Forward and Inverse Modeling for Synthetic Aperture Radar Observables in Bistatic Configuration. Applications in Forest Remote Sensing

Kurzfassung

Forest observation worldwide is paramount in many fields of study, not only regarding environment applications - whether at a global or local scale (carbon cycle and climate stakes, ecosystems health) - but also towards anthropogenic activities (forestry, detection related to military or rescue purposes). Though vast, with about one third of earth’s land surface, its importance lies rather in its highly sensitive nature versus climate or human pressure, which thereby strengthen the need of observation and monitoring. Within this scope, remote sensing technique can’t be overlooked and takes truly part in a better understanding of Earth System Science. Prominent among these, radar is favoured by supplying its own illumination (as a active system) and by the all-whether as well as medium penetration capabilities of microwaves. Radar remote sensing is thereby particularly relevant to probe forest structure, all the more with the Pol-InSAR acquisition which enables to localize by means of the interferometric height retrieval, the polarimetric information about the medium scattering nature, and this on top of the imaging product. As testified by the fertile number of campaigns as well as studies to cope with its current limitations, one can truly assert the golden age of SAR use, among which the bistatic configuration comes within this scope of growing improvements. Although revisited several times, its potential towards forest remote sensing and by means of biPol-InSAR acquisition haven’t been assessed so far. Besides, the current resurgence for bistatic, favored by noticeable technical advances and well illustrated by several successful airbone and hybrid spaceborne campaigns makes the well known geometrical, opportunistic and discretion advantages of a receive only sensor more and more attractive. For all that, such acquisition haven’t been achieved yet in the framework of forest remote sensing (especially at suitable frequencies such as P or L band), our investigation will be thus based forward electromagnetic simulation. Indeed, the latter finds a great importance to understand and foresee the scattering behaviour, all the more that bistatic as well as forest diversity widen considerably the possible set of configurations. For that purpose, MIPERS -Multistatic Interferometric and Polarimetric model for Remote Sensing- based on a coherent and discrete formulation, has been developed to cope not only with the multistatic geometry -and the subsequent imaging constrains- but also with possible sparse forests including man-made structures by means of an original multi-zone approach. Theoretical validations but also confrontations with experimental monostatic data with temperate forests have been performed and emphasize - jointly to a sensitivity analysis - the importance of specific ground truth elements. Moreover, simulations have been achieved in order to set forth specific scattering behaviour intrinsic to a given bistatic geometry. On top of symmetry properties and polarization effects which turn out to be relevant to discriminate coherent and distributed targets (coming thereby with interesting detection applications), the sensitivity of the scattering mechanisms towards the bistatic configuration has been used with the aim of improving the main retrieval methods currently used in monostatic, that is the P-HV approach or the Pol-InSAR one. For the latter, an ad-hoc inversion scheme has been developed to cope not only with the extension to bistatic but also with an improved description of the forest model, particularly for what concerns the structured medium and the coupling terms between volume and ground scatterers. Whether for this quantitative inversion approach involving descriptive parameters or for biomass retrieval directly based on polarimetric radiometry, optimal bistatic configurations have been set forth in the light of simulations analysis, coming thereby with the planning of experimental campaigns as further prospects.