Title of Invention: Method of estimating the attitude of a space device with improved precision Abstract: The invention provides a method of determining the attitude (φ, θ, ψ) of a spacecraft, the method comprising the steps consisting in: a) sensing the projections (xmi, ymi) of a plurality of stars on an optical star sensor mounted on the spacecraft; b) providing a reference focal length (fref) for said sensor, said focal length serving to convert the projections of the stars (xmi, ymi) as picked up on the sensor into sensor-star directions (wi) in a frame of reference associated with the sensor; and c) determining the angular values of the attitude of the spacecraft relative to the stars by solving a system of equations which has as its input data in particular the projections of the stars (xmi, ymi) on the sensor and the reference focal length of the sensor; the method being characterized by introducing, as unknowns in the system of equations, at least one parameter (D1, . . . , D7) defining a series of focal length errors, each of which errors has an influence on converting the projection (xmi, ymi) of a respective star into the corresponding sensor-star direction (Wi); and in step c), by determining said parameter(s) (D1, . . . , D7) simultaneously with determining the values for the attitude angles (φ, θ, ψ) of the spacecraft; in such a manner that taking said focal length error parameter(s) into account as unknowns in the system reduces the uncertainty introduced by incomplete knowledge of these errors on the results for the values of the attitude angles (φ, θ, ψ) of the spacecraft. Detailed Description: The invention relates to estimating the attitude of a spacecraft such as a satellite by means of a star sensor mounted on such a craft, and the invention relates more precisely to reducing errors that appear in such attitude estimation. More precisely, the present invention relates to reducing so-called “field of view” (FOV) errors. Nowadays, the performance expected of an observation satellite is very ambitious in terms of absolute localization: for example, accuracy of 10 meters (m) is required in 90% of cases and of 20 m in 99.7% of cases. The main factors contributing to such performance are error in determining attitude, and also thermoelastic deformation between the instrument line of sight and the attitude sensors. For example, it is possible to define and propose a “gyrostellar” attitude measuring system based on three very high performance fiber optic gyros (FOGs) and three wide-angle (≈25°) star sensors. Star sensors present various sources of error in measuring attitude, which can be classified depending on their time nature. Generally, distinctions are made between: bias, harmonics, FOV error, and white noise. The impacts of these various time categories on the performance of the attitude-determining subassembly are different. In particular, gyrostellar filtering serves to reduce the impact of the “noise” category. The harmonic category is essentially of thermoelastic origin, and its impact can be limited by controlling the temperature environment of the sensor. The bias category has small impact insofar as calibration means can be implemented. The FOV error of a star sensor has greatest impact on performance in determining the attitude of such a star sensor or of the spacecraft on which it is mounted. This leads to specifications being issued that are quite severe concerning star sensors and for which there has been no response until now. Indeed specifications have been relaxed in order to comply with that which is available. In this context, analysis of results shows that FOV error can be assumed to be 10 microradians (μrad) in a configuration comprising 3 FOGs+3 standard star trackers (SSTs), which error can be reduced to 6 μrad by the gyrostellar estimator filter. This error, combined quadratically with other factors, gives rise to an overall attitude and orbit control system (AOCS) end result of 11 μrad, for an allocation of 13 μrad. Configurations with 2 or 1 SSTs lead to degraded performance. These observations have been confirmed by statistical results obtained from simulations, and have led to a present tightening of FOV error specifications as issued to the suppliers of star sensors. FOV errors lead to phenomena such as incomplete knowledge of and fluctuation in focal length, distortion calibration residues, chromatic aberrations, and errors in knowledge about the absolute positions of stars (catalog errors, relativistic correction residues). They lead to errors in measuring the position of each star in the focal plane, said errors being of amplitudes and directions that vary as a function of the position of the star in the field of view. Wide-angle star sensors track a large number of stars simultaneously (typically in the range about ten to a few tens). The resulting attitude measurement error depends on the distribution of stars in the field of view and on the way it varies over time under the effect of the angular speed of the carrying satellite. In the context of geocentric pointing, and particularly for a satellite that is maneuvering, induced determination error can be modeled as noise filtered by a lowpass filter-having a time constant