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    • 专利摘要:
    N U ABSTRACT The present invention relates to a method and spacecraftarrangement for autonomous control of an angular momentum of aspacecraft, wherein the spacecraft comprises means for storingangular momentum, a set of at least two propulsion unitsarranged to independently provide thrust to the spacecraft,and a control unit operatively connected thereto, wherein themethod comprises the steps, calculate the angular momentum ofthe spacecraft, calculate a projected angular momentum,provide change of the attitude of the spacecraft an offsetangle and provide thrust by activating said propulsion unit,wherein the step of providing change of the attitude of thespacecraft must be started before the step of providing thrust by activating said propulsion unit is terminated. Publication picture: Figure 2a
    公开号:SE1150382A1
    申请号:SE1150382
    申请日:2011-05-02
    公开日:2012-11-03
    发明作者:Camille Chasset
    申请人:Svenska Rymdaktiebolaget；
    IPC主号:
    专利说明:

    NUMETHOD FOR AUTONOMOUS CONTROL OF THE ANGULAR MOMENTUM OF ASPACECRAFT IN CONNECTION WITH AN ORBIT TRANSLATIONAL MANOEUVREAS WELL AS A SPACECRAFT ASSEMBLYTECHNICAL FIELDThis invention relates in general to the field of autonomouscontrol of a spacecraft. Furthermore, the present inventionrelates specifically to a method and an assembly forautonomous control of the angular momentum of a spacecraft inconnection with an orbit translational manoeuvre.
    BACKGROUNDDuring the (S/C),several orbit translationallife of a spacecraft such as a geostationarytelecommunication satellite,manoeuvres need to be performed, and these orbit translationalmanoeuvres are commonly designated “repositioning manoeuvres”in connection with a geostationary orbit. A repositioningmanoeuvre usually comprises numerous steps of accelerating theS/C along the track (velocity) direction. Positive andnegative acceleration will decrease and increase,respectively, the drift with respect to the desired position.
    Repositioning manoeuvres are utilized in several phases duringthe life of the S/C:- First operational slot insertion, when it is not possibleor desirable to insert the S/C into its final operationalposition, which in connection with a geostationary orbitis called slot, with the launch vehicle directly.
    - Repositioning of the S/C from a first to a second slot,by exiting the old slot and then drifting along a numberof slots before insertion into the new slot.
    - At the end of the mission, perform an orbit raising thatwill cause the S/C to exit the slot and enter a higherorbit, a so called graveyard orbit.
    In order to perform repositioning manoeuvres the S/C comprisesa number of propulsion thrusters, which are used to exertexternal force on the S/C by means of mass flow.
    There are at least two known types of S/C propulsionthrusters, electrical and chemical. Chemical thrusters providethe required thrust for translating the spacecraft from aninjection orbit to a geostationary orbit and are capable ofexerting a substantial force on the S/C. However, chemicalthrusters expend a great amount of mass (propellant) during arepositioning manoeuvre in order to achieve a predeterminedon the other hand, createbut theyposition. Electrical thrusters,significantly less thrust than chemical thrusters,expend much less amount of mass (propellant) when activated.Thus chemical thrusters provide high thrust but with lowpropellant efficiency whilst electrical thrusters provide lowthrust but with high propellant efficiency.
    The force from the propulsion thrusters on the S/C is appliedin a direction not passing through the centre of mass of theS/C. However, a force applied to a S/C in a direction notpassing through the centre of mass position creates torque andThechange of angular momentum of the S/C will cause the S/C totherefore a change of the angular momentum of the S/C.rotate uncontrollably, however, in order to avoid uncontroll-able rotation said change of angular momentum is in the artstored by arrangements including means such as momentum wheelsand/or reaction wheels, or the like. However, in course oftime the angular momentum transferred onto said means causesthe angular speed of each of the reaction wheels and/ormomentum wheels to gradually increase until a maximum allowedangular speed is reached, a so called “saturation limit” ofthe reaction wheel.
    In EPl227037Bl a thrust system for S/C station acquisition,station keeping and angular momentum unloading is disclosed.This system utilizes special arrangements of the thrusters forthe purpose of angular momentum unloading.
