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

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 <strong>Manual</strong>s and Guides No 14 vol IV 47