associated with the mean time taken by a star to pass through the field of view. Because of the resulting relatively large time constants that stem therefrom (about 100 seconds), such noise is difficult to reduce by the attitude estimator filter which combines star measurements with gyro measurements. With inertial pointing (e.g. solar pointing), this substantially constant measurement error is no longer filtered at all by the attitude estimator algorithm. Furthermore, such a model of field of view errors is somewhat theoretical, and complete simulations associating fine modeling of the sensor with dynamic modeling would be needed to quantify such errors more precisely. In parallel, it therefore appears appropriate to pursue lines of investigation that enable this source of error to be reduced without impact on the hardware architecture of existing sensors, for example by means of an inline calibration algorithm. The invention seeks to reduce the amplitude of FOV errors in attitude-estimation methods. The invention achieves this object by a method of determining the attitude of a spacecraft, the method comprising the steps consisting in: a) sensing the projections of a plurality of stars on an optical star sensor mounted on the spacecraft; b) providing a reference focal length for said sensor, said focal length serving to convert the projections of the stars as picked up on the sensor into sensor-star directions in a frame of reference associated with the sensor; and c) determining the angular values of the attitude of the spacecraft relative to the stars by solving a system of equations which has as its input data in particular the projections of the stars on the sensor and the reference focal length of the sensor; the method being characterized by introducing, as unknowns in the system of equations, at least one parameter defining a series of focal length errors, each of which errors has an influence on converting the projection of a respective star into the corresponding sensor-star direction; and in step c), by determining said parameter(s) simultaneously with determining the values for the attitude angles of the spacecraft; in such a manner that taking said focal length error parameter(s) into account as unknowns in the system reduces the uncertainty introduced by incomplete knowledge of these errors on the results for the values of the attitude angles of the spacecraft. The invention also provides apparatus for determining the attitude of a spacecraft using an optical star sensor, the apparatus including such an optical star sensor arranged to sense the projections of a plurality of stars of said sensor, the apparatus further comprising calculation means suitable for determining the values of attitude angles of the spacecraft relative to the stars by solving a system of equations having as its inputs specifically the projections of the stars on the sensor and also a reference focal length of the sensor, said focal length serving to convert the star projections into sensor-star directions in a frame of reference associated with the sensor, the apparatus being characterized in that the calculation-means are arranged to take account of one or more parameters defining a series of focal length errors as one or more additional parameters in the system of equations, each of which errors has an influence on converting the projection of a respective star into the corresponding sensor-star direction, the calculation means being arranged to determine said parameter(s) defining a series of focal length errors simultaneously with determining the values for the attitude angles of the spacecraft. Other characteristics, objects, and advantages of the invention will appear on reading the following detailed description made with reference to the accompanying figures, in which: FIG. 1 shows a field of view facing a detector used for implementing the invention; FIG. 2 shows reference points used for implementing measurements and calculation steps in a preferred method of implementing the method; FIG. 3 shows a field of twelve stars used for analyzing the effects of the method of the invention; FIG. 4 shows the effects of noise on the results of attitude estimation using a conventional method; FIG. 5 shows the effects of noise and of incomplete knowledge of focal length on the results of a conventional method; FIG. 6 shows the effects of noise, of incomplete knowledge of focal length, and of distortion at the center of the field on the results obtained with a conventional method; FIG. 7 shows the effects of the same errors on the results of a method that includes estimating focal length at the center of the field; FIG. 8 shows the effects of an algorithm with distortion being estimated in accordance with the invention; FIG. 9 shows a plurality of star configurations used for testing a preferred method of the invention; FIGS. 10a to 10f show results obtained with and without the method of the invention in the various different star configurations; FIGS. 11a to 11f show results obtained with and without the method of the invention in making estimates of pointing errors; FIGS. 12a to 12f show results obtained with greater distortions; FIGS. 13a to 13f show results obtained with smaller