ModellinG thE GNSS reflectometry signal over land: sensitivity

Nazzareno Pierdicca1, Leila Guerriero2, Emanuele Santi3, Alejandro Egido4
Dept. Information Engineering, Electronics and Telecommunications, Sapienza Univ. of Rome, Italy
Dept. Computer Science, Systems and Production, Tor Vergata Univ. of Rome, Italy
CNR-IFAc, Sesto Fiorentino, (FI) Italy
STARLAB, Barcelona, Spain
The GNSS-R signal is the output of a signal correlation operation, and it is a function of the time delay
and of the Doppler shift, i.e., the so called delay-Doppler map. It has been mathematically described in a
fundamental work by Zavorotny and Voronovich [1], where the sea surface scattering was modeled
according to the Geometrical Optic approximation. The latter assumes that scattering from randomly
rough surface is only incoherent. On the contrary, when the observed surface is characterized by small
scale roughness with respect to wavelength, like in the case of bare soil or vegetated fields, the bistatic
signal consists of both a coherent and an incoherent component and, according to recent studies, the
coherent scattering in the specular direction can largely overpass incoherent scattering [2]. In the present
work, the Advanced Integral Equation Model [3] in the bistatic configuration will be considered to
represent the incoherent scattering component from land surfaces, and a formulation for the coherent
component of the bistatically scattered field from bare soil will also be introduced. The latter has been
derived by generalizing the theory in Fung and Eom [4], which was limited to the specular and the
backscattering directions only. Vegetation (crop and forest) scattering contribution has been also
considered through a discrete bistatic approach [5].
A simulator of the GNSS-R signal has been developed based on the above electromagnetic formulations
and including the actual receiver characteristics (i.e., antenna characteristics, observing geometry,
receiver correlation, and so on). The results of the simulator, presented in terms of delay-Doppler maps,
waveforms or simply received power peak, have been compared to real GNSS-R data collected in the
frame of two experiments funded by ESA. The experiments have consisted in the exploitation of a
GNSS-R receiver, manufactured by Starlab, operated both on a ground based (LEiMON project) and an
airborne (GRASS project) platform. The experiments and the simulations have single out the sensitivity
we can expect using GNSS-R as a sensor to retrieve Soil Moisture Content and vegetation biomass.
[1] Zavorotny, V.U., and A.G. Voronovich, "Scattering of GPS Signals from the Ocean with Wind
Remote Sensing Application", IEEE Trans. Geosci. Remote Sensing, pp. 951-964, 2000.
[2] Ferrazzoli P., Guerriero L., Pierdicca N., Rahmoune R., ``Forest biomass monitoring with GNSS-R:
theoretical simulations'', Advances in Space Research, pp. 1823--1832, 2011.
[3] Wu T.-D., and K.-S. Chen, “A Reappraisal of the Validity of the IEM Model for Backscattering
From Rough Surfaces”, IEEE Trans.Geosci.Remote Sensing, pp. 743-753, 2004.
[4] Fung A.K., and H.J. Eom, “Coherent Scattering of a Spherical Wave from an Irregular Surface”,
IEEE Trans.Antennas and Propagation, pp. 68-72, 1983.
[5] Bracaglia M., P. Ferrazzoli, and L. Guerriero, “A fully polarimetric multiple scattering model for
crops”, Remote Sens. Environ., pp. 170–179, 1995.