Centro di Ricerca Progetto San Marco
Università degli Studi di Roma "La Sapienza"
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AVIO - Space Avionics Lab
1. Work objective
Find a solution for the avionics system of a micro-satellite that is low-cost and easy to implement in a University laboratory.
2. Staff
Prof. F. Curti, Prof. M. Parisse, Giuseppe Russo (Research Engineer), Alessandro Baldoni (Consultant Engineer), Nishantha Costa (Consultant Engineer), Daniele Spano (undergraduate student).
3. Our activities
- Micro-satellite simulator (SACS)
- Momentum-bias mock-up
- Sensors package
- Control strategies
- Radio-Frequency Attitude Control System
- Optical Sensor
3.1. Micro-satellite simulator (SACS)
A complex model, named Satellite Attitude Control Simulator (SACS), has been developed in SIMULINK environment, in order to test several sensors and control strategies developed for a typical University micro-satellite. SACS is a modular package to design different satellite system and sub-system (Attitude GNC, Power, Telecommunications) and environment parameters, which can be easily modified. SACS can be also configured via a Graphical User Interface.
3.2. Momentum-bias mock-up
The objective of this activity has been to assemble a mock-up wheel for a typical University micro-satellite. To this end, SACS model has been used to evaluate possible disturbance torques, several existing devices have been analysed and the range of necessary control momentum has been assessed. SACS has been used again to test the control scheme and to find the optimal momentum. Finally, a trade-off was performed to find the optimal wheel inertia/motor angular rate, taking into account typical satellite size, and a mock-up of the wheel has been designed and assembled.
Further steps in the assembly of the wheel will be the implementation of the thermal control, the assembly of the case, the hardware implementation of the wheel drive electronics (to use the wheel in reaction wheel mode) and the development of an dedicated instrument to measure the wheel imbalances.
Wheel, motor and the mock-up during testing
3.3. Sensors package
The task of this activity has been to develop a compact and simple sensors package able to improve the on-board attitude estimation during all flight phases, which is ideal for a University micro-satellite. Such a package can be implemented by using standard magnetometers, which provide the Earth magnetic field direction, combined with units of colocated Sun and temperature sensors that are distributed optimally on the satellite faces to guarantee a complete coverage. During day orbit three non-coplanar Sun sensors with highest output are used to compute the Sun direction (in order to reduce the effect of noise and albedo) by using the cone intersection method, whereas during eclipse temperature sensors are used to reconstruct the Earth direction and to calibrate the Sun sensors measurements on-board.
Sensor package overview and during testing
3.4. Control strategies
Different attitude control strategies have been studied for the typical mission phases of a University micro-satellite with a 3-axis Earth pointing stabilization. After satellite injection, de-tumbling is executed, using a B-dot control law with magnetic coils, at the same time as the wheel start-up, in order to reduce the overall manoeuvre duration. Once the angular rates are below a fixed threshold, the Earth pointing acquisition manoeuvre is performed in two steps, using both the magnetic actuators and the wheel: first the satellite Z-axis is positioned in a direction orthogonal to the orbital plane and then the stabilized reference frame is acquired manoeuvering around the Z-axis. The Earth pointing attitude control is performed with a simple PD controller: when the system is not controllable and/or observable (during short time periods) the control is switched off and satellite stability resides in the wheel stiffness.
3.5. Radio Frequency Attitude Control System
Following a work of Ravelli that dates back to the 70's, we studied a simple satellite attitude control system based on a single onboard transmitting system and a ground receiving terminal, which can be implemented with a new in-house developed receiver plus a personal computer integrated with commercial boards and operated with dedicated software, thus reducing the onboard equipment complexity and the overall system cost.
The transmitting part of the sensor, located on the spacecraft, consists of a dual baseline antenna system: each baseline is composed by two antennas driven by properly modulated and coded signal, which carries the spacecraft attitude data that, after being received and processed by a ground system, will provide the view angles of the radiating baseline elements as seen from the ground antenna. Attitude data are processed on ground to create a control law: finally a signal with the control strategy is transmitted back to the satellite using the uplink command signal. At the same time the RF signal radiated by the sensor can be used as carrier to convey other information from spacecraft to ground. Preliminary tests run using a dedicated LabView model show that the RF attitude control system is able to provide the information on attitude angles with an accuracy of few tenth of a degree
RF sensor simulator in LabView environment
3.6. Optical Sensor
The scope of this activity is the development of an optical sensor system, suitable for attitude and position navigation of both rendezvous spacecrafts and landing probes. Several computer vision techniques have been investigated and the software implementation of the algorithms have been tested using simulated and real rendezvous videos. The feasibility study for the hardware implementation of the system has also been started, with the aim of reducing the navigation cycle and improving the sensor stability and robustness. Further studies should investigate the development of a hardware simulation environment, to test the sensor in both the rendezvous and landing scenarios.
4. Future developments
Our future activities will be focused on the implementation of the on-board computer mock-up, the completion of the wheel assembly and the implementation of an experimental platform to test the developed space avionics systems.
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