Tide gauging by GNSS buoy
With the development of data processing, the Global Navigation Satellite System (GNSS) provides new applications in the field of tide gauging. By placing a GNSS receiver on a floating buoy, one can determine water level at all times.
The use of GNSS buoys in the field of oceanography began in the 1990s. Australia, the USA and France were among the pioneering countries. Originally GNSS buoys were developed for the calibration of satellite altimetry missions. With the continuous improvement of GNSS measurement processing capabilities and computer processing power, GNSS buoys can now be used in oceanography, hydrography, tide gauging and more.
Today, there is a wide range of hydrography applications for GNSS buoys:
- Calibration of satellite altimetry missions;
- Calibration of coastal tide gauges;
- Absolute positioning of sea level measured in a global reference system: ellipsoidal height;
- Reduction of soundings (for nautical charts)
Examples of French GNSS buoys
France is one of the pioneering nations in the research and development of GNSS buoys. Today there are 4 types of buoys in use.
The GNSS Buoy at the Cote d'Azur Tide Station / Centre National d'Etudes Spatiales was the first to be installed in France. By the late 1990s, measurement campaigns were regularly conducted off Ajaccio in order to calibrate the satellite altimetry missions of Topex-Poseidon and Jason-1 and Jason-2.
The GNSS buoy of the Institut de Physique du Globe de Paris is substantially identical to the OCA-Géoazur/CNES buoy. But it is used for quite a different purpose. The GNSS buoy provides data that are compiled with other data, including pressure measurements on the seabed, in order to study the vertical deformation of the seabed related to seismic activity. Today we cannot globally map (in the marine environment) deformations of the Earth based on satellite signals alone. Ongoing studies aim to circumvent this problem.
LEGOS / DT-INSU Buoy
The GNSS buoy of the Laboratoire d'Études en Géophysique et Océanographie Spatiales built by the Institut National des Sciences de l'Univers (CNRS) consists of three floats composing the vertices of an equilateral triangle, with the GNSS antenna placed at centre of gravity. The LEGOS/DT-INSU GPS buoy is used to monitor instrument drift on tide gauges composing the ROSAME network. Because the ROSAME stations are located in the French Southern and Antarctic Lands, there are strict specifications on the weight and size of the unit.
The SHOM buoy is closer in shape to the LEGOS/DT-INSU buoy, with a base forming an equilateral triangle with a float at each end. However, it is larger and forms a tetrahedron with the geodetic antenna placed at the top of the last vertex, out of the water and up in the air.
The two models do not have the same constraints. The SHOM buoy seeks to achieve floating stability vis-à-vis the chop and swell. The GNSS antenna is raised above the waterline to avoid multipath as much as possible. Multipath produces adverse reflection effects during the acquisition.
This buoy was funded through loans from GRGS and ANR CECILE.
Comparison of sea level measurement systems on the Ile d'Aix in March 2012
A comparison between three buoys (IPGP GPS buoy, LEGOS/DT-INSU GPS buoy, SHOM GNSS buoy), a pressure tide gauge and the reference, the RONIM OptiFlex permanent radar tide gauge, was conducted from 27 to 29 March 2012 on the Ile d'Aix. The operation was organized by Dynamique du Littoral de l'Unité Mixte de Recherche (CNRS / Université de La Rochelle) LIENSs.
This comparison provided valuable information. The analyses are underway. Another comparison session is already scheduled for the years to come.
Other examples of GNSS buoys in the world
- Australian GNSS buoy
To find out more:
- Presentation by P. Bonnefond et al, January 2012. Corse: des marégraphes pour le suivi des altimètres radars. Tide gauge data exchange conference in France within the framework of the CRATANEM project
- Presentation of the IPGP GPS buoy
- Use of the IPGP GPS buoy
- Presentation of the LEGOS/DT-INSU buoy
- Presentation of the ROSAME network
- Article in the French newspaper Sud-Ouest on the system comparison on the Ile d'Aix in March 2012
- André G., B. Martin Miguez, V. Ballu, L. Testut, G. Wöppelmann (2013). Measuring sea level with GPS-Equipped Buoys: A multi-instruments experiment at Aix island. The International Hydrographic Review, n°10-2013.
- Born G.H., M.E. Parke, P. Axelrad, K.L. Gold, J. Johnson, K.W. Key, D.G. Kubitschek (1994). Calibration of the TOPEX altimeter using a GPS buoy. J. Geophys. Res., 99 (C12), pp.24517-24526.
- Watson C.S. , Coleman R., Handsworth R. (2008). Coastal tide gauge calibration : A case study at Macquarie Island using GPS buoy techniques. Journal of Coastal Research. 24(4) pp1071-1079 doi: 10.22112/07-0844.1.
Last updated: 11/07/2013