distortions; FIGS. 14a to 14f show results obtained with non-radial errors having an amplitude of 0.3″; FIGS. 15a to 15f show results obtained with non-radial errors having an amplitude of 1″; FIGS. 16a to 16f show the results obtained with non-radial errors having an amplitude of 5″; FIGS. 17a to 17c show the time-varying outputs of attitude as calculated by using conventional filtering; FIG. 18 shows a distribution of light rays, revealing distortion; FIGS. 19a to 19c show time-varying curves of attitude error as estimated by using conventional filtering; FIG. 20 shows a calculation reference point for a method in a variant of the invention; FIGS. 21a to 21f show results obtained with three algorithms, including two algorithms of the invention, for multiple fixed star configurations; FIG. 22 shows the impact of the stars moving on estimation performance; FIGS. 23a to 23f show the impact of the stars moving on estimation performance; FIG. 24 shows results obtained with a conventional algorithm for moving and stationary star fields; FIG. 25 shows the spectral densities of an angle φ when using conventional filtering; FIG. 26 shows the same elements as FIG. 25 but when using a distortion-estimating filter; FIG. 27 shows the spectral densities of the filtered outputs; FIGS. 28a to 28f show results of estimation with a star passing at a speed of 0.1°/s; FIGS. 29a and 29b show spectrum densities of distortion residue; FIGS. 30a to 30f show the result of estimation with a radial distortion relationship including a third-order parameter; FIGS. 31a to 31f show the result of estimating with a “vibration” type pointing error of 500 micrometers (μm); FIGS. 32a to 32f show the result of estimation with a “bias” type pointing error of 500 μm; FIGS. 33a to 33f show results obtained with distortion errors that are five times greater; FIGS. 34a to 34f show results obtained with distortion errors that are five times smaller; FIGS. 35a to 35f show results obtained with non-radial type errors of 1″; and FIGS. 36a to 36f show results obtained with non-radial type errors of 5″. Typical assumptions on the characteristics of a star sensor are those given below, in order to evaluate the proposed algorithm under representative conditions: field of view 25°×25°; focal length 30 centimeters (cm); number of stars tracked: ten; measurement errors for one star: noise: 37″ at three standard deviations (3σ); focal length fluctuation: 1.5″/K at the edge of the field with a temperature environment that is stable to within ±2° C.; distortion residue: 9″ at 3σ; chromatic aberrations: 3″ at 3σ; and catalog errors: 0.3″ at 3σ. In the description below, a preferred implementation of the invention is evaluated by means of simulations performed in the context of this study by using a tool for simulating a measurement error function of an instrument. The tool is known as COROT, it has been developed by the Applicant, and it makes it possible to represent the following: the designation in position and magnitude of stars present in the field of view; the fluctuation of real attitude about an inertial pointing reference; the measured positions of the stars on the detector, taking account of various sources of error; and the attitude-determining algorithm. Amongst possible sources of error, account is taken of the following: noise in measuring the center of gravity of the star spot, with standard deviation that depends on magnitude; interpolation errors; fluctuation over time in focal length at the center of the field (non-centered harmonic model); and the distortion that is specific to each star as a function of its position in the field, and that is represented as a focal length value. Attempts have been made to minimize modifications to the tool. In addition to installing the proposed algorithm, these modifications have consisted mainly in taking account of certain items of configuration data such as the size of the field of view, focal length, and error parameters. In particular, the number of pixels on the detector has been kept identical to that of the COROT case (4140×4140) so as to avoid modifying interfaces with the input catalog, in which the reference positions of stars are defined in pixels. The field of view is defined as being the circle in which the detector is inscribed, as shown. This leads to a “virtual” pixel of size 22.71 μm, i.e. 15.37″. The field of view and the focal length are the values defined in paragraph 2. Inertial pointing operation on a predefined field was maintained, thereby facilitating interpretation of the effects of errors and algorithms compared with circumstances in which the stars move through the field of view. For the same reason, account is taken of incomplete knowledge concerning focal length at the center of the field that remains constant over time. With star sensors, the error due to distortion is associated with a calibration residue for which no assumption is available concerning distribution over the field. It has been assumed that it is distributed randomly over the field, in application of a relationship that is uniform between 0 and the maximum calculated as follows. Nevertheless, it may be assumed that the effect of distortion residues and incomplete knowledge