Manual on sea level measurement and ... - unesdoc - Unesco

Sea Level Measurement and Interpretation8. New Techniques for Sea LevelMeasurements8.1 GPS on BuoysRadar altimetry is widely used for various sea-levelrelatedresearch. Owing to unknown post-launchbiases and the aging of satellite electronic systemcomponents, a consistent and long-term stable offshoreheight reference is needed. Some satellites arepassing offshore oil rigs equipped with tide gaugesand GPS. However, these locations are rare. Withthe increasing accuracy of GPS for offshore applications,various groups have developed GPS-equippedbuoys for the determination of the instantaneoussea-surface heights (iSSH). While in the past onlya few calibration experiments were carried out forradar altimetry, in 2001 GPS buoys were successfullyused during the calibration/validation campaigns ofJason-1 and ENVISAT. More recent applications are fortsunami detection (e.g. Kato et al. 2005).One of the earlier calibration experiments was successfullycarried out by Hein (Hein et al. 1990).Between 1999 and 2003, GPS water level measurementwas coordinated by a special study group of theInternational Association of Geodesy (see http://www.gfz-potsdam.de/pb1/op/altimetry/SSG_buoys/index.html and references herein).Depending on the type of application, logistic requirements,and sea state, different types of buoys are used.The most simple and straightforward types are lifesavertypebuoys fitted directly with a GPS antenna (Watsonet al., 2004; Figure 8.1), and towed by, for example, avessel. Larger buoys, such as toroid buoys (Schöne etal., 2001; Figure 8.3) or barrel buoys, can be mooredfor long-term applications, but are in need of additionalsensors to account for the buoy movement. Examplesof height time-series from GPS buoys are shown inFigures 8.2 and 8.4.To estimate the iSSH, GPS measurements are used ina differential mode. The computed value is furthercorrected for the dipping and the tilting of the buoy.The accuracy of GPS for the height component isbetter than a few centimetres, allowing very precisemeasurements of the iSSH. Even with a decrease inaccuracy with increasing distance from the GPS referencestation at the shore, the resulting iSSH values areprecise enough for calibrating radar altimeters.With the technological advances in hardware and newdevelopments in GPS processing, small lightweightsystems will be available to support the massive applicationin different fields. Fields of future research arethe application of precise point positioning techniquesto reduce the amount of data to be transferred, lowpowerconsumption equipment, and new data transfertechniques for offshore applications.IOC <strong>Manual</strong>s and Guides No 14 vol IV45