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A French Contribution to IGS TIGA-PP- Progress
Report -
Guy WOPPELMANN 1, Marie-Noëlle BOUIN 2, Loïc
DANIEL 3 and Ronan LE ROY 4 1 CLDG - Université
de La Rochelle, gwoppelm_at_univ-lr.fr 2 LAREG -
Institut Géographique National,
bouin_at_ensg.ign.fr 3 ENSG - Institut Géographique
National, daniel_at_ensg.ign.fr 4 EPSHOM , Section
Géodésie-Géophysique, rleroy_at_shom.fr
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.
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. This
poster draws up a status report on the
contribution led by Université de La Rochelle in
collaboration with the French national mapping
and geodetic agency (IGN), the French
hydrographic and oceanographic service (SHOM) and
Ecole Normale Supérieure (ENS). This contribution
to TIGA consists in ? seven observing
stations, ? a dedicated TIGA data
centre, ? an analysis centre.
MORS acoustictide gauge
? CGPS since July 1998. ? Precise levelling
performed yearly.
Trimble SSI with Dorne Margolin antenna
? Sea level station since 1806. ? CGPS since
October 1998. ? Distance about 350 m. ? Precise
levelling performed in 1999 and 2002.
Six French CGPS_at_TG stations participate into TIGA
Ajaccio (IGN), Brest (SHOM), Dumont dUrville
(ENS), La Rochelle (CLDG), Marseille (IGN), and
Saint Jean-de-Luz (IGN). Additional existing
CGPS_at_TG 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. 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).
Somewhere here
Acoustic tide gauge since 27/10/1998
? Sea level station since 1941, (also known
as La Pallice). ? CGPS since November 2001. ?
Distance less than 100 m. ? Precise levelling
performed in 2001.
Ashtech MicroZ with Dorne Margolin antenna
Krohne radartide gauge
? Sea level station since June 2001. ? CGPS since
January 2001. ? Distance less than 500 m. ?
Geodetic connection (GPS levelling)
performed in 2001.
? 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
Overview 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. (see http//op.gfz-potsda
m.de/tiga/index_TIGA.html) Reference frame
realisation. 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. The processing
is split into 4 global networks of less than 50
stations each. The map below shows one of these
networks.
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
CGPS_at_TG 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 CGPS_at_TG
(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.
8 Seismic Cruises between 1999 and 2002

• Present data centre features
• Equipment
• ? Internet link at 4 Mbps
• ? Dedicated computer room (LAN, stabilised t )
• ? Netserver II (Pentium II - 5 x 4 Go / 128 Mo)
• ? Nas Dell (400 Go on line disks - RAID5)

• Software
• ? Apache (http server)
• ? MySQL (Database)
• ? IGN Data Centre software package (based on
  mirror, perl, php, cron, NQS)

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