    Another document EPO780299Bl discloses gimballed thrusters forangular momentum unloading. By changing the direction of theNUapplied force through the gimballed thrusters the necessarytorque for angular momentum unloading can be created.
    The abovementioned prior-art solves the general problem ofangular momentum build-up through special arrangements of thepropulsion thrusters. However, these special arrangements ofthe propulsion thrusters render the S/C to be complicated andthe special arrangements adds mass to the S/C, increasing thelaunch costs, etc.SUMMARYThe present invention aims at obviating the aforementioneddisadvantages of previously known methods and arrangements forcontrolling the angular momentum of a S/C in connection withorbit translational manoeuvres, and at providing an improvedmethod for controlling the angular momentum in connection withorbit translational manoeuvres. A primary object of theinvention is to provide an improved method of the initiallydefined type that is both simple and needs no specialarrangements added to the S/C.According to the invention at least the primary object isattained by means of the initially defined method and S/Cassembly having the features defined in the independentclaims. Preferred embodiments of the present invention arefurther defined in the dependent claims.According to a first aspect of the present invention, there isprovided a method for controlling the angular momentum of aspacecraft in connection with orbit translational manoeuvresof the initially defined type, the spacecraft comprising:- means for storing angular momentum,- a set of at least two propulsion units arranged toindependently provide thrust to the spacecraft, and- a control unit operatively connected to the means forstoring angular momentum and to the at least two propulsionunits.
    NUThe method comprises the following set of steps which isperformed sequentially for each of the at least two propulsionunits:- calculate said angular momentum of the spacecraft,- calculate a projected angular momentum by projecting thecalculated angular momentum onto a direction that isperpendicular to the thrust direction of the propulsionunit, and perpendicular to a mean torque direction of saidset of at least two propulsion units,wherein the following two steps are performed after the twoabovementioned calculation steps,- provide change of the attitude of the spacecraft by thefollowing sub-steps:o calculate an offset angle about the thrust directionof the propulsion unit, which offset angle isproportional to said calculated projected angularmomentum,o cause the spacecraft to turn about the thrustdirection of the propulsion unit said offset anglein a first direction of rotation by manipulatingsaid means for storing angular momentum,- provide thrust by activating said propulsion unit,wherein the step of providing change of the attitude of thespacecraft must be started before the step of providing thrustby activating said propulsion unit is terminated.
    This method will enable the S/C to on-board autonomouslycontrol the angular momentum of the S/C during an orbittranslational manoeuvre, by means of turning the S/C an offsetangle about the thrust direction of the propulsion unit, thusno significant change in orbit and attitude is taken place.
    BRIEF DESCRIPTION OF THE DRAWINGSFigure la is a schematic view of a S/C in orbit around acelestial body, i.e. the earth,Figure lb is a schematic perspective view of the S/C of figurel showing a SLO-frame and a S/C-frame,NUFigure 2a is a schematic perspective view from above of theS/C,Figure 2b is a schematic perspective view from north-east ofthe S/C,Figure 2c is a schematic perspective view from south-west ofthe S/C,Figure 3 is a schematic illustration of the activation of apropulsion unit,Figure 4 is an illustration of the forces and vectors actingon the S/C,Figure 5 is a flowchart illustrating the inventive method.
    DETAILED DESCRIPTIONFigure la shows a schematic view of an S/C, generallydesignated l, in an orbit 2 around a celestial body 3, i.e.the earth. A common way to refer to objects in orbit around acelestial body is to use a coordinate system 4 fixed in space,but centred in the celestial body 3, a so called inertialframe. The orientation of the S/C l relative the inertialframe is called the inertial attitude of the S/C.
    Reference is now made to figure lb. In case one of the axes ofthe S/C is pointing towards the celestial body, it isconvenient to use a coordinate system centred in the S/C bodywith axes that are fixed relative the celestial body. Such acoordinate system is for example the S/C local orbit frame(SLO-frame). The SLO-frame is centred in the S/C, with the z-in aaxis pointing towards the celestial body 3, i.e.direction called the nadir direction, and the x-axis pointingin the velocity direction of the S/C, finally the y-axis isperpendicular to the x- and z-axis.Reference is now also made to figures 2a-2c, which showschematic perspective views of a S/C l. The S/C l comprises amain S/C body 5 with six panels 6-ll and two solar panels 12,l2'. thepanel designated 9 is called the south panel,The panel designated 8 is called the east panel,the paneldesignated ll is called the west panel and the paneldesignated lO is called the nadir panel.
