## 4 Potential Instrument, Suitable Platforms, and Technological Limitations  
After the survey of the suitable EO technologies in terms of the spatial resolution, swath, mass and power consumption, in this section, the suitable small commercial platforms and technological limitations of the potential sensors are identified. Tables 6 and 7 show the potential technologies studied in this work, with the suitable platforms and limitations with respect to the needs detected in the horizon 2020-2030. Platforms are selected according to their capacity to support the instrument mass and power consumption (available commercial platforms surveyed, Tables A1-A3). Additionally, it takes in to account the platforms with minor categorization (e.g., nano-, micro-, or mini-platforms), that satisfy both requirements. Special attention has been paid to the possibility to use new techniques and smaller platforms, focusing on the quality of the measurements as compared to the ones generated by full-fledged payloads onboard large spacecrafts. Indeed, since a small platform also means less volume, mass, power and data rate for the payload, the measurements are usually of reduced quality. Depending on the mission (i.e., environmental data), this may be compensated by more frequent data acquisitions (exchange between measurement quality and revisit time), yet to be evaluated on a case-by-case basis. A brief the potential instruments, suitable platforms, and technological limitations are explained below:  
* GNSS-R (1.4 kg, 12 W) instruments are suitable for nanosatellites (3U or 6U). Table 6 presents sample available commercial platforms for the SGR-ReSi [57], such as the Endeavour-3U [18] and the MAI-3000 [17]. Endeavour by Tyvak Nanosatellite Technology Inc. (San Luis Obispo, CA. United States of America), is a 3U platform with 15 W of average payload power, 3 deg of pointing control. MAI-3000 by Maryland Aerospace, is a 3U platform with 12 W of payload power and 3 kg of available payload mass. The main limitation of GNSS-R altimetry data is the poorer (decimetric) resolution and accuracy (\(\sim\)20 cm for SSH, and 2 m/s for wind speed) are offset by the much larger number of simultaneous observations from different specular reflection points [80]. * Another good example are microwave sounders on small-platforms such as EON-MW [33], for measuring the atmospheric pressure over the sea surface. However, the antenna system must be redesigned to achieve the spatial resolution required. For a 10 km spatial resolution, at 50 GHz, the require antenna aperture is 36 cm, from an altitude of 600 km. Table 6 summarizes a list of the available commercial micro-platforms suitable for this instrument. * Microwave imagers at X-, K-, Ka-, and W- bands are particularly well suited for implementation on small platforms (Table 6). TMI is a light instrument suitable for mini-satellites, with X-, K-, Ka-, and W- bands capable of measuring and covering the gaps for wind speed, sea ice cover, sea ice type, and sea surface temperature variables. For sea surface temperature, microwave radiometers improve the coverage in polar regions because of their all weather capabilities. In order to obtain a spatial resolution of 10 km at 18.7 GHz from 600 km height, a 2.2 m antenna is required. On the other hand, an SSM/I type of instrument with a modified antenna, could be implemented in a micro-platform in order to cover wind speed over the sea surface, sea ice cover, and sea ice measurements, with the required performance. L-band radiometers contribute to sea ice thickness monitoring, agriculture (soil moisture) and forestry measurements. Those instruments are suitable for mini-platforms (Table 6). The main limitation is their coarse resolution. Inflatable antennas must be used to reduce the footprint size, or aperture synthesis techniques could be implemented [81]. ELiTeBUS 1000 [10] by Thales Alenia Space (Cannes, France) is an available commercial small-platform suitable for this instrument.