transponder and UHF transceiver located in the Rover), while the Orbiter has its own X-band subsystem to support the Earth link in the cruise and the Mars operations, as well as its own UHF system to receive the data transmitted from the DM during the EDLP and the Rover during the Mars operations. Since the Orbiter data relay can be used in the descent and landing phase, the amount of data that can be returned to Earth in this phase is much greater than in the Soyuz configuration, and the 30-kpbs required data rate can be met. The option 1 ETO (European Telecommunication Orbiter) is composed of two modules, the ETO-Propulsiom Module and the proper ETO Orbiter . The ETO-PM is based on the Carrier Module (CM) used for the parallel Carrier+DM mission. It is a propulsion stage to carry the propellant for the escape and DSM burns, and it is separated after those burns have been performed. Thus, the structure can be simplified and the on-board avionics can be removed, leaving the commanding to the ETO orbiter. The main advantage of this approach is the improved dry mass budget available for the Orbiter, as the benefit of staging is greater than the penalty of duplicating the thruster equipment. The proposed design can accommodate up to 2200kg of propellant, 1540kg of which in the Carrier. All tanks are off-the-shelf. The configuration of the ETO-Orbiter is derived from a standard GEO platform. The structure is based on a shear web design with propellant tanks accommodated inside the web and the subsystem components mainly accommodated on the outer structural panels. The Unified propulsion system (MON-MMH) uses the same set of equipment as the Carrier, including a 400N engine, and tailored off-the-shelf tanks. The MTO avionic architecture is very close to that of the Carrier/DM spacecraft composite, with the objective of using the same FDIR logic and possibly similar AOCS on-board software. A standard CDMU design is envisaged, tailored with 12 Gbit mass memory and TT C boards adapted to support X-band and UHF communications. The AOCS sensor set comprises star trackers (two optical heads connected to a common electronics box), one internally redundant IMU unit, and six coarse sun sensors.
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