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:


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., O3, H2O and CO2). 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.


**Table 5.** Mapping of the potential technologies to cover measurements with gaps.

The data relevance of the instrument depends on its ability and limitations to obtain the measurements:

*<sup>a</sup>* Marginal relevance; *<sup>b</sup>* medium relevance; *<sup>c</sup>* high relevance.
