## 2 Survey of Commercial Small Platforms
### Active Optical  
Lidar  
Active Optical Instruments or Lidars use pulsed laser emissions to measure atmospheric profiles and Earth surface applications such as vegetation height. Due to the short wavelengths, the laser pulse propagation through the atmosphere is scattered and attenuated by air molecules and aerosols. On the Earth's surface, the vegetation and canopy also cause scattering. A small portion of the scattered light is sent back to the instrument which collects, and detects it. Subsequently, the electric signal is digitized through a Lidar signal numerical processing. Over the ocean, the variables that can be measured with Lidars are sea ice thickness, sea level and ocean dynamic topography.  
Lidars can be divided into two broad categories: (i) atmospheric profilers producing also the total column content for atmospheric composition, i.e., particles layers and key trace gases, and (ii) altimeters with decimeter to meter accuracy for topography retrieval and canopy vertical distribution. The objectives of relevant Lidars are:  
* Surface topography, ice sheet [75], and canopy [76] (e.g., ICESat-1).
* Climate and Radiation Budget by profiling clouds and aerosols optical and microphysical properties (e.g., NASA CALIPSO since 2006 [77], and ESA/JAXA EarthCARE [78], to be launched in 2021).
* Atmospheric dynamics or horizontal winds, (e.g., ESA Atmospheric Dynamic Mission ADM-Aeolus [79] was launched on 22 August 2018). Lidar instruments present the following main characteristics:
* Operating wavelengths in the UV, VIS, NIR, and SWIR; possible dual-wavelength, polarimetry, and two receivers (for Mie and Rayleigh scattering).
* Spatial resolution in the range of 100 m to a few tens of centimeters for LIDAR altimeters.
* Non-scanning, either nadir-viewing or oblique.  
Doppler LIDARs generally operate in the UV to track aerosol and air molecules and it are used for track aerosol and air molecules. Backscatter LIDARs are typically operated at one or two wavelengths (UV or VIS + NIR), often with amount of polarizations cross-talk into a succession of atmospheric backscatter measurements (rotatable half-wave plate) to discriminate between spherical and non spherical particles in the atmosphere, the nadir view brings the capability to measure aerosol profiles, cloud top height and atmospheric discontinuities, and the multi-beam to perform a large swath. Lidars altimeter operated at two wavelengths (VIS + NIR) can measure with very high vertical resolution and horizontal resolution (for sea-ice elevation, and ice boundaries). Differential absorption LIDARs (DIAL) operate at one wavelength centered on the absorption peak of one trace gas (e.g., O\({}_{3}\), H\({}_{2}\)O and CO\({}_{2}\)). The main limitation of this technology is the narrow swath. The variable with a gap that can be analyzed with Lidar is the sea ice thickness.  
Table 5 summarizes all technologies discussed in this section: radiometer imager, radiometer sounder, GNSS-R, AIS, scatterometers, altimeters, altimeter with SAR processing, SARs imager, Passive optical and Lidars. The measurements with gaps that can be measured for each technology are identified. The studied technologies are feasible on small platforms taking into account the survey of the commercial platform addressed in the previous section. Now, the best technology option needs to be analyzed, based on the future observations required by the Copernicus space infrastructure.