# CMAG: A Mission to Study and Monitor the Inner Corona Magnetic Field
## 2 Mission Concept
### Mission Profile
The main spacecraft, which also carries the occulter, will act as a communication relay to transmit the necessary CMAG data products to the Science Operation Centre. No dedicated communications transceiver between CMAG and the ground segment is necessary, although a low-bit-rate data link with the ground station is foreseen for housekeeping, telemetries, and telecommands. The primary spacecraft, which carries the two-meter radius occulter (making it the main and likely heavier of the two spacecrafts), can also accommodate remote or in situ instrumentation. However, this does not imply that communications with the ground will be established directly from CMAG. It can be devised as a stand-alone mission as well. The baseline for the CMAG platform is close to a service-like module with a weight of 300 kg and a maximum power consumption of 100 W, excluding any potential structural redesign or additional propellant needs. For the Attitude and Orbit Control System (AOCS), CMAG will carry star trackers and reaction wheels for a fine 3-axis-stabilized pointing mode, combined with coarse rate and Sun sensors for safe operations. A cold gas propulsion system will be used for both relative position control and reaction wheel unloading, ensuring formation flight. The CMAG spacecraft will be able to generate power when outside the occulter shadow and will have active thermal control. The battery capacity will be sized for the foreseen scientific operations, which take place within the shadow cast by the main spacecraft. A metrology sensor suite is key for mission robustness and for achieving the required performance. Metrology must be accurate enough to meet the mission's scientific requirements and robust enough, at the same time, to guarantee recovering formation flight from a completely lost state after any potential system failure and a smooth handover between the two spacecraft. Four key elements make these operations seamless: the Formation Flying Radio Frequency (FFRF) metrology, based on differential RF ranging from a reduced set of antennas, provides almost omnidirectional coverage with comparatively low accuracy; the Visual-Based System (VBS), placed on the occulter spacecraft and always pointing away from the Sun's direction, can detect a light pattern on CMAG and is capable of robustly providing lateral relative position estimation with relatively high accuracy, as well as coarse longitudinal estimation while maintaining a wide enough field of view to allow for both the Science and the Stand-By Modes (see below); the Fine Longitudinal System (FLS) laser-based metrology that reflects on a corner-cube retroreflector on the occulter spacecraft and provides high-accuracy longitudinal estimation; and the Shadow Position Sensor (SPS), a limb detector that corrects the lateral positioning of the CMAG spacecraft with respect to the occulter shadow. The last two systems guarantee a spacecraft's positioning within a 1 mm error margin in the perpendicular plane (lateral) to the line-of-sight and along it (longitudinal). Relative positioning stability (lateral) is 0.3 mm, while the absolute pointing error and stability are 2.5\({}^{\prime\prime}\) and 1\({}^{\prime\prime}\) in 1 min, respectively.