6th International Workshop on Retrieval of Bio- & Geo-physical Parameters from SAR Data for Land Applications

Agricultural Crop Structure by Means of Multi-Baseline Pol-InSAR and SAR Tomography

18 Nov 2015, 15:00

20m

Harwell, UK

Agriculture IV - Agriculture

Speaker

Ms Hannah Joerg (German Aerospace Center, ETH Zurich)

Description

Multi-parameter synthetic aperture radar (SAR) acquisitions can be employed for the quantitative estimation of biophysical parameters of agricultural vegetation (e.g. crop height and canopy structure). Polarimetric SAR Interferometry (Pol-InSAR) [1], by exploiting the variation of the interferometric coherence with polarisation, presents a valuable support to the extraction of crop parameters from SAR data. The estimation of these parameters is achieved via the inversion of electromagnetic scattering models relating the Pol-InSAR complex coherences to the physical properties of the agricultural scenario. The Random Volume over Ground (RVoG) scattering model, widely used for the inversion of forest height, assumes a 2-layer structure consisting of a cloud of particles with no preferred orientation (volume layer) on top of an impenetrable ground. Based on this simplified formulation, the scattering from the ground is polarisation-dependent, whereas the propagation through the volume is independent of polarisation [1]. However, orientation effects present inside the agricultural vegetation require to consider a polarisation dependency also for the volume layer. Hence, a generalisation of the RVoG model, the Oriented Volume over Ground (OVoG) model, has been developed. In this case, the volume particles have a non-uniform orientation distribution: as a result, the effective medium is not isotropic and the propagation constants vary with polarisation [2]. Due to the increased number of unknown parameters when compared to the RVoG case, the inversion of the OVoG model requires fully-polarimetric radar measurements from at least two spatial baselines [2, 3]. (Polarimetric) tomographic SAR techniques, as an extension of the Pol-InSAR principle, enable the analysis of the profile of the backscattered power along height and hence the 3-D characterization of the scattering mechanisms occurring in vegetated fields. SAR Tomography can help to interpret and separate ground and volume scattering signatures in an unambiguous and non-model based way yielding estimates of the scattering parameters, such as for instance, scattering phase centres, the ground-to-volume ratio as well as the multi-baseline volume coherences for every polarisation [4]. In this work, crop parameters such as vegetation height, extinction coefficients and ground-to-volume ratios are estimated by using both a Pol-InSAR OVoG inversion scheme [3] and a more generalised tomographic approach. The investigation is carried out on a data set of fully-polarimetric multi-baseline SAR acquisitions at X-, C- and L-band, acquired by DLR’s airborne sensor F-SAR in 2014. These measurements have been collected from May to August over an agricultural area in Germany, covering different stages of the plant growth cycle. A comparison of the results from Pol-InSAR and SAR Tomography is then performed to ascertain the potential and the limitations of these two techniques over different crop types and sensor frequencies. This comparison provides a big potential in fostering the understanding of the physical scattering behavior of agricultural crops and the implications for model based inversion, as for instance the benefit from particular a priori knowledge. [1] K.P. Papathanassiou, and S.R. Cloude, “Single-baseline polarimetric SAR interferometry”, IEEE Transactions on Geoscience and Remote Sensing, vol. 39, no. 11, pp. 2352-2363, Nov. 2001. [2] J.D. Ballester-Berman, J.M. Lopez-Sanchez, and J. Fortuny-Guasch, “Retrieval of biophysical parameters of agricultural crops using polarimetric SAR interferometry”, IEEE Transactions on Geoscience and Remote Sensing, vol. 43, no. 4, pp. 683-694, Apr. 2005. [3] M.Pichierri, I. Hajnsek, and K.P. Papathanassiou, "A multi-baseline Pol-InSAR inversion scheme for crop parameter estimation at different frequencies," IEEE Transactions on Geoscience and Remote Sensing (submitted). [4] H. Joerg, M. Pardini, I. Hajnsek, “Spatial and temporal characterization of agricultural crop volumes by means of polarimetric SAR Tomography”, in Geoscience and Remote Sensing Symposium (IGARSS), 2015 IEEE International, IEEE, 2015.