PATENT ABSTRACT
Described herein is a projective optical metrology system including: a light target equipped with a plurality of light sources having a pre-set spatial arrangement; an optoelectronic image sensor; an optical unit receiving a light signal coming from the light target and defining two different optical paths for the light signal towards the optoelectronic image sensor, the two optical paths being such as to cause simultaneous formation on the optoelectronic image sensor of at least two images of the light target; and an electronic processing unit coupled to the optoelectronic image sensor and determining a plurality of different quantities indicating the position and attitude of the light target with respect to the optical unit, on the basis of the two images.

PATENT DESCRIPTION
[0001]    The present invention relates to an optical metrology system of a projective type. In particular, the present invention regards a projective optical metrology system for determining attitude and position. 
       BACKGROUND OF THE INVENTION 
       [0002]    As is known, now available are numerous metrology systems, which find use, amongst other things, in the aerospace sector. In particular, in the aerospace sector, metrology systems are known having the function of determining the attitude and/or position of a satellite. Even more in particular, metrology systems are known which enable, given a pair of satellites, determination of the mutual position and mutual attitude of the two satellites. 
         [0003]    Determination of the attitudes and positions of satellites is of particular importance in the case of satellite systems the satellites of which are arranged in formation, i.e., in the cases where there is envisaged the determination of the attitude and position of each satellite as a function of the attitude and position of the other satellites. 
         [0004]    In practice, given two satellites, determination of the mutual attitude and mutual position requires determination of six degrees of freedom. In fact, assuming a first reference system and a second reference system fixed with respect to a first satellite and a second satellite, respectively, and formed, each, by a triad of perpendicular axes, the mutual attitude and mutual position of the first and second reference systems, and hence of the first and second satellites, can be expressed in terms of three (linear) displacements and three rotations (angles). In particular, the mutual position of the first satellite with respect to the second satellite can be expressed by means of a set of three displacements measured, respectively, along the three axes of the second reference system. Likewise, the mutual attitude of the first satellite with respect to the second satellite can be expressed by means of a set of three angles, equal to corresponding rotations of the first reference system with respect to the second reference system. 
         [0005]    This being said, in general optical metrology systems now available can be divided into so-called “coarse” systems and so-called “fine” systems, according to the accuracy and the field of application, the latter being given by the range of distances that can lie between the satellites without the levels of performance degrading significantly. 
         [0006]    In greater detail, fine metrology systems enable determination of the mutual position of two satellites with an accuracy lower than a centimetre, provided that the satellites are not set at a distance apart greater than about fifty metres. Some fine metrology systems even enable determination of the mutual position of two satellites with an accuracy of the order of one tenth of a millimetre, provided that the satellites are not set at a distance apart greater than one metre. 
         [0007]    Instead, coarse metrology systems are characterized by an accuracy not lower than about ten centimetres. However, they are able to operate also when the distance between the satellites is greater than fifty metres, for example also up to distances of twenty kilometres. 
         [0008]    By way of example, coarse metrology systems comprise metrology systems based upon the use of the satellite global positioning system (GPS), as well as metrology systems based upon the use of radio-frequency radiation, the latter resorting to considerably complex antenna networks. 
         [0009]    As regards, instead, fine metrology systems, known to the art are systems of an at least in part projective type, which envisage that, given two satellites, one of them is equipped with a target formed by a number N of light sources, and the other is equipped with an optical unit, which includes an optoelectronic sensor able to acquire an image of the target, on the basis of which, by means of post-processing, the optical unit itself determines one or more of the aforementioned degrees of freedom. 
         [0010]    By way of example, the patent application No. BP1986018 describes a system for determining the position and attitude of a system with six degrees of freedom, and where the number N of light sources of the target is equal to one. However, to enable determination of all six degrees of freedom, the system described in the document No. EP1986018 requires the individual light source to be formed by a coherent-light source such as, for example, a laser, and moreover requires that the optical unit will be able to carry out, in addition to processing of the images of the target, measurements of the power effectively received by the optoelectronic sensor and an angular measurement of rotation of the polarization of the beam of light emitted by the coherent-light source. 
         [0011]    In even greater detail, with reference to a first satellite and a second satellite, and assuming that the target is located on the first satellite, the system described in the document No. EP1986018 envisages that the optical unit on board the second satellite will be equipped with three optoelectronic detectors that are able to process images and detect, each, the power associated to the fraction of electromagnetic radiation emitted by the coherent-light source and effectively impinging upon the optoelectronic detector itself. Consequently, the system described in the document No. EP1986018 is not of a purely projective type. 
         [0012]    There are on the other hand known also fine metrology systems that do not envisage determination of measurements of power, i.e., metrology systems of a purely projective type. With respect to what is described in the document No. EP1986018, said systems of a projective type require the use of targets formed by a large number of light sources. An example of said metrology systems is provided in the document No. U.S. Pat. No. 7,561,262, where the light sources are formed by reflectors designed to be arranged on the first satellite, which are illuminated by radiation emitted by the second satellite. Moreover known are fine metrology systems of a purely projective type, where the target is formed by a particularly large number of light sources (for example, eight). 
         [0013]    The use of targets formed by a large number of light sources involves a greater complexity of construction, and moreover entails an increase in the consumption of electric power both in the case where the light sources are optically active (i.e., they emit light signals) and in the case where they are passive (i.e., they are formed, for example, by reflectors). In fact, in the latter case it is necessary to irradiate the satellite housing the target with a particularly wide electromagnetic-wave front in order to illuminate all the reflectors, with consequent expenditure of electromagnetic power. 
         [0014]    Moreover known are so-called “star-tracking” systems. For example, the document No. US2005/213096 describes a system designed to receive light signals coming from a stellar field and to generate a pair of images using an array of mirrors that can be actuated independently of one another. The mirrors are arranged in such a way that, given a stellar target that is to be identified, when this stellar target is framed by the system, the corresponding pair of images collapses into a common focal point, enabling recognition of the stellar target itself. This system hence operates on the hypothesis of light rays coming from infinity, and hence parallel to one another; moreover, it does not enable determination of any quantity regarding the position (distance) of the stars with respect to the system itself. 
         [0015]    Finally, systems of the type described in the document No. US2008/111985 are known, where a first image and a second image of a target are formed on two different photosensitive arrays. Quantities of interest are then determined on the basis alternatively of the first image or else of the second image. This system is hence characterized by a certain complexity since it requires the use of two different photosensitive arrays. 
       SUMMARY OF THE INVENTION 
       [0016]    The aim of the present invention is to provide a projective optical metrology system that will solve at least in part the drawbacks of the known art. 
         [0017]    According to the invention, a projective optical metrology system is provided as defined in claim  1 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For a better understanding of the invention, embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
           [0019]      FIGS. 1 ,  4  and  8  show block diagrams of satellite systems including the present metrology system; 
           [0020]      FIG. 2  is a schematic illustration of a light target; 
           [0021]      FIGS. 3 and 7  show block diagrams of an optical unit according to the present invention; 
           [0022]      FIG. 5  is a qualitative illustration of a front view of a detection surface of an optoelectronic sensor of the metrology system illustrated in  FIGS. 1 ,  4  and  8 ; and 
           [0023]      FIG. 6  shows an optical diagram regarding formation of an image. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 1  shows an optical metrology system of a projective type, hereinafter referred to for brevity as “metrology system  1 ”. The metrology system  1  is described moreover in what follows with particular reference to an application of a satellite type, even though it can find use in different spheres of application, as mentioned in the sequel. 
         [0025]    In detail, the metrology system  1  comprises a target  2  designed to be constrained to a first satellite sat 1  and an optical unit  6  designed to be constrained to a second satellite sat 2 . 
         [0026]    As shown in greater detail in  FIG. 2 , the target  2  comprises a first light source  4   a , a second light source  4   b , and a third light source  4   c , each formed, for example, by a corresponding light-emitting diode (LED). The first, second, and third light sources  4   a - 4   c  are arranged coplanar and at the vertices of a hypothetical triangle, for example an isosceles or equilateral triangle. In particular, on the hypothesis of point-like light sources, assuming a first reference system x 1 , y 1 , z 1  fixed with respect to the first satellite sat 1  and formed by three axes x 1 , y 1 , z 1  orthogonal to one another, and assuming moreover that the axis z 1  is perpendicular to the plane in which the first, second, and third light sources  4   a - 4   c  lie, and that the first and second light sources  4   a - 4   b  lie along the axis x 1 , it is possible to express the points of the space in which the first, second, and third light sources  4   a - 4   c  are located, respectively, as [X 0 ,0,0], [−X 0 ,0,0], and [0, y 0 ,0]. 
