Speaker
Description
Bistatic Radar for Earth Observation has been recently proposed for interferometric and urban applications, addressing bistatic geometry with relatively small spatial baselines, i.e., quasi monostatic configurations. In this work, we explore the scattering behaviour of land surfaces for arbitrary bistatic configuration, that is for any observing direction of the passive element, even out of the incidence plane and with large baselines. This study is based on electromagnetic model simulations with two main objectives: i) providing a characterization of the target scattering coefficient to support the technical design of a bistatic imaging system at L-band, ii) understanding its potential to infer bio-geophysical parameters, namely soil moisture and vegetation biomass, eventually in combination with conventional monostatic data.
The electromagnetic models adopted in this work for bare soil incoherent scattering are the Advanced Integral Equation Model (AIEM) and the Small Slope Approximation up to the second order (SSA2) [Johnson and Ouellette, 2013]. As far as vegetated targets are concerned, we have considered the Tor Vergata (TOV) model [Guerriero et al., 2013], which is a radiative transfer model with a discrete approach for vegetation elements. As for the coherent scattering from the rough soil surface, which is important when looking at the specular direction, we have adopted a revised version of the model in [Fung and Eom, 1983].
The performance analysis has been carried out by means of the Cramer Rao Lower Bound (CRLB), starting from the sensitivities predicted by the models to the different target parameters. Here it has been considered for the retrieval of soil moisture of both bare and vegetated soil, considering the effect of unknown roughness variations, and for that of biomass of a vegetated surface, considering the unknown variation of soil moisture. Possible limitations can be related to the noise equivalent sigma naught (NE0), i.e. the lowest measurable 0 due to system noise. The retrieval capability can be affected when the signal is below the noise floor, a condition that can be achieved in particular configurations. The results of this study show that the most effective configuration for the passive system is at an azimuth angle close to 90° with respect to the incidence plane, and at VV polarization. This is mainly because at HH polarization the signal is lower and the saturation due to the noise floor is more likely reached.
The availability of a bistatic measurement at VV polarization at about 90° azimuth angle improves also the accuracy of the SMC estimation in vegetated surfaces, with respect to monostatic measurements.
The CRLB tool has been used also to detect the azimuth anisotropy of the surfaces. A theoretical study has demonstrated a significant improvement in the retrieval of wind speed when active and passive systems flow on the same orbital plane, provided that the AT baseline is long enough.
Finally, L-band bistatic synthetic images with different land covers have been generated with the addition of the speckle noise. A retrieval exercise has been carried out using monostatic or multistatic images, i.e, simultaneous monostatic and bistatic acquisitions, to show the potential of a multistatic approach, although in a simulated scenario.