    In order to describe the properties and directions of the S/Cl a local coordinate system fixed to the geometry of the S/Cthis coordinateis used, system is commonly designated S/C-frame. The x-axis of the coordinate system is normallyoriented in the velocity direction of the S/C; the z-axis isnormally oriented in the nadir direction and finally the y-axis is oriented perpendicular to the x- and z-axis.
    It is often desirable to have one of the panels 6-ll of theS/C to point towards the celestial body 3; this panel can forexample include radio antennas or cameras, in this embodimentthe nadir panel lO facing the nadir direction is used for thispurpose.
    During a mission of the S/C, such as a geostationaryseveral orbit translationalInitially the S/C l needs totelecommunication satellite,manoeuvres needs to be performed.be placed into a final operational position, at some timeduring the mission the S/C may need to be repositioned and atthe end of the mission the S/C l needs to be removed from itsfinal operational position and placed in a so called graveyardorbit, these orbit translational manoeuvres are commonlydesignated “repositioning manoeuvres” in connection with ageostationary orbit. These orbit translational manoeuvres allneed external forces that acts on the S/C to cause positive ornegative acceleration, such external forces are created bymeans of a propulsion system.
    According to the invention the S/C comprises at least twopropulsion units. In the shown embodiment four propulsionunits l3-l6 are mounted on the east panel 8 of the S/C andfour propulsion units l7-20 are mounted on the west panel llof the S/C,directionsthe propulsion units having their thrustarranged around the nadir direction. During normaloperational conditions each propulsion unit presents thrustvector components in directions perpendicular to the orbitalplane and tangential to the S/C Velocity vector in theinertial frame.
    The direction of the thrust vectors of the propulsion units(CoM) ofS/C 1 is within a volume bounded by the thrust vectors ofare selected such that the lifetime centre of massthethe propulsion units, thereby as long as the CoM is withinthis volume there is always a possibility to exert torqueabout the CoM by the use of any of the propulsion units.Two different types of electrical propulsion (EP) thrustersare used in the S/C 1. The normally used propulsion units arethe EP thrusters designated 14, 15, 18, 19 of HEMP type (HighEfficient Multistage Plasma). The other propulsion units areEP thrusters designated 13, 16, 17, 20 of HET type (HallEffect Thrusters) and they are used for purpose of redundancyand are normally not used. It is a requirement that allorbital manoeuvres can be performed using a single type ofthrusters (i.e. only HET thrusters or only HEMP thrusters).The propulsion units are arranged in pairs of propulsionunits, a first pair of propulsion units is formed by thepropulsion units designated 16 and 17, a second pair ofpropulsion units is formed by the propulsion units designated14 and 19,propulsion units designated 13 and 20,a third pair of propulsion units is formed by theand a fourth pair ofpropulsion units is formed by the propulsion units designated15 and 18.propulsion thrust direction can be defined as well as a meanFor each of these pairs of propulsion units a meanthrust torque direction.
    In order to predict the build-up of the angular momentum ofthe S/C it is necessary to estimate the disturbance torquescaused by the offsets of the thrust vectors from the S/Ccentre of mass. Estimations of these torques are suitable toperform in the SLO-frame. The torque from each propulsion unitis computed in the SLO-frame under the following assumptions:- The thrust force from the propulsion unit is aligned tothe SLO frame x-axis (velocity direction).- The S/C frame z-axis is aligned with the SLO frame z-axis(nadir direction).whichreaction wheels and/orThe S/C comprises means for storing angular momentum,may be constituted by momentum wheels,control moment gyros. In the shown embodiment the means forstoring angular momentum comprises at least three reactionwheels each oriented in a separate direction from the other;thereby any angular momentum of the S/C can be projected ontosaid reaction wheels. Preferably the S/C comprises fourreaction wheels arranged in a tetrahedron configuration,wherein the fourth reaction wheel is used for redundancy.