         [0027]    As shown in  FIG. 3 , the optical unit  6  comprises an optoelectronic sensor  10 , a diaphragm  12  designed to receive the optical rays generated by the target  2 , a first optical beam splitter  14  and a second optical beam splitter  16 , first reflecting element  18  and a second reflecting element  20 , and a first intermediate lens  22  and a second intermediate lens  24 . Purely by way of example, the first and second intermediate lenses  22 ,  24  may be concave. 
         [0028]    The optical unit  6  further comprises an electronic processing unit  30 , which is electrically connected to the optoelectronic sensor  10 . 
         [0029]    In greater detail, the diaphragm  12 , the first optical beam splitter  14 , the first intermediate lens  22 , the second optical beam splitter  16 , and the optoelectronic sensor  10  are arranged in succession and optically aligned. In other words, the first optical beam splitter  14  is arranged between the diaphragm  12  and the first intermediate lens  22 , which in turn is arranged between the first and second optical beam splitters  14 ,  16 ; finally, the second optical beam splitter  16  is arranged between the first intermediate lens  22  and the optoelectronic sensor  10 . 
         [0030]    In greater detail, the optical axes of the diaphragm  12  and of the first intermediate lens  22  are approximately coincident and pass substantially through the centre of the optoelectronic sensor  10 . By way of example, in  FIG. 3  the (coincident) optical axes of the input lens  12  and of the first intermediate lens  22  are designated by OA; for simplicity, in what follows, they will be referred to as “system axis OA”. 
         [0031]    In practice, the diaphragm  12 , the first optical beam splitter  14 , the first intermediate lens  22 , and the second optical beam splitter  16  form a first optical circuit, which will be referred to hereinafter as “first optical train OT 1 ”. 
         [0032]    As regards, instead, the second intermediate lens  24 , it is arranged between the first and second reflecting elements  18 ,  20 , in such a way that the diaphragm  12 , the first optical beam splitter  14 , the first reflecting element  18 , the second intermediate lens  24 , the second reflecting element  20 , and the second optical beam splitter  16  form a second optical circuit, which will be referred to hereinafter as “second optical train OT 2 ”. 
         [0033]    Operatively, if the optical rays generated by the target  2  and received by the diaphragm  12  are referred to as “overall optical rays”, a first fraction of the overall optical rays is focused by the first optical train OT 1  on the optoelectronic sensor  10 , along the system axis OA, i.e., after traversing the first optical beam splitter  14 , the first intermediate lens  22 , and the second optical beam splitter  16 . The first fraction of the overall optical rays, which will be referred to hereinafter as “first optical beam”, is shown in  FIG. 4  in a purely qualitative way, where it is designated by R. In practice, the first optical beam R 1  comprises the optical rays that traverse, in addition to the input lens  12  and the first intermediate lens  22 , also the first and second optical beam splitters  14 ,  16 , without being reflected thereby. 
         [0034]    The first optical train OT 1  hence defines a first optical path, arranged between the diaphragm  12  and the optoelectronic sensor  10  and having a first length l 1 , understood as the distance that the first optical beam R 1  covers to reach, starting from the diaphragm  12 , the optoelectronic sensor  10 . 
         [0035]    As regards, instead, the second optical train OT 2 , the first reflecting element  18  is arranged so as to receive a second fraction of overall optical rays, comprising, in the case in point, the portion of overall optical rays that, after traversing the diaphragm  12 , are reflected by the first optical beam splitter  14 . The second fraction of overall optical rays, which will be referred to hereinafter as “second optical beam”, is shown in  FIG. 4  in a purely qualitative way, where it is designated by R 2 . In practice, the first optical beam splitter  14  operates so as to separate the first and second optical beams R 1 , R 2 . 
         [0036]    The first reflecting element  18 , the second intermediate lens  24 , and the second reflecting element  20  are moreover arranged in such a way that the second optical beam R 2  is reflected by the first reflecting element  18  in the direction of the second intermediate lens  24 . After traversing the second intermediate lens  24 , the second optical beam R 2  is received by the second reflecting element  20 , which reflects it in the direction of the second optical beam splitter  16 . Finally, the second optical beam splitter  16  reflects the second optical beam R 2  in the direction of the optoelectronic sensor  10 , once again along the system axis OA. In practice, the second optical beam splitter  16  operates so as to recombine the first and second optical beams R 1 , R 2 . 
         [0037]    Operatively, the second optical train OT 2  is able to direct the second optical beam R 2  towards the optoelectronic sensor  10 . The second optical train OT 2  hence defines a second optical path, arranged between the diaphragm  12  and the optoelectronic sensor  10  and having a second length l 2 , greater than the first length l 1  and understood as the distance that the optical rays of the aforementioned second optical beam R 2  cover to reach, starting from the diaphragm  12 , the optoelectronic sensor  10 . 
         [0038]    The second optical path does not traverse the first intermediate lens  22 , and moreover shares with the first optical path one and the same input point, defined by the diaphragm  12 , and one and the same terminal point, defined by the optoelectronic sensor  10 . 
         [0039]    In greater detail, each lens between the first and second intermediate lenses  22 ,  24  has a respective pair of principal planes, also known as front principal plane and rear principal plane, which, in the case where the lens is thin, collapse into a single principal plane. Consequently, assuming that the first and second intermediate lenses  22 ,  24  are effectively thin, they have, respectively, a first principal plane P 1  and a second principal plane P 2 , which pass through the optical centres of the first and second intermediate lenses  22 ,  24 , respectively. In addition, the first and second intermediate lenses  22 ,  24  have, respectively, a first focal length f 1  and a second focal length f 2 , the second focal length f 2  being greater than the first focal length f 1 . 
         [0040]    In even greater detail, assuming for simplicity that the first optical beam splitter  14  has negligible dimensions, the first and second principal planes P 1 , P 2 , and hence also the first and second intermediate lenses  22 ,  24 , are set apart from the first optical beam splitter  14 , respectively, by a first distance d 1  and a second distance d 2 , measured along the first and second optical paths, respectively. In other words, neglecting the inclination of the optical rays with respect to the system axis OA, the first optical beam R 1  propagates, to reach the first principal plane P 1  starting from the first beam splitter  14 , over a distance equal to d 1 . Likewise, the second optical beam R 2  propagates, to reach the second principal plane P 2  starting from the first optical beam splitter  14 , over a distance equal to d 2 . 
         [0041]    In practice, from an optical standpoint, the first and second principal planes P 1 , P 2  are set apart from the target  2 , respectively, by a distance d p1 =D sat +d 1  and d p2 =D sat +d 2 , where D sat  is the distance between the target  2  and the first optical beam splitter  14 . Consequently, from an optical standpoint, the first and second principal planes P 1 , P 2  are at different distances from the target  2 . Equivalently, the first and second principal planes P 1 , P 2  are at different distances from the diaphragm  12 , since they are at different distances from the first optical beam splitter  14 , and the first and second optical paths, between the diaphragm  12  and the first optical beam splitter  14 , coincide. In particular, if d c  is the distance between the diaphragm  12  and the first optical beam splitter  14 , the first and second principal planes P I , P 2  are set apart from the diaphragm  12  by a distance d c +d 1  and a distance d c +d 2 , respectively. It should be noted that d 1  and d 2  can also assume negative values, in the case where the first principal plane P 1  and/or the second principal plane P 2  are/is arranged between the diaphragm  12  and the first optical beam splitter  14 . 
         [0042]    In addition, the first and second principal planes P 1 , P 2 , and hence the first and second intermediate lenses  22 ,  24 , are set apart from the optoelectronic sensor  10 , respectively, by a third distance d 3  and a fourth distance d 4 , measured, respectively, along the first and second optical paths. In other words, neglecting the inclination of the optical rays with respect to the system axis OA, the first optical beam R 1  propagates, to reach the optoelectronic sensor  10  starting from the first principal plane P 1 , over a distance equal to d 3 . 
         [0043]    Likewise, the second optical beam R 2  propagates, to reach the optoelectronic sensor  10  starting from the second principal plane P 2 , over a distance equal to d 4 . 
         [0044]    In greater detail, as shown in  FIG. 5 , the optical sensor  10  can be formed, for example, by a charge-coupling device (CCD), and has a detection surface A having, for example, a square shape. The aforementioned third and fourth distances d 3  and d 4  express precisely the distances of the first and second principal planes P 1 , P 2  from the detection surface A. 
         [0045]    The detection surface A is formed by an array of M image elements generally known as pixels  11  and is arranged, as mentioned previously, in a direction perpendicular to the system axis OA, i.e., in a direction perpendicular to the optical axes of the diaphragm  12  and of the first intermediate lens  22 . Moreover, the geometrical centre of the detection surface A preferably lies along the system axis OA. In practice, the optoelectronic sensor  10  is a so-called image sensor. 
         [0046]    Once again with reference to the third and fourth distances d 3 , d 4 , preferably they are substantially equal, respectively, to the first and second focal lengths f 1  and f 2 . Consequently, given a hypothetical light source set at an infinite distance from the second satellite sat 2 , the corresponding first and second optical beams R 1 , R 2  are focused substantially in a single point of the detection surface A of the optical sensor  10 . 
         [0047]    As regards, instead, the target  2 , set inevitably at a non-infinite distance from the second satellite sat 2 , in a position corresponding to the optical sensor  10  a first image and a second image of the target  2  itself are formed. 
