Satellite Imagery Field Measurements and Data Extraction

2.2 Satellite Imagery

The RapidEye satellite system is a constellation of five identical earth observation satellites with the capability to provide large area, multi-spectral images with frequent revisits in high resolution 6.5 m at nadir. In addition to the Blue B 440- 510 nm, Green G 520-590 nm, Red R 630-685 nm and near-infrared NIR 760-850 nm bands, the sensor has a Red- edge RE 690-730 nm band, especially suitable for vegetation analysis s. Eitel et al. 2007. The RapidEye level 3A standard product covers an area of 25x25 km, is radiometrically calibrated to radiance values Anderson et al. 2013, and is ortho-rectified and resampled to 5 m spatial resolution. All the images used in this study Tab. 1 were calibrated to top of atmosphere reflectance. Due to atmospheric influences the Blue band of the scene corresponding to site-ID G1.1 was noisy and therefore treated with a noise removal based on the Maximum Noise Fraction approach.

2.3 Field Measurements and Data Extraction

In Germany, field data collection included leaf area index Licor LAI-2200 © , Delta-T Sunscan © and leaf chlorophyll meter readings at the upper canopy Minolta SPAD-502 © . SPAD measurements represent unit-less relative values, which have been proven to be positively correlated to chlorophyll content of wheat Reeves et al. 1993 and other crops Zhu et al. 2012. Ground reference data were gathered at winter wheat stem elongation 8th May and early ripening stages 22nd June. Sample locations were defined aiming at covering the entire crop variability within the field as described in Siegmann et al. 2013 Fig. 1. Due to extreme dry weather conditions in early summer 2011, some areas of the field were affected by premature ripening Fig. 1. Figure 1. Test site with the sampling locations measured on 8th of May dark points and 27th of June 2011 light points. The background image from the 27th of June shows the early ripening areas in brownish tones. In the USA, LAI and SPAD measurements were collected similarly and sample dates nearest to image acquisition were chosen. The average of twelve SPAD values from fully-expanded upper leaves and four LAI measurements within a 1 m 2 location at each ground point were used in analysis. Leaf nitrogen concentration N was also averaged from twelve top of canopy leaf samples processed by dry combustion. Due to the highly heterogeneous nature of the hilly Palouse, wide differences in SPAD, N and LAI were observed during these time periods Tab. 1. All sampling points were buffered with a 10-meter-radius circle, and the image reflectance values extracted from these areas were used to calculate mean GRE, YEL, PSRI, and NDVI values for each point.

2.4 Spectral Indices