## 2 Study Area and Data
### Data Sources
#### 2.2.2 Satellite Data  
The PROBA-V satellite has three co-aligned cameras. It can provide data at 300- and 100-m spatial resolution. The 300-m products combine the image stripes of the three cameras into a swath of 2295 km and a near-daily global coverage (approximately 90%), whereas the 100-m products were obtained solely from the central camera, which covers a much smaller swath (approximately 517 km) and has full global coverage every five days. Specific and detailed information of the PROBA-V mission was previously described [25; 26; 27]. The PROBA-V 100- and 300-m S1 (daily synthesis product) top of canopy (TOC) reflectance products were downloaded from VITO's product distribution portal [28] in the HDF5 file format and then converted to the ENVI file format using SPIRITS (Software for the Processing and Interpretation of Remotely sensed Image Time Series [29; 30; 31].  
Because of the lower latitude of the study areas, PROBA-V 100-m S1 products are available for approximately every 4-5 days. The quality of the PROBA-V data is largely influenced by severe clouds and haze in the NCP. The corresponding quality status map dataset (a quality state indicator for each pixel) indicates that only approximately 42% of the 300-m images during the whole growing season of winter wheat (October to June in the following year) had more than 90% good pixels, whereas the percentage was much lower for the 100-m data. Therefore, there are a lot of gaps (spatial) over the study areas (same for most of high latitude areas) if only the PROBA-V 100-m imageries are used for the classification and yield estimation. All the 100-m images of the 2014-2015 agricultural season covering the two study sites with good pixels exceeded 96% were selected and processed (Table 2), which mainly included trimming the study area borders and projecting to the Universal Transverse Mercator (UTM) Zone 50N projection.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Study Sites** & **Crop Type** & **Sample Date** & **Growing Stage** & **Number (N)** \\ \hline \multirow{3}{*}{Site 1} & \multirow{3}{*}{Winter WHEAT} & 16 April 2015 & Booting & 25 \\  & & 17 May 2015 & Flowering & 25 \\  & & 4 June 2015 & Harvest & 25 \\ \hline \multirow{3}{*}{Site 2} & \multirow{3}{*}{Winter Wheat} & 15 April 2015 & Booting & 15 \\  & & 17 May 2015 & Flowering & 15 \\ \cline{1-1}  & & 6 June 2015 & Harvest & 15 \\ \hline \hline \end{tabular} Note: the images marked in bold were employed as references to evaluate the predicted images.  
\end{table}
Table 1: The observations of winter wheat in the 2014-2015 agricultural year.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Crop Types**} & \multicolumn{2}{c}{**Site 1**} & \multicolumn{2}{c}{**Site 2**} \\ \cline{2-5}  & **Date** & **Good Pixels Ratio** & **Date** & **Good Pixels Ratio** \\ \hline \multirow{8}{*}{Winter wheat} & 2 November 2014 & 100\% & 2 November 2014 & 100\% \\  & 13 December 2014 & 100\% & 13 December 2014 & 100\% \\  & **18 December 2014** & **100\%** & **31 December 2014** & **100\%** \\  & **22 December 2014** & **100\%** & 9 January 2015 & 100\% \\  & 31 December 2014 & 100\% & 10 February 2015 & 100\% \\  & 19 January 2015 & 99\% & 27 March 2015 & 100\% \\ \cline{1-1}  & 10 February 2015 & 100\% & 10 April 2015 & 100\% \\  & 28 March 2015 & 98\% & **15 April 2015** & **100\%** \\  & 10 April 2015 & 99\% & 12 May 2015 & 97\% \\  & **15 April 2015** & **98\%** & **25 May 2015** & **99\%** \\  & **7 May 2015** & **98\%** & 30 May 2015 & 100\% \\  & 16 May 2015 & 100\% & - & - \\  & 30 May 2015 & 98\% & - & - \\ \hline \hline \end{tabular} Note: the images marked in bold were employed as references to evaluate the predicted images.  
\end{table}
Table 2: The Project for On-Board Autonomy-Vegetation (PROBA-V) 100-m S1 product used in the study.