         [0048]    In greater detail, as shown qualitatively in  FIG. 4 , the first and second focal lengths f 1  and f 2  are such that, when the first satellite sat 1 , and hence the target  2 , are at the distance D sat  from the second satellite sat 2 , or rather from the first optical beam splitter  14 , both the first optical beam R 1  and the second optical beam R 2  impinge effectively on the detection surface A of the optical sensor  10 , there concurring in determining a first distribution of M light intensities, as detected by the M pixels  11  of the optical sensor  10 . 
         [0049]    In particular, as shown in  FIG. 5 , the first optical beam R 1  forms on the detection surface A of the optical sensor  10  a first image I 1  of the target  2 , formed by a first image dot  4   a ′, a second image dot  4   b ′, and a third image dot  4   c ′ corresponding, respectively, to the first, second, and third light sources  4   a - 4   c . Moreover, also the second optical beam R 2  impinges upon the detection surface A of the optical sensor  10  and concurs, hence, to forming the first distribution of M light intensities. In particular, the second fraction R 2  of overall optical rays form on the detection surface A of the optical sensor  10  a second image I 2  of the target  2 , formed by a fourth image dot  4   a ″, a fifth image dot  4   b ″, and a sixth image dot  4   c ″, corresponding, respectively, to the first, second, and third light sources  4   a - 4   c . It should be noted that, for simplicity of exposition, in  FIG. 5  it is assumed that each image dot excites a single pixel, even though typically each image dot excites a number of pixels greater than one. 
         [0050]    On the basis of the first distribution of M light intensities, the electronic processing unit  30  is able to determine a first pixel, a second pixel, and a third pixel designated by  11   a ,  11   b , and  11   c , respectively, which correspond, respectively, to the first, second, and third image dots  4   a ′- 4   c ′. In addition, the electronic processing unit  30  is able to determine a fourth pixel, a fifth pixel, and a sixth pixel, designated by  11   d ,  11   e , and  11   f , respectively, which correspond to the fourth, fifth, and sixth image dots  4   a ″- 4   c ″, respectively. 
         [0051]    For example, the electronic processing unit  30  can identify the first, second, third, fourth, fifth, and sixth pixels  11   a - 11   f  by means of a search for the relative maxima between the M light intensities of the first distribution of M light intensities. In this case, in a way in itself known, the first, second, third, fourth, fifth, and sixth pixels  11   a - 11   f  are characterized in that, considering any one of them, the light intensity detected thereby is greater than the light intensities detected by the pixels surrounding it. 
         [0052]    In other words, the first, second, third, fourth, fifth, and sixth image dots  4   a ′- 4   c ′,  4   a ″- 4   c ″ can be identified electronically as distinct from one another, and in particular as corresponding to six different pixels. This means that both the first image I 1  and the second image I 2  are clear; namely, each of them can be processed electronically by the electronic processing unit  30  in such a way as to identify, in a way in itself known, a number of relative maxima of light intensity equal to the number of light sources present in the target  2  (in the case in point, three). 
         [0053]    Instead, if, hypothetically, the first, second, and third image dots  4   a ′- 4   c ′ were all to excite one and the same pixel, they would not be identifiable electronically as distinct from one another; hence, the first image I 1  of the target  2  would not be clear. 
         [0054]    In practice, the fact that a generic image formed on the detection surface A of the optical sensor  10  is clear depends, in a way in itself known, upon the dimensions of the target  2 , the distance of the target  2  from the second satellite sat 2 , the focal length of the corresponding optical train that has generated it, as well as the area of the detection surface A of the optical sensor  10 . In this connection, in what follows, it will be assumed, without this implying any loss of generality, that the detection surface A has a square shape of side Q, and that each pixel  11  has a square shape of side q. 
         [0055]    Moreover, strictly speaking, the fact that a generic image is clear or otherwise depends upon the (known) processing techniques adopted by the electronic processing unit  30  to process the distribution of M light intensities detected by the M pixels  11 . In fact, even though it is possible, as described previously, that the electronic processing unit  30  determines the relative maxima of light intensity directly on the basis of the light intensities detected by the pixels  11 , it is moreover possible that the determination of the relative maxima is carried out by means of the so-called centroid method, or similar methods. In particular, the centroid method, described for example in “Error analysis of CCD-based point source centroid computation under the background light”, by Xiaoyu Ma, Changhui Rao and Hanging Zheng, OPTICS EXPRESS Vol. 17, No. 10 and in “Improved Iteration Centroid Algorithm Based on Linear CCD Light-spot Location”, Shengjin Tang, Zhaofa Zhou, Xiaosong Guo, Yongchao Xiao, The Ninth International Conference on Electronic Measurement &amp; Instruments, ICEMI 2009, envisages interpolation of the light intensities detected by the pixels  11 , in themselves discretized in space, in order to determine space-continuous interpolating curves (for example, Gaussian curves), on the basis of which the relative maxima are determined. It should be noted that, in the case where the electronic processing unit  30  determines the relative maxima of light intensity merely on the basis of the light intensities detected by the pixels  11 , without operations of interpolation, a generic image is clear if the electronic processing unit  30  is able to identify, for each image dot of the generic image, a corresponding pixel that exhibits a relative maximum of light intensity. If this occurs, the electronic processing unit  30  associates the image dots to the co-ordinates of the corresponding pixels. For example, in the case of the first, second, third, fourth, fifth, and sixth image dots  4   a ′- 4   c ′ and  4   a ″- 4   c ″, they are, respectively, associated to the co-ordinates (described hereinafter) of the first, second, third, fourth, fifth, and sixth pixels  11   a - 11   f . Consequently, the maximum discretization that it is possible to achieve in determining the position, within the detection surface A, of possible image dots is equal to the side q of a single pixel. 
         [0056]    Instead, by adopting known interpolation techniques, such as for example the centroid method, it is possible to obtain a spatial discretization lower than q, i.e., it is possible to associate to the image dots co-ordinates with a discretization lower than q. In fact, for an image to be clear it is sufficient for the electronic processing unit  30  to be able to identify, on the basis of the M light intensities detected by the pixels  11  and in a way in itself known, a number of relative maxima of light intensity equal to the number of light sources present in the target  2 , irrespective of whether, on the basis of the light intensities detected by the pixels and of the search alone for the relative maxima between said light intensities, it is possible to identify a number of pixels corresponding to the number of light sources present in the target  2 . For example, the identification of a number of pixels corresponding to the number of light sources present in the target  2  can be prevented by the fact that this search supplies a number of relative maxima lower or higher than the number of light sources. 
         [0057]    For simplicity of description, in what follows it will be assumed, except where otherwise specified, that the electronic processing unit  30  determines the relative maxima of light intensity merely on the basis of the search for the relative maxima between the light intensities detected by the pixels  11 , without operations of interpolation, and hence that the first, second, third, fourth, fifth, and sixth image dots  4   a ′- 4   c ′ and  4   a ″- 4   c ″ are associated, respectively, to the co-ordinates of the first, second, third, fourth, fifth, and sixth pixels  11   a - 11   f.    
         [0058]    Once again with reference to the first and second images I 1  and I 2  shown in  FIG. 5 , assuming a second reference system x 2 , y 2 , z 2  formed by three axes x 2 , y 2 , z 2  orthogonal to one another, fixed with respect to the second satellite sat 2  and such that the detection surface A of the optical sensor  10  lies in the plane defined by the axes x 2  and y 2 , the co-ordinates of each of the first, second, and third pixels  11   a - 11   c  can be expressed as triads, respectively, of the type [X 1P , y 1P , 0], [X 2P , Y 2P , 0] and [X 3P , y 3P ,0]. Likewise, the co-ordinates of each of the fourth, fifth, and sixth pixels  11   e - 11   f  can be expressed as triads, respectively, of the type [X′ 1P , y′ 1P ,0], [X′ 2P , Y′ 2P , 0] and [X′ 3P , y′ 3P , 0]. 
         [0059]    Next, on the basis of the triads of co-ordinates associated to the first, second, third, fourth, fifth, and sixth image dots  4   a ′- 4   c ′,  4   a ″- 4   c ″, and on the basis of the shape of the target  2 , i.e., on the basis of the triads of co-ordinates of the first, second, and third light sources  4   a - 4   c , the electronic processing unit  30  determines six quantities for mutual attitude and mutual position of the first and second satellites sat 1 , sat 2 . In particular, the electronic processing unit  30  determines the angles θ, β, γ, a distance D, and the displacements Δx, Δy, which refer to rotations or translations that enable, starting from the six triads of co-ordinates associated to the first, second, third, fourth, fifth, and sixth pixels  11   a - 11   f , the co-ordinates of the first, second, and third light sources  4   a - 4   c  to be obtained. 
         [0060]    In greater detail, the angles θ, β, γ refer, respectively, to rotations about the axes x 1 , y 1  and z 1 , whilst the displacements Δx, Δy refer to translations along the axes y 1 . The distance D, instead, is the distance between the target  2  and the first optical beam splitter  14 , i.e., to a first approximation, the distance between the first and second satellites sat 1 , sat 2 . 
         [0061]    Still more in particular, in a way in itself known, assuming a mutual reference arrangement of the first reference system x 1 , y 1  and z 1  with respect to the second reference system x 2 , y 2  and z 2 , the angles θ, β, γ and the displacements Δx, Δy indicate corresponding rototranslations of the first reference system x 1 , y 1  and z 1  with respect to this mutual reference arrangement. 
         [0062]    Operatively, the electronic processing unit  30  resorts to the so-called homogeneous co-ordinates, in themselves known in the field of projective geometry. In particular, the electronic processing unit  30  uses a three-dimensional reference system in homogeneous co-ordinates, of a right-hand type, i.e., where the positive rotations are the rotations in a counterclockwise direction. Consequently, translations and rotations can be expressed, in a way in itself known, in terms of corresponding four-by-four matrices. With reference to the aforementioned angles θ, β, γ, the corresponding rotations can hence be expressed as 
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                              
                             
