ESA GNC Conference Papers Repository

The µHETSat project started in the 2016 at SITAEL premises, in order to demonstrate the in-orbit behavior and performances of a small Hall Effect electric thruster. The µHETsat space segment design shall maximize the re-use of flight proven elements from existing project, with the needed adaptations required by the use of an high power (relatively to the considered satellite class) Hall Effect based propulsion subsystem (HET). Motivated by this challenging and reduce the design and development costs the implementation of COTS components have been extensively used. Past developments made by SITAEL in the field of electric propulsion allowed the design of low power, high efficiency HETs that could enable the realization of all electric platforms in the mass range of 100-300 Kg, drastically increasing their expected lifetime and operability. Low power HET performances have already been demonstrated in being a suitable candidate for a dedicated In Orbit Validation that will move their current TRL up to 9. Since one of the main mission goals, is to validate the propulsion subsystem, during the mission it will be possible to: allow the use of the EPS for a minimum of 1000 hours, allow a minimum of 2000 ignition cycles, measure thrust effects and acquire and download an adequate number of parameters to monitor the EPS operation. In order to fulfil the above mentioned requirements a series of modifications with respect to the previous mission are foreseen in order to: maximize the power generation and energy storage without affecting the overall satellite dimensions, improve AOCS performances and allow an adequate transmission link with the ground station. The µHETSat design have been conceived in order to provide a distributed system with a centralized on board computer (OBDH), that it is in charge to manage the system during the nominal operation. The OBDH subsystem shares at system level the fault detection, recovery and isolation (FDIR) management with the TT&C subsystem. The distributed architecture has been designed to include a common processing hardware, based on an ARM Cortex M4 microcontroller and includes CAN BUS interface with CANOpen protocol. The only exceptions are the reaction wheels, which are procured as COTS components provided by an external supplier. As a matter of the fact, the AOCS subsystem is in charge to determine the spacecraft attitude and to maneuver and control the spacecraft attitude in order to achieve the mission purposes. The µHETSat sensors and actuators have been implemented as independent units, in order to follow the distributed architecture philosophy. The units implemented are: GNSS receiver, magnetometers in cold redundancy, Earth sensor in cold redundancy, sun sensors in cold redundancy, a set of coarse sun sensors, four reaction wheels and magnetic actuator (two for each axis) in cold redundancy. The main improvements of the AOCS subsystem, with respect to the previous mission implemented by SITAEL, are: manage different attitudes during sunlight and eclipse phase and use of RWs on three axis instead of MW. The sensors are able to perform the sun-pointing and the three-axis attitude determination as required by the mission phases. The angular rate measurement is deemed to a filter implemented in the on-board software where angular velocity are estimated starting from the measurement of magnetometers. The attitude actuators system is based on three orthogonal magnetic torquers, that is useful for attitude acquisition maneuvers and coarse attitude pointing and is based on a set of reaction wheels for fine pointing. The AOCS main control software is hosted by the OBDH subsystem and has been implemented as a dedicated task. The onboard SW is RTOS based and Real-Time executive for Multiprocessor Systems RTEMS has been selected to provide to the application the required support, in terms of multi-threading standardized access to the hardware resources. In this paper a complete description of the distributed design implemented in the µHETSat will be presented, focusing on the design solution adopted in the current small satellite and COTS components implemented, in order to fulfill the mission purposes.