*3.2. Passive Optical*

This type of technology has shown its feasibility for small missions [59,60]. For example, for an optical instrument in the visible part of the spectrum, with a ground resolution better than 10 m, and an aperture of 10 cm (CubeSat size), the altitude of the satellite should be less than 500 km.

The data provided by passive optical instruments, from the ultraviolet to the far-infrared wavelengths can be used for weather forecast, vegetation, atmosphere, ocean and land studies. The main limitation of optical sensors is that data cannot be acquired in night-time (visible and near infrared parts of the spectrum) or cloudy conditions, and cloudy weather is very frequent in Polar Regions.

In this manuscript, the classification of optical sensors as radiometer imager and atmospheric sounders, and its subclassification between multispectral and hyperspectral is studied. Radiometer imagers measure the intensity of electromagnetic radiation in the visible or infrared bands, and sounders measure the vertical distribution of atmospheric parameters such as pressure, temperature, and humidity. Multispectral instrument refers to a maximum number of tens of bands, and hyperspectral radiometers consist of hundreds of narrow and continuously distributed bands (10–20 nm).

#### 3.2.1. Radiometer: Multispectral and Hyperspectral

Table A8 presents the features of the available multispectral and hyperspectral radiometers instruments, in terms of wavelength, spatial resolution, aperture size, swath, mass, power consumption, and data rate. The variables of interest that can be measured with optical sensors for the Marine for Weather Forecast, Sea Ice Monitoring, Fishing Pressure, and Agriculture and Forestry: Hydric stress use cases are the Sea Surface Temperature (SST), atmospheric pressure over the sea surface, ocean chlorophyll concentration, ocean imagery and weather leaving radiance, Color Dissolved Organic Matter (CDOM), detection of water in crops, estimation of crop evapotranspiration and the sea ice cover.

A good example of multispectral radiometer on micro-platform is AVHRR/3 [61] and also has good performance, and it could support the measurements with detected gaps, such as SST, ocean chlorophyll concentration, ocean imagery and weather leaving radiance, CDOM, detection of water in crops, estimation of crop evapotranspiration, sea ice cover, and atmospheric pressure over the sea surface (it can be inferred through measurements in the infrared band).