                                 
                             
                              
                             γ 
                           
                         
                         
                           
                             cos 
                              
                             
                                 
                             
                              
                             γ 
                           
                         
                         
                           0 
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           0 
                         
                         
                           1 
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           0 
                         
                         
                           0 
                         
                         
                           1 
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       R 
                       x 
                     
                      
                     
                       ( 
                       ϑ 
                       ) 
                     
                   
                   = 
                   
                     [ 
                     
                       
                         
                           1 
                         
                         
                           0 
                         
                         
                           0 
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           
                             cos 
                              
                             
                                 
                             
                              
                             ϑ 
                           
                         
                         
                           
                             sin 
                              
                             
                                 
                             
                              
                             ϑ 
                           
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           
                             
                               - 
                               sin 
                             
                              
                             
                                 
                             
                              
                             ϑ 
                           
                         
                         
                           
                             cos 
                              
                             
                                 
                             
                              
                             ϑ 
                           
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           0 
                         
                         
                           0 
                         
                         
                           1 
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       R 
                       y 
                     
                      
                     
                       ( 
                       β 
                       ) 
                     
                   
                   = 
                   
                     [ 
                     
                       
                         
                           
                             cos 
                              
                             
                                 
                             
                              
                             β 
                           
                         
                         
                           0 
                         
                         
                           
                             
                               - 
                               sin 
                             
                              
                             
                                 
                             
                              
                             β 
                           
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           1 
                         
                         
                           0 
                         
                         
                           0 
                         
                       
                       
                         
                           
                             sin 
                              
                             
                                 
                             
                              
                             β 
                           
                         
                         
                           0 
                         
                         
                           
                             cos 
                              
                             
                                 
                             
                              
                             β 
                           
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           0 
                         
                         
                           0 
                         
                         
                           1 
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    whilst a generic translation, formed by a generic triad of displacements of the type [Sx, Sy, Sz], can be expressed as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       T 
                       xyz 
                     
                      
                     
                       ( 
                       
                         Sx 
                         , 
                         Sy 
                         , 
                         Sz 
                       
                       ) 
                     
                   
                   = 
                   
                     [ 
                     
                       
                         
                           1 
                         
                         
                           0 
                         
                         
                           0 
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           1 
                         
                         
                           0 
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           0 
                         
                         
                           1 
                         
                         
                           0 
                         
                       
                       
                         
                           Sx 
                         
                         
                           Sy 
                         
                         
                           Sz 
                         
                         
                           1 
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0063]    Given a generic rigid body the centroid of which initially has cartesian co-ordinates [X 01 , y 01 , z 01 ], and which is subjected to rotations and/or translations, it is thus possible to determine mathematically a new triad of cartesian co-ordinates [X 11 , y 11 , z 11 ], corresponding to the position assumed by the centroid of the rigid body following upon rotations and/or translations. For this purpose, it is possible to apply to the homogeneous co-ordinates [X 01 , y 01 , z 01 , 1], corresponding to the initial position of the rigid body, the aforementioned rotation matrix and/or translation matrix, in the same order with which the rotations and/or translations effectively occur. We have in fact 
         [0000]        [X   11   ,y   11   ,z   11 ,1]=[ X   01   ,y   01   ,z   01 ,1]· R   x (θ)· R   y (β)· R   z (γ)· T   xyz (Δ x,Δy,Δz )  (5)
 
         [0064]    In other words, the equation that links the final homogeneous co-ordinates assumed by the centroid of the rigid body to the initial homogeneous co-ordinates assumed by the centroid of the rigid body is 
         [0000]      [ X   11   ,y   11   ,z   11 ,1 ]=[X   01   ,y   01   ,z   01 ,1]· M (θ,β,γ,Δ x,Δy,Δz )  (6)
 
         [0000]    where the matrix M defines mathematically any rototranslation to which the rigid body is subject and has the following form 
         [0000]    
       