    The S/C l comprises a control unit operatively connected toand arranged to control the propulsion units and the means forstoring angular momentum. In another embodiment said controlunit of the S/C l is constituted by a separate control unitoperatively connected to and arranged to control thepropulsion units and by another separate control unitoperatively connected to and arranged to control the means forstoring the angular momentum.
    Besides the abovementioned structural conditions for the S/Cseveral other constraints exists, such as spacecraftregulations, these constraints will shortly be discussedbelow.
    Due to spacecraft regulations, autonomous orbit control isnormally not allowed on-board the S/C. Therefore these controlparameters need to be calculated remotely on a remote site,i.e. the earth, and then uploaded to the S/C. The controlparameters include the attitude pointing profile and thepropulsion units ON/OFF times and thus fully define theorbital manoeuvre. The orbit translational manoeuvres arecalculated, using for instance PRISMA Formation Flying Tools,based on the initial conditions of the S/C (for exampleposition, and the desired final conditionsof the S/Cspeed and attitude)(for example position, speed and attitude) togetherwith some desired intermediate way points as input parameters.The output from the calculation performed, by for instancePRISMA Formation Flying Tools, is the orbit translationalmanoeuvres expressed in the SLO-frame of the S/C as velocity(Av).changes The velocity changes need to be translated topropulsion unit on/off times, in order to be useable for theS/C.remote site by deriving the propulsion unit actuation ON/OFFcontrol of the This translation is performed at thetimes from the set of Av. Each Av is split into actuation oftwo propulsion units (i.e. one propulsion unit pair),thehaving acommon mean thrust torque direction. In such a way,angular momentum is controlled along the mean thrust torquedirection. The propulsion unit thrust direction is fullydefined after this step, and the S/C orbit parametersevolution is frozen.
    In connection with the technical implementation of thepropulsion system of the S/C several design considerationsneed to be assessed. Due to these considerations someconstraints exist for the propulsion units of the S/C. Theseconstraints may be the following:- It is not possible to actuate more than one propulsionunit at a time.
    - A propulsion unit cannot be activated more than threeconsecutive hours.
    - A time interval of at least three minutes is necessarybetween deactivation of a first propulsion unit andactivation of a second propulsion unit.
    - It is not possible to use a combination of differenttypes of propulsion units, such as HET and HEMPpropulsion units. Only one type of propulsion units at atime can be used for repositioning purposes.
    Other constraints imposed on the S/C come from the system thatcontrols the attitude and orbit of the S/C, commonlyNUNdesignated the “Attitude and Orbital Control System”the art,(AOCS) inthese constraints include:- Angular momentum management, the angular momentum storedin the means for storing angular momentum shall beconfined to a value lower than a maximum value.
    - Gyroscope torque compensation, when the S/C rotates withrespect to the inertial frame,of the S/C (fixed in the inertial frame)the S/C-frame.the total angular momentummust rotate inThus the angular momentum stored in themeans for storing angular momentum must change directionalong the orbit of the S/C in the inertial frame withrespect to the S/C-frame. This change of the angularmomentums direction in the S/C-frame is created by mani-pulating the means for storing angular momentum, i.e.according to a preferred embodiment of the inventionmanipulating the angular speed of at least one of thereaction wheels.- Star tracker blinding, the S/C comprises an arrangementfor tracking the stars, a so called star tracker device.The star tracking device can be blinded by either the sunor the earth. This effect must be minimized and this isattained by choosing which propulsion unit to activateand thereby choosing the direction of rotation of theS/C.
    A general strategy for repositioning manoeuvres can beformulated based on the above requirements and constraints.The general strategy comprises:Calculation of the propulsion units thrust profile, forinstance by means of the PRISMA Formation Flying Tools.
    Rotate the S/C around the nadir direction in order to addressthe constraints imposed by the propulsion units and the needfor propulsion thrust;typically, the number of rotations perorbit is in the range of 3 to 6 full S/C revolutions.
