J.D. Ballester-Berman

Rice Phenology Monitoring by Means of SAR Polarimetry at X-Band

The feasibility of retrieving the phenological stage of rice fields at a particular date by employing coherent copolar dual-pol X-band radar images acquired by the TerraSAR-X sensor has been investigated in this paper. A set of polarimetric observables that can be derived from this data type has been studied by using a time series of images gathered during the whole cultivation period of rice. Among the analyzed parameters, besides backscattering coefficients and ratios, we have observed clear signatures in the correlation (in magnitude and phase) between channels in both the linear and Pauli bases, as well as in parameters provided by target decomposition techniques, like entropy and alpha from the eigenvector decomposition. A new model-based decomposition providing estimates of a random volume component plus a polarized contribution has been proposed and employed in interpreting the radar response of rice. By exploiting the signatures of these observables in terms of the phenology of rice, a simple approach to estimate the phenological stage from a single pass has been devised. This approach has been tested with the available data acquired over a site in Spain, where rice is cultivated, ensuring ground is flooded for the whole cultivation cycle, and sowing is carried out by randomly spreading the seeds on the flooded ground. Results are in good agreement with the available ground measurements despite some limitations that exist due to the reduced swath coverage of the dual-pol HHVV mode and the high noise floor of the TerraSAR-X system.

First Results of Rice Monitoring Practices in Spain by Means of Time Series of TerraSAR-X Dual-Pol Images

Time series of dual-pol TerraSAR-X images have been acquired during the whole cultivation period over a rice site in Spain. The objective of this paper is to investigate the coherent co-polarized behavior of rice plants during the growing stages and to explore their information content for rice monitoring at high frequencies recently available through new SAR satellite missions. Among different observations, the backscattering coefficients at HH and VV channels and the HH/VV ratio have confirmed to show a temporal variation that has a significant correlation with the development of the plants during the vegetative and reproductive phenological phases. A physical interpretation in terms of the scattering mechanisms and extinction has been provided for this response. In addition, the information content of the HHVV complex coherence and a dual polarimetric target decomposition is investigated and discussed. All the information layers investigated are contributing to the discrimination of rice fields from other crop types. Apart of polarization, also the effect of high spatial resolution imaging for rice monitoring is of high interest for any kind of growth disturbances that may occur within one field for yield production.

Retrieval of biophysical parameters of agricultural crops using polarimetric SAR interferometry

An existing two-layer model for forest height estimation is adapted for agricultural crops in order to develop a retrieval algorithm based on polarimetric synthetic aperture radar interferometry. This new inversion scheme is specifically tailored for vertically oriented agricultural crops, with extinction coefficients dependent on the wave polarization. Physical parameters of the vegetation scene are estimated from the location of the measured coherences in the complex plane. The proposed inversion scheme is validated experimentally with indoor wide-band polarimetric measurements on samples of corn and rice fields. Results show that the estimates of the thickness of the vegetation layer and the ground topography are reasonably accurate for a wide range of frequencies and baselines. Moreover, some interesting results are also obtained when using only dual-polarized data, which brings up new applications for present and future spaceborne missions.

Applying the Freeman–Durden Decomposition Concept to Polarimetric SAR Interferometry

In this paper, the Freeman-Durden polarimetric decomposition concept is adapted to polarimetric SAR interferometry (PolInSAR) data. The covariance matrix obtained from PolInSAR observations is decomposed into the three scattering mechanisms matrices proposed by Freeman and Durden for polarimetric SAR (PolSAR) data. The objective is to describe each interferometric cross correlation as the sum of the contributions corresponding to direct, double-bounce, and random volume scattering processes. This procedure enables the retrieval not only of the magnitude associated with each mechanism but also of their location along the vertical dimension of the scene. One of the most important features of this algorithm is the potential to isolate more accurately the direct and volume contributions which usually cannot be correctly separated by means of PolSAR measurements. In addition, it is also possible to distinguish between direct scattering responses originated either at ground or produced by upper layers of vegetation. The proposed algorithm has been tested with simulated data from PolSARProSim software, laboratory data from maize and rice samples, and airborne data from a test site with different scenarios.

First Demonstration of Agriculture Height Retrieval With PolInSAR Airborne Data

A set of three quad-pol images acquired at the L-band in interferometric repeat-pass mode by the German Aerospace Center (DLR) with the Experimental SAR (E-SAR) system, in parallel with the AgriSAR2006 campaign, has been used to provide, for the first time with airborne data, a demonstration of the retrieval of vegetation height from agricultural crops by means of polarimetric SAR interferometry (PolInSAR)-based techniques. Despite the low frequency of the data, hence providing a weak response from the vegetation volume in contrast to the ground, accurate estimates of vegetation height at field level have been obtained over winter rape and maize fields. The same procedure does not yield valid estimates for wheat, barley, and sugar beet fields due to a mismatch with the physical model employed in the inversion and to the specific crop condition at the date of acquisition. These results show the value of the information provided by both interferometry and polarimetry for some agriculture monitoring practices.

