Abstract:
A method for determining the state variables of a moving rigid body in space, in particular for determining the position and attitude of a spacecraft during an approach and docking maneuver. The task of finding a new possibility for determining the state variables of a moving rigid body, which delivers a simplification and ensures a higher level of reliability in calculating the state variables is accomplished according to the invention in the case of a method for determining the state variables of a moving rigid body by determining the state variables of the moving body with the aid of a state observer, in which case measured data of scannings of a plurality of retroreflectors in space are processed by an internal memory model to produce the desired output signal of the state variables of the moving body, starting from set initial values and known system parameters implemented in operators, a current state vector of the moving body being calculated as an estimate, and being adapted to the actual attitude characteristic and movement characteristic of the body with each measuring cycle by a correction, such that after a specific number of measuring and calculating cycles the estimated values track the actual movement data of the body in space and are used as state variables of the body.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    a) Field of the Invention  
           [0002]    The invention relates to a method for determining the state variables of a moving rigid body in space, in particular for determining the position and attitude of a spacecraft during an approach and docking maneuver.  
           [0003]    b) Description of the Related Art  
           [0004]    In space flight and in robotics, it is frequently necessary to determine the position and attitude of two bodies relative to one another. This is of particular importance in space flight during coupling of spacecraft. Thus, at least six variables are required to control the docking operation:  
           [0005]    a) the distance (in three coordinates x, y, z) and  
           [0006]    b) the three Euler angles (roll angle, pitch angle, yaw angle—as in the case of an aircraft),  
           [0007]    which must be determined in the simplest possible way.  
           [0008]    Radar engineering supplies the simplest method of determining the distance and the direction of view from one body to another. In this case, laser radar sets have increasingly become established recently for the very short range, from a few hundred meters down to a few decimeters. The short wavelength of the light permits high accuracies in the variables of range, azimuth and elevation which are to be measured (in spherical coordinates) or in the components in Cartesian coordinates.  
           [0009]    The radar set emits a transmitted beam which scans the field in which the target object (also: target, for example space station, spacecraft) is expected, and is reflected by the target object. The reflection, and thus the reliability of the measurement can be substantially improved when use is made of so-called retroreflectors which retroreflect the incident beam exactly in the opposite direction.  
           [0010]    The measuring beam leaves the transmitter optics and is guided in linewise fashion over the observation field by a coordinated movement of azimuth and elevation mirrors. When the measuring beam strikes a rectroreflector, it is retroreflected in the opposite direction and therefore reaches the receiver optics of the radar set. At this moment, the measured data are determined by determining the mirror positions, and the range is calculated from the travel time of the beam from the transmitter to the receiver.  
           [0011]    If it is desired to determine not only the position of the target, but also its attitude, it is necessary to use at least three retroreflectors which are arranged in a defined fashion in the coordinate system of the target. The position and the attitude of the target can be determined uniquely with reference to the coordinate system of the measuring device with the aid of such an arrangement composed of an active range finder (for example radar set, rendezvous sensor) and three retroreflectors such as has been proposed, for example, by Möbius, B. et al. (in: Development and Application of a Rendezvous and Docking Sensor “RVS”, International Conference on Space Optics, Toulouse, 1997). For this purpose, bearings are taken on the rectroreflectors simultaneously or sequentially in time, in order in each case to measure the range, azimuth and elevation. This means that it is possible in this way to determine the position (for example expressed by Cartesian coordinates x, y, z) and attitude (for example by specifying Euler angles) of the two coordinate systems relative to one another. If the active sensor—frequently also called rendezvous sensor (RVS)—and the retroreflector arrangement (as system components of the measurement process) are fixed, and thus do not move, it is possible to use geometrical relationships to determine from the nine measured values the total of six parameters for position and attitude of the two coordinate systems relative to one another.  
           [0012]    In the case of the movement of at least one of the two system components (for example during approach and docking of two spacecraft), a problem arises with evaluating the measured values by virtue of the fact that during the measurements at the three retroreflectors of the body scanned by the RVS, both the relative position and the attitude are changing and the measurement results can no longer be assigned with complete ease. Thus, in order to determine position and attitude use has been made to date of approximate solutions on the basis of triangular relationships which take account of estimated values of the speeds.  
