**4.3.2 Rice fields data collection**

192 Remote Sensing of Planet Earth

Considering the satellites in view, we pointed our antenna towards 170° of azimuth respect to geographical North. We successfully track reflected signals coming from GPS satellite 16 and GPS satellite 30. The geometry of acquisition was determined computing the iso-delay lines with ½ C/A code chip step and the specular reflection points, shown in Fig. 11 (in red for the 16th and in green for the 30th) together with the antenna footprint (depicted in light blue). All the points have been superimposed on ©Google static Maps and georeferenced in UTM. For both satellites the relative delay-doppler maps were computed over 1 s of noncoherent integration time and normalized from 0 to 1. Results are shown in Fig. 12. For satellite 30 (Fig. 12 (left)), the map is characterized by a very low noise, since the expected scattered signal is almost coherent and limited to one iso-range area, with no successive returns with delay greater than 1 chip (8 samples rising to the maximum, 8 samples going

Fig. 11. Specular reflection point and iso-delay lines superimposed in UTM on ©Google

Samples

Doppler [Hz]

Fig. 12. Delay-Doppler maps for satellite 16 (right) and satellite 30 (left)

static Maps (satellite 16 in red, satellite 30 in green)

down to the noise floor).

During the second data collection of May 2011, an experiment performed flying over an area placed in the Piedmont region (north west part of Italy), the receiving system was placed on board a small aircraft in order to track reflections from rice fields. Since rice fields are flooded during this month, they are a perfect scenario to study reflection phenomena.

Like in the previous experiment, the geometry of reflections was analyzed and all the satellites with elevation lower than 33° were discarded, since below this elevation the specular reflections did not enter inside the -3 dB beam-width of the LHCP nadir looking antenna.

The signal to noise ratio detected from the reflected signal was normalized respect to the correspondent direct signal; moreover, we compute all the specular reflection points visible and to each point we associate the relative normalized signal to noise ratio.

On board the aircraft, a video camera was placed to see which fields were really flooded during the acquisition. The panoramic view extracted from the video was superimposed on ©Google Maps, together with the specular reflection points (Fig. 13). After the superposition, we have noticed a good agreement between the fields' state and the received power (see Fig. 13 and 14). The minimum received power correspondent to a low normalized signal to noise ratio is clearly associated to not flooded fields.

Fig. 13. Specular reflection points tracks for satellite 8 and 26 over Piedmont rice fields with relative normalized signal to noise ratio. See Fig. 13 for the red rectangle zoom.

Furthermore, we compare the signals of two different satellites with similar elevation but different azimuth; we notice a high correlation between the two specular reflection point tracks both from the qualitative (Fig. 13, Fig. 14) and the quantitative (Fig. 15) point of view. The quantitative comparison is performed considering the reflected power coming from the same longitude, considering a bean of 0.01°. Further investigations on this behavior are under development.

GNSS Signals: A Powerful Source for Atmosphere and Earth's Surface Monitoring 195

as long as some tricks are taken into account. In particular it has to be outlined that, in order to make neutral atmospheric tomography more effective, the choice of the GNSS network topology is a key aspect. A good horizontal receiver's distribution guarantees a good retrieval of horizontal gradients. A good vertical receiver's distribution guarantees also a good retrieval of vertical gradients. Even if our network topology was not optimal for tomographic purposes, the inclusion of measurements (even if not very accurate) performed by two receivers placed at higher heights and of the low elevation observations, demonstrate this aspect. Since a suitable vertical receiver distribution is difficult to implement, the availability of quasi-horizontal observations is necessary. Then, limb sounding Radio Occultation observations are necessary in order to guarantee good observations coming also from low elevation angles (this aspect has already been

GNSS signals reflected off the Earth surface which represent an error source for navigation purposes, are instead useful for characterizing land and sea surfaces both from a monitoring and early-warning point of view. In particular the possibility of extracting information about the sea height and roughness, the soil moisture content, the snow and ice cover state have been successfully proven. Presently, no operative missions exist but many experimental activities have been carried out and the interest of national space agencies is constantly growing. From our point of view, we put some efforts in developing an instrument capable of collecting reflected GNSS signals, since we believe in the potentialities

We definitely believe that the "expansion" of GNSS sources expected when also the European GALILEO, the Indian IRNSS and the Chinese BEIDOU navigation satellite systems will be deployed, together with the consequent availability of Radio Occultation observations, and the consequent availability of "vertical" and "horizontal" observations,

For matherial contained in sections 2 and 3, authors are grateful to ESA for supporting the work in the framework of the METAWAVE project (Contract: ESTEC 21207/07/NL/HE). Work described in section 4 was supported by Regione Piemonte for funding the SMAT-F1 project, ISMB and Digisky for the development of the experimental campaign over rice

Askne, J. & Nordius, H. (1987). Estimation of tropospheric delay for microwaves from surface weather data*. Radio Science*, Vol.22, No.3, pp. 379-386, ISSN 0048-6604 Anthes, R. A.; Ector, D.; Hunt, D. C.; Kuo, Y-H.; Rocken, C.; Schreiner, W. S.; Sokolovskiy, S.

*Meteorological Society*, Vol. 89, pp. 313-333, doi: 10.1175/BAMS-89-3-313

V.; Syndergaard, S.; Wee, T-K.; Zeng, Z. P.; Bernhardt, A.; Dymond, K. F.; Chen, Y.; Liu, H.; Manning, K.; Randel, W. J.; Trenberth, K. E.; Cucurull, L.; Healy, S. B.; Ho, S-P.; McCormick, C. T.; Meehan, K.; Thompson, D. C. & Yen N. L. (2008). The COSMIC/FORMOSAT-3 Mission: Early Results. *Bulletin of the American* 

demonstrated by Foelsche and Kirchengast, 2001 and Notarpietro et al., 2008).

will improve definitively all the techniques here presented.

fields and NAVSAS group for providing the NGene receiver.

of this technique.

**7. References** 

**6. Acknowledgements** 

Fig. 14. Zoom of the specular reflection point tracks along the rice fields on ©Google Maps

Fig. 15. Quantitative comparison of normalized signal to noise ratio for satellite 8 (red) and 26 (blue) considering reflection points with the same longitude