    NUHIn figure 3 the S/C rotation around the nadir direction 30 andthe propulsion units thrust evolution 31 is disclosed in acase of a negative acceleration of the S/C. A switch of activepropulsion unit occurs at angles close to 135 ß and 225 Vdegrees, respectively, from the velocity direction 32, i.e.+/- 45 degrees from a mean thrust direction 33 arrangedopposite to the velocity direction 32. The available thrustfrom the propulsion unit can be calculated using the formulabelow:1 88.51nin[TEP -c0s(45 -wn - d: = 816% - TE,1.5 minTEf z 90 minWhere TEÜ denotes the effective thrust from a specificpropulsion unit with the nominal thrust THH This integralevaluates to 87.6% of the nominal thrust from the propulsionunit. This calculation includes the necessary intervalsbetween activation of the different propulsion units as wellas the effect of rotation by taking into account the cosinevalue of the rotation angle taken in relation to said meanthrust direction.
    All activation times, as well as the propulsion units thrustdirection profiles are calculated and planned from a remoteIn this way the orbit and attitudesite, such as the earth.control is performed by the remote site only. The inherentuncertainties come from modelling errors only.
    The next step in the general strategy performed at the remotesite after the calculations of the necessary speed changes,i.e. using PRISMA Formation Flying Tools, is to divide eachcalculated speed change between the propulsion units in aselected pair of propulsion units, and compute the exactattitude profile that maintains the angular momentum in aallowable region of operation of the means for storing angularmomentum.
    Due to reasons of symmetry one can conclude that the torquesfrom each propulsion unit in a pair of propulsion units arealmost equal but opposite each other, therefore the angular12momentum created by activation of one propulsion unit will becancelled by activation of the other propulsion unit in thepair. Small differences in torque magnitude can be handled byThedifferences in torque direction can be compensated by rotatingmeans of unequal activation times of the propulsion units.the S/C an angle about the thrust direction of the propulsionunit.
    In figure 4 the resulting forces and torques about the centreof mass (CoM) of the S/C l is illustrated for a scenario whenone of the propulsion units provide a thrust force F to theS/C in a propulsion unit thrust direction and thus a torque Iabout the centre of mass, due to the momentum arm that isformed by the perpendicular component rp of the vector rpointing from CoM of the S/C to the position 40 of thepropulsion unit. A vector D is perpendicular both to the forceF and to the torque T, and the direction of said vector D iscalled a controllable direction; this direction is fixed inthe S/C-frame for each of the propulsion units 13-20. However,since the SLO-frame rotates inertially in an orbit around thecelestial body 3 the controllable direction in the inertialframe varies along the orbit as the inertial attitude of theS/C in relation to the celestial body 3 changes, i.e. thenadir direction of the S/C-frame always points towards thecentre of the celestial body 3. This implies that during oneorbit all directions in the inertial frame are covered by acontrollable direction. The controllable direction for eachpropulsion unit will be used for the purpose of unloading theangular momentum of the S/C.Figure 5 shows a flowchart of the inventive control algorithmfor autonomous control of the angular momentum of a S/C whichis performed sequentially for each of said at least twopropulsion units.
    The first step 501 is to calculate the angular momentum of theS/C.momentum comprises at least three reaction wheels,In a preferred embodiment the means for storing angularwherein the13speed thereof is used in the calculation of the angularmomentum of the S/C.
    The second step 502 is to calculate a projected angularmomentum by projecting the calculated angular momentum onto adirection that is perpendicular to the thrust direction of thepropulsion unit, and perpendicular to a torque direction ofsaid propulsion unit. The projected angular momentum of theS/C is denoted Lprojected.
    The third step 503 is to provide a change of attitude of theS/C by performing the following sub-steps:- Calculate an offset angle Q about the propulsion unitthrust direction.
    - Cause the S/C to turn about the propulsion unit thrustdirection said calculated offset angle Q in a firstdirection of rotation. The turning of the S/C about thethrust direction of the propulsion unit is caused bymanipulating the means for storing the angular momentum.In a preferred embodiment the sub-step of manipulatingsaid means for storing angular momentum comprisesmanipulating the speed of at least one of said at leastthree reaction wheels.
    The fourth step 504 is to provide thrust by activating thepropulsion unit.
    The mutual order of the third step 503 and the fourth step 504is arbitrary and not essential for the invention however it isessential that the third step must be started before thefourth step is terminated. In one embodiment of the inventionthe third step is started before the fourth step is started,in order to utilize most of the thrust to unload the means forstoring the angular momentum.