concerning focal length is greater at the edge of the field. The relationship providing a first-order connection between fluctuation in focal length df and measurement error at the edge of the field dθ is: df = 2 · f FOV · d ⁢ ⁢ θ Account is thus taken of thermoelastic fluctuation in the focal-length of 10 μm per kelvin, and a maximum value for the distortion of 60 μm. No account was taken of interpolation error, and for simplification purposes, each star had position measurement noise allocated thereto that was distributed quadratically along two axes, giving a standard deviation of 9″ or 0.6 pixels per axis, regardless of its magnitude. There follows a description of a preferred method of estimating the attitude of a sensor or a spacecraft on which it is mounted (in this case a satellite). A three-dimensional frame of reference associated with the sensor is defined as follows: Zs is the normal to the detector going into space; Xs and Ys lie in a plane parallel to the plane of the detector as defined by the above scheme. Also defined (see FIG. 2) is a two-dimensional frame of reference associated with the detector, having its origin at the point of intersection between the optical axis and the plane of the detector. The axes XCCD and YCCD are respectively parallel to Xs and Ys. Each star given an index i is associated with a reference vector Vi normed in the sensor frame of reference. For a reference focal length at the center of the field fref, this vector Vi is defined as follows, from the reference position (xi, yi) of the star in the detector frame of reference. The following apply: V i = ( vix viy viz ) ⁢ ⁢ x i = f ref · Vi · X s Vi · Z s = vix viz ⁢ ⁢ and ⁢ ⁢ y i = f ref · Vi · Y s Vi · Z s = viy viz and also: V i = 1 1 + ( x i f ref ) 2 + ( y i f ref ) 2 ⁢ ( x i f ref y i f ref 1 ) The “reference” frame of reference of the sensor Rc is such that in this frame of reference the directions of the stars being tracked coincide with the vectors Vi. The real frame of reference of the sensor Rs is such that the directions of the target stars in the sensor frame of reference become vectors Wi. Determining attitude consists in estimating the angular differences between the real frame of reference of the sensor and its reference frame of reference. The following rotation convention is defined: R c ⁢ → ψ ⁢ ⁢ Z _ c ⁢ R ′ ⁢ → θ ⁢ ⁢ Y ′ ⁢ R ″ ⁢ → ϕ ⁢ ⁢ X _ ″ ⁢ R s Wi can be expressed as a function of Vi as follows: Wi = ( cos ⁢ ⁢ ψcos ⁢ ⁢ θ sin ⁢ ⁢ ψcos ⁢ ⁢ θ - sin ⁢ ⁢ θ cos ⁢ ⁢ ψsin ⁢ ⁢ θsinφ - sin ⁢ ⁢ ψcosφ sin ⁢ ⁢ ψsin ⁢ ⁢ θsin ⁢ ⁢ φ + cos ⁢ ⁢ ψcos ⁢ ⁢ φ cos ⁢ ⁢ θsin ⁢ ⁢ φ cos ⁢ ⁢ ψsin ⁢ ⁢ θcos ⁢ ⁢ φ + sin ⁢ ⁢ ψsin ⁢ ⁢ φ sin ⁢ ⁢ ψsin ⁢ ⁢ θcos ⁢ ⁢ φ - cos ⁢ ⁢ ψsin ⁢ ⁢ φ cos ⁢ ⁢ θcos ⁢ ⁢ φ ) · Vi In the absence of measurement error, the ideal measurements associated with Wi are as follows: xm i = f ref · Wi · X s Wi · Z s ⁢ ⁢ and ⁢ ⁢ ym i = f ref · Wi · Y s Wi · Z s The components of Wi can be reconstructed from these measurements as follows: W i = 1 1 + ( xm i f ref ) 2 + ( ym i f ref ) 2 ⁢ ( xm i f ref ym i f ref 1 ) The link between the position of the star on the detector and the associated vector in sensor axes thus relies on knowledge of a focal length. The focal length thus serves to transform the coordinates of an optical projection of a star, which projection is obtained on a projection surface of the sensor, into a sensor-star direction relative to the sensor. The least squares method relies on linearizing measurement equations about the reference. It is then assumed that the angles φ, θ, ψ are small relative to 1, giving, to the first order: Wi ≈ ( 1 ψ - θ - ψ 1 φ θ - φ 1 ) · Vi Thus, still to the first order: xm i f ref = ⁢ - vix + ψ · viy - θ · viz viz + θ · vix - φ · viy ≈ ⁢ - ( vix viz + φ · vix · viy viz 2 - θ · ( 1 + vix 2 viz 2 ) + ψ · viy viz ) ym i f ref = ⁢ - viy - ψ · vix + φ · viz viz + θ · vix - φ · viy ≈ ⁢ - ( viy viz + φ · ( 1 + viy 2 viz 2 ) - θ · vix · viy viz 2 - ψ · vix viz ) Whence, for each target, two measurement equations: Δ i = ( xm i - x i f ref ym i - y i f ref ) = ( vix · viy viz 2 - ( 1 + vix 2 viz 2 ) viy viz ( 1 + viy 2 viz 2 ) - vix · viy viz 2 - vix viz ) · ( φ θ ψ ) = J i · ( φ θ ψ ) By combining the equations associated with n targets, it is thus possible to construct the Jacobian matrix J of dimension 2n×3 such that: ( ⋮ Δ i ⋮ ) = ( ⋮ J i ⋮ ) · ( φ θ ψ ) = J · ( φ θ ψ ) In the presence of measurement errors, this set of linear equations is solved by the least squares method. The solution which minimizes the following criterion: C = ∑ ( ( x ⁢ ⁢ m i - x i f ref ) 2 + ( y ⁢ ⁢ m i - y i f ref ) 2 ) is given by: ( φ θ ψ ) = [ ∑ i ⁢ ( ⋯ ⁢ ⁢ J i T ⁢ ⁢ ⋯ ) · ( ⋮ J i ⋮ ) ] - 1 · ∑ i ⁢ ( ( ⋯ ⁢ ⁢ J i T ⁢ ⁢ ⋯ ) · ( ⋮ Δ i ⋮ ) ) It is also possible to introduce weighting for the variations measurements. In particular, for a given star, it is possible to envisage different weights σxi and σyi on the x and y measurements. Under such circumstances, the solution is provided by the same equation, in which the following formal replacements are made: x ⁢ ⁢ m i - x i f ref ⁢ ⁢ by ⁢ ⁢ x ⁢ ⁢ m i - x i f ref · σ ⁢ ⁢ x i ⁢ ⁢ and ⁢ ⁢ y ⁢ ⁢ m i - y i f ref ⁢ ⁢ by ⁢ ⁢ y ⁢ ⁢ m i - y i f ref · σ ⁢ ⁢ y i Finally, it is also possible to use this method in