# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 5 L2-CalSat Operation
### Calibration Strategy  
In order to describe the calibration, first, it is needed to define the positioning maneuvers. L2-CalSat must be inside the FoV of the CMB-Sat during the calibration time and move away at finishing. Consequently, four impulses are required for each calibration manoeuvre (two to start movement and two to stop the calibration satellite). Figure 10 shows a diagram of the relative locations between L2-CalSat and CMB-Sat during the different phases of the mission.  
Three mission phases are considered and constitute the timeline of the mission and its operations. The main of them is the Formation Flying Phase (FFP) that is active once L2-CalSat is in the vicinity of the CMB-Sat, after its deployment from the service platform. As it has been previously noted, the calibration satellite is expected to be equipped with RF ranging sensors, allowing the reconnaissance of the main spacecraft, and also the accurate determination of the relative position between both satellites.  
In this FFP, three sub-phases are defined. The first of them is the Stand-by orbit (FFP-SBO). During the periods between calibrations, L2-CalSat will be in a waiting orbit, out of the field of view of CMB-Sat. In this phase, the relative position of the satellites will be maintained with a loose accuracy. The range of variation of the relative distance will be defined in order to minimize the need of propellant, while ensuring safety of both spacecrafts and a quick transition to the Calibration phase (FFP-CAL). In this second sub-phase, L2-CalSat is accurately placed with respect to a previously defined reference position of CMB-Sat, and maintained in a relative position and orientation, enabling L2-CalSat to periodically enter in the FoV of CMB-Sat. These periods are used for calibration of the CMB-Sat instrumentation. The calibration satellite design will ensure metre and arcmin relative position accuracy, which guarantees a proper a priory knowledge of the received signal, provided that the calibration payload constantly measures its output power. These measurements, properly time-stamped, together with AOCS and housekeeping information, will be transmitted to CMB-Sat which will send them to the ground for post-processing and calibration of the scientific data. The last Formation Flying sub-phase is the Positioning (FFP-POS). This is a transition phase in which L2-CalSat either enters FFP-CAL from FFP-SBO, or abandons FFP-CAL returning to FFP-SBO. This phase is entered either by ground command or by a timed-tagged or orbit-driven event. The manoeuvres needed for permitting transferring from FFP-SBO point to FFP-POS point and vice versa, will be optimized taking into account the specific L2 environment.  
Additionally to FFP, in every phase of the mission, a Safe Escape Maneouvre (SEM) will be available. The design of Fault Detection, Isolation and Recovery (FDIR) of L2-CalSat will enable constant monitoring of the ranging sensors and the health status of the communications. In case of failure of any of them, or whenever a close approach to CMB-Sat is detected, a SEM will be triggered by making use of the remaining active positioning systems. The propulsion module will be commanded by the on-board computer (OBC) in order to provide a pre-determined thrust in a safe direction, thus avoiding collision with CMB-Sat or interference with its measurements. This manoeuvre will also be reachable by ground command.  
Finally, the Disposal phase will ensure reaching a final orbit diverging from the one of CMB-Sat, so that avoidance of collision is ensured even beyond the lifetime of the satellites.  
Figure 10: Diagram of the relative locations between the CMB satellite and L2-CalSat during the different phases of the mission.  
This phase will be reached by ground command, either following the end of the useful life of the satellite, or upon determination of a non-recoverable failure.