         
           
             
               
                 
                   M 
                   = 
                   
                       
                     
                       [ 
                       
                         
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               γcos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               γcos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               
                                 - 
                                 sin 
                               
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 ϑsin 
                                  
                                 
                                     
                                 
                                  
                                 βcos 
                                  
                                 
                                     
                                 
                                  
                                 γ 
                               
                               - 
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 γcos 
                                  
                                 
                                     
                                 
                                  
                                 ϑ 
                               
                             
                           
                           
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 γsin 
                                  
                                 
                                     
                                 
                                  
                                 βsin 
                                  
                                 
                                     
                                 
                                  
                                 ϑ 
                               
                               + 
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 γcos 
                                  
                                 
                                     
                                 
                                  
                                 ϑ 
                               
                             
                           
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               ϑcosβ 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 ϑsin 
                                  
                                 
                                     
                                 
                                  
                                 βcos 
                                  
                                 
                                     
                                 
                                  
                                 γ 
                               
                               - 
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 γsin 
                                  
                                 
                                     
                                 
                                  
                                 ϑ 
                               
                             
                           
                           
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 γsin 
                                  
                                 
                                     
                                 
                                  
                                 βcos 
                                  
                                 
                                     
                                 
                                  
                                 ϑ 
                               
                               - 
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 γsin 
                                  
                                 
                                     
                                 
                                  
                                 ϑ 
                               
                             
                           
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               ϑcosβ 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               x 
                             
                           
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               y 
                             
                           
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               z 
                             
                           
                           
                             1 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0065]    It is moreover known that, as shown schematically in  FIG. 6 , given a generic thin lens with focal length f, if a point-like object having cartesian co-ordinates [X 01 , y 01 , z 01 ] is set in front of the thin lens and at a distance D s  therefrom, it generates, in a hypothetical plane arranged behind the thin lens and at a distance d s  approximately equal to the focal length f, a corresponding image dot having co-ordinates approximately equal to X 01P =X 01 ·f/D s  and Y 01P =Y 01 ·f/D s . This physical phenomenon is expressed by the well-known equation of optical projection 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       P 
                       ′ 
                     
                      
                     
                       ( 
                       
                         
                           X 
                           
                             01 
                              
                             P 
                           
                         
                         , 
                         
                           Y 
                           
                             01 
                              
                             P 
                           
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       P 
                       ′ 
                     
                      
                     
                       ( 
                       
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             
                               X 
                               01 
                             
                           
                           
                             D 
                             s 
                           
                         
                         , 
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             
                               Y 
                               01 
                             
                           
                           
                             D 
                             s 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0066]    Substituting the equation of optical projection (8) in Eq. (6), we obtain 
         [0000]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           X 
                           
                             
                               11 
                                
                               P 
                             
                             , 
                           
                         
                          
                         
                           Y 
                           
                             11 
                              
                             P 
                           
                         
                       
                       , 
                       0 
                       , 
                       
                         f 
                         
                           D 
                           s 
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         f 
                         
                           D 
                           s 
                         
                       
                        
                       
                         [ 
                         
                           
                             X 
                             01 
                           
                           , 
                           
                             Y 
                             01 
                           
                           , 
                           0 
                           , 
                           1 
                         
                         ] 
                       
                     
                      
                     
                         
                       
                         [ 
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 γ 
                                  
                                 
                                     
                                 
                                  
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 γ 
                                  
                                 
                                     
                                 
                                  
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               
                                 
                                   - 
                                   sin 
                                 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   ϑsin 
                                    
                                   
                                       
                                   
                                    
                                   β 
                                    
                                   
                                       
                                   
                                    
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   γ 
                                 
                                 - 
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γ 
                                    
                                   
                                       
                                   
                                    
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                               
                             
                             
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γsin 
                                    
                                   
                                       
                                   
                                    
                                   βsinϑ 
                                 
                                 + 
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   γcos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                               
                             
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 ϑcosβ 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                    
                                   
                                       
                                   
                                    
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   βcosγ 
                                 
                                 - 
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γsinϑ 
                                 
                               
                             
                             
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γsinβcosϑ 
                                 
                                 - 
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   γsin 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                               
                             
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 ϑcos 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 x 
                               
                             
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 y 
                               
                             
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 z 
                               
                             
                             
                               1 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0000]    which sets in relation the co-ordinates [X 01 , y 01 , z 01 ] of the centroid of the rigid body prior to the rototranslation with the co-ordinates [X 11P , y 11P , z 11P ] of the image dot for the same centroid, after the rigid body has undergone a rototranslation. More in particular, the co-ordinates [X 01 , y 01 , 0] regard a reference system such that Z 01 =0, whereas the co-ordinates [X 11P , y 11P , 0] regard a reference system such that Z 11P =0. 
         [0067]    This being said, with reference to the reference systems x 1 , y 1 , z 1  and x 2 , y 2 , z 2 , if we assume that the angle θ is zero and we apply Eq. (9) with reference to the first light source  4   a  and to the first image dot  4   a ′, we obtain 
         [0000]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           X 
                           
                             
                               1 
                                
                               P 
                             
                             , 
                           
                         
                          
                         
                           Y 
                           
                             1 
                              
                             P 
                           
                         
                       
                       , 
                       0 
                       , 
                       
                         
                           f 
                           1 
                         
                         
                           D 
                           + 
                           
                             d 
                             1 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         
                           f 
                           1 
                         
                         
                           D 
                           + 
                           
                             d 
                             1 
                           
                         
                       
                        
                       
                         [ 
                         
                           
                             X 
                             0 
                           
                           , 
                           0 
                           , 
                           0 
                           , 
                           1 
                         
                         ] 
                       
                     
                      
                     
                       [ 
                       
                         
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               γcos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               γcos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               
                                 - 
                                 sin 
                               
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             
                               
                                 - 
                                 sin 
                               
                                
                               
                                   
                               
                                
                               γ 
                             
                           
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               γ 
                             
                           
                           
                             0 
                           
                           
                             0 
                           
                         
                         
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               βcos 
                                
                               
                                   
                               
                                
                               γ 
                             
                           
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               γsin 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               x 
                             
                           
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               y 
                             
                           
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               z 
                             
                           
                           
                             1 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0068]    Likewise, if we assume once again that the angle θ is zero, and we apply Eq. (9) with reference to the second light source  4   b  and to the second image dot  4   b ′, we obtain 
         [0000]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           X 
                           
                             
                               2 
                                
                               P 
                             
                             , 
                           
                         
                          
                         
                           Y 
                           
                             2 
                              
                             P 
                           
                         
                       
                       , 
                       0 
                       , 
                       
                         
                           f 
                           1 
                         
                         
                           D 
                           + 
                           
                             d 
                             1 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         
                           f 
                           1 
                         
                         
                           D 
                           + 
                           
                             d 
                             1 
                           
                         
                       
                        
                       
                         [ 
                         
                           
                             - 
                             
                               X 
                               0 
                             
                           
                           , 
                           0 
                           , 
                           0 
                           , 
                           1 
                         
                         ] 
                       
                     
                      
                     
                       [ 
                       
                         
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               γcos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               γcos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               
                                 - 
                                 sin 
                               
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             
                               
                                 - 
                                 sin 
                               
                                
                               
                                   
                               
                                
                               γ 
                             
                           
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               γ 
                             
                           
                           
                             0 
                           
                           
                             0 
                           
                         
                         
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               βcos 
                                
                               
                                   
                               
                                
                               γ 
                             
                           
                           
                             
                               sin 
                                
                               
                                   
                               
                                
                               γsin 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             
                               cos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               x 
                             
                           
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               y 
                             
                           
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               z 
                             
                           
                           
                             1 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0069]    Rendering explicit the matrix forms and subtracting Eq. (11) from Eq. (10) member by member, we obtain the pair of equations 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             
                               X 
                               
