The next generation of spatial missions for Earth observation are designed to improve the understanding of the complex phenomena that affect the life of our Planet, and to provide valuable information for a broad class of applications, such as, e.g., oceanography, climatology, and the cryosphere erosion. Spaceborne platforms generally make use of different technologies, mainly based on the observation and analysis of the signal re-irradiated by the illuminated surface, whose acquisition cannot be considered an instantaneous process. It spans, indeed, over a finite acquisition time window and, therefore, fluctuations of the scattering phenomenon taking place during the radar observation might either be the source of additional information or lead to an increase of noise and uncertainty. Even if this problem is well-known, very few efforts have been devoted to the modeling and understanding of the signal variation at the receiving antenna considering heterogenous soils, especially when observed from space by a receiver spatially separated by the transmitter.
This project proposes an accurate study of the temporal fluctuations of the scattering generated by natural surfaces when observed at microwaves by a moving platform. Both analytical and numerical modeling will be proposed and validated, and an investigation on the spatial decorrelation of the electromagnetic field scattered by different soils, including different configurations and realistic topography, will be conducted.
The proposed research could represent an important step towards the assessment of the capabilities offered by bistatic radar observations, to understand the signal generated by sources of opportunity, and to gather valuable information for the understanding of data produced by current and future spatial missions. This project completes and corroborates the 1-year research assignment of the principal investigator, whose renewal has been approved by the Department Council on June 12th, 2019.
As mentioned, understanding the scattering generated by ocean and land is nowadays considered fundamental for a wide class of interdisciplinary sciences. Bistatic satellite systems (e.g., Tandem-X for extraction of Digital Elevation Models) are presently employed for interferometric applications based on small baselines (i.e., the distance between platforms), whereas applications with large baselines, which entail large observation windows, are presently scarcely investigated. The understating of temporal coherence of data gathered with innovative bistatic radar configurations and through the exploitation of sources of opportunity, is an original distinctive feature of the research topic here under investigation.
The possibility of validating the proposed approach directly in real conditions, as detailed next, by processing raw data provided by past and current missions designed for the exploitation of the GNSS reflections, should be considered a significant added value to assess the accuracy and the effectiveness of the analytical and numerical modeling, which are optimized to characterize the temporal fluctuations of the field collected at different heights, including important non-ideal elements.
It should be stress that one of the innovative goals of the proposed research would consist in the design of a simple but effective analytical formula providing the temporal decorrelation (i.e., the loss of coherence during the acquisition time) of the field collected at the receiving antenna as a function of the roughness profile (described by the surface rms heights and the correlation length) as well as of the platform height an speed. This formula will extend the simple equation that is usually taken as reference, which only rigorously valid for a simple set of uncorrelated point-like scatterers.
The use of full-wave efficient numerical tools will be anyway required to validate the modeling and to understand the temporal coherence in the presence of topography (see Figs. 2 and 3) and inhomogeneities. Therefore, a further innovation delivered by the research project is connected to the understanding of the role of the surface profile (enabled by the introduction of a digital elevation model) for the bistatic scattering generated by bare soils. The full-wave solution of the problem would also allow for analyzing the coherence of the scattered field in the presence of realistic, non-planar, surface profiles, thus enabling a significant advancement in the field of remote sensing based on signals of opportunity.
As a matter of fact, it is still not clear how and when the nature of the scattering field can be considered coherent and if the fluctuations observed analyzing the first experimental results from spaceborne platforms can be effectively mitigated and filtered out. Also, their connection with the topography and the electric features of the illuminated region is not fully understood, and it is not clear which is the impact of the temporal decorrelation on the quality of the final product potentially delivered through the observation of bistatic reflections.
More applications could emerge in the course of the research activity; the proposed modeling can potentially provide new useful tools to further investigate and understand newer application domains in the future.