Estimating Phenology of Agricultural Crops fron Space
Lopez-Sanchez, Juan M.1; Vicente-Guijalba, Fernando1; Ballester-Berman, J. David1; Cloude, Shane R.2
1University of Alicante, SPAIN; 2AEL Consultants, UNITED KINGDOM
The use of SAR polarimetry (PolSAR) in agriculture applications is based on the known sensitivity of microwaves to crop structure (size, shape, and orientation of leaves, stalks, and fruits), dielectric properties of the canopy (related to the water content), and the physical properties of the underlying soil (roughness and moisture). Crop structure and plant water content vary as a function of crop type, growth stage and crop condition. Consequently, different crop types, or the same type at different growth stages, produce different polarimetric signatures which can be identified in the acquired images and used to establish classification algorithms. Moreover, the dependence of PolSAR observables on scene characteristics can be exploited to estimate physical parameters of interest, such as soil moisture, plant water content, biomass, etc. [McNairn04, Lopez-Sanchez09].
Most of these applications have made use of quad-pol data but recently several studies have been carried out using compact polarimetry, where a single transmit (usually circular) and dual linear coherent receive is used. These have shown some promising advances in agriculture applications [Charbonneau10, Ballester-Berman12], for instance combining high classification accuracy from the polarimetric information with wide swath coverage.
Among all these applications, the estimation of the phenological stage of crops has not been widely approached in the past due to the lack of time series of satellite images with short revisit time (a clear requirement from the application point of view) and due to the necessity of carrying out intensive ground campaigns along the whole cultivation cycles (3-4 months) for validation purposes, which are very expensive in terms of human resources.
In the framework of the ESA funded POLSAR-Ap project, we have studied the potential of PolSAR for this Earth observation product by exploiting the data gathered during the AgriSAR2009 campaign in a test site located in Indian Head, Canada, where 5 crop types (wheat, barley, oat, canola and field pea) were surveyed. A set of 24 quad-pol Radarsat-2 images, acquired at different incidence angles from 22 to 39 degrees, has been analysed for this purpose.
The polarimetric response of these crop types, in terms of a wide set of observables, has been represented as a function of phenology, providing a direct way to identify the main features of each phenological stage and the most sensitive radar observables for each of them. Then, simple retrieval algorithms in form of hierarchical trees have been proposed and tested for all crop types. The input of these algorithms consists in a single acquisition without any other auxiliary information, whereas the output is the current phenological stage at every multi-looked pixel from a set of 3-5 possible intervals. A validation at field level (according to the available reference data) has been carried out, showing very good results in most cases.
This study has obtained different degrees of sensitivity for different crop types, as well as different polarimetric requirements (i.e. necessity of dual or quad-pol data) for each of them. The influence of incidence angle has been also assessed, since it affects differently some of the observables and also depends on the crop type and phenological stage.
[Ballester-Berman12] J. D. Ballester-Berman and J. M. Lopez-Sanchez, "Time series of hybrid-polarity parameters over agricultural crops", IEEE Geosci. Remote Sensing Letters, vol. 9, no. 1, pp. 139-143, Jan. 2012.
[Charbonneau10] F. J. Charbonneau, et al. "Compact Polarimetry Overview and Applications Assessment", Can. J. Remote Sensing, Vol. 36, Suppl. 2, 2010.
[Lopez-Sanchez09] J. M. Lopez-Sanchez and J. D. Ballester, "Potentials of polarimetric SAR interferometry for agriculture monitoring", Radio Science, Vol. 44, RS2010, March 2009. DOI: 10.1029/2008RS004078
[McNairn04] H. McNairn and B. Brisco, "The application of C-band polarimetric SAR for agriculture: a review", Can. J. Remote Sensing, Vol. 30, No. 3, pp. 525-542, 2004.