                                 1 
                                  
                                 P 
                               
                             
                             - 
                             
                               X 
                               
                                 2 
                                  
                                 P 
                               
                             
                           
                           = 
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 X 
                                 
                                   12 
                                    
                                   P 
                                 
                               
                             
                             = 
                             
                               
                                 
                                   2 
                                    
                                   
                                     f 
                                     1 
                                   
                                 
                                 
                                   D 
                                   + 
                                   
                                     d 
                                     1 
                                   
                                 
                               
                                
                               
                                 X 
                                 0 
                               
                                
                               cos 
                                
                               
                                   
                               
                                
                               γ 
                                
                               
                                   
                               
                                
                               cos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               Y 
                               
                                 1 
                                  
                                 P 
                               
                             
                             - 
                             
                               Y 
                               
                                 2 
                                  
                                 P 
                               
                             
                           
                           = 
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 Y 
                                 
                                   12 
                                    
                                   P 
                                 
                               
                             
                             = 
                             
                               
                                 
                                   2 
                                    
                                   
                                     f 
                                     1 
                                   
                                 
                                 
                                   D 
                                   + 
                                   
                                     d 
                                     1 
                                   
                                 
                               
                                
                               
                                 X 
                                 0 
                               
                                
                               sin 
                                
                               
                                   
                               
                                
                               γ 
                                
                               
                                   
                               
                                
                               cos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
         [0070]    Operating on the pair constituted by the first light source  4   a  and the fourth image dot  4   a ″ and on the pair constituted by the second light source  4   b  and the fifth image dot  4   b ″ in a way similar to what is described, respectively, with reference to the pair constituted by the first light source  4   a  and the first image dot  4   a ′ and to the pair constituted by the second light source  4   b  and the second image dot  4   b ′, we obtain the pair of equations 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             
                               X 
                               
                                 1 
                                  
                                 P 
                               
                               ′ 
                             
                             - 
                             
                               X 
                               
                                 2 
                                  
                                 P 
                               
                               ′ 
                             
                           
                           = 
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 X 
                                 
                                   12 
                                    
                                   P 
                                 
                                 ′ 
                               
                             
                             = 
                             
                               
                                 
                                   2 
                                    
                                   
                                     f 
                                     2 
                                   
                                 
                                 
                                   D 
                                   + 
                                   
                                     d 
                                     2 
                                   
                                 
                               
                                
                               
                                 X 
                                 0 
                               
                                
                               cos 
                                
                               
                                   
                               
                                
                               γ 
                                
                               
                                   
                               
                                
                               cos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               Y 
                               
                                 1 
                                  
                                 P 
                               
                               ′ 
                             
                             - 
                             
                               Y 
                               
                                 2 
                                  
                                 P 
                               
                               ′ 
                             
                           
                           = 
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 Y 
                                 
                                   12 
                                    
                                   P 
                                 
                                 ′ 
                               
                             
                             = 
                             
                               
                                 
                                   2 
                                    
                                   
                                     f 
                                     2 
                                   
                                 
                                 
                                   D 
                                   + 
                                   
                                     d 
                                     2 
                                   
                                 
                               
                                
                               
                                 X 
                                 0 
                               
                                
                               sin 
                                
                               
                                   
                               
                                
                               γ 
                                
                               
                                   
                               
                                
                               cos 
                                
                               
                                   
                               
                                
                               β 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0071]    In practice, the pairs of equations (12) and (13) are obtained on the basis, respectively, of the first and second images I 1  and I 2  of the target  2 , as supplied, respectively, by the first and second optical trains OT 1  and OT 2 , which have different focal lengths equal, respectively, to the first and second focal lengths f 1 , f 2  of the first and second intermediate lenses  22 ,  24 . In addition, the first and second intermediate lenses  22 ,  24  are arranged at different optical distances from the target  2 . 
         [0072]    On the basis of the two pairs of equations (12) and (13), it is thus possible to obtain 
         [0000]    
       
         
           
             
               
                 
                   
                     γ 
                     = 
                     
                       arctan 
                        
                       
                         ( 
                         
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               Y 
                               
                                 12 
                                  
                                 P 
                               
                             
                           
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               X 
                               
                                 12 
                                  
                                 P 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     D 
                     = 
                     
                       
                         
                           
                             f 
                             1 
                           
                            
                           
                             d 
                             2 
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           
                             X 
                             
                               12 
                                
                               P 
                             
                             ′ 
                           
                         
                         - 
                         
                           
                             f 
                             2 
                           
                            
                           
                             d 
                             1 
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           
                             X 
                             
                               12 
                                
                               P 
                             
                           
                         
                       
                       
                         
                           
                             f 
                             1 
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           
                             X 
                             
                               12 
                                
                               P 
                             
                           
                         
                         - 
                         
                           
                             f 
                             2 
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           
                             X 
                             
                               12 
                                
                               P 
                             
                             ′ 
                           
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     β 
                     = 
                     
                       ar 
                        
                       
                           
                       
                        
                       
                         cos 
                         ( 
                         
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               
                                 X 
                                 
                                   12 
                                    
                                   P 
                                 
                               
                                
                               
                                 ( 
                                 
                                   D 
                                   + 
                                   
                                     d 
                                     1 
                                   
                                 
                                 ) 
                               
                             
                           
                           
                             2 
                              
                             
                               f 
                               1 
                             
                              
                             
                               X 
                               0 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   a 
                                    
                                   
                                       
                                   
                                    
                                   
                                     tan 
                                      
                                     
                                       ( 
                                       
                                         
                                           Δ 
                                            
                                           
                                               
                                           
                                            
                                           
                                             Y 
                                             
                                               12 
                                                
                                               P 
                                             
                                           
                                         
                                         
                                           Δ 
                                            
                                           
                                               
                                           
                                            
                                           
                                             X 
                                             
                                               12 
                                                
                                               P 
                                             
                                           
                                         
                                       
                                       ) 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0073]    Once the angles γ, β and the distance D are known, the displacements Δx, Δy can be determined on the basis of the following pair of equations 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       x 
                     
                     = 
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           X 
                           
                             12 
                              
                             P 
                           
                         
                       
                       - 
                       
                         
                           
                             f 
                             1 
                           
                           
                             D 
                             + 
                             
                               d 
                               1 
                             
                           
                         
                          
                         
                           X 
                           0 
                         
                          
                         cos 
                          
                         
                             
                         
                          
                         γcos 
                          
                         
                             
                         
                          
                         β 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       y 
                     
                     = 
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           Y 
                           
                             12 
                              
                             P 
                           
                         
                       
                       - 
                       
                         
                           
                             f 
                             1 
                           
                           
                             D 
                             + 
                             
                               d 
                               1 
                             
                           
                         
                          
                         
                           X 
                           0 
                         
                          
                         sin 
                          
                         
                             
                         
                          
                         γcos 
                          
                         
                             
                         
                          
                         β 
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0074]    Finally, the angle θ can be determined by applying Eq. (9) with reference to the third light source  4   c  and to the third image dot  4   c ′, as well as on the basis of the angles γ, β and of the displacements Δx, Δy just determined. In particular, applying Eq. (9) with reference to the third light source  4   c  and to the third image dot  4   c ′ we obtain 
         [0000]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           X 
                           
                             
                               3 
                                
                               P 
                             
                             , 
                           
                         
                          
                         
                           Y 
                           
                             3 
                              
                             P 
                           
                         
                       
                       , 
                       0 
                       , 
                       
                         
                           f 
                           1 
                         
                         
                           D 
                           + 
                           
                             d 
                             1 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         
                           f 
                           1 
                         
                         
                           D 
                           + 
                           
                             d 
                             1 
                           
                         
                       
                        
                       
                         [ 
                         
                           0 
                           , 
                           
                             Y 
                             0 
                           
                           , 
                           0 
                           , 
                           1 
                         
                         ] 
                       
                     
                      
                     
                         
                       
                         [ 
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 γ 
                                  
                                 
                                     
                                 
                                  
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 γ 
                                  
                                 
                                     
                                 
                                  
