Francois Barlier

A new global Earth's gravity field model from satellite orbit perturbations: GRIM5‐S1

A new model of the Earths gravity field, called GRIM5-S1, was prepared in a joint German-French effort. The solution is based on satellite orbit perturbation analysis and exploits tracking data from 21 satellites to solve simultaneously for the gravitational and ocean tide potential and tracking station positions. The satellite-only solution results in a homogeneous representation of the geoid with an approximation error of about 45 cm in terms of 5×5 degree block mean values, and performs globally better in satellite orbit restitution than any previous gravity field model. The GRIM5 normals, which were generated taking into account the latest computational standards, shall be the reference for use during the coming geopotential satellite mission CHAMP and should provide new standards in computing orbits of next altimetric missions like Jason and ENVISAT. The GRIM5-S1 normals also give the basis for the tracking/surface data combined solution GRIM5-C1.

French transportable laser ranging station: scientific objectives, technical features, and performance

The French Transportable Laser Ranging Station (FTLRS) is a highly mobile satellite laser ranging (SLR) system unit that weighs 300 kg and is housed in eight containers. This telemetry laser station is dedicated to the tracking of geodetic satellites equipped with retroreflectors. There are fascinating uses in the geosciences for such a system: in tectonics, oceanography, terrestrial reference frames, and precise positioning. The idea is to use a very small 13-cm-diameter telescope installed on a motorized mount and derived from a geodetic motorized theodolite of high precision. The laser is also compact, and the use of an avalanche photodiode makes detection possible at a single photoelectron level. On-site installation of this new SLR system is fast, and the systems routine operation is quite automated. It started its operational phase in late 1996. At present, it can track satellites at altitudes of as much as 3000 km and is designed to range to the Laser Geodynamic Earth Orientation Satellite (LAGEOS) at 6000 km in a further step. The standard error of individual measurements during the first observation campaign in Corsica is estimated to be of the order of 2-3 cm. Since then, significant improvements have been implemented. A technical description of the FTLRS is given, and the main results of the Corsica campaign are summarized.

Stability control of range biases on the French laser ranging stations

Space geodetic techniques of artificial satellites tracking have been greatly improved since this last decade. Particularly, the successful development of radio tracking systems like GPS, DORIS, and PRARE has to be emphasized, concerning precise positioning, orbit determination, and gravity field determination. All these techniques are able to operate under all weather conditions and are of very easy use. In fact, all the complexity of systems is to be found in space on board the satellites. To some extent, for laser satellite tracking, the situation is the opposite. It is weather dependent and comparatively complex and costly at the ground level. However, this technique is the most straightforward in concept for accurate measuring of the Earth-satellite distance. But, it is true, provided that instrumental biases can be well understood and precisely estimated. As a result, the role of laser tracking has to be reassessed and the control of range biases is nowadays of the utmost importance. Therefore, the international organization for laser cooperation has been reconsidered and the purpose of this paper is to present the effort made in this field at the Grasse, France observatory.

New Performances of the French Transportable Laser Ranging Station (FTLRS)

The French Transportable Laser Ranging Station (FTLRS) is a highly mobile laser ranging station dedicated to geodetic and oceanographic satellite tracking. Its performances were estimated during a probatory experiment in Corsica (from October 1996 to February 1997). The results showed that the 1-sigma rms was close to 2 cm for 80% of the passes. But, about 20% of the passes exhibited significant variations in the signal quality (as much as 10 cm) and a large bias. The FTLRS has been considerably improved to reach the 1 cm accuracy. That is the level necessary for JASON-1 validation and calibration for which the FTLRS is a main component. The instrument evolutions concern all the FTLRS fundamental components: laser, detection devices, timing chain, and calibration process. Thanks to laboratory measurements and to some comparisons with the Grasse laser ranging fixed station with similar characteristics, the FTLRS new performances are satisfactory: the ranging accuracy may be of about 11 mm, with 9 mm due to the station. The first results of this new collocation experiment are presented.