## 3 Mission Overview
### Measurement Sequence and Coverage  
A typical measurement sequence for the nadir LiDAR and limb sounding observations lasts for a total of 540.6 seconds; this is depicted in Figure 2. The sequence comprises the time for nadir LiDAR observations, tracking and locking of a retroreflector, calibration of the IPDALLS system and finally the limb measurement itself. The sequence is designed so that the nadir and limb measurements are roughly collocated (within 200 km; see Figure 4 and the discussion thereof, below). In order not to overstretch power and heat evacuation requirements and to maximize the outgoing pulse energy, the nadir and limb instruments are powered sequentially. The sequence begins at time \(t_{0}\) with the nadir LiDAR operating at a nominal pulse repetition frequency (PRF) of 25 Hz for 324 s, covering a ground track of 2,244 km, the midpoint of which corresponds to the tangent point of the following limb sounding and approximately to the orbital crossing of the primary and secondary spacecraft. At time \(t_{1}\) a search and lock procedure (described in Section 5.4) is initiated to establish the optical link between the instrument and the retroreflector; this optical communication procedure can take no longer than 122 s. Once the link is established, it will be maintained by an internal tracking mechanism within the IPDALLS instrument. The limb occultation measurement starts at time \(t_{2}\) with 'free space' calibration measurements taken from a height of 250 km down to 100 km (\(t_{3}\)). Scientific data is gathered for 35 s between \(t_{3}\) and \(t_{4}\), from a height of 100 km down to ground level, although in practice the link will be terminated at optically thick cloud top height (estimated between 5 and 15 km according to the latitude and season), and data may not all be downlinked due to communication constraints. During the idle time of 35 s between limb sounding termination and the next collocated nadir sequence, the system will resume and dwell in nadir sounding mode, thereby maintaining a continuous laser operation to increase transmitter lifetime, and providing further science data. Figure 2: Sequence of one set of nadir and limb measurements, with the time for each part of the sequence shown in seconds. The observed region is shown hatched. The outermost orbit is that of the primary spacecraft (PSC; 582 km altitude, shown traveling clockwise), with the orbit of the retroreflector spacecraft just inside (RSC; 550 km altitude, traveling anticlockwise). At time \(t_{0}\), the primary spacecraft begins making nadir measurements. At time \(t_{1}\) the nadir measurement ceases, and the primary spacecraft searches for and locks onto the retroreflector. Limb measurements begin at \(t_{2}\) with calibration, then the collection of scientific data from \(t_{3}\) to \(t_{4}\). There is then some free time before the start of the next measurement sequence as the next retroreflector comes into view (shown in grey). Ray bending due to atmospheric refraction is unaccounted for. Figure 3(a) shows the simulated coverage for one day of measurements with five retroreflectors, with the measurement tracks coloured according to time of day (UTC). Observations are global, with the largest orbit step or measurement gap of about 2500 km around the equator, and highest sampling density over the poles (although a certain proportion of these will be lost when the instrument is pointing directly at the Sun). Figure 3(b) shows the coverage over 10 days, and Figure 3(c) over 21 days of operations. Three weeks' sampling produces almost global coverage, with the largest gaps at the equator reduced to around 500 km, and simultaneously the sub-diurnal sampling has been improved. Coverage will always be limited by the narrow ground track of the LiDAR system. In the case of the loss of one retroreflector, global coverage after 21 days is not significantly affected (Figure 3(d)).