality ground receivers, GPS-based approaches to EOP determination seem likely to achieve similar or better accuracies, with improved temporal resolution. Additional satellite methods make use of laser tracking to the Starlette or to the two Etalon spacecraft. The objective of the paper is to compare results obtained from these various techniques, using a common software package developed at ESOC for high precision orbit determination. The main emphasis is on experience gained during the IGS 1992 Campaign, one of the objectives of which was to assess the capability of GPS for fast, routine, high accuracy estimation of earth orientation.

Global geodynamics from Lageos and Starlette combination solutions

Abstract Continuing previous work which emphasised polar motion, tidal models and plate motion improvements from separate Lageos and Starlette laser data sets, the paper discusses combination solutions, in which the strength of each satellite for estimation of the various geophysical and geodetic parameters is utilised. The consistency of the individual and combined solutions is assessed.

Polar motion and earth rotation series from LAGEOS

Abstract As an Associate Analysis Centre for the MERIT campaign ESOC has processed the complete data set of LAGEOS laser ranging normal points for the 14 months period of the campaign (September 1983 to October 1984), using independently developed software. Geocentric coordinates have been obtained for a network of 20 laser stations, and series for the pole, UT1R and length of day have been derived with 5 day resolution. Discrepancies in the node and inclination estimates between successive monthly arcs are attributed to mismodelled tidal effects. Detailed comparisons have been made with solutions obtained at other processing centres based on VLBI and SLR techniques.

Potential Interests of the IGS for the Galileo System

Galileo is the name of a future worldwide satellite navigation system that could be developed soon by Europe. Its technical characteristics should be close to those of GPS, allowing in the near future possible dual-capability GPS-Galileo receivers for all types of users. Even if several years may be required to obtain a fully operational constellation, it seems legitimate to envision already the potential interest of Galileo to the scientific GPS user community and more specifically to try to define a possible medium- and long-term strategy for the International GPS Service (IGS). After a brief description of the present key characteristics of the Galileo project and a general overview of its deploymet timetable, we will focus more on the possible short-term and medium-term actions that could be conducted within the IGS community to prepare future dual use of the GPS and Galileo satellite signals for precise scientific applications. In all cases a close cooperation between IGS and the Galileo project could bring mutual benefits. Some examples of such possible actions are described in this paper.

Orbital and other products of the international GPS service for geodynamics (IGS)

Abstract The paper reviews the current status of the routine products of the International GPS Service for Geodynamics (IGS) from the point of view of one of the many participating institutions. Some recent developments are highlighted. These include monitoring of the earths ionosphere.

ERS-1 orbits: The routine operation and high precision products

Abstract The objective of the paper is to summarise the experience of one year of orbit determination and prediction for the ERS-1 spacecraft: firstly, the results of the operational solutions based on S-band ranging and doppler plus fast delivery altimetry will be discussed, and subsequently, higher precision solutions using data from the global laser ranging network. Steady progress has been achieved in improving the latter, in particular through better solutions for station positions, drag, gravity and tides. The development of this offline capability has strengthened considerably the routine orbit determination, and gives high confidence that the requirements for the latter are being satisfied. The paper attempts to quantify the accuracies which have been reached the ESOC solutions, and to answer the question: how far are we still from achieving 10 cm radial accuracy for altimeter satellites at 800 km?