           [0013]    A further disadvantage of this solution is the extensive use of trigonometric functions, which can only be implemented in onboard computers expensively and can cause ambiguities.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0014]    It is the primary object of the invention to find a new possibility for determining the state variables of a moving rigid body which is accompanied by simplification and ensures a higher level of reliability of the calculation of the state variables of the moving body. A widened object of the invention is to ensure the above-named aims even in the case of serial acquisition of measured values without increasing the outlay on evaluation.  
           [0015]    According to the invention, the object is achieved in the case of a method for determining the state variables of a moving rigid body, such as relative position and attitude as well as translational and rotational speeds, in particular for a movement of approach to another body employing measured data provided by an active sensor, the measured data being derived from at least three rectroreflectors whose configuration on the other body is known and is assigned in a defined fashion to a body-specific coordinate system of the other body, by virtue of the fact that the state variables of the moving body are determined with the aid of a state observer which processes the measured data of scannings of the individual rectroreflectors in space by means of an internal memory model to produce the desired output of the state variables of the moving body, in which case, starting from set initial values and known system parameters implemented in operators, a current state vector of the moving body is calculated as an estimate, and a correction determined by linkage with the measured data is used to adapt this estimate with each measuring cycle to the actual attitude characteristic and movement characteristic of the moving body, such that after a specific number of measuring and calculating cycles the estimated values calculated in the state observer track the current movement data of the moving body in space to a very good approximation, and the calculated estimated values are used as state variables of the moving body.  
           [0016]    The measured data are advantageously compared with the estimated values present in the state observer, a difference being determined from a measurement vector y(k) and a current estimate y*(k) of the measurement vector on the basis of an estimated state vector x*(k), k being a counting index of the running measurement clock and calculation clock, and being used to correct the estimated values. It proves to be expedient that an estimated measurement vector y*(k) for providing an estimated reference variable is generated from the currently estimated state vector x*(k) by means of an output matrix which contains the effect of the real state vector x(k) on the measurement vector y(k), a subsequent state of the state vector x*(k) is calculated from the currently estimated state vector x*(k) via a system matrix which takes account of influences of the system upon transition from a current state to a following state, and this subsequent state is corrected by means of the difference by using a return matrix which contains a rule for converting the difference into a correction of the current state, and after this correction a subsequent state vector x*(k+1) is yielded which is taken over into the current state vector x*(k) for the next clock. It is particularly advantageous in this case when the state observer is designed as a filter, in which case the output matrix and the return matrix are linked with the system matrix to generate an operator for generating a subsequent state which describes the relationship between the measurement vectors and two sequential estimated state vectors x*(k) and x*(k+1) without knowledge of an estimated measurement vector, the subsequent state of the current estimated state vector x*(k) is calculated therefrom by means of said operator, the generated subsequent state of the state vector is corrected with the aid of the measurement vector converted via the return matrix, and is transferred into the subsequent state vector x*(k+1), and the subsequent state vector x*(k+1) is provided optionally parallel to the still present current state vector x*(k) as output of the state observer.  
           [0017]    A particularly simple variant of the invention consists in processing the measured data of a plurality of simultaneously scanned retroreflectors in the form of measurement vectors in the state observer simultaneously with the measurement clock frequency of the retroreflector scannings, a cycle clock of the calculations in the state observer corresponding to the measurement clock of the scanning of the retroreflectors.  
           [0018]    Another configuration of the method according to the invention, which is advantageous because of savings in weight and volume, proceeds from the fact that the measured data are processed in the form of measurement vectors of a plurality of sequentially scanned retroreflectors in the state observer with a cycle clock frequency whose period corresponds to the duration of a series of scannings of all the retroreflectors with a measurement clock frequency, the measurement clock frequency being an integral multiple of the cycle clock frequency as a function of the number of retroreflectors used. In the case of this variant, the measurement vectors of the sequentially scanned retroreflectors are advantageously buffered in the state observer, weighted as a function of their temporal sequence and simultaneously processed in a cycle clock after termination of a series of scannings of all retroreflectors.  
           [0019]    For this purpose, the measurement vectors y i  arriving sequentially in time are advantageously buffered in a state observer designed as a filter, and are weighted differently as a function of the measuring instant by means in each case of an associated return matrix and a weighting matrix which takes account of a forgetting rate dependent in terms of time on the age of the individual measurement vectors y 1 , and the specifically weighted and subsequently combined measurement vectors y i  are used to correct the current state vector x*(k), which was previously multiplied by a matrix which embodies the memory over n measurement clocks.  
