3.1.1. Microwave Imagers (MWIm)

The main applications of Microwave Imagers (MWIm) are atmospheric (X, K, Ka, and milimiter waves bands), oceanographic (C, X, K, and Ka bands), vegetation and soil moisture monitoring (P, L, S, C and X bands). High frequency microwave radiometers are particularly well suited for small platforms because of the antenna size constraints. These types of instruments can measure: wind speed [26,27], sea ice thickness [28,29], and sea ice cover [30], among other variables. Table A4 presents the features of some microwave radiometers, in terms of frequency bands, spatial resolution, antenna size, swath, mass, power consumption, and data rate. Assuming only one payload per platform, the affordable platforms (nano, micro, mini, and large) for the instruments are identified according to the power and mass requirements. This information is valuable in order to choose the potential instruments that will complement the Copernicus Space segment, trying to make them compatible with the smallest possible platforms, while fulfilling the user requirements. The measurement gaps that can be covered with this technology are: horizontal wind speed over the sea surface (MWIm with channels around 7, 10, 19, 37 GHz or 19 and 37 GHz), sea ice monitoring (cover, type, drift, MWIm with channels around 7, 10, 19, 37, and 90 GHz), sea ice thickness (MWIm with channels around 1.4 GHz), soil moisture (MWI with channels around 1.4 GHz, or 7 GHz, or 11 GHz), and sea surface temperature (MWIm with channels around 7 and/or 10 GHz).

According to Table A4, two microwave imagers capable of measuring the variables with gaps have been identified. These are selected because they are suitable for small platforms and present good data quality, to cover the user requirements.


## 3.1.2. Microwave Sounders (MWS)

In the last few years, intensive work has been conducted to develop missions to prove the feasibility of using microwave sounders on nano-platforms, such as MicroMas [32], and the Earth Observing Nanosatellite-Microwave (EON-MW) [33]. The measurement with gaps that can be analyzed with this technology is the atmospheric pressure over the sea surface.

Table A5 presents a survey of the representative current and future missions with microwave sounders capable of measuring the atmospheric pressure over the sea surface. The gaps for this variable are the revisit and the latency times. To fill these gaps, a constellation of microwave sounders based on CubeSats missions could observe fast weather phenomena requiring high revisit time (3 h or less). A good example of CubeSat mission is EON-MW. The payload is a dual-reflector radiometer with a mass of 4 kg, an antenna size of 11 cm, and spatial resolution of 30 km on altitude of 600 km at 54 GHz.
