Proba-3 mission
The Proba-3 mission will provide an opportunity to validate the metrology and actuation techniques and technologies and to develop and dynamically verify the guidance strategies and navigation and control algorithms necessary for formation flying during future science, astronomy and Earth observation missions. It will also test a wide range of command and control strategies and provide development and verification tools and facilities.
Financial and launcher considerations have helped focus the design on a two spacecraft mission and the development effort on the fine control system, meanwhile providing valuable scientific return from the mission.
In the concept considered, the spacecraft pair will fly a highly elliptical orbit divided between periods of accurate formation flying, when payload observations will be possible, and periods of free flight. The length of the formation control period will be the result of a trade-off involving the amount of fuel needed to maintain the orbits when in formation. For the perigee (nearest to Earth) segment of the orbit, the spacecraft will revert to normal, gravitationally determined orbits to reduce fuel consumption, with the thrusters then being used only for collision avoidance.
The perigee pass will also be used for demonstration of formation flying configurations required for low Earth orbit observation missions.
With this orbit and control regime it is expected that a science lifetime of nearly two years can be achieved with the fuel that can be carried within the launch mass constraints.
Guidance and control system
The relative positions of the two craft are determined by S-band radio metrology, which functions for separations between 5 metres and 8 kilometres, with an accuracy of a few centimetres. Increased accuracy for separations up to 500 metres will be obtained using optical laser techniques having both coarse and fine sensors to refine the relative position measurements to an accuracy of hundreds of microns.
Multiple thrusters, using either cold gas or ion technology, will be used to maintain the required relative positions.
Both spacecraft will carry GPS receivers to provide timing synchronisation. It is known that the GPS signals can be used at least 25 000 kilometres from Earth, so the signals should be useful for about one third of the proposed orbit. Each spacecraft will employ a star-tracker for absolute attitude determination.
