**4.2 State of the art**

190 Remote Sensing of Planet Earth

Fig. 9. Shape of the correlation function for non-coherent reflections (black); each triangle

The basic observables are the delay of the reflected signal respect to the direct one, and the received power after reflection. Both observables are retrieved looking at the correlation function of the reflected signal and eventually comparing or normalizing it with the

The delay is used to determine the surface height, so is considered in case of GNSS signals reflected off water surfaces (Martin-Neira et al., 2001; Hajj & Zuffada, 2003). The height of the surface respect to the observer is retrieved in eq. 14 through the relative delay Δτ, the

On the other hand, the reflected power is used to determine the surface reflectivity and the

The surface reflectivity belongs to the coherent part of the scattered power that is measurable from the specular part of the received echo; it is used to determine the reflection coefficient that is related to the incident angle and dielectric constant. The dielectric constant is related to the soil composition and to its moisture content following empirical models or

The surface can be characterized looking at its roughness from the scattering cross section, since it contains the non-specular part of the reflected power. In this case we consider reflected power part calculated from the amplitude and the gradient of the correlation

In order to retrieve surface winds over the sea, the shape of the non-specular echo is compared with a simulated one obtained using a sea surface model (Zavorotny &

cΔτ = 2h sin γ (14)

refers to the signals received by an isorange, 8 samples equal to 1 C/A chip

correspondent correlation of the direct signal.

scattering cross section (Masters et al., 2004).

carefully calibrating the data (Masters et al., 2004).

function on the right side of its maximum.

Voronovich, 2000; Elfouhaily et al., 2002).

speed of light *c* and the elevation angle of the reflection γ.

Winds retrieval and altimetry are the more consolidated applications, while soil moisture and ice monitoring are under-development.

Many instruments were developed up to now (Nogues-Correig et al.,2007) and the techniques of retrieval have been tested through many experimental activities. The early experiments deal basically with altimetry; measurements were collected either from a static position (Martin-Neira et al, 2001), from balloon (Cardellach et al., 2003) or from aircraft (Lowe et al, 2002). Other set of experiments were developed to retrieve the ocean surface state (Garrison et al., 2000), such as wind or sea roughness. Last but not least, the technique was demonstrated on board a small satellite, the UK-DMC (Gleason et al., 2005).

Nevertheless, nowadays no operative missions exist in this field.

From our point of view, during the SMAT-F1 project we developed a prototype based on a Software Defined Radio solution, using a navigation software receiver (Tsui, 2005). This is the NGene SW receiver, developed by NAVSAS group of Politecnico di Torino (Fantino et al., 2009). The instrument is highly reconfigurable, since collects raw I and Q IF samples of the incoming signals (direct and reflected). A sampling frequency of 8.1838 MHz is used, giving about 8 samples per C/A code chip.

Moreover, the small hardware architecture is made up of cheap COTS (Commercial Of The Shelf) components, with very low overall weight and power consumptions. These features make the system suitable to be easily placed on board aircrafts, also small U.A.V.s (Unmanned Aerial Vehicle) (Cucca et al., 2010).
