Changes between Version 14 and Version 15 of TBR/UserApp/Space/Proba_2

Nov 29, 2011, 9:02:34 PM (8 years ago)


  • TBR/UserApp/Space/Proba_2

    v14 v15  
    5 [[TOC(TBR/UserApp/Space/Proba_2, depth=2)]]
     4d==ESA Proba-2==
    76'''Proba-2''', Proba stands for PRoject for OnBoard Autonomy. The Proba satellites are among the smallest spacecraft
    87ever to be flown by ESA, but they are making a big impact in the field of space technology. Proba-2 is the
    98second of the series, building on nearly eight years of successful Proba-1 experience.
    6060of attitude maneuvers. Furthermore, it performs all required navigation and maneuvering computations
    6161onboard. The spacecraft platform provides full redundancy
    62 = PROBA 2 platform =
    63 = Mechanical and thermal =
     63[wiki:File:PROBA2_Auto11.jpeg File:PROBA2 Auto11.jpeg]
     64PROBA 2 block diagram
     66= 2.1. PROBA 2 platform =
     67= 2.1.1. Mechanical and thermal =
    6569The PROBA 2 structure is derived from the PROBA 1 structure and is compatible with launchers such as
    8589[wiki:File:PROBA2_Auto14.jpeg File:PROBA2 Auto14.jpeg]
     90Figure 3 PROBA 2 internal structure and accommodation
     922.1.2. Attitude control and Navigation system
     93The PROBA 2 ACNS is strongly based on the PROBA 1 ACNS. The latter was a complex system providing (i)
     943-axis attitude control including high accuracy pointing and maneuvering capabilities in different pointing
     95modes, (ii) full spacecraft attitude control based only on target oriented commands and (iii) the demonstration of
     96new technologies. Furthermore, it was developed relying heavily on the use of Computer-Aided Software
     97Engineering tools. The PROBA 2 ACNS includes the full PROBA 1 ACNS, with the additional functionality
     98to support the solar observation mission. This includes an improved Sun-model and the possible inclusion of a
     99sun-sensor in the control loop. Furthermore, the ACNS incorporates a technology demonstration of a series of
     100new algorithms:
     101 *  low-cost determination of the attitude and orbit using temperature, light and/or magnetic-field
     103 * the use of a Square-Root Unscented Kalman Filter (SR-UKF) for attitude and orbit
     105 *  autonomous, high-precision, recurrent largeangle manoeuvre capability during the Sun-
     106Observation Mode to avoid star-sensor blinding by the Earth
     108 Finally, the ACNS functions support automatic “image paving” for the Sun-Imaging instrument (SWAP)
     109 in order to increase its actual field of view. PROBA 2, as PROBA 1, has been fitted with a highaccuracy
     110 double head star tracker, with GPS receiversand with a set of reaction wheels for the nominal ACNS
     111operation. This set of sensors and actuators is complemented with the magnetotorquers and 3-axis
     112magnetometers. As explained above, PROBA 2 carries as well an additional star tracker, an additional GPS, an
     113additional magnetometer and a Sun Sensor as technology demonstrations.
     115As on PROBA 1, the star tracker is the main attitude determination sensor. It provides full-sky coverage and
     116achieves the high accuracy required for Sun pointing. The sensor can autonomously reconstruct the
     117spacecraft’s inertial attitude starting from a “lost in space” attitude with a performance of a few arc-seconds
     118up to an arc-minute. The attitude can be reconstructed at relatively high inertial rates, which allows the ACNS
     119software to perform gyro-less rate measurements sufficiently accurately to control large-angle precise and
     120stable manoeuvres. The model selected to fly on PROBA 2 is the micro-autonomous stellar compass (m-
     121ASC), a next generation of the star tracker to that flown onboard PROBA 1. It requires less electrical power,
     122has a lower mass and smaller volume, can connect to 4 camera heads instead of to 2 (although only 2 are used
     123in PROBA 2) and provides attitude output at 4 Hz instead of 2 Hz. The star tracker is provided by the
     124Technical University of Denmark. Orbit and time knowledge is acquired autonomously
     125from measurements performed by a GPS receiver. As a technology demonstration, PROBA 2 flies a redundant
     126set of Phoenix GPS receivers provided by DLR.
     128[wiki:File:PROBA2_Auto2.jpeg File:PROBA2 Auto2.jpeg]
     129Phoenix GPS architecture for PROBA-2
     131It is a miniature receiver specifically designed for high dynamics space applications. It is based on SigTech’s
