## 3 Design
### System Technical Requirements
#### 3.1.1. Mosaic Coating  
As for the acquisition of wavelength, the current spaceborne instruments mainly adopt the forms of filter beam splitting and prism beam splitting. As far as the splitting of the filter is concerned, there are many limitations in the commonly used realization method of the filter wheel. The filter wheel relies on mechanical rotation to switch different filters to filter light of a specific wavelength. This mechanical movement process is relatively slow, resulting in a serious restriction on the imaging speed. Especially in the face of rapidly changing observation scenes, due to the difficulty of improving the imaging frame rate, it is only suitable for shooting tasks for stationary targets. For example, in the observation scene of some surface fixed geomorphological features, the filter wheel may also meet the basic needs. However, once dynamic targets such as fast-moving clouds and dynamic ocean surface phenomena are involved, it is difficult to capture continuous and clear image sequences, resulting in missing key information and unable to meet the demands of high-precision real-time observation. Although the prism splitting technology can decompose the mixed light into beams in different directions according to the wavelength difference to achieve the splitting effect, it requires multiple corresponding detectors to receive these signals separately, which directly leads to the overall structure of the instrument becoming complicated and bloated. Multiple detectors not only occupy a lot of space, but also need to support complex signal transmission and processing lines, which greatly increases the volume of the instrument. This huge instrument architecture, whether in the satellite's limited carrying space layout, or in the subsequent equipment maintenance, update and upgrade process, is facing a lot of inconvenience and high cost. In view of the above drawbacks exposed by filter beam splitting and prism beam splitting, in order to effectively reduce the resource and cost costs of multi-spectral images in the acquisition and transmission stages, this paper introduces mosaic coating technology in the system, that is, a pixel-level coating process [18], as shown in Figure 4a. Specifically, we cover the image sensor with a specially designed multilayer dielectric film, which exhibits different spectral transmission characteristics at different locations, achieving selective transmission of light in specific spectral bands. The feasibility of the method has been proven by simulation analysis [19]. This technique allows each pixel of the image sensor to respond to only one specific spectral band, thereby obtaining multiple spectral images in the working band range of 400 nm to 900 nm at one time. The bandwidth of each band is precisely controlled at 20 nm (except for the 865 nm band where the bandwidth is 40 nm). Through this array detection technology, we can obtain high-resolution imaging data and realize the so-called mosaic pixel coated multi-spectral array detector measurement technology. Finally, this technique can produce high-resolution multi-spectral imaging data. In this way, it not only avoids the slow imaging of the filter wheel and the redundancy of the instrument caused by the prism splitting, but also significantly improves the efficiency and convenience of multi-spectral image acquisition and successfully solves the long-term problem of multi-spectral image acquisition and transmission. The problem provides a new solution for the performance optimization and function expansion of spaceborne optical instruments. The ground resolution of MEPI is 10 km, and each pixel unit of its detector is composed of a \(3\times 3\) pixel array, so that the total number of pixels required for a single image is \(1274.2\times 1274.2\) units. In view of the four-channel architecture of the system, there are four Earth images with the same size but different polarization information, so the total number of pixels required by the whole system is \(2548.4\times 2548.4\) units.