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

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Sea Level Measurement and InterpretationSea Level Measurement and InterpretationFigure 8.1 The University of Tasmania GPSbuoy (C. Watson, personal communication).Figure 8.2 Height time-series (SSG 2.194, 2003).Figure 8.3 The GFZ Potsdam GPS buoy.Figure 8.4 Height time-series from a GPS buoy;the time-series is dominated by sea state. Thesmoothed curves are the running mean filteredtime-series; the dot is the actual RA measurementused for comparison.8.2 GNSS ReflectometryOnce the European satellite constellation GALILEO startstransmission of navigation signals in 2008, an infrastructureof three global satellite navigation systemswill be available for commercial and scientific applications.GALILEO, together with the US Global PositioningSystem (GPS) and the Russian GLONASS (Global’nayaNavigatsionannaya Sputnikovaya Sistema) constellation,offers novel opportunities for remotely sensing the Earth’satmosphere and oceans with dense spatial and temporalcoverage.The high reflectivity of GPS signals in the L-band frequencyrange (1.2276 and 1.57542 GHz) at water andice- or snow-covered surfaces allows for the detectionand analysis of reflected GNSS (Global Navigation SatelliteSystem) signals. The passive reflectometry and interferometrysystem (PARIS) was the first concept proposed forocean altimetry using GNSS (Martín-Neira, 1993). In theFigure 8.5 The PARIS concept.46IOC ong>Manualong>s and Guides No 14 vol IV

PARIS scheme, direct and ocean-reflected signals aredetected by spaceborne receivers, and altimetric heightinformation is extracted from the delay in arrival timesof the reflected signals relative to the direct signals(Figure 8.5).Using dedicated GNSS receiver instruments, sea levelheights accurate up to ~5 cm were determined in anumber of airplane and balloon experiments (Garrisonand Katzberg, 2000; Rius et al., 2002; Ruffini et al.,2004). In ground-based GNSS reflection experimentsabove an artificial pond, Martín-Neira et al. (2002)achieved an accuracy of 1 cm, and at an altitude ofabout 500 m above Crater Lake (Oregon, USA) altimetricheight values accurate to 2 cm were obtained (Treuhaftet al., 2001). Anderson (2000) reported on 12-cm accuracyin near-surface measurements at heights between7 and 10 m. In addition, the dependency of the codecorrelation function on the slope characteristics ofthe reflecting surface can be used to infer sea-surfaceroughness as well as wind speed and direction (GNSSscatterometry) (Katzberg et al., 2001; Cardellach et al.,2003; Germain et al., 2004).First spaceborne observations of signal reflections aredescribed by Pavelyev et al. (1996) and Lowe et al.(2002); later, signatures of coherent GPS reflections atgrazing incidence angles were found in radio occultationdata observed by the GPS/MET, CHAMP andSAC-C satellites (Beyerle et al., 2002; Hajj et al., 2004).CHAMP and SAC-C are both already supplied withnadir-looking antennas to detect reflected GPS signals;efforts are now being made to establish space-basedGNSS altimetry as a viable remote-sensing technique(e.g. Hajj and Zuffada, 2003).IOC ong>Manualong>s and Guides No 14 vol IV 47

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