           [0020]    In a particularly preferred variant of the invention, the measured data are processed in quasi-real time, with short-term buffering, in the form of measurement vectors of sequentially scanned retroreflectors in the state observer with a scanning frequency which corresponds to the mean duration of the scannings of each of the retroreflectors. In this case, the measurement vectors, which are in each case picked up in a defined sequence of the scanned retroreflectors, are processed without delay, in each case an individual measurement vector y i (k) advantageously being weighted in the state observer with the aid of a return matrix which is appropriately matched to an output matrix specific to each retroreflector, and a subsequent state, which is instantly the initial state for processing the next measurement vector y i+1 (k+iΔ) of the subsequently scanned retroreflector, being calculated in each case from the weighted measurement vector y i (k) and the present current state vector x*(k), which is multiplied by a matrix representing the memory via a measurement clock. In this advantageous variant, the estimated state vector x* is available after each measurement clock.  
           [0021]    The fundamental idea of the method according to the invention is based on the presupposed provision of measured values from a space-scanning range-finding device (for example laser radar) of the moving body, and on the knowledge of a configuration of measurement points (retroreflectors) on the other body (target).  
           [0022]    The essence is the determination of the continuously changing position and attitude between an active sensor and target with the aid of the currently scanned target measurement points, this being done by simulating the movement of the measuring device and target by means of processing the current measured values. In this process, sequential measurements are combined during the approach in a so-called state observer such that, starting from suitably selected initial values, the following state variables are calculated as estimated values immediately after each individual measurement of the retroreflectors:  
           [0023]    the relative position in Cartesian coordinates, that is to say a position vector p between the coordinate systems of the active sensor and the target (3 variables),  
           [0024]    the velocity components in the three coordinate directions (3 variables),  
           [0025]    the quaternions relating to the attitude description (3 independent q 1 , q 2 , q 3 , and one dependent variable q 4 ) which, instead of Euler angles, describe the attitude of a body with reference to a fixed inertial system, and  
           [0026]    the angular speeds about the coordinate axes (3 variables).  
           [0027]    In this method, consequently, 12 state variables of the moving body are estimated and are continuously corrected and updated by means of a comparison with the measured data of the scanning of the retroreflectors.  
           [0028]    It is possible by using the method according to the invention to determine the state variables of a moving rigid body, this type of determination representing a simplification and a higher level of reliability of the calculation of the state variables of the moving body. Moreover, the method according to the invention succeeds without increasing the outlay on evaluation even in the case of serial measured value acquisition.  
           [0029]    The invention is to be explained in more detail below with the aid of exemplary embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    In the drawings:  
         [0031]    [0031]FIG. 1 shows the relationship between the measurement vector r 1 , target vector t 1  and position vector p;  
         [0032]    [0032]FIG. 2 shows a measuring principle according to the invention consisting of a controlled system  1  and state observer  2 ;  
         [0033]    [0033]FIG. 3 shows a view of the state observer  2  as a filter;  
         [0034]    [0034]FIG. 4 shows a state observer  2  for sequential measurements at three retroreflectors Ri of a target, and simultaneous evaluation at the end of a cycle of measurements of all the retroreflectors Ri;  
         [0035]    [0035]FIG. 5 shows a state observer  2  composed of three partial observers for the successive processing of measurement results within a measuring cycle;  
         [0036]    [0036]FIG. 6 shows the transient response of the state observer; and  
         [0037]    [0037]FIG. 7 shows the subsequent response of the state observer, illustrated with reference to the example of two quaternions. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    In order to explain the method according to the invention, the following boundary conditions are to be adopted as given, so as to facilitate the description.  
         [0039]    It is immaterial for the functioning of the method according to the invention whether it is the body with the active sensor which moves or the other body (termed target subsequently) with the retroreflectors. It is assumed here expediently that an active rendezvous sensor is mounted on a moving spacecraft and that the target (for example a space station or another spacecraft) “is stationary”, that is to say a movement of the target is of no significance for the relative approach of the spacecraft. It is possible in this way for the determined state variables of the spacecraft to be used as results of the method according to the invention for directly controlling the moving spacecraft. Without limiting generality, the active rendezvous sensor is denoted below in a simplified fashion as a laser radar, since it is preferred to use such a device. Moreover, in order to simplify the explanations the number of retroreflectors is taken as three, since these suffice as a rule to define a surface in space (for example the docking surface of the other body) exactly in position (range) and attitude, when the distribution of the retroreflectors is suitably chosen and their configuration is known.  