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               
                                 
                                   - 
                                   sin 
                                 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   ϑsin 
                                    
                                   
                                       
                                   
                                    
                                   β 
                                    
                                   
                                       
                                   
                                    
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   γ 
                                 
                                 - 
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γ 
                                    
                                   
                                       
                                   
                                    
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                               
                             
                             
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γsin 
                                    
                                   
                                       
                                   
                                    
                                   βsinϑ 
                                 
                                 + 
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   γcos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                               
                             
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 ϑcosβ 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                    
                                   
                                       
                                   
                                    
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   βcosγ 
                                 
                                 + 
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γsinϑ 
                                 
                               
                             
                             
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γsinβcosϑ 
                                 
                                 - 
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   γsin 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                               
                             
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 ϑcos 
                                  
                                 
                                     
                                 
                                  
                                 β 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 x 
                               
                             
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 y 
                               
                             
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 z 
                               
                             
                             
                               1 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
         [0075]    Rendering explicit Eq. (16), we obtain 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             X 
                             
                               3 
                                
                               P 
                             
                           
                           = 
                           
                             
                               
                                 
                                   f 
                                   1 
                                 
                                  
                                 
                                   Y 
                                   0 
                                 
                               
                               
                                 D 
                                 + 
                                 
                                   d 
                                   1 
                                 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   ϑsin 
                                    
                                   
                                       
                                   
                                    
                                   βcos 
                                    
                                   
                                       
                                   
                                    
                                   γ 
                                 
                                 - 
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   γcos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                                 + 
                                 
                                   
                                     Δ 
                                      
                                     
                                         
                                     
                                      
                                     x 
                                   
                                   
                                     Y 
                                     0 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             Y 
                             
                               3 
                                
                               P 
                             
                           
                           = 
                           
                             
                               
                                 
                                   f 
                                   1 
                                 
                                  
                                 
                                   Y 
                                   0 
                                 
                               
                               
                                 D 
                                 + 
                                 
                                   d 
                                   1 
                                 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   sin 
                                    
                                   
                                       
                                   
                                    
                                   ϑsin 
                                    
                                   
                                       
                                   
                                    
                                   βsin 
                                    
                                   
                                       
                                   
                                    
                                   γ 
                                 
                                 + 
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   γcos 
                                    
                                   
                                       
                                   
                                    
                                   ϑ 
                                 
                                 + 
                                 
                                   
                                     Δ 
                                      
                                     
                                         
                                     
                                      
                                     y 
                                   
                                   
                                     Y 
                                     0 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
         [0076]    The trigonometric system ( 17 ) can be transformed into a pair of parametric equations imposing the conditions 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           m 
                           = 
                           
                             sin 
                              
                             
                                 
                             
                              
                             ϑ 
                           
                         
                       
                     
                     
                       
                         
                           n 
                           = 
                           
                             sin 
                              
                             
                                 
                             
                              
                             β 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
         [0000]    and collecting the known term adopting the following notations 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             a 
                             1 
                           
                           = 
                           
                             cot 
                              
                             
                                 
                             
                              
                             γ 
                           
                         
                       
                     
                     
                       
                         
                           
                             a 
                             2 
                           
                           = 
                           
                             tan 
                              
                             
                                 
                             
                              
                             γ 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
             
               
                 
                   
                     P 
                      
                     
                       ( 
                       D 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         f 
                         1 
                       
                        
                       
                         Y 
                         0 
                       
                     
                     
                       D 
                       + 
                       
                         d 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
           
         
       
     
         [0000]    and 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       b 
                       1 
                     
                     = 
                     
                       
                         
                           X 
                           
                             3 
                              
                             P 
                           
                         
                         
                           
                             P 
                              
                             
                               ( 
                               D 
                               ) 
                             
                           
                            
                           sin 
                            
                           
                               
                           
                            
                           γ 
                         
                       
                       - 
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           x 
                         
                         
                           
                             Y 
                             0 
                           
                            
                           sin 
                            
                           
                               
                           
                            
                           γ 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       b 
                       2 
                     
                     = 
                     
                       
                         
                           Y 
                           
                             3 
                              
                             P 
                           
                         
                         
                           
                             P 
                              
                             
                               ( 
                               D 
                               ) 
                             
                           
                            
                           cos 
                            
                           
                               
                           
                            
                           γ 
                         
                       
                       - 
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           y 
                         
                         
                           
                             Y 
                             0 
                           
                            
                           cos 
                            
                           
                               
                           
                            
                           γ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   21 
                   ) 
                 
               
             
           
         
       
     
         [0000]    so that the trigonometric system ( 17 ) becomes 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       
                                         a 
                                         1 
                                       
                                        
                                       mn 
                                     
                                     - 
                                     
                                       b 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                               = 
                               
                                 1 
                                 - 
                                 
                                   m 
                                   2 
                                 
                               
                             
                           
                         
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       
                                         - 
                                         
                                           a 
                                           2 
                                         
                                       
                                        
                                       mn 
                                     
                                     + 
                                     
                                       b 
                                       2 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                               = 
                               
                                 1 
                                 - 
                                 
                                   m 
                                   2 
                                 
                               
                             
                           
                         
                       
                       ⇒ 
                       
                         
                           ( 
                           
                             
                               
                                 a 
                                 1 
                               
                                
                               mn 
                             
                             - 
                             
                               b 
                               1 
                             
                           
                           ) 
                         
                         2 
                       
                     
                     = 
                     
                       
                         
                           
                             ( 
                             
                               
                                 
                                   - 
                                   
                                     a 
                                     2 
                                   
                                 
                                  
                                 mn 
                               
                               + 
                               
                                 b 
                                 2 
                               
                             
                             ) 
                           
                           2 
                         
                         ⇒ 
                         m 
                       
                       = 
                       
                         
                           
                             b 
                             1 
                           
                           + 
                           
                             b 
                             2 
                           
                         
                         
                           n 
                            
                           
                             ( 
                             
                               
                                 a 
                                 1 
                               
                               + 
                               
                                 a 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   22 
                   ) 
                 
               
             
           
         
       
     
         [0000]    The angle θ is hence equal to 
         [0000]    
       
         
           
             
               
                 
                   ϑ 
                   = 
                   
                     arcsin 
                      
                     
                       ( 
                       
                         
                           
                             b 
                             1 
                           
                           + 
                           
                             b 
                             2 
                           
                         
                         
                           
                             sin 
                              
                             
                               ( 
                               β 
                               ) 
                             
                           
                            
                           
                             ( 
                             
                               
                                 a 
                                 1 
                               
                               + 
                               
                                 a 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   23 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Summarizing, we hence have 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           ϑ 
                           = 
                           
                             arcsin 
                             ( 
                             
                               
                                 
                                   
                                     X 
                                     
                                       3 
                                        
                                       P 
                                     
                                   
                                   
                                     
                                       P 
                                        
                                       
                                         ( 
                                         D 
                                         ) 
                                       
                                     
                                      
                                     sin 
                                      
                                     
                                         
                                     
                                      
                                     γ 
                                   
                                 
                                 + 
                                 
                                   
                                     Δ 
                                      
                                     
                                         
