## 5 Mission Technical Implementation
### Orbit Control and Launch Options  
Active orbit control and maintenance is performed by each of the satellites to prevent the counter-rotating orbits drifting apart. Since maintaining the inclination is a crucial issue, a precise \(\Delta\)v budget based on data from [47] has been determined. This includes correction of inclination after orbit injection from the launch vehicle, altitude control by drag make-up, perturbations caused by radiation, and correction maneuvers to maintain the orbit inclination within the required \(\pm\)0.01 degrees for a nominal mission lifetime of 4 years. The \(\Delta\)v budget additionally includes a maneuver to de-orbit the satellite after end-of-life. For the retroreflector spacecraft, an additional \(\Delta\)v item for distribution of the satellites within the orbit is estimated at 11 m/s. This maneuver is performed by altitude change via a Hohman transfer, drift period and return transfer.  
Hydrazine monopropellant has been identified as the best choice. It guarantees a high specific impulse of 200 s, high reliability and low complexity in system design. The \(\Delta\)v requirement analysis resulted in propellant masses of 280 kg for the primary spacecraft and 16 kg for the retroreflector spacecraft. The propulsion subsystem consists of standard components from space heritage manufacturers, including the pressurization system, pressure regulator, pyro-valves, propellant tank, flow control valves and the monopropellant thruster. In the case of the primary spacecraft, four thrusters are employed to achieve redundancy. Dry masses of the propulsion subsystems amount to 32 kg for the primary spacecraft and 2.2 kg for the retroreflector spacecraft.  
As the primary spacecraft and the constellation of retroreflector spacecraft are in counter-rotating orbits, two launchers are required, and we propose using Soyuz for the launch of the primary spacecraft and Dnepr for the five retroreflector spacecraft. In order to distribute the constellation of retroflector spacecraft in orbit, several options can be considered:  
1. Use of a dispenser to position the constellation, such as the one for SWARM [74]. The dispenser would distribute all the retroreflector spacecraft using its own propellant. This option offers an optimal mission lifetime but incurs the cost and design of a dispenser.
2. Use of the two-tier layout in the fairing of Dnepr but no dispenser. In this option, three spacecraft would be placed on the first floor of the fairing, and two spacecraft on the second floor. The spacecraft would have to use their own propellant to achieve the correct distribution. This option shortens the mission lifetime but is cheaper as it does not incur the cost of a dispenser.
3. The final option is a combination of the previous two. Two dispensers, one containing three spacecraft and another containing two spacecraft would be put on the two floors of Dnepr. The dispensers would then distribute the constellation. This option offers an optimal mission lifetime but incurs the cost of two dispensers. The advantage of using this configuration would be a reduced total amount of propellant for the dispensers and hence a potential cost saving for the mission. This option would offer an intermediate solution: same lifetime mission as option 1 but potentially cheaper, a longer mission lifetime than option 2 but more expensive.