## 3 Mission Overview
### Data Retrieval and Use  
Observations from this novel system are designed to deliver higher resolution and higher sensitivity water vapour measurements than currently exist for the UTLS and above. It will help to quantify and understand the differences between current water vapour measurements, and will provide collocated active and passive measurements of water vapour, clouds and aerosol for process studies. Full exploitation of the potential of this combination of measurements will require further research, but much progress has already been made in understanding and assimilating limb observations. Inversion algorithms for the nadir water vapour and aerosol elastic backscatter LiDAR will follow standard procedures adopted for DIAL measurements. The inversion of an integrated path DIAL signal is conceptually rather straightforward, though specific considerations for an occultation geometry will complicate the picture somewhat: chromatic ray bending, refractive dilution and scintillations (see [30] and references therein, for the GOMOS stellar occultation measurements), a number of considerations related to the use of narrow absorption lines (see [48] and references therein, for a water vapour DIAL system), as well as background radiance from sunlight scattering and spontaneous emission (e.g., from  
Figure 4: Representation of the ground-track offset between limb and nadir measurements, for \(0\) to \(20^{\circ}\) latitude. The offset arises because of the rotation of the Earth between nadir and limb measurements.The nadir LiDAR will be inclined to minimise returns from specular reflection; this inclination can be utilised to minimise the offset (5\({}^{\circ}\) inclination shown). auroras). We expect that well-established retrieval approaches for limb sounding systems, such as 'onion peeling' implemented, e.g., for SCIAMACHY solar occultations [28] will be adapted for this system. These algorithms require the use of a reference atmosphere, and the quality of the results will depend on the appropriateness of that reference. However, [PERSON] _et al_. [28] find that this dependence is greatest at low altitudes and is negligible at an altitude of 35 km. Long path inhomogeneities will need to be deconvolved from the limb measurements. With a nominal PRF of 25 Hz for the limb sounding, we avoid signal ambiguities at the maximum path length between the instrument and the retroreflector (\(\sim\)5,500 km). This means that the previous pulse returns before the following is emitted. At a vertical descent rate of the optical link through the atmosphere of approximately 2.8 km/s, the vertical sampling resolution is therefore constrained to roughly 110 m. Single signals may need to be integrated to increase the SNR, which is typically proportional to the square root of the number of shots, and the actual vertical resolution will be degraded accordingly. We expect to find single shot SNRs from the reflected pulse returns large enough to make large averaging sets unnecessary, though this has yet to be confirmed by a detailed end-to-end simulation (see Section 4 for the preliminary calculations). In addition, with every pulse or burst, 4 DIAL wavelengths are emitted, yielding three pairs for averaging, even if two of them will generally be inadequate for the water vapour concentration at a given altitude. Forward radiative transfer modelling of brightness temperatures and reflectance as a function of cloud parameters across the relevant wavelengths will be used to generate look-up tables for the exploitation of the radiometer data. By matching the observed data to the look-up table values, users can recover the cloud properties. Benefits of this novel system are likely to come from the improvement of model routines and parameterizations through process studies of clouds and convection. Assimilation of high resolution, accurate water vapour measurements within models would lead to improved water vapour convergence estimates [4].