iterative manner. Work performed by the Applicant on defining a measurement error function for an instrument known as COROT has shown that this method gives results that are similar to those obtained by a QUEST type algorithm without linearization, even in the presence of pointing errors, and that iterations do not provide any particular advantage. The work carried out on COROT has shown that it is possible to reduce significantly the impact of uncertainty concerning focal length on attitude measurement error. The means for doing this consist in formulating attitude measurement equations internal to the sensor, which equations include focal length uncertainty as an unknown. This is possible so long as the number of equations (equal to twice the number of measured stars) remains greater than or equal to the number of unknowns (i.e. 4 for small angles). This assumes that approximate knowledge is available concerning attitude (for example the reference attitude for a guided satellite). This also assumes that measurements can be processed for each star. The method has been evaluated by simulation and has given results that are very satisfactory for two or more tracked stars. To do this, starting from the above-described method, it is possible to include additional unknowns such as fluctuation in focal length, by appropriately adapting the measurement equations. δfsf is defined as follows: δ ⁢ ⁢ fsf = f v - f ref f ref or indeed: fv=fref·(1+δfsf) Assuming that φ, θ, ψ, and δfsf are small compared with 1, the following equations are established for each star: Δ i = ⁢ ( x ⁢ ⁢ m i - x i f ref y ⁢ ⁢ m i - y i f ref ) = ⁢ ( vix · viy viz 2 - ( 1 + vix 2 viz 2 ) viy viz vix viz ( 1 + viy 2 viz 2 ) - vix · viy viz 2 - vix viz viy viz ) · ⁢ ( φ θ ψ δ ⁢ ⁢ fsf ) = ⁢ J i · ⁢ ( φ θ ψ δ ⁢ ⁢ fsf ) Combining the equations associated with the set of N stars enables the Jacobian matrix J of dimension 2N×4 to be constructed, and enables the linear system to be solved by the least squares method. An extension to this algorithm is proposed herein covering estimating the focal length specific to each star (i.e. distortion). If it is considered that distortion is equivalent to a dispersion in focal length over the field of view (generally as an increasing function of distance from the center of the field), the above method can be extended to each star. The vector of the unknowns is then of dimension 3+N: Each star is associated with a local value for focal length fi, and δfsfi is defined as follows: δ ⁢ ⁢ fsf i = f i - f ref f ref Each tracked-star enables the following measurement equations to be established: Δ i = ⁢ ( x ⁢ ⁢ m i - x i f ref y ⁢ ⁢ m i - y i f ref ) = ⁢ ( vix · viy viz 2 - ( 1 + vix 2 viz 2 ) viy viz 0 ⁢ ⁢ … ⁢ ⁢ 0 vix viz 0 ⁢ ⁢ … ⁢ ⁢ 0 ( 1 + viy 2 viz 2 ) - vix · viy viz 2 - vix viz 0 ⁢ ⁢ … ⁢ ⁢ 0 viy viz 0 ⁢ ⁢ … ⁢ ⁢ 0 ) · ⁢ ( φ θ ψ ⋮ δ ⁢ ⁢ fsf i ⋮ ) = ⁢ J i · ⁢ ( φ θ ψ ⋮ δ ⁢ ⁢ fsf i ⋮ ) Combining the equations associated with the set of N stars enables the Jacobian matrix J of dimensions (2N)×(3+N) to be constructed, and enables the linear system to be solved by the least squares method. At least three stars are necessary in order to have as many-equations as there are unknowns. Since this method relies on geometrical modeling, it can be expected that it also makes it possible to reduce all of the errors compatible with the model (radial coupling between axes in the focal plane). Nevertheless, this method has the drawback of requiring a matrix to be inverted, where the dimension of the matrix increases with the number of stars observed, being (3+N)×(3+N). Using the assumptions and the tools described above, the effects of this method of determining attitude are studied in the manner described below. Initially the description is restricted to the specific case of a configuration of stars in the field of view under inertial pointing. The time signature in attitude of the various errors at star level, i.e. noise, incomplete knowledge concerning focal length at the center of the field, and distortion residue, is validated for a conventional attitude estimation algorithm. Then the contribution of algorithms for estimating focal length and distortion is shown. Finally, the advantages and the limits of the algorithm using distortion estimates are evaluated by analyzing robustness. To begin with, the analysis is carried out on a field of 12 stars having the distribution in the field as shown in FIG. 3. It is recalled that these stars remain stationary in the field of view throughout the simulation. In order to identify the effects of various errors and to validate the approach, we begin by examining the behavior of the simple algorithm for determining attitude by the least squares method. Work carried out on COROT has shown that this algorithm can be considered as a “conventional” algorithm, giving results that are equivalent to those obtained by the QUEST method. Simulations were carried out on about 12,600 shots. It has thus been verified that with inertial pointing and a conventional algorithm, the effect of time noise on measuring star positions does indeed lead to noise in attitude estimation, and that so-called “field of view” errors lead to bias. For each shot, noise was added in