1. Rationale and Objectives
The motivation of SONEL is to get more from several mono-thematic permanent observing networks, either geodetic or sea-level oriented. This will come from integrating them into an efficient data collection and dissemination scheme, and by combining their measurements in order to provide specific products. This synergy is indeed the only way to get a clear and unambiguous picture of what is actually going on at a specific coastal site: is the sea level rising or is the land sinking over long term time scales?
Up to now several groups have already carried out local or regional campaigns in order to tie the tide gauge benchmarks into a global reference network using space geodetic techniques. Nevertheless, very few groups were able to complete the necessary work and thus provide a centimetre precise ellipsoidal height for the zero reference point of the tide gauge expressed in a well defined and maintained global geodetic reference frame. Moreover, determining the vertical velocity of a tide gauge with a level of accuracy better than 1 mm/yr remains a very challenging problem.
In many countries, tide gauge operations are managed by marine organisations or harbour services and not by the national geodetic or surveying agency. In some places the personnel in charge of geodesy survey the tide gauge benchmarks; in others the personnel responsible for tide gauges carry out the work. Sometimes the different organisations communicate well, sometimes not. However, many other reasons can explain the lack of available results: new techniques, uncertainties concerning the methodology to be used in observation or computation strategies, specific coastal problems, and the multi-type data involved in the process.
The ancillary tide gauge or geodetic information frequently requested by users to obtain the expected final results is either unavailable or is obtained by unreliable methods. This stresses the importance of an integrated sea level monitoring system like SONEL. Consequently, SONEL aims to be : (i) an integrated sea level monitoring system (different types of data, sources, time and space scales); (ii) a sea level information system handling the data measured by different observation networks; (iii) a national component of international sea level observing systems (GLOSS, ESEAS etc.); (iv) a national interface for related European and Global projects and databases (PSMSL, TIGA etc.); and (v) a service based on long term commitment as most sea level studies on climate change require time series spanning a long period of time.
2. Joint Programme
Expertise on the various aspects of tide gauges, levelling and space geodesy exists within the cooperating organisations (see section 3 below). This provides a favourable technical framework in which each organisation can make its specific contribution to establish and maintain fully integrated tide gauge - GPS stations. In particular :
to install, operate and maintain the different observing devices,
to guarantee high quality measurements according to the current international standards,
to asses the accuracy of the data,
to perform repeated local ties between GPS and tide gauge benchmarks on a routine basis, depending on the local topography and geology,
to gather and make available the required data and metadata.
3. Participating Organisations
- SHOM (Service Hydrographique et Océanographique de la Marine) :
SHOM is the French hydrographic and oceanographic service. Among its many roles, SHOM is in charge of tidal analysis and predictions, in particular for nautical chart datum definition and sounding reduction. To carry out its hydrographic and tidal activities, SHOM is implementing a modern network of tide gauges called RONIM. RONIM is based on acoustic and radar probes, as well as modern data communications. In addition to its invaluable expertise and experience on tide gauges, SHOM operates a permanent GPS station at the historical sea level station of Brest. (The first sea level observations were performed by the astronomers La Hire and Picard in Brest in 1679).
- IGN (Institut Géographique National) :
IGN is the French national mapping and geodetic agency. This agency is familiar with GPS techniques. It has been involved in IGS and EUREF since the very beginning (Global data center, national permanent GPS network…). The ground segment of the French DORIS positioning system is under the responsibility of IGN (installation and maintenance of a worldwide network of about 50 tracking beacons). IGN is also involved in IERS (Terrestrial Reference Section). Last but not least, IGN has operated a secular tide gauge in Marseille since 1885. The scientific activities in geodesy are carried out in the IGN lab called LAREG.
- CLDG (Centre Littoral de Géophysique) :
CLDG is the Earth Science lab of La Rochelle University. The topics of interest in CLDG are related to the ocean - land interface. Geophysical and geodetic techniques are deployed in order to carry out its research activities; in particular CLDG has gained experience in gravity measurements and is now developing expertise in GPS. It contributes to the EUREF and IGS networks through the permanent GPS stations in Chizé and La Rochelle. CLDG is involved in the TIGA IGS pilot project . It hosts the SONEL data centre and coordinates the activities of the various participants.
- LEGOS (Laboratoire d’Etudes en Géophysique et Océanographie Spatiale) :
LEGOS is a pluridisciplinary research unit with three affiliations : the CNES (Centre Nationale des Etudes Spatiales), CNRS (Department of Science de l’Univers) and the Université Paul Sabatier. LEGOS maintains a tide gauge network in the Subantarctic and Antarctic zones of the Indian Ocean. The network is called ROSAME and contributes to the World Ocean Circulation Experiment (WOCE).
Although SHOM and IGN represent important governmental agencies where many observations are made, relatively small but numerous and independent organisms perform either tide gauge or individual GPS measurements in a permanent mode (Harbors, Maritime Services, labs etc.).
In order to ensure that SONEL fulfills most of the needs of the scientific community, an informal steering group has been set up. The present chairman is Dr. Paolo Pirazzoli (Laboratoire de Géographie Physique, CNRS) and the secretary is Dr. Guy Wöppelmann (CLDG). Various options are currently being examined in order to move from an informal to a recognized group with a national official framework.
4. Structure of SONEL
The main components of SONEL are :
- networks of permanent stations that are either geodesy or sea-level oriented,
- a data gathering, archiving and distribution system,
- specific and partner operational data centres,
- specific and associate analysis centres for data quality control, reductions, comparisons and combinations,
- a steering committee that will include representatives willing to contribute to one of the components of SONEL : users, partners, policy makers.