## 6 Significant Challenges  
As with any new measurement technique that has not yet been demonstrated in space, substantial technological and technical development efforts and risks are associated with the mission concept described in this paper. The main challenge is likely to be the design of the transmitter and the resilience of the lasers, as evidenced by existing and planned spaceborne LiDAR missions. Aside from the obvious stress on delicate optical elements and alignment during launch, laser damage to optical coatings is an issue, and the large power consumption and heat dissipation requirements during operations are challenging, especially with respect to the cooling of optical components. The design of a high power pulsed laser system generating the required water vapour wavelengths with extreme frequency accuracy and stability, spectral purity, output energy and temperature control requirements will require significant development and testing. Much research has already been conducted into potential optical layouts for the development of new water vapour DIAL systems in general, and of the WALES airborne platform and spaceborne proposal in particular. Since lasers are prone to failure, transmitter redundancy with limb/nadir switching capabilities would have to be implemented with associated impacts on mass budget, dimensions and costs. Aiming the laser beam precisely onto a target satellite that passes at a distance of about 5,000 km and at a relative speed of roughly 15 km/s constitutes a further major technical challenge. However, there have been substantial developments in the pointing, acquisition and tracking (PAT) technology in the last 50 years, since the first attempt of space-to-ground laser communications during the Gemini 7 mission in 1965. In the mid 1980s, ESA embarked on the SILEX programme, demonstrating both space-to-ground laser communications between the Artemis satellite and a ground-based tracking station, and inter-satellite laser communication between ARTEMIS and the Earth Observation satellite SPOT-4. SILEX demonstrated for the first time that the stringent PAT requirements associated with the extremely low divergence of optical communication beams (7 \(\mu\)rad in the case of SILEX) can be reliably mastered in space. In 2008, TerraSAR-X and the NFIRE satellites tested data transmission in space at a range of about 5,000 km with a laser beam divergence of approximately 3 \(\mu\)rad [75]. Today, DSP-I satellites carry a laser communications package that enable the satellites to relay information to each other with a similar beam divergence. Although the previous examples did not have to deal with the signal spectral quality requirements that arise for a differential absorption measurement mission, they demonstrate that the extremely demanding PAT requirements associated with optical wavelengths can be reliably mastered, and the knowledge gathered will be extremely valuable for the concept proposed in this paper. PAT will be further complicated due to the very narrow FOV of the limb telescope required to minimize background radiation. The instrument design proposed herein foresees a flat mirror of the same size as the main telescope mirror to steer the FOV, and to enable the tracking in both pitch (to perform the occultation measurement) and yaw (to compensate for orbit divergence). Even if the respective angular velocities remain small, the mass of the mirror will induce a significant angular momentum which will need to be compensated for by attitude control. This ties in with the very tight requirements on accurate spacecraft positioning, on orbit maintenance and attitude control, for both the primary spacecraft and allthe retroreflector spacecraft, as well as the orbit insertion of the entire constellation during two successive launches. Since the success of this novel measurement technique relies on optimally returned signals, issues associated with the design of the retroreflectors must not be neglected.