## 3 Mission Overview  
### Instruments  
We propose a novel approach which combines in a single instrument a water vapour Differential Absorption LiDAR (DIAL) in nadir-viewing mode [4, 42] with a monostatic IPDA LiDAR in limb sounding by occultation geometry (\"IPDALLS\"). Looking through the limb of the atmosphere, the IPDALLS system will sample the atmosphere at high vertical resolution with a long integration path. Several wavelength pairs with varying online absorption cross sections will be used to increase the observations' dynamic range within the UTLS and above. Measurements in this region were beyond the scope of the WALES candidate explorer mission [4], hence this new concept would lead to a more complete and accurate characterization of the water vapour profile. A secondary payload of a medium spatial resolution multispectral radiometric imager would allow wide-swath cloud and aerosol imaging. The IPDALLS system could be implemented with a bistatic instrument, where transmitter and receiver are on separate platforms. This is the configuration proposed for the ACCURATE mission. However, since two satellites in prograde and retrograde orbits within the same orbital plane could generate at the most four occultations per revolution, the basis of our mission concept is rather a monostatic transmitter-receiver spacecraft flown in formation with multiple spaceborne retroreflectors. An image of the mission concept is shown in Figure 1.  
The nadir LiDAR will provide range-resolved vertical profiles of water vapour and aerosols, as proposed for the WALES mission, albeit at slightly relaxed requirements, as the signal will not have to penetrate into the optically thick lower troposphere. The same (or a redundant) transmitting and the same receiving units are used for the limb IPDA LiDAR (see Sections 3.3 and 5.1 for proposed operation and instrument design, respectively). Even if the IPDALLS system, by its implementation, will in principle provide range-gated water vapour and aerosol data in addition to the integrated values, in practice, we estimate that any molecular backscatter signals will probably remain undetectable over the large limb distances involved. Synergistic data processing--nadir-limb matching--will allow for the simultaneous exploitation of the high along-track resolution of the range-resolved nadir signal with the enhanced sensitivity of a path-integrated measurement from the IPDALLS system. The latter shall also improve the relatively poor vertical resolution that could be achieved with a nadir DIAL signal alone when operating in upper atmospheric regions characterized by low backscatter coefficients. The proposed resolution and  
Figure 1: Illustration of the mission concept. The primary spacecraft is flown in an orbit which is counter-rotating with respect to a constellation of retroreflectors. The primary spacecraft carries a water vapour Differential Absorption LiDAR in nadir-viewing mode, and a monostatic IPDA LiDAR for limb sounding by occultation geometry (IPDALLS), using the retroreflectors as hard targets. The primary spacecraft also carries an imaging radiometer optimised for wider-swath cloud and aerosol imaging. sensitivity improvement through the use of multiple observation methods is conceptually similar to panchromatic sharpening of multispectral imagery. By design, the multiple-wavelength bidirectional LiDAR instrument, core element of the mission's payload, will emit pulses at water-vapour absorption line-specific wavelengths for the DIAL retrieval. According to studies summarized in [42], three distinct water vapour absorption bands are required for probing the entire atmospheric column from the boundary layer up to the lowermost stratosphere (see Section 5), in addition to a reference offline wavelength in the wing of the absorption spectrum. The four wavelengths used in the WALES proposal were \(\lambda_{\mathrm{on1}}\) = 935.6845 nm, \(\lambda_{\mathrm{on2}}\) = 935.5611 nm, \(\lambda_{\mathrm{on3}}\) = 935.9065 nm, \(\lambda_{\mathrm{off}}\) = 935.4122 nm.