         [0040]    [0040]FIG. 1 illustrates the relationship between the measured data, state variables and the different coordinate systems of the moving body and the target which is to be approached (and possibly docked at). Here, a measurement vector r 1  constitutes the actual measured variable of the range to a first retroreflector R 1 . The measurement vector r 1  specifies in the coordinate system (x C , y C , z C ) of the laser radar where the retroreflector R 1  is located. Also specified in the coordinate system (x C , y C , z C ) of the laser radar is a position vector p which points to the origin of a coordinate system (x T , y T , z T ) of the target. In the coordinate system (x T , y T , z T ) of the target, target vectors t i  are specified which point from the origin of coordinates thereof to the retroreflectors R i . For the purpose of simplification, only the target vector t 1  relating to the retroreflector R 1  is illustrated in FIG. 1. The arrangement of the retroreflectors R i  in the coordinate system (x T , y T , z T ) is fixed with a corresponding assignment of further target vectors t i .  
         [0041]    If the two above-named coordinate systems can be transferred into one another by simple translation, the simple relationship  
           r   1   =p+t   1   (1)  
         [0042]    holds for the measurement vector r 1  pointing to the retroreflector R 1  in the vector representation in accordance with FIG. 1.  
         [0043]    If the two coordinate systems are rotated with respect to one another, an attitude matrix Q must be included. It indicates how the target vector t 1 , which is specified in the coordinate system (x T , y T , z T ) of the target, appears in the coordinate system (x C , y C , z C ) of the laser radar. The result of this is  
           r   1   =p+Qt   1   (2)  
         [0044]    Equation (2) shows how the state variables of the position with p(x, y, z) and attitude with Q (q 1 , q 2 , q 3 , q 4 ) feature in the measurement result. Here, q 1 , q 2 , q 3 , q 4  are quaternions which, instead of the Euler angles, can describe the attitude of a body with reference to a fixed coordinate system. In addition to the Euler angles, quaternions constitute an equivalent possibility for representing the attitude of a body with a coordinate system fixed in the body with reference to another coordinate system. They have the advantage that they can be used to represent the attitude matrix Q in equation (2) without trigonometric functions. The influence of the associated speeds is given by the temporal dependence, and is detected by repeated measurement of the retroreflectors R i .  
         [0045]    It follows from equation (2) that of the twelve state variables to be determined only six state variables (x, y, z, q 1 , q 2 , q 3 ) determine the 3 components of the individual measurement vector r 1 . Consequently, a specific number of measurements must be carried out simultaneously and sequentially in time at different and identical retroreflectors R i  in order to compensate the information deficit of only one measurement.  
         [0046]    Further equations analogous to equation (2) can be set up mutatis mutandis for the other retroreflectors R i .  
         [0047]    The method according to the invention for determining said state variables, as illustrated in FIG. 2, is based on a principle of control engineering (Luenberger observer [Föllinger, O.: Regelungstechnik [Control engineering], Hüthig Buch Verlag, Heidelberg, 1990, pages 501-517]).  
         [0048]    The upper part of FIG. 2 can be interpreted in terms of control engineering as a controlled system  1  (spacecraft with laser radar set). Belonging to this system are an input vector u, an input matrix B, a system matrix A, a state vector x, an output matrix C and an output vector y.  
         [0049]    The input vector u, in which the drive signals and possible noise signals are combined, determines the state vector x of the moving spacecraft via the input matrix B. It is to be seen that the new state at the instant (k+1) comprises the current state x(k), weighted with the system matrix A, and the current input vector x, which is multiplied by the input matrix B. A downstream transfer element  11  with the unit matrix I symbolizes by the operation I z −1  that the subsequent state x(k+1) after a scanning clock is transferred into the current state vector x(k). Combined in the output vector y are the measurement results r 1 , r 2 , r 3  for the adopted three rectroreflectors R 1 , R 2  and R 3 , which can be measured on the basis of the current state variables x(k) of the moving spacecraft. It is expedient to convert the measurement results of range, azimuth and elevation into Cartesian coordinates.  