    It shall be pointed out that the turn of the S/C about thepropulsion unit thrust direction the offset angle Q in the14first direction of rotation is performed in a directionopposite to the direction of the projected angular momentum.
    The calculation of the offset angle Q can be performed in manyways but it is essential for the present invention that theoffset angle is proportional to the projected angular momentumLproj ected:oc<>projectedAccording to a preferred embodiment of the inventive methodthe offset angle is calculated as:(X = (X0 _ CGI/lst 1 Lprojectedwhere ao is a deviation angle between a nominal torquedirection of the propulsion unit and a mean torque directionof said set of at least two propulsion units. The negativepart of the equation is an angle calculated as the product ofa constant and the projected angular momentum. The constant inthe equation above may be calculated as:Const =PEP ' AIEPWhere Tmf is the projected torque of the propulsion unit inthe SLO-frame, and AtH>is the propulsion unit thrust time.
    In a preferred embodiment of the invention it is not possibleto instantly turn the S/C from O degrees to the offset angleQ, the turn of the S/C about the propulsion units thrustdirection is performed in a smooth way with the followingconstraints:- maximum angular acceleration about the propulsion unitthrust direction is 5XlO¿ deg/s2, and/or- maximum angular speed about the propulsion unit thrustdirection is 0.0l deg/s, and/or- maximum offset angle Q about the propulsion unit thrustdirection is 20 deg.
    As mentioned earlier there is a requirement not to have anyautonomous orbit control on-board the S/C. Therefore, theorbit translational manoeuvres cannot be modified on-board.
    But the additional rotation about the propulsion unit thrustdirection a small offset angle Q, that is computed for eachsingle activation of the propulsion units is completely de-correlated from the attitude profile, and is thus allowable.In this way the angular momentum of a spacecraft in connectionwith an orbit translational manoeuvre may be autonomouslycontrolled on-board, by exploiting the controllable directionthat is fixed in the SLO-frame. But as the SLO-frame rotateswith respect to the inertial frameorbit),(one full rotation perthe controllable direction D changes over time in theOver a half orbit (12 hours)and the angular momentum of the S/Cinertial frame. all directions inspace can be controlled,can be controlled/unloaded within the required operationalrange of the means for storing angular momentum.
    In a preferred embodiment of the invention the step ofproviding change of the attitude of the spacecraft and thestep of providing thrust by activating said propulsion unitat least partly overlap each other, wherein the step ofproviding change of the attitude of the spacecraft comprises athird sub-step causing the S/C to turn about the thrustdirection of the propulsion unit said offset angle Q in asecond direction of rotation that is opposite said firstdirection of rotation by manipulating said means for storingangular momentum. It shall be pointed out that the turn of theS/C about the thrust direction of the propulsion unit in thesecond direction of rotation may be different from said offsetangle Q in order to correct the attitude of the S/C.
    权利要求:
    Claims (11)
    [1] l. Method for autonomous control of an angular momentum of aspacecraft, wherein the spacecraft comprises,- means for storing angular momentum, - a set of at least two propulsion units arranged to independently provide thrust to the spacecraft, and - a control unit operatively connected to said means forstoring angular momentum and said at least two propulsionunits, wherein the method comprises the following set of steps which is performed sequentially for each of said at least two propulsion units: - calculate the angular momentum of the spacecraft, - calculate a projected angular momentum by projecting thecalculated angular momentum onto a direction that isperpendicular to the thrust direction of the propulsion unit, and perpendicular to a torque direction of saidpropulsion unit, wherein the following two steps are performed after the two abovementioned calculation steps, - provide change of the attitude of the spacecraft by thefollowing sub-steps: o calculate an offset angle about the thrust directionof the propulsion unit, which offset angle isproportional to said calculated projected angularmomentum, o cause the spacecraft to turn about the thrustdirection of the propulsion unit said offset anglein a first direction of rotation by manipulatingsaid means for storing angular momentum, - provide thrust by activating said propulsion unit, wherein the step of providing change of the attitude of the spacecraft must be started before the step of providing thrust by activating said propulsion unit is terminated. 17
    [2] 2. Method according to claim l, wherein said means for storing angular momentum comprises at least three reaction wheels, andwherein the sub-step of manipulating said means for storingangular momentum comprises manipulating the speed of at leastone of said at least three reaction wheels.
