## 2 Sensors
### Camera  
Cameras are central in almost any MMS design. A camera senses a specific or multiple parts of the electromagnetic spectrum. Cameras could be classified based on different technical criteria, one of which is capturing configuration that makes two classes of global and roller shutter cameras [57]. The first category deals with simultaneously reading the pixel grid, while the second refers to sequential readings. Global shutters are more desirable for MMS applications since they are less sensitive to movement of a platform [58]. Most MMS cameras are equipped with an external trigger mechanism (wired or wireless) that enables time-accurate image capturing [59]. Accurate triggering is absolutely necessary for precise synchronization of MMS components. Cameras' optical system design deals with the placement and configuration of a set of lenses to adjust certain optical parameters such as field of view, etc. Usually, a fixed-lens camera is more suitable for an MMS since geometric properties of the perspective geometry are preserved in comparison to a variable focal-length lens system that is more complex and inaccurate [60].  
In camera systems used in MMSs, usually several electromagnetic bands are observed. Multi-band cameras are usually designed in three fundamental approaches. In the first approach, a multi-spectral filter array is used to take a series of exposures using a tunable filter [61]. The second approach is to use a camera array, where each camera is assigned to take records of a specific band in the electromagnetic spectrum [62]. In the third approach, multiple pixels are designed to capture different bands of an approximate pixel location [63].  
In addition to having multi-band settings, designing a suitable viewing angle is an important factor to be considered in MMSs with cameras. One way to achieve wide viewing angles is by employing multi-camera systems that are used when a single camera is not sufficient to cover a designed field of view or not suitable to cover the designed electromagnetic spectrum. Instead, a set of cameras is employed for applications such as wide imaging [64]. Former specifications enabled MMS designers to employ complex multi-band multi-directional cameras that brought unseen possibilities for tasks such as environmental monitoring and material classification.  
Geometric calibration of a general multi-camera is possible by employing the following collinearity equation  
\[\mathbf{F}\Big{(}(\mathbf{IOP})_{\mathrm{i=1:\mathbf{x}}},\mathbf{R}_{( \zeta,\mathbf{n},\Psi)_{\mathrm{i=2n}}},\mathbf{A}_{i=2:\mathbf{x}},\mathbf{R} _{(\omega,\Phi,\kappa)_{\mathrm{i=1:\mathbf{x}}_{\mathrm{i}}},\mathbf{X}_{( \mathrm{1:\mathbf{x}}\mathrm{O})},\mathbf{x}_{(\mathrm{1:\mathbf{x}}\mathrm{II })}\Big{)}=0, \tag{1}\]  
where \((\mathbf{IOP})_{\mathrm{i}}\) is the interior orientation parameters of \(i^{th}\) camera, \(\mathbf{R_{i}}\) is relative orientations of \(i^{th}\) camera in the camera frame, \(\mathbf{R_{i}}\) is relative orientation of the camera system at time (t), \((\mathbf{X}_{0})_{\mathrm{t}}\) is the relative position of center of the local coordinate system defined for the camera system at time (t), and finally, \(\mathbf{X}\) and \(\mathbf{x}\) are object coordinates and image coordinates, respectively.  
Using cameras in MMSs provides a type of location awareness, since images could be processed to estimate a local positioning. In this sense, cameras could be placed in the same category as IMUs, because image locations and orientations could be accurately estimated with some degrees of uncertainty. Techniques such as SfM, aerial triangulation, or stitching could be employed to provide a trajectory of a sequence of images.