application of a Gaussian relationship. The effect of measurement noise on its own is shown in FIG. 4. The resulting attitude noise is practically centered (mean of about 100th of a second of arc), with a standard deviation on the cross axes of about 2.7″. On applying the following multiplying factors: {square root}12 associated with the number of stars; and {square root}2 associated with combining two cross axes; to go from 1σ to 3σ, a value of 39″ is indeed found which is close to the measurement noise value used as an initial assumption at star level. Error concerning knowledge about focal length is applied to the entire simulation. The results are shown in FIG. 5. It does not affect standard deviation, but, as could be expected with inertial pointing, it does lead to perceptible bias on the cross axis (in this case dtheta). The link between the value u bias and incomplete knowledge concerning focal length depends on the configuration of the stars in the field. The effect of the distortion is shown in FIG. 6. The same typical errors are observed as with “noise only”, but as expected, in this case also there can be seen relatively large amounts of bias on all three axes, as a function of the distortion local to each star and as a function of the configuration of the stars in the field. FIG. 7 shows the results of determining attitude using the same errors as above (noise, incomplete knowledge concerning focal length and distortion, and with all shots reproduced identically), and with the algorithm that includes focal length in the vector of unknowns, as described above. The standard deviations remain unchanged, but bias can be seen to vary, mostly getting worse. This is attributed to the relative importance of distortion compared with incomplete knowledge about focal length in this simulation. Thermoelastic fluctuation in focal length is of little importance in terms of distortion residue, given the relatively accurate control over the temperature environment of the sensor. Such an algorithm is thus of some advantage but it would be desirable to improve it. FIG. 8 shows the effects of the algorithm in which distortion is estimated. There can be seen a clear reduction in bias compared with estimating attitude alone, bias being reduced to a level comparable with that observed with noise type errors only. However standard deviations are degraded by a factor of under 2. Nevertheless, the attitude estimation noise introduced by the SSTs is well filtered by the gyrostellar estimator. This approach which reduce the impact of FOV errors to the detriment of noise thus essentially has a clear advantage. It is desirable to multiply star fields in order to assess the behavior of the algorithm in the presence of different configurations of stars. It is then desirable to study robustness, and in particular: in the face of pointing error (insofar as the method relies on linearizing measurement equations about a reference attitude); in the face of assumptions about the relative weight of distortion-compared with measurement noise; and in the face of other sources of FOV error. The simulations were repeated for 100 configurations of stars. In each case, the positions of ten stars in the field of view were drawn randomly in application of a uniform relationship, as was the associated distortion residue. In order to reduce the time required for stimulation, the number of shots on which bias and standard deviation were evaluated was reduced to 6000. The first five cases (distinguished by the symbols represented the respective stars) are shown in FIG. 9. Thereafter, the biases and the standard deviations on the attitude as determined with the conventional algorithm were compared with those for attitude as determined using the algorithm with distortion estimation. The results are shown in FIG. 10, where the maximum bias value obtained with the algorithm estimating distortion is given at the bottom of each curve. The results are also summarized in the following table, in which there can be distinguished the effects on the cross axes (marked ⊥) and the effects on the SST aiming axis (marked //). In these figures, crosses represents the results obtained with the simple least squares method, and dots represent the results obtained with the least squares method obtained with distortion estimation (6000 shots, nominal case). In the nominal case, the following results were obtained: Maximum values over 6000 shots Mean ⊥ Std. dev. Mean // Std. dev. in (″) axes ⊥ axes axes // axis Conventional algorithm 1.8 3.5 8.0 32.5 Algorithm with distortion 0.2 7.8 1.0 45.0 estimation The maximum value of the bias obtained with the conventional algorithm is close to 2″, which is compatible with the worst case FOV error found in the previously evaluated results using 3 SSTs. With the distortion estimating algorithm, there is observed a reduction in said bias compared with the conventional algorithm by a factor that is close to 10, on all three measurement axes, thereby demonstrating the effectiveness of the proposed algorithm. However, the standard deviation for noise on the cross axes is degraded by a factor of under 2 on average. The worst cases of degradation are correlated with the worst results of the conventional