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

Timestamp:

11/29/11 21:02:34 (13 years ago)

Author:

Iliyankatsarski

Comment:

TBR/UserApp/Space/Proba_2

@@ -2 +2 @@
 
 
-
-[[TOC(TBR/UserApp/Space/Proba_2, depth=2)]]
+d==ESA Proba-2==
 
 '''Proba-2''', Proba stands for PRoject for OnBoard Autonomy. The Proba satellites are among the smallest spacecraft
 ever to be flown by ESA, but they are making a big impact in the field of space technology. Proba-2 is the
 second of the series, building on nearly eight years of successful Proba-1 experience.
+
 = PROBA 2 MISSION SUMMARY =
 
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 of attitude maneuvers. Furthermore, it performs all required navigation and maneuvering computations
 onboard. The spacecraft platform provides full redundancy
-= PROBA 2 platform =
-= Mechanical and thermal =
+
+[wiki:File:PROBA2_Auto11.jpeg File:PROBA2 Auto11.jpeg]
+PROBA 2 block diagram 
+
+= 2.1. PROBA 2 platform =
+= 2.1.1. Mechanical and thermal =
 
 The PROBA 2 structure is derived from the PROBA 1 structure and is compatible with launchers such as
@@ -84 +88 @@
 
 [wiki:File:PROBA2_Auto14.jpeg File:PROBA2 Auto14.jpeg]
+Figure 3 PROBA 2 internal structure and accommodation
+
+2.1.2. Attitude control and Navigation system
+The PROBA 2 ACNS is strongly based on the PROBA 1 ACNS. The latter was a complex system providing (i)
+3-axis attitude control including high accuracy pointing and maneuvering capabilities in different pointing
+modes, (ii) full spacecraft attitude control based only on target oriented commands and (iii) the demonstration of
+new technologies. Furthermore, it was developed relying heavily on the use of Computer-Aided Software
+Engineering tools. The PROBA 2 ACNS includes the full PROBA 1 ACNS, with the additional functionality
+to support the solar observation mission. This includes an improved Sun-model and the possible inclusion of a
+sun-sensor in the control loop. Furthermore, the ACNS incorporates a technology demonstration of a series of
+new algorithms:
+ *  low-cost determination of the attitude and orbit using temperature, light and/or magnetic-field
+sensors;
+ * the use of a Square-Root Unscented Kalman Filter (SR-UKF) for attitude and orbit
+determination;
+ *  autonomous, high-precision, recurrent largeangle manoeuvre capability during the Sun-
+Observation Mode to avoid star-sensor blinding by the Earth
+
+ Finally, the ACNS functions support automatic “image paving” for the Sun-Imaging instrument (SWAP)
+ in order to increase its actual field of view. PROBA 2, as PROBA 1, has been fitted with a highaccuracy
+ double head star tracker, with GPS receiversand with a set of reaction wheels for the nominal ACNS 
+operation. This set of sensors and actuators is complemented with the magnetotorquers and 3-axis
+magnetometers. As explained above, PROBA 2 carries as well an additional star tracker, an additional GPS, an
+additional magnetometer and a Sun Sensor as technology demonstrations.
+
+As on PROBA 1, the star tracker is the main attitude determination sensor. It provides full-sky coverage and
+achieves the high accuracy required for Sun pointing. The sensor can autonomously reconstruct the
+spacecraft’s inertial attitude starting from a “lost in space” attitude with a performance of a few arc-seconds
+up to an arc-minute. The attitude can be reconstructed at relatively high inertial rates, which allows the ACNS
+software to perform gyro-less rate measurements sufficiently accurately to control large-angle precise and
+stable manoeuvres. The model selected to fly on PROBA 2 is the micro-autonomous stellar compass (m-
+ASC), a next generation of the star tracker to that flown onboard PROBA 1. It requires less electrical power,
+has a lower mass and smaller volume, can connect to 4 camera heads instead of to 2 (although only 2 are used
+in PROBA 2) and provides attitude output at 4 Hz instead of 2 Hz. The star tracker is provided by the
+Technical University of Denmark. Orbit and time knowledge is acquired autonomously
+from measurements performed by a GPS receiver. As a technology demonstration, PROBA 2 flies a redundant
+set of Phoenix GPS receivers provided by DLR.
+
+[wiki:File:PROBA2_Auto2.jpeg File:PROBA2 Auto2.jpeg]
+Phoenix GPS architecture for PROBA-2 
+
