3.2.2. Sounder: Multispectral and Hyperspectral

A good example of hyperspectral infrared sounder capable of measuring atmospheric pressure over the sea surface on CubeSat is EON-IR [62]. This instrument is under development with spatial resolution comparable to legacy sounders such as Infrared Atmospheric Sounding Interferometer (IASI), Atmospheric Infra-Red Sounder (AIRS), and Cross-track Infrared Sounder (CrIS).

Table A9 presents the details of the available multispectral and hyperspectral sounders instruments, in terms of spatial resolution, aperture size, swath, mass, power consumption, and data rate. For each optical sensor, it classifies (nano-, micro-, mini-, and large-satellite) according to the payload power and mass that can support the available commercial platforms summarized in Table 2.

## *3.3. Active Microwave*

Several missions have been launched with active microwave instruments that can be grouped into three main families: Scatterometers, Synthetic Aperture Radars (SAR), and Radar Altimeters (RA). This section describes the variables of interest that can be measured with satellite-based active microwave sensors: wind speed, and direction over the sea surface using radar scatterometers, SAR and SAR altimeters; sea level, significant wave height, wave and wind speed using RA; and dominant wave direction, significant wave height and sea ice cover by SAR. Then, each variable is presented with the available active microwave technology, and the new trends of these sensors in small satellites.
