3.3.1. Real Aperture Radar Altimeter

Radar altimeters measure the distance of the Earth's surface underneath the spacecraft by measuring the time between transmitting the signal and receiving the echo. Microwave radar altimeters have been used for a wide range of applications that can be grouped as: (a) geodesy and geophysics, study the Earth's shape and size, on the ground as well as on the sea surface [63]; (b) ocean applications (ocean surface currents, wind speed, significant wave height); (c) ice sheets and sea ice (sea ice thickness, and glacier topography) [64]; (d) climate (ocean topography and the heat exchange with the atmosphere); and (e) hydrology.

Nowadays, altimeter constellations on small platforms are deemed important, since they bring improved temporal resolution, and some ocean phenomena can only be perceived if subject to an almost continuous observation. At the same time, a shorter revisit time represents an increase in the spatial coverage and a finer spatial sampling grid. Equally, SSO should be avoided because of the errors associated with solar tidal effects.

Examples of recent altimetry missions are presented in Table A10. Typical requirements are: 100 W average power consumption, 1.2 m antenna diameter, 61 kg payload mass. The implementation on nano- platforms for radar altimeters may partially degrade the quality of the measurements. Additionally, nadir looking altimeters do not provide a wide swath. In this way, constellations of small satellites embarking a compact nadir altimeter [65] could improve the temporal/spatial sampling and therefore closing the gap with current planned missions.

#### 3.3.2. Real Aperture Radar Scatterometers

Current and planned scatterometers missions have been identified and are summarized in Table A11. Earth Observation missions based on scatterometers typically operate at C-, and Ku-bands, and present spatial resolutions from 10 to 50 km. Current and future contributing missions to the Copernicus system with radar scatterometer are: ASCAT and SCA, ASCAT/Metop-A/B/C (2007 to 2021), with global coverage every 1.5 days and 12.5 km spatial resolution for basic sampling, SCA/Metop-SG-B1/B2/B3 (from 2022 to 2030) with near global coverage every 1.5 days, from 15 to 20 km of spatial resolution with sampling at 6.25 km intervals.

The main variables derived from radar scatterometer data are wind speed and vector over sea surface [66], but scatterometers are also capable to obtain surface soil moisture indices [67], leaf area index [68], snow water equivalent, snow cover [69], and sea ice extent measurements [70] Table A11 shows the characteristics of the radar scatterometer. The power consumption of these sensors is in the range of 210–540 W, and mass is in the range from 260 to 600 kg. According to the requirements of power consumption, size and mass, this payload can be carried over mini- or large-satellites.
