## 5 Mission Technical Implementation
### Payload Design: IPDALLS System and nadir LiDAR
The stringent requirements on frequency stabilisation for water vapour detection, particularly relevant in the stratosphere where absorptions lines are very narrow, can be met by the use of seed lasers used in conjunction with a multi-pass water vapour absorption cell (for reference) and wavemeters. [PERSON] _et al._[42] gives a good overview of techniques implemented for the monitoring and diagnostics of the source's very high frequency stability and spectral purity (typically \(>\)99.9%). A particular challenge of active laser sounding in limb geometry with retroreflection will arise from spacecraft motion-induced Doppler shifts. Doppler shifts can be precisely modelled from orbit geometry and are expected to vary by less than 1 pm over the sounding altitudes range. However, absolute wavelength shifts on the order of 43 pm for diverging spacecraft and extremely narrow water vapour absorption peaks imply that emitted online sounding wavelengths will need to be generated and calibrated outside the actual absorption peaks. Furthermore, wavelengths will be shifted twice, on the forward as well as on the backward path, implying that peak absorption can only occur over one of them and effectively reducing the lower detection limit. The transmitter's heat-generating elements are cooled by a heat pipe assembly linked to the primary spacecraft's temperature control subsystem. Temperature control should provide a stable regime at an equilibrium temperature to minimize thermal stress on the transmitter. As mentioned above, it may be necessary to operate a single transmitter unit continuously and to switch its output between the nadir and limb systems. Switching must be designed to avoid single failure points and to guarantee the maintenance of accurate optical alignments over the mission lifetime. It may also be necessary to implement fully independent redundant transmitter units as back-up in case of premature failure. The use of redundant units must be traded-off against thermal requirements and overall weight and size. The receiving optics consist of a separate, slightly cross-track off-nadir telescope and a limb telescope. Our preliminary design calculations were based on limb measurements with a 0.5 m diameter telescope, while the required nadir optics were specified with a larger 1 m diameter main mirror, in order to collect sufficient backscatter radiance from the lowermost atmospheric region determined by the mission objectives. A fine-pointing mechanism within the limb optical path is used for scanning the laser beam within the telescope's FOV. Before beam collimation of the limb telescope signal, a small proportion of the received power (above 1,000 nm) is projected onto an acquisition and tracking CCD in the image plane, using a beam splitter. This image is used to localize the retroreflecting spacecraft within the telescope's FOV for a faster and more efficient searching and centralizing procedure. Aside from this, the same spectral separation and detection unit can be used for both nadir and limb instruments (see Figure 8); it may however be preferable to use two different units optimized for their respective sounding principle, providing redundancy and/or limiting signal losses due to beam combining. Since the four water-vapour wavelengths are sent off sequentially, one single gated detector could in theory be used for all of them (see [42]). In practice, the optimal pulse separation time (order of 100 \(\mu\)s) will be much smaller than the maximum return time for spaceborne nadir (order of \(<\)4 ms) and limb (order of \(<\)40 ms) sounding, which makes wavelength separation or demultiplexing mandatory. In [4], wavelength separation through Fabry-Perot etalons tuned to the respective frequencies was proposed, but other spectral separation can be envisaged as well. Individual detectors will be shielded from solar background noise via narrow-band (nominally 1 nm) interferometric filters.