Speaker
Description
The number of applications using SAR backscatter time series for crop production and monitoring has increased in recent years, and understanding of the radar signal’s interaction with different crop types in experimental and theoretical studies has improved. However, there is still a lack of information concerning different parameters, such as row orientation, especially for Sentinel-1. The early studies from ERS-1 [1, 2], Radarsat-2 [3], and portable scatterometers [4, 5] have proven the impact of row orientation in backscatter time-series analysis. It has been reported that the backscatter is higher when the SAR signal targets row orientation perpendicular and lower when targets parallel (σ0 of θ90° > σ0 of θ0°). Moreover, according to the experiment of Ulaby et al. in 1976 [5] with multifrequency scatterometers, the look direction dependence was observed in fields of wheat, soybeans, and corn for frequencies below 4.25 GHz for co-polarized (hh, vv, up to ∆=6dB) and not present for cross-polarized at all frequencies even as low as 1 GHz [5]. On the other hand, Auquière et al. [2] reported backscatter (σ0) change for ERS-1 (5.3 GHz, ∆=3dB) due to row orientation (on 33 corn fields). In our study, we also observed the trend for backscatter (γ0) retrieved from Sentinel-1 (5.405 GHz) in three study areas (39 fields) and four crop types (rapeseed, winter wheat, spring barley, and corn), which we attribute to the impact due to row orientation. However, the results in our work are slightly divergent from other works listed above: apart from that the trend being present for co-polarized (vv) backscatter, the trend was also observed for rapeseed in cross-polarized (vh) backscatter. The results suggest that the attenuation on crop stems occurs at θ90°(/θ0°) and θ[30/60°±10]. look directions. We hypothesize that the maximum of attenuation occurs at 30°(/60°) of the look direction (when the maximum exposure of the number of stems is directed toward the sensor) and less attenuation at 0° of the look direction (parallel to row orientation) in compare to 90° (perpendicular to row orientation) of look direction (0° < 90° < [30/60°±10]). In other words, it is related to the different distances between plant rows in orthogonal and perpendicular directions and the width of the crop steam. According to Ulaby et al. in 1976 [5] experiment, the dependence on row orientation is coupled with incidence angle, the influence of which is being explored at the moment. The in-situ data was not available to verify the results; however, the trend was observed in three study areas and, therefore, considered valid.
In this talk, we will give a detailed description of the method used to generate results and discuss our interpretation and possible factors underlying this effect.
This study is funded by the German Federal Ministry for Economic Affairs and Energy (FKZ: 50EE1901) to develop an application for crop monitoring based on Sentinel-1 data and is carried out in collaboration with CLAAS E-Systems GmbH.
REFERENCES
[1] Wegmüller U; Santoro M; Mattia F; Balenzano A; Satalino G; Marzahn P; Fischer G; Ludwig R; Floury N. (2011): Progress in the understanding of narrow directional microwave scattering of agricultural fields. Remote Sensing of Environment. 115(10):2423–2433. doi:10.1016/j.rse.2011.04.026.
[2] Auquière E.; Defourny P.; Baltazart V.; Guissart A. (1997): ERS SAR time series analysis for maize monitoring using experimental and modeling approaches. In Proceedings of the ESA ERS Conference, Florence, Italy, 5 March 1997; pp. 147–152.
[3] Moran MS; Alonso L; Moreno JF; Cendrero Mateo MP; La Cruz DF de, Montoro A. (2012): A RADARSAT‐2 Quad‐Polarized Time Series for Monitoring Crop and Soil Conditions in Barrax, Spain. IEEE Trans.
Geosci. Remote Sensing. 50(4):1057–1070. doi:10.1109/TGRS.2011.2166080.
[4] McNairn H; Duguay C; Boisvert J; Huffman E; Brisco B. (2001): Defining the Sensitivity of Multi‐Frequency and Multi‐Polarized Radar Backscatter to Post‐Harvest Crop Residue. Canadian Journal of Remote Sensing. 27(3):247–263. doi:10.1080/07038992.2001.10854941.
[5] Ulaby, Fawwaz T.; Long, David G.; Blackwell, William J.; Elachi, Charles; Fung, Adrian K.; Ruf, Chris et al. (2014): Microwave radar and radiometric remote sensing. Ann Arbor: The University of Michigan Press.