algorithm and are therefore attributed to the geometrical configuration of the stars. Noise degradation is not noticeable on the pointing axis. These results, obtained under a very wide variety of geometrical configurations, confirm the conclusions of the preceding paragraph. It is clearly shown that the value of the bias-depends on the geometrical configuration, which confirms that variation of the star field in view as the satellite-moves will lead to “low frequency” variation over time in attitude measurement error. It is thus to be expected that there will be a reduction in error amplitude due to distortion residue when using a moving star field. It is recalled that this method relies on inline processing of measurements and does not rely on any filtering. Pointing error has been taken into account on a random basis in application of a distribution relationship that is uniform over the range ±500 μrad on all three axes. The result is shown in FIG. 11 (6000 shots with pointing errors, crosses=simple least squares, dots=least squares with distortion estimation) and these results are summarized in the table below. With pointing error: Maximum values over 6000 shots Mean ⊥ Std. dev. Mean // Std. dev. in (″) axes ⊥ axes axes // axis Conventional algorithm 1.8 3.5 8.0 32.5 Algorithm with distortion 0.2 7.8 1.0 45.0 estimation For both algorithms, the order of magnitude of biases and standard deviations remains unaffected. The effectiveness of the algorithm with distortion estimation is comparable. The algorithm is therefore robust in the face of pointing errors of the order of magnitude that is to be expected. Simulations were repeated using assumptions that were successively five times greater and five times smaller for distortion error, other sources of error remaining unchanged (noise, pointing error). The respective results are shown in FIG. 12 (6000 shots, distortions ×5, crosses=simple least squares, blue dots=least squares with distortion error), and FIG. 13 (6000 shots, distortions /5, crosses=simple least squares, dots=least squares with distortion estimation), and they are summarized in the following tables. Greater distortion: Maximum values over 6000 shots Mean ⊥ Std. dev. Mean // Std. dev. in (″) axes ⊥ axes axes // axis Conventional algorithm 8.5 3.5 48.0 33.0 Algorithm with distortion 0.2 7.5 0.9 45.0 estimation Smaller distortion: Maximum values over 6000 shots Mean ⊥ Std. dev. Mean // Std. dev. in (″) axes ⊥ axes axes // axis Conventional algorithm 0.7 3.5 2.0 33.0 Algorithm with distortion 0.2 7.5 0.9 45.0 estimation Attitude error standard deviations are insensitive to distortion assumptions, they depend only on star position measurement noise and on algorithm. The reduction in bias introduced by the algorithm increases with the assumption that distortion is large compared with noise. There thus exists a lower limit for distortion residue relative to noise, below which the distortion estimating algorithm does nothing other than degrade noise without significantly improving bias. The geometrical meaning of the underlying equations causes the distortion estimator algorithm to be well adapted to determining radial type errors, i.e. errors characterized by radial coupling of the two components of the position of a star in the focal plane. It is important to evaluate the robustness of the distortion estimator algorithm faced with different geometrical distribution errors. The category of FOV errors includes other sources of errors depending on the star or on its position in the field: chromatic aberration, catalog errors, interpolation errors. Their impact on attitude measurements under inertial pointing also leads to bias associated with the configuration of the stars in the field. The impact of chromatic aberrations on the performance of star position measurement as identified in a review of sensors varies greatly from one sensor to another. It lies in the range 30% to 150% of the distortion factor, from which it is not always clearly distinguished. To the first order, it appears that in an optical system, axial type chromatic aberrations do not have any effect on the centers of gravity of star spots, and that lateral type chromatics has an effect that is characterized by radial coupling like distortion. Nevertheless, this needs to be confirmed with the manufacturers of sensors as a function of the optical designs used. Depending on manufacturer, the amplitude of catalog errors is evaluated as lying in the range negligible to 10% of distortion residue. The errors associated therewith may be considered as being distributed randomly as a function of star and along both axes in the focal plane. Interpolation errors are not always distinguished from other FOV errors. It can be assumed that they are of arbitrary distribution in the plane of the detector if inertial pointing is perfect. These errors nevertheless give rise to a source of “high frequency” coupling between attitude measurement and pointing that make their effects difficult to assess. They are often treated as being time-varying noise. The robustness of the algorithm has therefore been assessed in the face of error that is distributed randomly in amplitude and in direction as a function of each