## 4 Preliminary Performance Considerations for IPDALLS
### Potential Performance Limitations  
The results presented above have been obtained without considering the turbulent nature of the atmosphere and related multiple diffraction effects, which arise in addition to the continuous refractive dilution and chromatic refraction. Random anisotropic and isotropic irregularities in upper air density, originating from internal gravity waves, wave breaking, wind shear and other instabilities, result in inhomogeneity of the refractive index which leads to temporal and spatial fluctuations in the recorded intensity of radiant energy: scintillations. Scintillations have been successfully exploited in stellar occultation for gathering data on atmospheric turbulent dynamics (e.g., [57, 58, 59]), however they severely affect the absorption measurements of the GOMOS instrument (e.g., [60]), the most relevant comparable payload on orbit, and may be even more detrimental for laser (coherent) radiative transfer due to the generation of speckle patterns. The only analogue to our proposed measurement technique is an inter-orbit optical communications experiment between a geostationary and a LEO platform, which maintained a narrow-beam link through the atmosphere during an occultation event [61, 62]. In this case, the combined action of a smaller continuous wave laser divergence and the very long distance to the geostationary orbit made the technique more susceptible to angular beam deflections (wander) than the one described in this paper. Both terminals experienced strong fluctuations of the received power, a non-monotonic decrease of the normalized intensity log-normal probability density mode from about 50-40 km downwards, first detector saturation due to refractive lens effects at about 35 km, and a termination of the optical link in the best case at 15 km. For GOMOS retrieval corrections [60], this steady decline of the average power has been factored into the total atmospheric transmission as an altitude-dependent transmittance, in which refractive dilution is modulated by scintillation estimated from measured intensity fluctuations. The average decrease in signal strength is simultaneously accompanied by temporal amplitude fluctuations due to scintillations which often exceed the GOMOS instrumental noise, hence impacting the SNR and therefore the measurement error in a twofold way. According to the available scintillation measurements, the scintillation variance is saturated below 25-30 km at a root-mean-square level of about 100% of the mean signal. Such a level of scintillation variance within the UTLS can also be expected for the IPDALLS principle. Even if two spectrally adjacent channels are affected by turbulence in virtually the same way, this may not improve the single shot SNR in the individual channels, and scintillations will still degrade the measurement accuracy. Since wavelength separation for water vapour is very small (less than 0.06%), the light at the different wavelengths travels essentially through the same air density irregularities, if emitted simultaneously. In combination with short laser pulses, this allows us to expect that the impact of scintillations on differential absorption in water vapour channels is very small. According to simulationsfor ACCURATE [63; 64], we can expect the root-mean-square error in differential transmission due to scintillations to remain less than 1% down to 5-10 km for the proposed system. On the other hand, [PERSON] _et al._[65] point out that the correlation between multi-chromatic signal fluctuations is minimal when speckle dominates over other sources of noise, the speckle pattern between different wavelengths itself is decorrelated, and the mean number of integrated speckles per pulse is small, which can be expected in classic hard target low-divergence LiDAR systems. Pure retroreflection speckle has been observed during laser ranging due to the use of reflector arrays [66], but avoided by design for absorption measurements through the use of a single cube-corner retroreflector [55], such as proposed in our preliminary design.