# A New Orbiting Deployable System for Small Satellite Observations for Ecology and Earth Observation
## 2 Small Satellites for VIS, TIR, and MW Remote Sensing
### Current Status of Small Satellite Payload for Earth Observation
#### 2.2.2 Microwave Payloads  
Earth imaging in the microwave range is mainly achieved using the synthetic aperture radar (SAR) technology, which synthesizes the elevation by processing a tailored signal (e.g., [57; 70]). This technology allows to increase the spatial resolution using a smaller antenna than in the real aperture radar (RAR) system. Due to its day, night, and (almost) all-weather capabilities, SAR is a key tool for a broad range of EO applications, including the observation of oceans (currents, winds, waves, pollution, etc.), the cryosphere (coverage and extension of ice and snow, navigation routes, etc.), and lands (coverage and extension of vegetation, evolution of geological and tectonic structures, utilization of terrains for agriculture, etc.) [57; 71; 72; 73].  
The SAR system was the main payload of large satellites such as Sentinel-1 (ESA) and TSX (BMBF, DLR), medium-large satellites such as COSMO-SkyMed (first and second generations, ASI) (cf. Table S1 in the Supplementary Materials), down to the smaller sizes (\(<\)500 kg) such as TecSAR (Israe's MoD) and NovaSAR-1 (SSTL/UKSA) (cf. Table 2). Similar to small satellites with optical payloads, constellations of SAR-carrying satellites can improve observation revisiting time (e.g., ICEYE (ICEYE Ltd. of Espoo), cf. Table 2).  
Passive instruments operating in the microwave range are termed as microwave radiometers (MWRs). They collect microwave radiation naturally emitted by the Earth, guaranteeing continuous, large-scale, all-day, and almost all-weather Earth Observations, which are of paramount importance for environmental monitoring, for instance soil moisture and sea surface salinity [74; 75].  
MWRs can be built by means of the \"aperture synthesis\" approach too [76], allowing spatial resolutions that otherwise would require the use of antennas with a larger aperture [77]. Synthetic aperture microwave radiometers (SA-MWRs) are based on the cross-correlation of signals collected by different pairs of antennas, placed at varying antenna-pair spacings [76]. Such baselines represent the visibility function in the frequency domain, which must then be transposed into the brightness temperature map in the spatial domain through an inversion algorithm (e.g., Fourier transform) [78].  
MWRs can be designed to allow measurements at a given combination of frequency and polarization (single-channel MWRs), such as the Soil Moisture Active Passive (SMAP) radiometer and the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), which is part of the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission, or they can call for multi-frequency and multi-polarization capabilities (multi-channel MWRs). The latter category includes instruments that can perform measurements at different channels (i.e., using different frequency/polarization combinations), e.g., (i) the Special Sensor Microwave Imager (SSM/I), which covers four frequencies (ranging from 19.35 GHz to 85.5 GHz) and seven channels (i.e., all the frequency channels, with theexception of the 23.235 GHz one, allow for collecting measurements at both vertical and horizontal polarizations); (ii) the Special Sensor Microwave Imager Sounder (SSMIS), which is a 24-channel, microwave radiometer with channel frequencies ranging from 19 GHz to 183 GHz; (iii) the 12-channel Advanced Microwave Scanning Radiometer 2 (AMSR2); and (iv) the 10-channel Microwave Radiation Imager (MWRI) [79]. Although MWRs are mesoscale sensors, a number of products have been also proposed to retrieve parameters on a regional scale [79; 80; 81].