     132commercial-off-the-shelf MG5001 receiver board but operates a proprietary firmware developed by DLR.
     133Though originally designed for automotive applications, the receiver board has been qualified for space use in a
     134series of thermal-vacuum, vibration and total ionization dose tests. The receiver employs a GP4020 baseband
     135processor which combines a 12 channel GP2021 correlator and an ARM7TDMI microprocessor kernel.
     136At a power consumption of less than one Watt and a board size of 50 x 70 mm the receiver is among the
     137smallest of its kind and particularly well suited for satellites with limited onboard resources. The Phoenix
     138receiver is extensively used in European sounding rocket missions and has been selected for various other
     139micro-satellite missions in low Earth orbit (LEO) such as X-Sat, ARGO, Flying Laptop and PRISMA. Specific
     140features of the Phoenix receiver software for LEO applications include optimized tracking loops for high
     141accuracy code and carrier tracking, precision timing and integer ambiguities for carrier phase based relative
     142navigation, a twoline elements orbit propagator for signal acquisition aiding, and an attitude interface to
     143account for non-zenith pointing antennas in the channel allocation process. A pulse-per-second signal enables
     144synchronization to GPS (or UTC) time with an accuracy of better than 1ms. Noise levels of 0.4 m (pseudorange)
     145and 0.5 mm (carrier phase) at representative signal conditions (C/N0=45dB-Hz) have been demonstrated in
     146signal simulator and open air tests which render the receiver suitable for precise orbit determination. While
     147the instantaneous (kinematic) navigation solution is restricted to an accuracy of roughly 10m (3D rms) due
     148to broadcast ephemeris errors and unaccounted ionospheric path delays, an accuracy of about 0.5-1m
     149can be achieved in a ground based precise orbit determination.
     151The orbital information allows pointing of the  spacecraft towards any point on Earth (by using as well
     152an onboard Earth-rotation ephemeris calculator), to autonomously determine the optimal moments for a
     153high-angle maneuver to avoid sensor blinding by the Earth and to perform accurate Sun-pointing.
     154The generation of control torques is by means of four reaction wheels (Dynacon, Canada) mounted in a
     155tetrahedron configuration. Their inertia capacity is 0.65 Nms and their maximum torque capacity is 30 mNm.
     156The reaction wheels are an evolution of those used on the Canadian MOST mission.
     158All ACNS sensors and actuators are controlled by the ACNS software running on the central LEON based
     159computer and provides functions including:
     160 *  Navigation (NAV) which consists in the onboard Kalman filter based autonomous
     161estimation of the orbit using GPS measurements and the on-board autonomous
     162determination of the attitude using data from the star tracker, digital Sun sensor and
     163magnetometers. The navigation function also includes the prediction for all the mission
     164related orbital events (eclipses, next Earth target passages, next ground station flybys, Earth exclusion angle etc…).
     165 *   Guidance (GDC) which consists in the onboard autonomous generation of the commanded reference attitude profiles and
     166manoeuvres, depending on the spacecraft operational mode. The guidance function also includes the computation of the control error,
     167the difference between the desired and the current, estimated, dynamical state.
     168 *   Control (CTL) which consists in the determination and execution of the necessary control commands that will bring the current
     169dynamical state of the spacecraft coincident with the desired state. The control function also includes the maintenance of internal
     170dynamic variables within specified boundaries (e.g. reaction wheel speed).
     171 *   Failure Detection & Identification (FDI) which consists in monitoring the inputs, the internal
     172and output variables and parameters of the AOCS software to test them for numerical and/or physical validity.
     173Furthermore, to increase the pointing accuracy of the SWAP instrument, the AOCS SW also provides inflight
     174compensation of thermo-elastic misalignments of the star tracker relative to the instrument.
    86206= Providing flight opportunities =