## 3 Mission Overview
### Instruments
In this preliminary study, we find that the same number of wavelengths is required for long path integrated limb sounding from the upper troposphere upwards, and we retain these four wavelengths as a starting point for the performance assessment, even if a posterior optimization study may well lead to a more favourable selection. See Section 4 for more detailed discussion and calculations. The pump laser's first (1,064 nm) and second (532 nm) harmonic wavelengths will also be used for retrieval of atmospheric aerosol loading, via backscatter and extinction measurements from the elastic backscatter and retroreflected signals at these two widely spaced laser wavelengths. A measurement of the depolarization ratio will also be made for aerosol characterization. Inevitably, the presence of clouds will limit the number of IPDALLS measurements, as the technique can only be applied above cloud top height, below which the optical link between the instrument and retroreflector will be lost. However, in its function as an aerosol elastic backscatter LiDAR and due to its bidirectional viewing geometry, in these situations the system will act as a combined range-gated nadir-limb ceilometer, especially for thin high altitude cloud decks with a sufficiently large horizontal spread. It will thus provide information on the vertical extent of high altitude, optically thin translucent clouds, such as (contrail) cirrus, polar stratospheric and notilucent clouds. Given a typical cirrus cloud thickness on the order of 1-2 km, achieving a high vertical resolution from the combined data is crucial. A fairly conventional radiometric imager will be used for the retrieval of cloud macro- and microphysical properties at cloud-resolving scale, following the operational method of [PERSON] and [PERSON] [43]. Clouds are detected using the brightness temperature difference between visible and thermal infrared channels; cirrus ice clouds and water clouds can be distinguished by the brightness temperature difference between particular thermal infrared channels, which can also be used for the retrieval of cloud top temperatures (and heights). The radiative impact of cirrus clouds is dependent on certain physical cloud parameters: effective particle radius (\(R_{e}\)), ice water path (IWP), the optical depth (\(\tau\)), and the vertical position of the cloud, manifested in its cloud top temperature and cloud top height. The radiometer spectral bands have been chosen to be sensitive to these parameters. The visible (VIS) and short-wave infrared (SWIR) bands (0.66 \(\mu\)m and 1.66 \(\mu\)m ) are highly sensitive to \(R_{e}\), IWP and thus \(\tau\). The 10.8 \(\mu\)m and the 12.0 \(\mu\)m bands are also sensitive to the same parameters, but need to be combined with additional bands for accurate results: VIS/SWIR bands during daytime and the 3.9 \(\mu\)m mid-infrared (MIR) band during night. The thermal infrared (TIR) bands are sensitive to cloud top temperature and height [44]. The five bands specified above are the minimum requirements to recover the desired cloud parameters. Additionally, two SWIR bands, 1.38 \(\mu\)m and 1.24 \(\mu\)m, are incorporated into the radiometer design. The 1.38 \(\mu\)band enables to detect upper level cirrus clouds, especially over land [45]. Furthermore, the ratio of the 1.38 \(\mu\)m band over the 1.24 \(\mu\)m band is very effective in discriminating upper level cirrus clouds from the lower level aerosols and dust [46]. The horizontal distribution of column integrated water vapour will be retrieved from a further, dedicated band at 6.3 \(\mu\)m, and will be useful for the context interpretation of the nadir LiDAR data and their cross-validation.