Ocean Remote Sensing Using GNSS-R - GNSS Remote Sensing: Theory, Methods and Applications - page 214

Chapter 9
Ocean Remote Sensing Using GNSS-R

This chapter is organized in two main sections, to tackle the essential Oceanic
application of the GNSS reflectometry: sea surface altimetry and surface roughness.
Other potential areas of interest for oceanography and marine applications are the
retrievals of dielectric properties of the surface (related to temperature and salinity);
and ship detection. Although the ratio between the co-polar and the cross-polar
components of the Fresnel reflection coefficients for sea water presents up to 10 %
sensitivity to salinity conditions ( Cardellach et al. 2011 , Fig. 7), the effects of the
roughness are not well modelled yet. These applications are still immature and no
further details are given in this Chapter.

9.1 Altimetry

The concept GNSS-R was proposed in 1993 by ESA ( Martín-Neira 1993 ) to provide
additional measurements of the sea surface to increase the spatial and temporal reso-
lution of the Radar Altimeters (RA), which typically only measure the ocean height
at the sub-satellite point (mono-static nadir-looking systems). As it will be shown
in Sect. 9.1.3 , the single-measurement precision-level of the LEO-based GNSS-R
is expected to be lower than standard space-based microwave RA. However, and
as seen in Chap. 8 , the potential temporal and spatial coverage/resolution of the
concept suggests its complementarity with mono-static RA. This passive wide-
swath altimeter concept lends itself to new applications, such as the detection of
tsunamis, Ocean eddies or other meso-scale features in ocean height. Significant
impact of GNSS-R data into ocean circulation models was demonstrated in Le Traon
et al. ( 2002 ) through assimilation of simulated data from a single GNSS-R satellite
mission. In order to achieve user required precision levels, the altimetric application
is more demanding than other GNSS-R applications, in terms of required SNR,