Expected results, prospects
Acquiring observation data is the bedrock of any research activity and progress in understanding the variations in sea level is no exception. However, there is as yet no permanent agency dedicated to making available to the scientific community the observations, or the results of geodetic surveillance, of French tide gauges. SONEL is fragile even if it does already provide many services, as was demonstrated at the SONEL symposium organised in April 2006 in La Rochelle. The development of a system founded on "best effort" has its limitations. It is ill adapted both to a research observatory type of infrastructure for long-term phenomena and to France’s commitments as part of the associated world programmes (GLOSS in particular). Now that the value of SONEL is recognised by the international community, its long-term survival must be ensured with renewable and pluriannual funding, for example through a homologation by INSU (CNRS). It should be noted that the ocean is a turbulent environment with modes of slow fluctuations that range from decades to centuries. Consequently, given the lack of a perfect model, continuous observation over a long period is indispensable to describe the ocean and predict how it will behave.
The most demanding application in terms of metrology is undoubtedly the study of the climatic variations of sea level in coastal zones. This requires a specific long-term observation system which is on the same scale as the issue being tackled. Thus, SONEL is here for the foreseeable future and represents a concentration in France of the European system ESEAS and the world system GLOSS. Long-term fluctuations in sea level are of great scientific interest due to the fact that these questions dovetail with issues from numerous other fields. However, it should be underlined that the impacts of a rise in sea level will not occur gradually but in fits and starts, during coastal flooding triggered by storms. The study of storm surges, and their frequency, is therefore essential to predict changes and the environmental risks in coastal regions that are vulnerable to flooding or coastal erosion. Again, these studies cannot be carried out satisfactorily without an analysis of long time series of coastal tide gauges.
The 2009 meeting of the group of experts of the world programme GLOSS gave a timely reminder of the difficulties that are currently being encountered in obtaining an estimation of the vertical movements of the land using spatial geodetic techniques, particularly GPS. The same measures processed by different teams, with different software, gave appreciably different results, even with time series of several years. Further studies must be undertaken on the geophysical (troposphere), physical (antennae) and geodetic phenomena (creation and long-term stability of the bench marks using GPS solutions). In this uncertain context, the constitution of a data bank of all the relevant measurements and metadata would appear essential. Clearly this includes GPS measurements but also tide gauge measurements and levelling, together with all additional data that contribute to our understanding of the geophysical signals. The idea is to create an archive of the measurements as rigourously as possible in order to process them, or reprocess them, as soon as the phenomena that affect the vertical GPS component are better understood. This is not an original approach, but as modest as it may appear, it has often been found to be crucial (Tycho Brahé, Lalande etc.).
In March 2010, during its meeting of data centres, the GLOSS programme adopted the GPS component of SONEL as the new GLOSS data centre specialised in GPS data co-located with tide gauges. However France is expected to make a pluriannual (at least four years) commitment in order to play this role within the world programme. A demand for homologation by SOERE was made in January 2010.
Maintaining a regular scientific and technical activity such as SONEL over a long period of time is difficult for French university research laboratories, which are obliged to continually publish new work, to the detriment of slower, more routine activities like observing, collecting, controlling, archiving and publishing data that cannot be interpreted over a short time span. In contrast, a system of measurement and surveillance like SONEL is closely linked to a research activity. We are at the metrological limits of tide gauges and spatial geodesy for certain scientific applications. A satisfactory development of SONEL will not be possible in the long term without recognised scientific and technical support. An observation system homologation by INSU would appear to be the ideal national framework to perpetuate SONEL.