Model Limitations and Parameter-Estimation Methods for Agricultural Applications of Polarimetric SAR Interferometry

Application of polarimetric synthetic aperture radar interferometry to the retrieval of geophysical parameters from vegetated scenes is based on simple direct models of such scenes. The first part of this paper presents an analysis of the correspondence between these simple models, namely, the random volume over ground and the oriented volume over ground (OVoG), and experimental data from samples of two agricultural crops (maize and rice) acquired in controlled conditions. Although an overall agreement between model and data is clear, some discrepancies have been found as a consequence of two assumptions in the model formulation: vertical homogeneity of the vegetation volume and absence of multiple scattering effects inside the volume. This paper presents the shape and location of the visible region of the experimental coherences on the complex plane and compares it with the feasible region predicted by the model. This comparison has also pointed out the low sensitivity of the direct model to extinction coefficients. In the second part, two different strategies for a complete inversion (i.e., estimation of all model parameters) of the OVoG model are proposed and compared, using the same data set. The first one is based on a combination of geometrical and numerical approaches (genetic algorithms) and the second one on a dual-baseline configuration. In all cases, ground topography is accurately estimated, with a maximum error of 10 cm. Vegetation height estimates are accurate up to 30 cm, with some bands and baseline configurations providing errors below 15 cm. However, results obtained for the extinction coefficients are not stable with frequency and exhibit high variability.

Coherence Loci for a Homogeneous Volume Over a Double-Bounce Ground Return

The formulation of the polarimetric interferometric coherences derived by simple homogeneous-volume-over-ground models is revisited for the case of ground returns dominated by the double-bounce terms. The differences between single-transmit (single-tx) and alternate-transmit modes of the interferometer are analyzed by inspecting the positions of the coherences on the complex plane. An additional volume-decorrelation term appears when the interferometer is operated in a single-tx mode, which adds complexity in the potential inversion of the model parameters. This effect is even more noticeable for the cross-polar channels in the oriented case, since the phase of the coherence with an infinite ground-to-volume ratio does not correspond to the topographic phase, as observed for the copolar channels

Indoor wide-band polarimetric measurements on maize plants: a study of the differential extinction coefficient

A study of the wide-band polarimetric backscatter of maize plants, measured in laboratory conditions, is presented. The backscatter slant-range profiles in both linear (H-V) and Pauli basis manifest a higher extinction coefficient in the vertical channel due to the dominant vertical orientation of the structure of corn plants. The difference between the horizontal and vertical range profiles as the wave penetrates into the volume is employed to retrieve the differential extinction coefficient. In addition, the polarimetric target decomposition, as proposed by Cloude and Pottier, is used to obtain range profiles of alpha, entropy, and anisotropy. All these results reveal important features to be accounted for in the retrieval of the biophysical parameters of agricultural crops by means of polarimetric synthetic aperture radar interferometry.

Combination of Direct and Double-Bounce Ground Responses in the Homogeneous Oriented Volume Over Ground Model

Complex interferometric coherence expressions for the homogeneous (random or oriented) volume-over-ground model are originally derived in a simplified way by neglecting one of the two possible contributions of the ground response: direct return from the ground surface or double-bounce interaction with stems or trunks. The influence of these depends on both system and scene parameters, and in many cases, none of them should be ignored a priori. Therefore, a more general formulation accounting for the simultaneous presence of both ground contributions should be developed. This letter derives this formulation and analyzes the combined effect of both ground returns on the complex coherences. In addition, the implications for a subsequent inversion procedure based on this model are also discussed.

Time Series of Hybrid-Polarity Parameters Over Agricultural Crops

An example of the potential of hybrid-polarity architecture parameters for agricultural monitoring is presented in this letter. Time series of such descriptors have been generated from quad-polarization radar airborne measurements at the L-band acquired during the AgriSAR 2006 campaign. An assessment on the sensitivity of these observables for monitoring the cultivation of winter wheat, maize, and winter rape crops has been addressed. All possible correlations among these polarimetric descriptors and crop coverage, phenological stage, leaf area index, wet biomass, and crop height have been analyzed for the whole growth season. The results of this study reveal that hybrid-polarity systems provide sensitivity in many cases to retrieve both quantitative information and qualitative information from agricultural crops.