Joelle Nicolas

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.

The Role of Laser Ranging for Calibrating Jason-1: The Corsica Tracking Campaign

The French Transportable Laser Ranging System (FTLRS), a highly transportable Satellite Laser Ranging (SLR) instrument, was set up in Corsica (from January to September 2002) for participating to the JASON-1 altimeter verification phase. In addition to the tracking of oceanographic satellite missions and in order to perform an accurate positioning, the FTLRS also acquired laser ranging data on geodetic satellites, STARLETTE and STELLA essentially. The paper describes the analysis strategy mainly based on the use of a short-arc orbit technique to compute accurate 1 cm local orbits, and then the geocentric positioning (2–3 mm relative to GPS). Finally, we established the JASON-1 absolute calibration value, based on 9 SLR short-arcs (between cycles 1 and 26), at 108.2 ± 8.7 mm; the 10-day repeatability is of 26.1 mm showing that a great accuracy has been reached.

An estimate of the influence of loading effects on tectonic velocities in the Pyrenees

Surface displacements due to temporal changes in environmental mass redistributions are observable in the coordinate time series of many Global Navigation Satellite System (GNSS) sites. In this study, we investigated the effect of loading on estimates of tectonic velocity computed from campaign-style GNSS observations. The study region is in the Pyrenees mountain range between France and Spain (ResPyr campaigns). In this area, seismic activity is continuous and moderate and the expected amplitude of the horizontal tectonic velocity is less than 0.5 mm/yr. In order to determine the velocity, 4 sparse GNSS campaigns were carried out from 1995 to 2010. Considering this small rate of deformation, loading phenomena can contribute a non-negligible artifact to the velocity computation that could affect our geodynamical interpretation. In this investigation, we specifically considered the atmospheric, hydrological, and non-tidal ocean loading phenomena. The computed loading deformations for this region show the horizontal displacements are dominated by the non-tidal ocean loading (maximum 4 mm for the North and 3.1 mm for the East components); the main vertical contributions are due to the atmospheric and continental water storage loading (maximum 14.3 for the atmosphere and 8.1 mm for the hydrology, respectively). We have found that the dominant loading effect on the horizontal velocity is the non-tidal ocean loading (mean of 0.11 mm/yr), whereas the vertical component is dominated by the hydrological loading (mean of 0.21 mm/yr). Since the study area is in a mountainous region, we also analyzed the difference between the atmospheric and the topography dependent atmospheric loading models at our GNSS campaign sites. We did not find any significant difference between the two atmospheric loading models in terms of horizontal velocity. Finally, we performed simulations to identify the optimum timing and frequency of future GNSS campaigns in this area that would minimize the loading effects on tectonic velocity estimates.

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.

Impact of Loading Phenomena on Velocity Field Computation from GPS Campaigns: Application to ResPyr GPS Campaign in the Pyrenees

To quantify the present-day tectonic deformation in the Pyrenees mountain range area where the seismic activity is continuous and moderate, different GPS ResPyr campaigns were performed between 1995 and 2008. Considering the expected rate of deformation of about 1 mm/yr, we can wonder what would be the impact of the different loading effects on velocity field computed from GPS campaigns and therefore on the characterization of the deformation.

Ocean Loading in Brittany, Northwest France: Impact of the GPS Analysis Strategy

In this contribution, we analyze the impact of different GPS processing strategies on ocean tide loading estimation. We use continuous GPS data acquired during a 4-month campaign performed in 2004 in Brittany, Northwest France. Since the expected geodynamical signal in the estimated positions is exceeding the typical GPS data analysis noise, this data set can be used to compare the results obtained with different analysis software packages. Moreover, in this specific case we need short sub-daily solutions to study short-period signals instead of classical 24 h-solutions. The GPS capability for measuring 3D ocean tide loading deformation has already been assessed, but since we are looking for the finest signal as the one induced by the shallow water constituents, it is essential to be sure that the position time series represent a geodynamical signal and are not biased by the data processing strategy used. To analyze the possible effect of the methodology used on the geodynamical results, we compare different solutions computed with different strategies (Double Differencing and Precise Point Positioning) with various GPS analysis software packages (Bernese, GAMIT, GINS, and GIPSY/OASIS). We show that the different solution consistency is at the level of 1–3 mm. We also show that the data processing strategy has a mean effect of about 10–20% of the ocean tide loading signal amplitude.

First Results of the 2005 Seismology – Geodesy Monitoring Campaign for Volcanic Crustal Deformation in the Reykjanes Peninsula, Iceland

The Reykjanes Peninsula, Iceland, presents en-echelon volcano-tectonic systems trending oblique to the normal to the NAM-EUR separation. To better understand the tectonic of the Reykjanes Peninsula’s oblique spreading, a combined geodetic/seismologic campaign was performed from April to August 2005. For the seismologic part, 18 3-component seismometers of short period (1, 2 Hz) were deployed in an area of 30 km E–Wx20 km N–S. To detect small tectonic displacements less than 1 mm across postulated active faults, we used classical topometric measurements instead of GPS measurements, with high precision instrumentation: tacheometer, corner cube retro-reflectors, and centering system with hydraulic locking. The geodetic measurements were performed on 4 local networks in the studied area, on both sides of the fractures with points of about 10–100 m apart. In this contribution, we focus on the geodetic part of the campaign. We first present the geodetic networks and the reference frame setting used. Then, we describe the processing strategy, and show first results of the observed crustal deformations and time series analysis. Finally, a first comparison with the seismologic results is given.

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.