## 2 Materials and Methods
### Data Preparation
#### 2.2.2 Remote Sensing Data  
The optical remotely-sensed data in this study included the images of Chinese Gaofen-1 high-resolution satellite Wide Field Camera (GF-1 WFV), the China environment and disaster monitoring and forecasting small satellite constellation 1A/B satellites charge coupled device (HJ-1A/B CCD) and the Landsat 8 Operational Land Imager (Landsat-8 OLI). The primary sensor characteristics are shown in the Table 1. Compared with the commonly-used optical remote sensing data, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), these images have good spatial resolutions of less than 30 m, which makes it more suitable to map winter wheat LAI with relatively fragmented areas in Hengshui city. The combination of these multiple sensor images makes it possible to obtain more optical images to cover the study area during the growth periods of winter wheat. In this study, 16 images covering the study area with a high overall quality between March and June in 2014 were acquired, which are listed in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sensor**} & \multirow{2}{*}{**Band**} & **Wavelength Range (\(\mu\)m)** & **Radiometric Resolution (bi)** & **Spatial Resolution (m)** & **Swath (km)** & **Revisit Period (days)** \\ \hline \multirow{4}{*}{GF-1 WFV} & Blue (1) & 0.450–0.520 & & & & \\  & Green (2) & 0.520–0.590 & & & & \\  & Red (3) & 0.630–0.690 & & & & \\  & Nir (4) & 0.770–0.890 & & & & \\ \hline \multirow{4}{*}{HJ-1 CCD} & Blue (1) & 0.430–0.520 & & & & \\  & Green (2) & 0.520–0.600 & & & & \\  & Red (3) & 0.630–0.690 & & & & \\  & Nir (4) & 0.760–0.900 & & & & \\ \hline \multirow{4}{*}{Landsat-8 OLI} & Blue (2) & 0.450–0.515 & & & & \\  & Green (3) & 0.525–0.600 & & & & \\  & Red (4) & 0.630–0.680 & & & & \\  & Nir (5) & 0.845–0.885 & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: The GF-1 WFV (Wide Field Camera), HJ-1 CCD (Charge Coupling Device)and Landsat-8 OLI (Operational Land Imager) specifications.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Data** & **Sensor** & **Time UTC** & \(\mathbf{\theta_{sum}}\) & \(\mathbf{\varphi_{sum}}\) & \(\mathbf{\theta_{Sensor}}\) & \(\mathbf{\varphi_{Sensor}}\) \\ \hline
1 & 4 March 2014 & HJ-1B CCD2 & 01 h 58 min & 55.97 & 315.70 & 12.75 & 97.00 \\
2 & 23 March 2014 & HJ-1B CCD2 & 01 h 45 min & 49.80 & 310.44 & 32.00 & 97.00 \\
3 & 3 April 2014 & GF-1 WFV4 & 03 h 42 min & 55.67 & 162.79 & 63.44 & 285.08 \\
4 & 4 April 2014 & HJ-1B CCD2 & 01 h 55 min & 45.97 & 306.82 & 15.00 & 98.00 \\
5 & 8 April 2014 & HJ-1B CCD2 & 01 h 58 min & 44.76 & 305.55 & 12.70 & 98.00 \\
6 & 13 April 2014 & Landsat-8 OLI & 02 h 54 min & 34.85 & 141.18 & 0 & 90 \\
7 & 14 April 2014 & GF-1 WFV1 & 03 h 12 min & 58.12 & 149.88 & 63.41 & 101.48 \\
8 & 29 April 2014 & Landsat-8 OLI & 02 h 54 min & 38.09 & 136.81 & 0 & 90 \\
9 & 5 May 2014 & HJ-1B CCD2 & 01 h 55 min & 37.93 & 296.64 & 22.0 & 98.00 \\
10 & 7 May 2014 & HJ-1A CCD2 & 01 h 55 min & 37.05 & 296.71 & 24.00 & 97.00 \\
11 & 12 May 2014 & HJ-1A CCD1 & 02 h 23 min & 32.81 & 298.83 & 13.00 & 283.00 \\
12 & 15 May 2014 & Landsat-8 OLI & 02 h 54 min & 25.33 & 131.37 & 0 & 90 \\
13 & 17 May 2014 & GF-1 WFV2 & 03 h 18 min & 68.51 & 144.80 & 74.43 & 101.31 \\
14 & 21 May 2014 & GF-1 WFV1 & 03 h 16 min & 68.27 & 141.16 & 63.39 & 101.47 \\
15 & 29 May 2014 & HJ-1B CCD1 & 02 h10 min & 31.14 & 293.58 & 12.40 & 282.00 \\
16 & 5 June 2014 & HJ-1B CCD2 & 01 h 50 min & 33.43 & 286.39 & 13.30 & 98.00 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Image acquisition information (\(\theta\): zenith angle; \(\varphi\): azimuthal angle).  
A series of pre-processing steps was performed using ENVI 5.3 software (Exelis Visual Information Solutions Inc., 4990 Pearl East Cir, Boulder, CO, USA), including radiometric calibration, atmospheric correction and geometric correction, to convert the radiance to reflectance with the correct geographic information.