+It is a miniature receiver specifically designed for high dynamics space applications. It is based on SigTech’s
+commercial-off-the-shelf MG5001 receiver board but operates a proprietary firmware developed by DLR.
+Though originally designed for automotive applications, the receiver board has been qualified for space use in a
+series of thermal-vacuum, vibration and total ionization dose tests. The receiver employs a GP4020 baseband
+processor which combines a 12 channel GP2021 correlator and an ARM7TDMI microprocessor kernel.
+At a power consumption of less than one Watt and a board size of 50 x 70 mm the receiver is among the
+smallest of its kind and particularly well suited for satellites with limited onboard resources. The Phoenix
+receiver is extensively used in European sounding rocket missions and has been selected for various other
+micro-satellite missions in low Earth orbit (LEO) such as X-Sat, ARGO, Flying Laptop and PRISMA. Specific
+features of the Phoenix receiver software for LEO applications include optimized tracking loops for high
+accuracy code and carrier tracking, precision timing and integer ambiguities for carrier phase based relative
+navigation, a twoline elements orbit propagator for signal acquisition aiding, and an attitude interface to
+account for non-zenith pointing antennas in the channel allocation process. A pulse-per-second signal enables
+synchronization to GPS (or UTC) time with an accuracy of better than 1ms. Noise levels of 0.4 m (pseudorange)
+and 0.5 mm (carrier phase) at representative signal conditions (C/N0=45dB-Hz) have been demonstrated in
+signal simulator and open air tests which render the receiver suitable for precise orbit determination. While
+the instantaneous (kinematic) navigation solution is restricted to an accuracy of roughly 10m (3D rms) due
+to broadcast ephemeris errors and unaccounted ionospheric path delays, an accuracy of about 0.5-1m
+can be achieved in a ground based precise orbit determination.
+
+The orbital information allows pointing of the  spacecraft towards any point on Earth (by using as well
+an onboard Earth-rotation ephemeris calculator), to autonomously determine the optimal moments for a
+high-angle maneuver to avoid sensor blinding by the Earth and to perform accurate Sun-pointing.
+The generation of control torques is by means of four reaction wheels (Dynacon, Canada) mounted in a
+tetrahedron configuration. Their inertia capacity is 0.65 Nms and their maximum torque capacity is 30 mNm.
+The reaction wheels are an evolution of those used on the Canadian MOST mission.
+
+All ACNS sensors and actuators are controlled by the ACNS software running on the central LEON based
+computer and provides functions including:
+ *  Navigation (NAV) which consists in the onboard Kalman filter based autonomous
+estimation of the orbit using GPS measurements and the on-board autonomous
+determination of the attitude using data from the star tracker, digital Sun sensor and
+magnetometers. The navigation function also includes the prediction for all the mission
+related orbital events (eclipses, next Earth target passages, next ground station flybys, Earth exclusion angle etc…).
+ *   Guidance (GDC) which consists in the onboard autonomous generation of the commanded reference attitude profiles and
+manoeuvres, depending on the spacecraft operational mode. The guidance function also includes the computation of the control error,
+the difference between the desired and the current, estimated, dynamical state.
+ *   Control (CTL) which consists in the determination and execution of the necessary control commands that will bring the current
+dynamical state of the spacecraft coincident with the desired state. The control function also includes the maintenance of internal
+dynamic variables within specified boundaries (e.g. reaction wheel speed).
+ *   Failure Detection & Identification (FDI) which consists in monitoring the inputs, the internal
+and output variables and parameters of the AOCS software to test them for numerical and/or physical validity.
+Furthermore, to increase the pointing accuracy of the SWAP instrument, the AOCS SW also provides inflight
+compensation of thermo-elastic misalignments of the star tracker relative to the instrument.
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 = Providing flight opportunities =