         [0050]    In the lower part of FIG. 2, is a model of the moving body, a so-called state observer  2 . The system parameters, that is to say the system matrix A and the output matrix C, of the controlled system  1  of the moving body and of the state observer  2  are assumed to be identical. The special adaptation of the state observer  2  to the measurement problem of the prescribed controlled system  1  consists in that an input vector u is not used in the state observer  2 , since it is not available. Consequently, estimated values y*(k) of the output signal are generated in the state observer  2 ; they correspond in the ideal case to the measurement results r 1 , r 2 , r 3  of the laser radar. However, since the input vector u is not available in the state observer  2 , the next state is calculated from a currently estimated state vector x*(k) and a correction e(k), which includes the deviation of the real measurement vector y(k) from the measurement vector y*(k) estimated in the state observer  2 . Thus, the influence of the input vector u from the controlled system  1  is also contained in this correction e(k).  
         [0051]    The estimated measurement vector y*(k) is generated from the estimated state vector x*(k) via an output matrix C which includes the effect of the real state vector x(k) on the measurement vector y(k) measured by the laser radar. The correction e(k) obtained by forming the difference between the real measurement vector y(k) and the estimated measurement vector y*(k) is thereafter applied to the current estimated state vector x*(k) via a return matrix H, which contains a rule as to the extent to which the correction e(k) acts on the estimated state vector x*(k), the next estimated state, a subsequent state vector x*(k+1) resulting therefrom. The subsequent transfer element  22  symbolizes, as already in the controlled system  1  with I z −1 , that after one clock the subsequent state vector x*(k+1) is transferred into the current estimated state vector x*(k).  
         [0052]    If the state vector x(k) of the controlled system  1  (spacecraft) corresponds with the estimated state vector x*(k) of the state observer  2  (computing model of the spacecraft), that is to say x(k)=x*(k), the real measurement vector y(k) and the measurement vector y*(k) estimated by the state observer  2  also correspond. The state variables of the moving spacecraft can then be read off directly at the output of the state observer  2 .  
         [0053]    If this correspondence does not obtain, the state vector x*(k) of the state observer  2 , is corrected with the aid of a correction vector e(k) by making use of the return matrix H already mentioned above, until the error, that is to say the deviation between x(k) and x*(k), vanishes. This is performed on the basis of further scannings of the retroreflectors R 1  to R 3 , and so the actual changes in position and attitude of the moving body (controlled system  1 ) always also participate directly. In this way, the state variables of the approaching body can already be read off directly at the output of the state observer  2  after a few scanning and calculating cycles.  
         [0054]    When the data from the state observer  2  are being used, there is also in each case the option of deciding whether to make use, as the best approximated state, from the states of two sequential clocks, of the estimated or calculated state vector x*(k) or of the latter&#39;s subsequent state vector x*(k+1).  
         [0055]    Dispensing with the representation of the controlled system  1  from FIG. 2 produces a simplified block diagram illustrated in FIG. 3, which specifies the relationship between the real measurement vectors y(k) (input  21  of the state observer  2 ) and the estimated state vectors x*(k) and x*(k+1) (output  23  of the state observer  2 ).  
         [0056]    The structure illustrated in accordance with FIG. 3 corresponds to that of a digital filter. The input variables of this filter are the measured values of the laser radar r 1 , r 2 , r 3 , which are combined as a real measurement vector y(k), and the desired state vectors x*(k) and x*(k+1) represent the output variables of the state observer designed as a filter.  
         [0057]    [0057]FIG. 3 shows how the (new) subsequent state vector x*(k+1) results from the current state vector x*(k), multiplied by an operator  24 , which results from the matrices already introduced above and whose calculating rule can be specified with the matrix operations A−HC and from the simultaneously given measurement results y(k), which are weighted with the return matrix H.  
         [0058]    Such a filter is to be connected downstream of the laser radar set, which supplies the measured values r i , in order to be able to determine the desired state variables by evaluating the available sequential state vectors x*(k) and x*(k+1).  
         [0059]    The matrices used and their meaning are summarized once again in the following table for the purpose of understanding the operator  24  and of facilitating the explanations, based thereon, of the preferred exemplary embodiments of the invention.  
