## 4 Preliminary Performance Considerations for IPDALLS
### First Order Estimation of the Link Budget and Measurement Accuracy
[PERSON] _et al._[55], on the other hand, report no significant influence of sunlight in laser ground-space absorption measurements in IR wavelengths. Exposure to direct sunlight has not been considered, since it is likely to damage the detector and needs to be avoided. Figure 7(a) shows the contribution of the various sources of noise to the total detector noise current for the on- and offline wavelengths: signal shot noise, Rayleigh-scattered solar background in the baseline case of 0\({}^{\circ}\) azimuth and 60\({}^{\circ}\) zenith solar angles with respect to the occultation plane and detector dark current noise from surface and bulk origins. The maximum signal photocurrent of 6 mA at the top of the atmosphere (TOA) falls well within the maximum peak rating of the detector (Table 1). Figure 7(b) shows the CNR with altitude: the potential of the IPDALLS technique is evident from the very high CNRs achieved throughout the stratosphere and into the tropopause layer, with TOA peak ratios on the order of 700. Similar values have recently been reported for a comparable mission concept described in [41]. The kink around the tropopause originates from the refractive dilution. Above roughly 25 km, where atmospheric transmission is high, the CNR for all wavelengths is clearly signal noise power limited (see [56]). Consequently, few improvements can be expected from reducing detector noise or telescope FOV, and the stringent requirements with respect to the latter could be relaxed, making search and tracking easier. The same holds for the lower atmospheric layers, where the signal shot noise remains consistently larger than the background and detector noise at the online wavelength corresponding to the relevant height range, although scintillation variance is likely to lie above the shot noise. Equally, the range of possible occultations could be increased to include configurations with higher solar zenith angles than the baseline. Note however that neither the background from auroras, stellar radiation and non-Rayleigh scattering, nor interference speckle have been considered, and that the actual received signal power and shot noise are likely to be lower due to larger optical depths in the real atmosphere and less favourable parameters in the real system. Also, the full assembly of detector and amplification circuit would need to be considered to evaluate the real optical receiver noise current. Figure 6: Outcome of preliminary performance assessment for IPDALLS. Altitude is given for the tangent point of the limb sounding optical path, and values below 5 km have not been computed. (**a**) Atmospheric transmission due to absorption on the forward path (thick lines) and on the backward path (thin lines). The shaded area shows the range over which the relative error approximately doubles with respect to its minimum at a transmission of 0.33. (**b**) Atmospheric transmission calculated for Rayleigh scattering alone (black lines) and geometric mean of the forward and backward refractive dilution factor for the offline wavelength (magenta). (**c**) Radiative signal pulse energy returned to receiver and incident upon detector. Figure 7(c) shows the relative random error at maximum sampling resolution for single shot measurements. For each wavelength pair, the random error increases at low altitude because of a weak CNR and at high altitude because of decreasing differential absorption. Over the entire measurement range defined in the mission objectives, the modelled random error on the two-way path-averaged water vapour optical depth varies between 0.2% and 3%, with a significant contribution of refractive dilution to the error within the UTLS. Optimizing the choice of wavelengths may still improve these results and narrow the current gaps between the optimal retrieval ranges. Since the model atmosphere used to simulate transmission after molecular absorption held about 6 ppmv of water vapour towards the upper  
Figure 7: **(a)** Noise currents modelled for the IPDALLS receiving system.