## 5 Mission Technical Implementation
### Spacecraft Tracking  
Laser ranging from available ground station networks is performed continuously to accurately track the retroreflector spacecraft. This information is used for updating contact schedules with the primary spacecraft. The retroreflector spacecraft will be positioned in the direction of the primary spacecraft using its attitude control system and positioning data. Retroreflector spacecraft tracking, by the primary \begin{table}
\begin{tabular}{l c c c c} \hline  & **Primary Spacecraft** & **Retroreflector Spacecraft** \\ \hline
**Component** & **Mass (kg)** & **Power (W)** & **Mass (kg)** & **Power (W)** \\ \hline Payload & 1,100 & 1,500 & 44 & – \\ Spacecraft Bus & 1,280 & 700 & 45 & 16 \\ Margin & 600 & 530 & 21 & 4 \\ Spacecraft Dry Mass & 2,980 & – & 110 & – \\ Propellant & 280 & – & 16 & – \\ \hline
**Total** & **3,260** & **2,730** & **126** & **20** \\ \hline \end{tabular}
\end{table} 
Table 2: Mass and power budgets for the primary and the retroreflector spacecraft. spacecraft, commences when the nadir measurements have been completed and continue until the retroreflector spacecraft disappears behind the Earth horizon, approximately 215 seconds later (as shown in Figure 2). The primary spacecraft will receive the positional data of the retroreflector spacecraft to within 0.1 m (via the ground station) and GPS data for its own position. The primary spacecraft performs a search for the retroreflector spacecraft (using its scheduling data) to establish the initial Line of Sight (LOS). The PAT system may incorporate a separate, low-power, high-divergence laser to perform broad search pattern techniques until a signal is received, at which time, measurement can commence using the low-divergence laser. A small proportion of the received power (above 1000 nm) is used to localize the retroreflecting spacecraft within the telescopes FOV for a fast, closed loop efficient tracking procedure. The attitude control system of the primary spacecraft may need to perform coarse movement of the primary spacecraft if a signal is not received but this should be minimised. Fine alignment of the laser is achieved by the movable mirror in the attached assembly, similar to the test bed proposed and tested by [PERSON] _et al._[73]. During tracking minimal mass is moved, avoiding distortions and the need for excessive attitude control. An added restriction is that both telescopes must avoid direct sun light and sun glare from the surface of the earth. This may be achieved by accurate scheduling data provided to the primary spacecraft. The design of the spacecraft is such that the nadir telescope always faces perpendicular to Earth. From the initial design process it is proposed that the limb telescope is positioned beside the nadir telescope facing perpendicular to Earth also. Attached to the limb telescope is an assembly consisting of a movable mirror angled to allow signal to and from the retroreflector (Figure 8). The limb telescope has a very narrow FOV, which is required in order to minimize atmospheric background noise. Because of this restriction, the attached mirror assembly has to be able to pitch up and down along the orbital plane, with microrad accuracy, so that the retroreflector remains in the centre of the FOV. This scanning configuration ensures that the primary spacecraft or its limb telescope do not have to move position every time it needs to track a retroreflector spacecraft, since the retroreflector should be centred within the FOV of the limb telescope via the movable mirror in the attached assembly. An important feature to note about this measurement configuration is that the constellation will never remain in exactly the same orbital plane. Small deviations in inclination from a polar orbit within the specified injection precision may induce a diverging precession of primary and retroreflector spacecraft orbits, imposing additional yaw steering capabilities of the limb FOV.