                                           Matrix   Symbol   Meaning   Comments                   System   A(12 ×   describes the influence of   for relatively large Euler       matrix   12)   the current state x(k) on   angles (&gt;10°), x(k + 1) =               the subsequent state   A x(k) is to be replaced               x(k + 1) in accordance with   by a function x(k + 1) =               equation (3)   f[x(k)]       Output   C(3n ×   describes the effect of the   for relatively large Euler       matrix   12)   state vector x on the   angles (&gt;10°), y(k) = C               measurement vector y in   x(k) is to be replaced by               accordance with equation   the function y(k) = g               (2), n is the number of   [x(k)]               retroreflectors R i         Return   H(12 ×   describes how the error   determines the stability       matrix   3n)   signal is used to correct the   and the convergence               state vector in the state   speed of the state               observer 2, n is the number   observer               of retroreflectors R i                    
 
         [0060]    The dynamic response of the state observer  2  in accordance with FIG. 2 or  3  can be expressed with the aid of the equation  
           x *( k+ 1)= Ax *( k )− HCx *( k )+ Hy ( k )  (3)  
         [0061]    Equation (3) can be applied directly in accordance with FIG. 3 when, for example, the positions and attitudes of the three retroreflectors R 1 , R 2  and R 3  are measured simultaneously with three laser beams. In this case, the output vector has the form of y=(r 1 ′, r 2 ′, r 3 ′)′, the apostrophe respectively denoting the transposed vector. This is normally an excessively high technical outlay. For this reason, it is desired to execute the measurement of the three (or more) retroreflectors R i  sequentially in time with the aid of a laser radar.  
         [0062]    The mode of procedure according to FIG. 2 or  3  cannot be used directly for a volume- and weight-orientated achievement of the object according to the invention, since  
         [0063]    the scanning of the retroreflectors R i  (i=1, 2, . . . , n; n≧3) is performed sequentially in time, and so the time-discrete system has two scanning frequencies, a measurement clock frequency f p  of the serial scanning of the retroreflectors R i  and a cycle clock frequency of the calculation of the state vectors, which both are related via the number n of the retroreflectors R i  used, f p =n f C ;  
         [0064]    the retroreflectors R i  are located at different positions, and so the output matrices C i  are different for the n retroreflectors R i ;  
         [0065]    a suitable return matrix H i  must be selected for each of the retroreflectors R i , and  
         [0066]    the Eigen values of the state observer  2  are to be set as a function of the operating mode of the laser radar.  
         [0067]    In the phase of larger range, the state variables are to be measured with lower accuracy, the state observer  2  must be able to settle more quickly, that is to say the estimated measurement vectors y*(k) must be approximated more quickly to the real measurement vectors y(k). Right close to the docking operation, the approaching speed is low, and it is therefore sensible to inertly dimension the state observer  2  so as to obtain a higher accuracy through effective averaging.  
         [0068]    For the purpose of applying linear relationships between the real state vector x(k) of the moving body (controlled system  1 ), on which the laser radar set is located, and the measurement vector y(k)=(r 1 ′, r 2 ′, r 3 ′)′, it is assumed that the Euler angles, which describe the relative attitude of the two approaching bodies with respect to one another, are small enough (&lt;10°). Otherwise, the measurement vector y(k) would need to include nonlinear functions as a function of the state vector x(k), and this is possible without difficulty in principle and is to be taken into account for specific applications.  
         [0069]    Starting from the above equation (3), the principle of the state observer  2  is modified in the following way in the case of three retroreflectors R 1 , R 2 , R 3  to be scanned sequentially in time:  
           x *( k +Δ)=( A   S   −H   1   C   1 ) x *( k )+ H   1   y   1 ( k ),  
           x *( k+ 2Δ)=( A   S   −H   2   C   2 ) x *( k +Δ)+ H   2   y   2 ( k +Δ)  
           x *( k+ 1)=( A   S   −H   3   C   3 ) x *( k+ 2Δ)+ H   3   y   3 ( k+ 2Δ).  (4)  
         [0070]    In a first section of the cycle clock, Δ=1/3, a new estimated value is calculated on the basis of an initial value x*(0) of the state vector x*(k) to be estimated, by using the measurement result y 1 (k)=r 1  at the retroreflector R 1 . This resulting estimated state vector x*(k+Δ) is improved with the aid of the next measurement y 2 (k+Δ)=r 2  at the retroreflector R 2 , the result being x*(k+2Δ). Finally, in the third and last section of the cycle clock the measurement result y 3 (k+2Δ)=r 3  of the retroreflector R 3  is used to determine a further estimated value x*(k+3Δ) as new state vector x*(k+1). The next cycle of the three cycle sections just described begins therewith.  
