# _remote sensing_
## 3. HORUS Multiangle and Multispectral Observation Module  
To cover a sufficiently large off-nadir angle with appropriate optical characteristics, multiple cameras can be used, with appropriate boresight angle (off-nadir-tilt in the along-track direction). In the implementation selected for HORUS, the complete angle coverage is obtained with four sets of four identical cameras, or camera quadruplet, as shown in Figure 3. The Type (a, b, c, d) within the camera quadruplet respectively indicates the position in the sequence in the off-nadir order, starting from forward off-nadir and ending with afterward off-nadir camera pointing. Each camera HFOV is aligned in the along-track direction, covering a FOV of 33.4 degrees. All the cameras are mounted on the spacecraft with boresight angles of \(\pm 16.7^{\circ}\) and \(\pm 50.1^{\circ}\). This arrangement allows continuous coverage of a full range of nadir angles from 66.8 degrees forward to 66.8 degrees afterward. The relation between the on-board off-nadir boresight angle and the view angle at the observation point depends on the satellite orbital height, by straightforward geometrical considerations, indicated in Figure 4:  
\[\sin\eta=R_{e}/\left(R_{e}+h\right)\ \sin\sigma\]  
Figure 3. The HORUS camera quadruplet (Types a-b-c-d) FOVs and off-nadir boresight angles (not to scale). Figure 2. 2D representation of HORUS iFOV. The camera arrangement in Figure 3 guarantees that all the NASA-JPL MISR view angles fall within the view angles reached by HORUS, as also schematically shown in Figure 5 (not to scale). Table 2 indicates the off-nadir boresight angles obtained at various orbital heights, corresponding to \(\sigma=70.5^{\circ}\). All of them are within the HORUS camera system total off-nadir field of view of \(\pm 66.8\) degrees. To obtain multispectral acquisitions, four camera quadruplets are used, each one with a suitable optical filter in the required optical band. In the HORUS implementation, we have four spectral bands, namely Red, Green, Blue (RGB) and NIR, therefore we have four camera quadruplets, summing to a total of 16 cameras. To simplify the system implementation, the same sensor type and optical system is used for all of the 16 cameras, except for individual optical filters mounted on each camera. The selected CMOS-based sensor is provided with Global Shutter to expose all the images at the same time, 'freezing' the moving object in place. Different types of windowing are allowed, in case only portions of the scene shall be acquired [48]. The features of a typical commercial CMOS sensor, selected as a reference for the present analysis ([49]), are listed in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Orbital Height** & **300 km** & **400 km** & **500 km** & **600 km** & **700 km** \\ \hline Boresight angle for \(\sigma=70.5^{\circ}\) & 64.2\({}^{\circ}\) & 62.5 & 60.9 & 59.5 & 58.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Off-nadir angles corresponding to the view angle of 70.5\({}^{\circ}\) at various orbital heights. Figure 4: Geometrical relation between view angle and on-board off-nadir boresight angle. Figure 5: Schematic representation of NASA-JPL MISR view angles, all within the view-angle range reached by HORUS (not to scale). The selected effective focal length (EFL) of the lens and the effective aperture diameter ensure the best compromise to minimize the instrument volume, while keeping the system's performances within the required spatial resolution. Table 4 illustrates the main features of the selected optics. Table 5 illustrates the main features of the selected COTS filters. The acquired data will be preprocessed on-board the spacecraft through a dedicated processing unit, mainly to obtain a set of lossless compressed data, stored on a high-capacity solid state data recording (SSDR) unit, sized to increase the operations' flexibility and avoid the constrain given by the total amount of data that can be downloaded every day. A summary of the main features of the HORUS instrument is provided in Table 6.