## 5 Mission Technical Implementation  
### Payload Design: IPDALLS System and nadir LiDAR  
Based on the aforementioned measurement configuration, the mission's primary payload comprises a multi-wavelength bidirectional transmitting system, receiving optics, a spectral separation and detection unit, a transient recorder data acquisition unit, a control unit and a scanning and tracking facility based on novel (albeit proven) optical communication technology. A simplified conceptual block diagram (Figure 8) illustrates the payload subdivision into nadir and limb pointing instruments. The transmitter constitutes a critical driver of the overall mission concept. Since the concept draws extensively from the heritage of the WALES mission proposal, and because of necessary further  
Figure 8: Conceptual layout of the main payload. Emitted beams are in red; optical path of incoming radiation is in blue. Redirection of the outgoing beams into either limb or nadir sounding geometry in case of breakdown of one of the redundant transmitter units is drawn as rotatable unit switching mirrors (US). Sampling of the emitted pulses is via very low transmission beam splitters (BS). Limb telescope field-of-view scanning can be performed in pitch and in yaw using the flat scanning mirror (S) and the rotating drum (D), respectively. In order to maintain the very narrow divergence of the outgoing limb beam with a realistic beam quality parameter and limited diffraction, pulses are routed through the main telescope for beam expansion. Switching between transmission and reception is conceptually performed by a fast mirror galvanometer (GM). Planar scanning of the outgoing limb beam within the telescopes FOV is performed using a fine-pointing mechanism (FPM). Detectors can be shielded from direct sunlight exposure using the field stop and shutter (FS). A fraction of the returned signal (above 1,000 nm) is directed onto a planar acquisition and tracking (AT) CCD through the PAT beam splitter (PBS), to facilitate retroreflector locking before the measurement sequence. The continuous-wave beacon laser beam with higher divergence than the measurement beam, potentially necessary for the PAT system and turbulence estimation, can be steered using a piezoelectrically-controlled mirror (P), and is not further described in this paper. For further details on PAT system layouts for optical communication, we refer to [70]. industrial feasibility studies, a detailed description of the laser source is beyond the scope of this study. A comprehensive account on water vapour differential absorption LiDAR transmitter choices can be found in [69]. In addition, retrieving profiles of atmospheric aerosol size distribution, backscatter and extinction coefficients from elastic backscatter LiDAR signals generally relies on at least two widely spaced laser wavelengths and their polarisation. DLR's airborne WALES precursor described in [42] uses two Nd:YAG lasers in master oscillator/power amplifier configuration. The radiation of the WALES pump lasers is frequency doubled by a second-harmonic generating crystal (532 nm) and then converted to values in the vicinity of 935 nm by an Optical Parameter Oscillator (OPO). The output of the OPO is switched between two wavelengths, using calibrating seed laser diodes that can be tuned over a range of about 1 nm via temperature or current control. In order to provide the four wavelengths that have been used in our preliminary performance studies, two identical chains of lasers and non-linear conversion stages would need to be synchronized in a temporally interleaved fashion, at least during nadir operation. Industrial research into most mature transmitter options for the spaceborne WALES ESA candidate mission converged however on a high-energy Ti:Sapphire oscillator in a ring cavity configuration, pumped by a frequency-doubled Nd:YAG laser and injected by four tunable seed lasers to set the frequencies [4].