         [0071]    It follows from the system of equations (4) that  
         [0072]    each retroreflector R 1 , R 2 , R 3  has its own output matrix C 1 , C 2 , C 3 ;  
         [0073]    the corresponding return matrix H 1 , H 2 , H 3  is to be determined for each output matrix C 1 , C 2 , C 3 ;  
         [0074]    a system matrix A s  is required which belongs to the clock section Δ=1/3f C , the factor 3 in the denominator resulting from the number n=3 of the retroreflectors R i ;  
         [0075]    the intrinsic behavior, and thus the convergence of the state observer  2  is determined by the matrices C 1 , C 2 , C 3 , H 1 , H 2 , H 3  and A s , and  
         [0076]    the updated estimated value of the state vector x* is available at each measurement clock.  
         [0077]    The equations specified above can be used to determine a filter structure derived from FIG. 3, and this is illustrated in FIG. 4. In this case  
         [0078]    the matrix  25  signifies a resulting system matrix M of the state observer  2 , whose Eigen values determine the stability and convergence of the simulated approach process of the moving body, where  
           M=[A   s ( k )− H   3   C   3   ][A   s ( k )− H   2   C   2   ][A   s ( k )− H   1   C   1 ]  (5)  
         [0079]    and  
         [0080]    the weighting operators  251 ,  252  and  253  signify a special weighting for each of the temporally clocked measurement vectors y 1 , y 2  and y 3  arriving at separate inputs  211 ,  212  and  213 .  
         [0081]    In this case, the weighting operators  251 ,  252 ,  253  include  
         [0082]    weighting matrices M i , which take account of the different instants ( . . . , k, k+Δ, k+2A, k+3Δ=k+1, . . . ) and therefore the up-to-dateness of the measurement vectors y i , where  
           M   1   =[A   s ( k )− H   3   C   3   ][A   s ( k )− H   2   C   2 ],  
           M   2   =[A   s ( k )− H   3   C   3 ] 
           M   3   =I, (I=unit matrix),   (6)  
         [0083]    and  
         [0084]    return matrices H 1 , H 2 , H 3 , which specify which influence is exerted by the measurement vectors y 1 , y 2 , y 3  on the correction of the state vector x*.  
         [0085]    The layout in accordance with FIG. 4 for implementing this filter structure operates with the cycle clock frequency f C , which corresponds in the case of three retroreflectors to the threefold measurement clock frequency f p  of the laser radar set (f C =3f p ). The filter structure therefore includes settable system parameters which specify  
         [0086]    how it is possible to use the matrix  25  to stipulate the transient response, the dynamics, of the state observer  2 ,  
         [0087]    how the temporal sequence of the measurement vectors y i  contributes to the state vector x* at the output  23  of the state observer  2 .  
         [0088]    [0088]FIG. 4 therefore specifies a solution in the case of which temporally sequentially occurring measured values (measurement vectors y i ) are simultaneously processed after a short buffering, and a subsequent state vector x*(k+1) is output in each case at the end of a cycle of n measurement clocks.  
         [0089]    The state observer  2  according to the system of equations (4) is illustrated in the form of a program loop  26  in FIG. 5. By contrast with FIG. 4, this shows that exactly one updated state vector x* is available after each measurement clock. Use is made of three retroreflectors R 1 , R 2 , R 3  in this example as well. The conditions for a linear state observer  2  (small Euler angle) are to be fulfilled.  
         [0090]    If the process begins with the starting value x*(0), the first updated state vector x*(k+Δ), k=0, is calculated as the result of the first measurement clock in accordance with the equation in the first block  261 , using y 1 (k), k=0. The instruction in the second block  262  is executed with this state vector x*(k+Δ) and the measurement y 2 (k+Δ), k=0. The third block  263  is processed in a similar way, and the first cycle is thereby terminated. The result is the estimated value x*(k=1). This estimated value is used as starting value for the next cycle, in which it now holds that k=1. In this sequence, the further measurements [measurement vectors y 1 (k)] are used to estimate the respective next subsequent state vector x*(k+1) of the moving spacecraft.  
