ESA GNC Conference Papers Repository
Prisma AOCS subsystem design with high agility and pointing performance for EO missions
PRISMA (PRecursore IperSpettrale della Missione Applicativa) is an Earth Observation satellite for the Italian Space Agency combining hyperspectral and panchromatic sensors in a single camera. The payload acquires images of the Earth simultaneously in a large number of spectral bands using an imaging spectrometer in a push-broom mode of operation. The spacecraft has a launch mass of about 800 kg and it is placed on a frozen sun-synchronous repetitive orbit at 620 Km of altitude. Basing on the heritage of AGILE for the initial attitude acquisition logic and safe mode, AOCS subsystem architecture has been designed in order to achieve a much higher pointing performance and agility with an increase of more than one order of magnitude (APE below 100 arcsec 3sigma), with the fine control modes to support payload activities. Control performance has been upgraded with the final version of the PRISMA design that will lead to its launch in mid-2018, which includes a great step forward on the agility of the platform, that is now able to acquire multiple target in sequence with the payload in an enlarged worldwide area of interest, with a much more performing re-pointing capability able to achieve the worst case Sun-Target pointing maneuver (and vice versa) in less than 4 minutes. Platform also includes an increased momentum storage that allows larger maneuver than before and long fine pointing period without the need of wheel damping. Design upgrade also includes an achievement of the geo-pointing and geo-location requirement of the Payload products with the concurrent use of AOCS sensors (APS star sensors, gyroscopes, GPS, magnetometers and sun sensors) in the different navigation modes in the AOCS algorithms, and actuators (four reaction wheels in pyramidal configuration, magnetic torquers, thrusters), to achieve control capability against all disturbance torque and reduced error during maneuver guidance following phase. Requirements of PRISMA mission and of other missions currently under study in OHB-Italia have been merged to create an envelope on which to base an AOCS high level architecture design. This design can be used for a range of different missions, when high pointing performance and a high agility is required, as for Earth and Deep Space Observation missions. This is done through an appropriate design of the AOCS logics implemented in the AOCS state machine, which has been ideated to support this wide range of possible missions.