                                     
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                   ( 
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         [0077]    In practice, the metrology system  1 , of a bifocal type, enables determination in an analytical and non-ambiguous way of all six degrees of freedom that characterize the position and attitude of the target  2  with respect to the optical unit  6 . This is due to the fact of having available two different images of the target  2 , obtained by means of two optical systems (in the case in point, the first and second optical trains OT 1  and OT 2 ) that share one and the same input point of the optical rays, have principal planes that are at different distances from the target and have different focal lengths. In this way, two different images of the target  2  are formed on the detection surface A. 
         [0078]    In general, in order to enable clear formation of the first and second images I 1 , I 2  when the target  2  is set at a design distance D prog  from the optical unit  6 , it is possible to determine the first and second focal lengths f 1 , f 2  as described hereinafter. 
         [0079]    In this connection, it is premised that, given any distance between the first and second satellites sat 1  and sat 2 , the first and second optical trains OT 1  and OT 2  produce, in a hypothetical plane extending at infinity and containing the detection surface A, two different images of the target  2 . In particular, the image produced by the second optical train OT 2  has greater dimensions than that produced by the first optical train OT 1 , because the second focal length f 2  is greater than the first focal length f 1 , and hence the second optical train OT 2  is characterized, as compared to the first optical train OT 1 , by a greater enlargement. 
         [0080]    This being said, it is possible to determine the first and second focal lengths f 1 , f 2  on the basis of the target  2  and of the detection surface A, so that, when the target  2  is at the design distance D prog , the first and second images I 1 , I 2  are contained within the detection surface A and are clear. In this connection, it may be noted that, given for simplicity of calculation a simplified target (not shown) formed by just two light sources set at a distance O from one another, the corresponding first image is certainly clear if it occupies at least two pixels of the optical sensor  11 . For this purpose, it is possible to impose f 1 =(D prog −H q )/O, where H q  is equal to 2·q (where q is the length of the side of a pixel). Since the first image is clear, also the corresponding second image will be clear, in so far as it is obtained with an optical train having greater enlargement; however, the second focal length f 2  must be such that the second image is contained entirely within the detection surface A. For this purpose, it is possible to impose f 2 &lt;(D prog ·Q)/O, where Q is precisely the length of the side of the detection surface A. 
         [0081]    As shown in  FIG. 7 , moreover possible is an embodiment in which the optical unit  6  comprises a first optical unit  42  and a second optical unit  44 , each of which is formed by a number of lenses. Purely by way of example, in the embodiment shown in  FIG. 7  the first optical unit  42  comprises, in addition to the first intermediate lens  22 , a third intermediate lens  42   a  and a fourth intermediate lens  42   b , arranged between which is the first intermediate lens  22 . Likewise, the second optical unit  44  comprises, in addition to the second intermediate lens  24 , a fifth intermediate lens  44   a  and a sixth intermediate lens  44   b , arranged between which is the second intermediate lens  24 . In the case where there are the first and second optical units  42 ,  44 , as on the other hand in the case where the first and second intermediate lenses  22 ,  24  are not thin, the first and second optical trains OT 1  and OT 2  each have a respective pair of principal planes. In this case, as shown qualitatively in  FIG. 7 , the first optical unit  42  defines, in addition to the first principal plane, here designated by P 42a , a third principal plane P 42b . Likewise, the second optical unit  44  defines, in addition to the second principal plane, here designated by P 44a , a fourth principal plane P 44b . Moreover, as shown once again in  FIG. 7 , the first and second principal planes P 42a , P 44a  do not necessarily pass through the optical centres of the first and second intermediate lenses  22 ,  24 . 
         [0082]    Purely by way of example, in the embodiment shown in  FIG. 7  the first principal plane P 42a  lies between the first optical beam splitter  14  and the third intermediate lens  42   a , whilst the second principal plane P 44a  lies between the first reflecting element  18  and the fifth intermediate lens  44   a . Moreover, the third principal plane P 44b  lies between the fourth intermediate lens  42   b  and the second optical beam splitter  16 , whilst the fourth principal plane P 44b  lies between the sixth intermediate lens  44   b  and the second reflecting element  20 . 
         [0083]    In practice, the first and third principal planes P 42a , P 42b  form, respectively, the front principal plane and the rear principal plane of the first optical unit  42 , whilst the second and fourth principal planes P 44a , P 44b  form, respectively, the front principal plane and the rear principal plane of the second optical unit  44 . 
         [0084]    In a way in itself known, each between the first and the second optical units  42 ,  44  moreover has a so-called front effective focal length and a rear effective focal length. 
         [0085]    If the rear effective focal lengths of the first and second optical units  42 ,  44 , respectively, are referred to as “first and second equivalent focal lengths f e1 , f e2 ”, and if d 3  and d 4  are the distances of the third and fourth principal planes P 42b , P 44b  from the optoelectronic sensor  10 , the latter can be approximately equal, respectively, to the first and second equivalent focal lengths f e1 , f e2 . 
         [0086]    In greater detail, it is again possible to designate by d 1  and d 2  the distances of the first and second principal planes P 42a , P 44a  from the first optical beam splitter  14 . Moreover, the first and third distances d 1 , d 3  are once again measured along the first optical path, whilst the second and fourth distances d 2  and d 4  are once again measured along the second optical path. This being said, in the case of the embodiment illustrated in  FIG. 7 , Eqs. (10-24) are modified accordingly, by replacing the first and second focal lengths f 1 , f 2 , respectively, with the first and the second equivalent focal lengths f e1 , f e2 , without any decay of the levels of performance of the optical unit  6 . In addition, the presence of the first and second optical units  42 ,  44  enables an improvement of the immunity of the optical unit  6  to optical phenomena such as, for example, aberration. 
         [0087]    Moreover possible are embodiments in which the optoelectronic sensor  10  comprises an additional detection surface, for example equal to the detection surface A, and where the second optical path terminates on this additional detection surface, instead of on the detection surface A. In this case, the second image I 2  of the target  2  is formed on the additional detection surface. 
         [0088]    Still different embodiments are moreover possible, where the target has a number N s  of light sources, with N s  other than three and where the optical unit  6  is provided with further optical trains in such a way that it is designed to form, on the detection surface A, a number N I  of images, where N I  is the total number of optical trains present in the optical unit  6 . For example, we may have N s =2 and N I =3. In fact, by increasing the number of images, it is possible to reduce the number of light sources of the target, without foregoing determination of any of the aforementioned quantities. It is moreover possible that the electronic processing unit  30  will determine quantities different from the angles θ, β, γ, the distance D, and the displacements Δx, Δy, such as for example quantities referenced to non-cartesian reference systems, or else a subset of these quantities. 
         [0089]    Furthermore, as shown in  FIG. 8 , the present metrology system can be used within a satellite system  60  including the first and second satellites sat 1 , sat 2 , and where the second satellite sat 2  houses a system for controlling attitude and position  62 , electronically coupled to the electronic processing unit  30 . The attitude and position control system  62  thus receives the quantitieth that, in time, are determined by the electronic processing unit  30 , iterating the operations described previously. The attitude and position control system  62  can hence operate, in a way in itself known, on the basis of the quantities supplied by the electronic processing unit  30 . 
         [0090]    The advantages that the present metrology system  1  affords emerge clearly from the foregoing description. In particular, it enables an analytical and non-ambiguous determination of all six degrees of freedom that characterize the position and attitude of the target  2  with respect to the optical unit  6  using a target having a limited number of light sources. Moreover, since both the first optical path and the second optical path start in one and the same point, i.e., in a position corresponding to the diaphragm  12 , phenomena of stereoscopic vision are not created. 
         [0091]    Finally, it is evident that modifications and variations may be made to the metrology system  1  described herein, without thereby departing from the scope of the present invention. 
         [0092]    For example, instead of the diaphragm  12 , an input lens may be present. Moreover, the first and second intermediate lenses  22 ,  24  may be of a type different from what has been described. 
         [0093]    The first and second optical beam splitters  14 ,  16  may be formed by corresponding prisms, or else may be of any known type. 
         [0094]    Again, the light sources of the target may be arranged at the vertices of a scalene triangle and/or may be different from what has been described. For example, the light sources may be passive, each comprising a mirror, for example of the so-called “corner cube” type. In this case, the optical unit  6  further comprises an illuminator designed to illuminate the target in such a way that the mirrors that form the light sources reflect radiation towards the optical unit  6  itself. 
         [0095]    Finally, as mentioned previously, the metrology system  1  can find uses in sectors other than the aerospace sector. For example, the target  2  can be mounted on a first object, whilst the optical unit  6  can be mounted on a second object, the first and second objects being mechanically coupled in such a way that the metrology system enables determination of the mutual positioning of the first and second objects. For example, said first and second objects can be two different elements of a telescope, mechanically connected together.