         [0091]    The reliable mode of operation of the method in accordance with FIG. 5 depends substantially on the dimensioning of the matrices used:  
         [0092]    The system matrix A S  describes the relationship of the state variables between two successive instants when no further measurement vectors y 1  act on the inputs  211 ,  212 ,  213  (so-called autonomous system).  
         [0093]    The output matrices C 1 , C 2 , C 3  specify the relationship between the state variables and the three components of the real state vector x(k) or of the calculated state vector x*(k) from the origin of coordinates of the laser radar set to the relevant retroreflector R 1 , R 2  or R 3 .  
         [0094]    The matrices H 1 , H 2 , H 3  are suitably dimensioned weighting matrices. They determine how the deviation between the measured measurement vector y(k) and the measurement vector y*(k) estimated in the state observer  2  is used to correct the estimated state vector x*(k).  
         [0095]    If the Euler angles become relatively large, the linear relationships are also to be replaced in FIG. 5 by functions of the form  
           x*:=f   1 ( x*, y   1 ),  x*:=f   2 ( x*, y   2 ),  x*: f   3 ( x*, y   3 )  
         [0096]    They represent combinations of the partial functions specified in the table.  
         [0097]    After approximately three cycles, that is to say three passes of the closed-loop control circuit  26  with successive inclusion of the measurement vectors y 1 , y 2 , y 3  provided successively by the ordered scanning of the retroreflectors R 1 , R 2  and R 3 , the state variables of the moving body can be obtained with sufficient accuracy for many cases. A further advantage of this variant of the method according to the invention consists in that it is also possible to use the estimated state variables to calculate the approximate attitude of the next retroreflector R i , and this signifies an improvement in the measurement operation through suitable selection of the field of view of the laser radar.  
         [0098]    [0098]FIG. 6 shows the settling process of the state observer  2  for the last example of the successive processing of the measurement vectors y i . The spacecraft and state observer  2  have different initial conditions at the starting instant of the measurement. After approximately three cycles of three clocks in each case, the error in the state vectors x*(k)−x(k) is virtually zero. This becomes particularly clear in this example with the quaternion q 1 , the associated angular velocity and the translational speed in the x direction. The remaining curves represent the deviations of the other state variables.  
         [0099]    [0099]FIG. 7 illustrates the way in which the state observer in accordance with FIG. 5 tracks the changing state variables of the moving spacecraft. If, for example, the quaternion q 1  (which corresponds to the roll angle) changes virtually with a jump, the state observer follows in a relatively short time by overshooting. The quaternion q 2  (which corresponds to the pitch angle) changes simultaneously, in an approximately sinusoidal fashion, and the state observer can track very accurately. The estimated value for the quaternion q 2 * generated by the state observer is virtually equal to the real value q 2 . The other state variables likewise exhibit a virtually identical response, as a result of which the efficiency of the last variant of the method according to the invention is impressively verified.  
         [0100]    Similar results are also obtained, however, for the fundamental variant of the invention. This also applies to the design according to FIG. 4, in which—on account of the necessity for a respective complete series of scannings of the retroreflectors—the subsequent response of the state observer has only a coarser time grid in outputting the state variables.  
         [0101]    While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.  
                                         List of reference symbols used                                1   Controlled system (spacecraft)       11   Transfer element       2   State observer       21   Input       211, 212, 213   Clocked inputs       22   Transfer element       23   Output       24   Operator (A-HC)       25   Matrix M       251, 252, 253    Weighting operators (M i H i )       26   Closed-loop control circuit       261, 262, 263   Block       A   System matrix       B   Input matrix       C   Output matrix       H   Return matrix       R i     Retroreflectors       p i     Position vector       q 1 , q 2  . . .   Quaternions       r 1     Measurement vector (relative to the retroreflector R 1 )       t 1     Target vector       (x C , y C , z C )   Coordinate system (of the moving body)       (x T , y T , Z T )   Coordinate system (of the target)       x   Real state vector       x*(k)   Estimated (calculated) state vector       x*(k + 1)   Subsequent state vector       y,/y*   Real/estimated measurement vector