Aucun titre de diapositive - SOEST

Aucun titre de diapositive - SOEST

A French Contribution to IGS TIGA-PP
- Progress Report Guy WOPPELMANN 1, Marie-Nolle BOUIN 2, Loc DANIEL 3 and Ronan LE ROY

4

CLDG - Universit de La Rochelle, [email protected]
2
LAREG - Institut Gographique National, [email protected]
3
ENSG - Institut Gographique National, [email protected]
4
EPSHOM , Section Godsie-Gophysique, [email protected]
1

INTRODUCTION
Determining rates of vertical crustal motion in a well-defined global reference frame with an accuracy better
than 1 mm/yr is a very challenging problem in Geodesy today (see for instance ITRF2000 results). A major
scientific application is the removal of the crustal motion signals from long historical time series of sea level
change recorded by tide gauges, and further to derive the absolute or climate related change in mean sea level.
For this purpose scientists have begun building continuous GPS (CGPS) observing stations at tide gauges
around the world. But even with a decade of CGPS observations, the problem remains challenging. The vertical
component is highly correlated with parameters like water vapour content or reference frame scale definition.
MORS acoustic
tide gauge

Sea level station since 1806.
CGPS since October 1998.
Distance : about 350 m.
Precise levelling performed
in 1999 and 2002.

Trimble SSI with
Dorne Margolin antenna

This poster draws up a status report on the contribution led by Universit de La Rochelle in collaboration with the
French national mapping and geodetic agency (IGN), the French hydrographic and oceanographic service (SHOM)
and Ecole Normale Suprieure (ENS). This contribution to TIGA consists in :
seven observing stations,
a dedicated TIGA data centre,
an analysis centre.

OBSERVING STATIONS

As close as possible to the tide gauge was the major specification in CGPS installation.
This constraint is optimally achieved at Marseille and Saint Jean-de-Luz. To establish the link
between both GPS and tide gauge and assess the site stability, precise levelling connections are
scheduled at regular intervals depending on the local context (geology, topography).

Sea level station since 1941,
(also known as La Pallice).
CGPS since November 2001.
Distance : less than 100 m.
Precise levelling performed
in 2001.

Since 1885...

CGPS since July 1998.
Precise levelling performed yearly.

Six French [email protected] stations participate into TIGA : Ajaccio (IGN), Brest (SHOM), Dumont
dUrville (ENS), La Rochelle (CLDG), Marseille (IGN), and Saint Jean-de-Luz (IGN).
Additional existing [email protected] stations may join TIGA soon. These are : Kerguelen (L. Testut,
LEGOS), Noumea (S. Calmant, IRD) and Tahiti (L. Duquesne, CNES). Toli-Toli (ENS) aims at
tectonic processes studies, but it appears that an interesting tide gauge operates in the vicinity.

Somewhere here

Krohne radar
tide gauge

The most adequate way to handle it seems to be a global scale approach, a scale which is coherent with the size of
the problem. In 2001, the International GPS Service established a pilot project called TIGA (GPS Tide Gauge
Benchmark Monitoring) to analyze GPS data from stations at or near tide gauges on a continuous basis.

Acoustic tide gauge
since 27/10/1998

Ashtech MicroZ with
Dorne Margolin antenna

Sea level station since June 2001.
CGPS since January 2001.
Distance : less than 500 m.
Geodetic connection (GPS +levelling)
performed in 2001.

Tide Gauge
GPS

Sea level station since 1997.
Sea level station since 1942.
CGPS installation scheduled for
2002, delayed to 2003.
Distance : CGPS on top of the tide
gauge building.

Pressure sensor

CGPS since December 1997.
Distance : about 500 m.
Geodetic connection (GPS +levelling)
performed in 2001.

Tide gauge station

ANALYSIS CENTER

DATA CENTER
Overview

Overview
The IGS TIGA project data centre is hosted at CLDG (University of La Rochelle). It is an extension of SONEL
research data centre infrastructure. SONEL capabilities are enhanced to cope with the additional amount of data
and workload generated by a world-wide coastal sea-level station network.
Retrieve, archive and make available data for as many as possible [email protected] sites is the foreseen objective.
Data is meant as observations and products, both from CGPS and tide gauges, as well as their connections and
any worthy ancillary information.
The data centre currently retrieves CGPS observations of the 55 core stations proposed by the TIGA
committee, plus 109 [email protected] (not core). Mean sea level data from these stations are also collected from
PSMSL (when available), as well as hourly sea level data from UHSLC. The aim is to propose a coherent,
simple and unique access point. Implicitly, the data centre acts as a geographical abroad backup for PSMSL
and UHSLC.

The analysis centre is a type I class in TIGA terminology, i.e. it aims at processing current and future data
with a latency of 460 days, and re-computing a selected subset of IGS network data for improved long-term
stability of the reference frame since the inception of the IGS.
Processing is carried out at CLDG on a Linux Pentium III - 33MHz with 750Mbytes of memory. The
analysis centre comes from a close cooperation between CLDG and IGN geodetic lab (LAREG). It highly
benefits from the following scientific software :
GAMIT, used for processing GPS measurements (R.King, MIT)
CATREF, used for combination of GPS coordinate solutions (Z.Altamimi, IGN)

Data Analysis Strategy
General
Common standards proposed by TIGA are adopted.
8 Seismic Cruises between 1999 and 2002

(see http://op.gfz-potsdam.de/tiga/index_TIGA.html)

Present data centre features

Reference frame realisation.

Equipment
Software
Internet link at 4 Mbps
Apache (http server)
Dedicated computer room (LAN, stabilised t )
MySQL (Database)
Netserver II (Pentium II - 5 x 4 Go / 128 Mo)
IGN Data Centre software package
Nas Dell (400 Go on line disks - RAID5)
(based on mirror, perl, php, cron, NQS)

Selection of 22 (red diamonds) out of the 55 proposed core stations, satisfying the following criteria :
1- Stations available in ITR2000 from at least 3 individual GPS solutions.
2- Standard deviations less than 3 mm (positions), less than 1/mm/yr (velocities), in ITRF2000.
3- weighted rms less than 1,5 mm (horizontal positions), less than 2,5 mm (vertical positions),
less than 1.4 mm/yr (horizontal velocities), less than 2.5 mm/yr (vertical velocities), in ITRF2000.
4- Minimum of 2 years of continuous GPS data.

Data servers
No offline storage, simple access to any file by anonymous
ftp (ftp.sonel.org) and http (under construction).

The processing is split into 4 global networks of less than 50 stations each. The map below shows one of
these networks.
Each network includes the 22 selected core
stations. Network solutions are combined in
order to build daily solutions using the
Minimal Constraint Approach to realise the
reference frame [Altamimi, 2002].

Current structure of FTP server

Daily solutions are then combined to produce
weekly and monthly solutions. The analysis
centre is still setting up and evaluating its
processing strategy. Solutions will be
provided soon (before end of 2002).
Note:
Sea-level data is organized within data sources directories (shom,
legos, psmsl, uhslc) because of present lack of standards. A data
type structure would be more desirable however (as for GPS).

Workshop on Vertical Crustal Motion and Sea Level Change

September 17-19, 2002
Toulouse, France

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