## 1 Introduction  
Forests are key components of surface-atmosphere interactions as demonstrated by previous studies [1; 2; 3; 4; 5]. They are sinks or sources for many atmospheric compounds that play major roles on the atmospheric chemistry such as the ozone cycle and the secondary organic aerosol formation [6; 7; 8; 9]. Representing 80% of the continental biosphere carbon stock, forests also sequester large carbon dioxide from the atmosphere [9] and act as conservators of the biodiversity [10; 11]. The biodiversity of forests depend on the three-dimensional (3D) distribution of canopy structures, which contains a substantial amount of information about the state of development of plant communities [12; 13; 14; 15; 16]. However the 3D canopy structures are not yet available at the global scale until now. Spaceborne observation, including passive and active remote sensing systems, is a good approach to get such information of canopy structures. Passive multispectral and hyper-spectral sensors produce two-dimensional atmosphere/ground information, whereas their ability to represent 3D spatial patterns is limited. For instance, their use for tropical dense forests is not adequate because the remote sensor has difficulty to penetrate the upper canopy layer [17]. On the other hand, active sensors, including lidar and radar, have been shown to be valuable tools for 3D vegetation mapping and characterization. Recent developments in P-band radar technology and data processing techniques allow accurate estimates with a large spectrum of forest biomass [18; 19; 20]. For instance, the BIOMASS Earth Explorer mission selected by ESA in the framework of its Living Planet program is envisaged as a P-band spaceborne Synthetic Aperture Radar (SAR) satellite. This future space mission aims at providing consistent global estimates of the forest biomass, disturbances and re-growth. However, radar gives more information about the volume and is less efficient to restitute the forest vertical structure than lidar. Lidar therefore appears to be a complementary instrument for both short and medium-term spaceborne missions in the future. Previous studies have been performed to evaluate the potential of spaceborne lidar systems [21; 22; 23; 24]. Nevertheless, these authors did not take into account the orbits and the related atmospheric contribution in their studies. Moreover, the instrumental characteristics were not explicitly analyzed. A spaceborne backscatter lidar could provide a full description of the forest vertical structures [25], and forest dynamics resulting from deforestation (clear-cut), forest management, or climate change [26; 27; 28]. Lidar technology is now very mature, as demonstrated by the LITE experiment [29], the operational CALIPSO mission [30; 31], and the ICESat mission [32]. Furthermore, the MOLI (Multi-Footprint Observation LiDAR and Imager) project selected by JAXA [33] and the GEDI (Global Ecosystem Dynamics Investigation Lidar) project selected by NASA [34] would involve embedding on the International Space Station (ISS) a vegetation lidar system for forest studies. A large-footprint lidar (a few tens of meters' footprint at the ground level) can accurately map canopy structures and aboveground biomass [35; 36; 37; 38; 39; 40], even in high-biomass ecosystems where passive optical and active radar sensors typically fail to do so. In this paper, a detailed link budget for a canopy lidar system onboard satellite is presented. It complements the previously published works [21; 22], by using an original approach coupling airborne lidar observations and an end-to-end simulator. This work has been conducted based on field experimentsspecifically conducted over French forests between 2008 and 2014. It aims at a pre-feasibility study of a spaceborne lidar dedicated to forest survey, required before a future Assessment Phase (Phase-0) by the Centre National d'Etude Spatial (CNES).