Thermal Environment The bus needs to cope with a wide range of thermal environment, from Near Earth conditions with Sun and Earth contributions (hot case) to Mars conditions where eventually eclipses and reduced solar flux give rise to cold case issues. The average solar flux at Mars orbit is 589 W/Sq.m, or about 42% of what is experienced by an Earth-orbiting spacecraft. As a result of the eccentricity of Mars orbit, however, the solar flux at Mars varies by +/- 19% over the Martian year, which is considerably more than the 3.5% variation at Earth. Radiation Environment The main frame bus elements and payloads are basically designed for interplanetary missions capable of operating in Earth Burn Manoeuvres (EBN), Mars Transfer Trajectory (MTT) and Martian Orbit (MO) environments. The bus unit components are selected with respect to a cumulated dose of 6 krads below 22 AWG aluminium shielding. Parts have been considered as directly suitable, if they have been evaluated successfully up to 12 krads (margin factor of 2). Communication Systems The communication systems for the Mars mission are responsible for the challenging task of communication management up to a distance of 400 million km. It consists of Telemetry, Tracking and Commanding (TTC) systems and Data transmission systems in S-band and a Delta Differential One-way Ranging (Δ-DOR) Transmitter for ranging. The TTC system comprises of coherent TTC Transponders, TWTAs (Travelling Wave Tube Amplifiers), a near omni coverage antenna system, a High Gain Antenna system, Medium Gain Antenna and corresponding feed networks. The High Gain Antenna system is based on a single 2.2 meter reflector illuminated by a feed at S-band. Power System One of the major challenges in the design of power system is due to the larger distance of the satellite from the Sun. The power generation in Mars orbit is reduced to nearly 50% to 35% compared to Earth’s orbit. The power bus configuration comprises of a single wing of solar array with 7.56 m2 area generating about 840 W during sunlit and normal incidence in Martian orbit, and a 36 Ampere-Hour Lithium-Ion battery supports the power load during launch phase, initial attitude acquisition, eclipse, Earth burns, MOI, safe mode and data transmission phases. Proplusion System Proplusion System consists of one 440N Liquid Engine and 8 numbers of 22N thrusters. The propellant tanks have combined storage capacity up to 852 kg propellant. The 22N thrusters are used for attitude control during the various activities of the mission like, orbit raising using liquid engine, attitude maintenance, Martian orbit maintenance (if any) and momentum dumping. As the critical operation of Martian Orbit Insertion with Liquid Engine burn occurs after 10 months of launch, suitable isolation techniques are adopted to prevent fuel/ oxidiser migration issues. On-board Autonomy Given that the Round-trip Light Time (RLT) from Earth to Mars can vary anywhere between 6 to 43 minutes, it would be impractical to micromanage a mission from Earth. Due to this communications delay, mission support personnel on Earth cannot easily monitor and control all the spacecraft systems in real-time basis. Therefore, the configuration includes the use of on-board autonomy to automatically manage both the nominal and non-nominal scenarios on-board the spacecraft.
Posted on: Mon, 22 Sep 2014 13:26:48 +0000