Influence of Incidence Angle in the Correlation of C-band Polarimetric Parameters with Biophysical Variables of Rain-fed Crops

Abstract

A multi-temporal field experiment was conducted within the Soil Measurement Stations Network of the University of Salamanca (REMEDHUS) in Spain in order to retrieve useful crop information. The objective of this research was to evaluate the potential of polarimetric observations for crop monitoring by exploiting a time series of 20 quad-pol RADARSAT-2 images at different incidence angles (i.e., 25º, 31º and 36º) during an entire growing season of rainfed crops, from February to July 2015. The time evolution of six crop biophysical variables was gathered from the field measurements, whereas ten polarimetric parameters were derived from the images. Thus, a subsequent correlation analysis between both datasets was performed. The study demonstrates that the backscattering ratios (HH/VV and HV/VV), the normalized correlation between HH and VV (γHHVV), and the dominant alpha angle (α1), showed significant and relevant correlations with several biophysical variables such as biomass, height, or leaf area index (LAI) at incidence angles of 31º or 36º. The joint use of data acquired with different beams could be exploited effectively to increase the refresh rate of information about crop condition with respect to a single incidence acquisition scheme. RÉSUMÉ: Sur le site du Réseau des Stations de Mesure des Sols de l'Université de Salamanca (REMEDHUS) en Espagne, un suivi multitemporelle de diverses cultures a été réalisée de radar à ouverture. L'objectif de cette recherche est d'analyser le potentiel des observations polarimétriques pour la surveillance de cultures exploitant une série temporelle d'images quad-pol RADARSAT-2 sous différents angles d'incidence (c'est-à-dire 25º, 31º et 36º) durant toute la période de croissance. Vingt images de RADARSAT-2 ont été obtenues dans la zone d'étude entre les mois de février et juillet 2015. Six variables biophysiques ont été mesurées sur le terrain et dix paramètres polarimétriques ont été dérivés des images. Une analyse de corrélation a été réalisée entre les ensembles de données. L'étude montre que les relations de rétrodispersion (HH/VV et HV/VV), la corrélation normalisée entre HH et VV (γHHVV) et l'angle alpha dominant (α1) présentent des corrélations significatives et importantes avec divers variables biophysiques comme la biomasse, la hauteur ou l'indice de surface foliaire (LAI) pour des angles d'incidence de 31º à 36º. On pourrait profiter de l'utilisation conjointe de ces deux angles pour augmenter le taux d'information sur l'état des cultures par rapport à un seul schéma d'acquisition d'incidence.

Multi-Temporal Dual- and Quad-Polarimetric Synthetic Aperture Radar Data for Crop-Type Mapping

“…The present research is aimed at extending the use of a time series of polarimetric observables for land-cover classification, mainly focussed over agricultural fields. This possibility was previously identified [42] when several crop parameters were tested against radar measurements. The difference found in the correlations for different crop types suggested that specific crops, mainly cereal types, could be classified using SAR observables.…”

“…Since Sentinel-1 is dual-pol, only one scenario was explored (G) for this satellite. Scenario E was created after previous research [42], where it was found that the differences in the correlation between SAR observables and biophysical parameters among crop types suggested the possibility of classifying crops with these SAR observables. Scenario F was generated to compare the dual-pol data from RADARSAT-2 against Sentinel-1.…”

Crop Monitoring and Classification Using Polarimetric RADARSAT-2 Time-Series Data Across Growing Season: A Case Study in Southwestern Ontario, Canada

“…The uniform image acquisition beam mode As shown in Table 1, seven fully polarimetric C-band RADARASAT-2 images in FQ10W (Fine-Quad Wide) covering the entire growing season from April to September in 2015 were employed. The uniform image acquisition beam mode offers the advantage of reduced complexity caused by varying SAR incidence angles for multitemporal stud-ies [35][36][37]. In particular, except for the date on 30 May, when the satellite did not overpass our study area to acquire imagery due to user conflicts, the time interval between data acquisitions was 24 days.…”

“…On the other hand, the data in some cases are acquired using different beam modes with different spatial resolutions and incidence angles (e.g., RADARSAT-2 launched by the Canadian Space Agency). In particular, incidence angle has shown to affect the polarimetric parameters and classification performance [35][36][37]. Moreover, the number and types of selected PolSAR parameters for investigating temporal variation are also limited, such as SAR backscattering and ratios, polarimetric decomposition parameters, polarimetric coherences and phase differences.…”

UAV Multispectral Imaging Potential to Monitor and Predict Agronomic Characteristics of Different Forage Associations

“…Other parameters were also estimated (e.g., plant height, chlorophyll) but were discarded here as they were collected at plant level. Table 1 listed the selected parameters, which aimed to describe both the state of the plant and their rate of activity [34], and have been largely used in remote sensing applications to agronomy [35][36][37]. Two replicates of plant samples were taken at association level over a circular fenced area of 0.125 m 2 , together with zenithal photographs taken at a same height of 1.5 m, following the protocol of Sánchez et al [38].…”