## 2 Motivation and Background
### A New Measurement Technique in Space: Active Limb Sounding  
The observational requirements for upper air water vapour, as described above, are challenging to meet. The measurement technique needs to combine very high vertical resolution with a high signal-to-noise ratio for the detection of very low water vapour concentrations. The technique we propose here is an active limb sounding system, based on Integrated Path Differential Absorption (IPDA) LiDAR. Limb sounding for water vapour is an attractive option because the measurements at high altitudes are not contaminated by the signal from the humid, optically thick troposphere. Current limb sounders (as described above) have resolutions on the order of km in the upper troposphere and above, which is not high enough to resolve the strong vertical gradients in water vapour around the tropopause. Solar occultation techniques, with larger radiative flux densities to analyze, have limited measurement opportunities per orbit. By moving to an active technique, not only can this resolution be improved but the threshold of detection should be lowered, leading to greater sensitivity and a better characterization of the low concentrations of water vapour in the UTLS and above. The concept of hard target Integrated Path Differential Absorption (IPDA) LiDAR has been around for several decades. Recent studies into spaceborne instruments for trace greenhouse gas detection propose solid ground and cloud surfaces for the reflection of a nadir-emitted pulse [33]. Along with the difficulty of estimating the differential reflectivity over heterogeneous surface types, the signal-to-noise ratio (SNR) of such an instrument will be severely affected by overall low ratios of backscattered radiation and by speckle noise from interference originating at an uneven ground surface. The technique itself is also ill-suited for exploiting the ranging or profiling advantages of a standard LiDAR system, especially above the middle troposphere. Differential Optical Absorption Spectroscopy (DOAS) is one of the most extensively applied methods for measuring trace species in the open atmosphere [34], producing total path measurements of chemical components from simultaneous observations of spectrally resolved electromagnetic radiation. The technique for the retrieval of multiple species path-integrated number densities has been pioneered amongst others by [PERSON] _et al._[35] in the form of long-path DOAS measurements with an active continuous light source, though common passive configurations use natural light as the source, either from direct solar or stellar radiation or from diffuse sunlight. Active long path systems have been proposed both as monostatic and bistatic designs. Monostatic systems use a collocated transmitter and receiver and rely on arrays of retroreflectors from which light is picked up by a telescope, while a bistatic system has a separate transmitter and receiver. Optimal design considerations are discussed in [36]. Since DOAS relies on continuous spectra, laser sources are not common in active set-ups. Nevertheless, an obvious advantage of a laser source is the concentration of the radiant energy within a narrow spectral band and limited beam divergence, albeit at the expense of spectral coverage. A further advantage of targeting only a few selected wavelengths is the possibility to implement significantly higher gain detectors with high quantum efficiency and low noise levels. For accurate measurements of water vapour, we propose a differential absorption system utilizing elastic backscatter and integrated path LiDAR, centred on wavelength \(\lambda_{\text{on}}\), an isolated vapour absorption feature in the near infrared, compared to a nearby off-peak \(\lambda_{\text{off}}\). Integrated absorption of \(\lambda_{\text{on}}\) over the limb path offers a much higher sensitivity (at the expense of spatial resolution) and increases the dynamic range of vapour detection.