Ocean Remote Sensing Using GNSS-R - GNSS Remote Sensing: Theory, Methods and Applications - page 233

instrumental-correction parameters. Depending on the model used for the fit,
the geophysical parameters can be 10-m altitude wind speed, or sea surface
slopes' variance (mean square slopes-MSS). Some of the works done with this
methodology are: (e.g. Katzberg et al. 2001 ; Garrison et al. 2002 ; Cardellach
et al. 2003 ; Komjathy et al. 2004 )
Multiple-satellite DM-fit: extends the DM-fit inversion to several simultaneous
satellite reflection observations, which resolves the anisotropy (wind direction or
directional roughness ( Armatys 2001 ; Komjathy et al. 2004 )).
DDM-fit: The fit is performed on a delay-Doppler waveform ( Germain et al.
2004 ). In this way, anisotropic information can be obtained from a single satellite
observation.
Trailing-edge: As suggested from theoretical models in Zavorotny and
Voronovich ( 2000 ) and Garrison et al. ( 2002 ) implements in real data a technique
in which the fit is performed on the slope of the trailing edge, given in dB.
Delay and Doppler spread: Elfouhaily et al. ( 2002 ) developed a stochastic the-
ory that results in two algorithms to relate the sea roughness conditions with the
Doppler spread (IDS in Sect. 8.10 ) and the delay spread (IDM in Sect. 8.10 )ofthe
reflected signals. The technique was applied to LEO-based GNSS-R observations
taken from one of the UK-DMC satellites ( Gleason 2006 ), where 5 GNSS-R
measured IDS correlated with the MSS records taken by nearby Buoys.
Scatterometric-delay: For a given geometry, the delay between the range of the
specular point and the range of the peak of the reflected delay-waveform is nearly
linear with MSS. This fact is used to retrieve MSS ( Nogués-Correig et al. 2007 ;
Rius et al. 2010 ).
DDM Area/Volume: Simulation work in Marchan-Hernandez et al. ( 2008 ) indi-
cates that the volume and the area of the delay-Doppler maps are related to the
changes in the brightness temperature of the ocean induced by the roughness.
The hypothesis has been experimentally confirmed in Valencia et al. ( 2009 ). This
approach might be valuable for potential use of GNSS-R observations in support
to Oceanic L-band radiometric missions.
Discrete-PDF: When the bi-static radar equation for GNSS signals is re-
organized in a series of terms, each depending on the surface's slope Z 0 ,the
system is linear with respect to the Probability Density Function (PDF) of the
slopes. Discrete values of the PDF(Z') are therefore obtained. This retrieval does
not require an analytical model for the PDF (no particular statistics assumed).
When the technique is applied on delay-Doppler-maps, is it possible to obtain
the directional roughness, together with other non-Gaussian features of the PDF
(such as up/down-wind separation Cardellach and Rius ( 2008 )). An example is
given in Fig. 9.10 .
Coherence-time: Finally, when the specular component of the scattering is
significant (very low altitude observations, very slant geometries, or relatively
calm waters), the coherence-time of the interferometric complex field depends on
the sea state. It is then possible to develop the algorithms to retrieve significant
wave height ( Soulat et al. 2004 ; Valencia et al. 2010 ).