    [3] 3. Method according to claim 2, wherein the step of calcu-lating the angular momentum of the spacecraft comprisesmeasuring the rotational speed of each of said at least threereaction wheels, and calculating the angular momentum of thespacecraft based on the measured rotational speed of each ofsaid at least three reaction wheels.
    [4] 4. Method according to claim l, wherein the step ofcalculating the offset angle about the thrust directioncomprises calculating a sum of: - a deviation angle between a nominal torque direction ofthe propulsion unit and said mean torque direction ofsaid set of at least two propulsion units, - an angle calculated as the product of a constant and theprojected angular momentum.
    [5] 5. Method according to claim 4, wherein said constant iscalculated as the inverse of the product of: - a projection of a torque of the propulsion unit onto amean torque direction of the set of at least twopropulsion units, and - an actuation time of the propulsion unit.
    [6] 6. Method according to claim l, wherein said sub-step ofturning the spacecraft about the thrust direction is limitedby at least one of the following values: - maximum angular acceleration about the propulsion unitthrust direction is 5XlO¿ deg/så - maximum angular speed about the propulsion unit thrust direction is 0.0l deg/s, and- maximum offset angle about the propulsion unit thrust direction is 20 deg. 18
    [7] 7. Method according to claim l, wherein said offset angle is added to an attitude profile, said attitude profile comprisescalculated propulsion unit activation ON/OFF times and calculated spacecraft orientation, and said attitude profileis uploaded to the spacecraft from a remote site.wherein the step of providing started before the
    [8] 8. Method according to claim l,change of the attitude of the spacecraft isstep of providing thrust by activating said propulsion unit isstarted.
    [9] 9. Method according to claim l, wherein the step of providingchange of the attitude of the spacecraft and the step ofproviding thrust by activating said propulsion unit at leastpartly overlap each other, wherein the step of providingchange of the attitude of the spacecraft comprises a thirdsub-step: - causing the spacecraft to turn about the thrustdirection of the propulsion unit said offset angle in asecond direction of rotation that is opposite said firstdirection of rotation by manipulating said means for storing angular momentum.
    [10] 10. Spacecraft assembly for autonomous control of an angular momentum of a spacecraft, wherein the spacecraft assembly comprises, - means for storing angular momentum, - a set of at least two propulsion units arranged to independently provide thrust to the spacecraft, and - a control unit operatively connected to said means forstoring angular momentum and said at least two propulsionunits, wherein the control unit comprises: - means arranged to calculate said angular momentum of thespacecraft, - means arranged to calculate a projected angular momentum by projecting the calculated angular momentum onto a direction 5 N U U that is perpendicular to the thrust direction of the propulsion unit, and perpendicular to a torque direction ofsaid propulsion unit, wherein the control unit is arranged to perform the following two steps based on the two abovementioned calculations, - provide change of the attitude of the spacecraft by thefollowing sub-steps: o calculate an offset angle about the thrust direction of the propulsion unit, which offset angle isproportional to said calculated projected angularmomentum, o cause the spacecraft to turn about the thrustdirection of the propulsion unit said offset anglein a first direction of rotation by manipulatingsaid means for storing angular momentum, - provide thrust by activating said propulsion unit, wherein the step of providing change of the attitude of thespacecraft must be started before the step of providing thrustby activating said propulsion unit is terminated.
    [11] ll. Spacecraft assembly according to claim lO, wherein thespacecraft assembly comprises means for storing angularmomentum such as at least three reaction wheels, and whereinthe sub-step of manipulating said means for storing angularmomentum comprises manipulating the speed of at least one of said at least three reaction wheels.
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    SE536353C2|2013-09-03|
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    SE1150382A|SE536353C2|2011-05-02|2011-05-02|Method for autonomously controlling the amount of movement of a spacecraft in conjunction with a translational runway operation as well as a spacecraft composition|SE1150382A| SE536353C2|2011-05-02|2011-05-02|Method for autonomously controlling the amount of movement of a spacecraft in conjunction with a translational runway operation as well as a spacecraft composition|
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