# _remote sensing_
## 2 Scientific Background
#### Acquisition Methodology  
Push broom scanners are commonly used in large satellite optical observation systems. Examples of EO satellites using push broom sensors are SPOT, IKONOS, QuickBird, and TERRA [43; 44; 45; 46]. This technology, also known as line imager, is based on a linear arrangement of detector elements covering the full swath width. The related read-out process delivers one line after another along the ground track, and each detector element corresponds to a pixel on-ground. In this case, the ground pixel size and the velocity of  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Application Type** & **View Angle/View Angles** & **Required Band** & \
\begin{tabular}{c} **Associated Spatial** \\ **Resolution** \\ \end{tabular} \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Ocean color \\ Surface classification \\ Land aerosols \\ \end{tabular} } & \multirow{4}{*}{Nadir (0 degrees)} & Blue (main) and green (secondary)/improvements by using NIR & \multirow{4}{*}{275 to 550 m} \\  & & red/NIR (main) and green/blue (secondary) & & \\  & & & Blue and red (main) and green/NIR (secondary) & \multirow{4}{*}{1.1 km} \\  & \(\pm\)45.6/26.1/0 degrees (secondary) & green/NIR (secondary) & & \\  & All & Green (main) and blue/red/NIR & & \\  & & (secondary) & & \\  & \(\pm\)60 degrees (main) and & & Red/NIR (main) & 1.1 km \\  & \(\pm\)45.6/26.1/0 degrees (secondary) & & & \\  & \(\pm\)60/70.5 degrees (main) and & & & \\  & \(\pm\)45.6/26.1/0 degrees (secondary) & & & \\  & All angles (especially stereo & & & \\  & images at \(\pm\)26.1) & & Red (secondary) & 1.1 km \\ \hline \hline \end{tabular}
\end{table}
Table 1: HORUS main applications and requirements in terms of angle, band, and spatial resolution.  
the sub-spacecraft point define the integration time [47]. The most recent technological developments in optical sensors, combined with power, mass and volume constraints typical of CubeSats, suggest using COTS imagers based on CMOS sensors, also known as step-and-stare scanners, based on a NxM (image rows x columns) arrangement of detector elements that can be used as a matrix imager to acquire pictures from orbiting spacecraft. Each detector element corresponds to a pixel on-ground. The ground pixel size and the sub-satellite-point velocity determine the integration time [47].  
HORUS optical payload is based on CMOS sensors and each line of pixels will be used as a separate push broom-like sensor, collecting the light backscattered from the ground from a specific area at the same time. CMOS sensors are typically arranged in a rectangular shaped detector matrix, originating a nonsymmetric field of view. The FOV along the larger side of the rectangle is referred to as horizontal field of view (HFOV), the FOV along the smaller side of the rectangle is referred to as vertical field of view (VFOV). In the HORUS configuration, the HFOV is aligned along track and the VFOV cross-track, as shown in Figure 1.  
The information collected within each row of the sensor will be characterized by a different instantaneous field of view (iFOV) per pixel, as shown schematically in Figure 2, in which the Earth is considered as a sphere, and no scale is properly kept. This configuration ensures the required coverage, depending on the camera HFOV and the integrated number of cameras. With this implementation, observations can be obtained at as many off-nadir angles as the number N of pixel (\"rows\") in the sensor array.  
Figure 1: Acquisition geometry of a matrix imager, with HFOV aligned along-track.