star. This has been done using a function for stimulating interpolation error. In the simulation tool, it is represented in the form of a sinusoidal spatial function of one-pixel period along both axes. Various amplitude values for this error have been taken into account: 0.3″, 1″, and 5″. The results are shown respectively in FIG. 14 (6000 shots, interpolation errors of 0.3″ amplitude, crosses=simple least squares, dots=least squares with distortion estimation), FIG. 15 (8000 shots, interpolation errors of 1″ amplitude, crosses=simple least squares, dots=least squares with distortion estimation), and FIG. 16 (6000 shots, interpolation error of 5″ amplitude, crosses=simple least squares, dots=least squares with distortion estimation), and they are summarized in the following tables: Non-radial error of 0.3″: Maximum values over 6000 shots Mean ⊥ Std. dev. Mean // Std. dev. in (″) axes ⊥ axes axes // axis Conventional algorithm 2.0 3.5 9.0 33.0 Algorithm with distortion 0.3 7.0 2.5 45.0 estimation Non-radial error of 1″: Maximum values over 6000 shots Mean ⊥ Std. dev. Mean // Std. dev. in (″) axes ⊥ axes axes // axis Conventional algorithm +1-628-841-3478 Algorithm with distortion 1.1 7.5 7.8 45.0 estimation Non-radial error of 5″: Maximum values over 6000 shots Mean ⊥ Std. dev. Mean // Std. dev. in (″) axes ⊥ axes axes // axis Conventional algorithm 4.8 3.5 27.0 33.0 Algorithm with distortion 4.8 7.5 34.6 45.0 estimation The effects of amplifying noise as introduced by the distortion estimator algorithm are unchanged. However a loss of effectiveness in reducing bias is observed in the presence of non-radial errors. This reduction is no more than a factor of 3 in the presence of a non-radial error of 0.3″, and is no longer perceptible in the presence of a non-radial error of 5″. It is recalled that the assumed distortion residue is 9″ at the edge of the field. The performance of the algorithm is therefore limited little by the presence of errors whose spatial distribution is not radial. The simulations described above show the potential advantage of an algorithm estimating the distortion of a star sensor for the purpose of reducing the resulting FOV attitude measurement error. The reduction that is observed is by a factor of slightly less than 10 on the cross axes with the assumptions that were adopted, which assumptions are representative of a typical sensor. A corresponding drawback lies in an increase in attitude noise by a factor that is close to 2, but its impact can be attenuated by the gyrostellar filter. The proposed algorithm is simple. It can be implemented inline in processing measurements at star level. Compared with a conventional attitude-determining algorithm, it implies an increase in the size of the matrix for inversion that is linear with increasing number of stars to be processed. It preferably also takes advantage of approximate knowledge about the attitude that is to be measured, in a manner that is compatible with required pointing specifications. Given the results obtained, the advantage of this algorithm is preferably restricted to a certain amplitude of noise level compared with distortion residue. Simulations have also shown that although this algorithm does enable the impact of FOV errors in radial distortion in the field to be reduced, its advantage is more limited by FOV error sources that have non-radial distribution. It is therefore desirable to make a fine assessment of the geometrical nature of all FOV error sources and their amplitudes. The advantage of the algorithm also lies in the context of a complete system for gyrostellar attitude estimation taking account of realistic movements of stars in the field of view. Following the above description of a first method for reducing “field of view” type errors by an algorithmic method applied directly to star outputs, there follows a description of an algorithm based on knowledge about the form of FOV errors in the field. Thereafter there follows an analysis of this algorithm which has been performed not only with inertial pointing, but also while taking account of the simple satellite dynamics that are to be found in geocentric pointing. In the method described above, the effects of distortion appear directly as fluctuations in the focal length at each point of the field. The distribution in the field of non-calibrated distortion residue is a random function in application of a relationship that is uniform in value and direction in the focal plane. The algorithm was therefore based on no assumption about how to model the errors. Unlike the treatment described above, the description below is based on the fact that an assumption is available on the way distortion residue varies over the field. This fundamental assumption makes it possible to adapt the estimation algorithms so as to identify one or more coefficients that are characteristic of the distortion and so as to make the algorithm more effective by limiting the number of parameters that need to be estimated. The description below proceeds as follows: description of how errors are modeled; validation of time signatures for each modeled error and demonstration of the advantage of the algorithm for a given star configuration with inertial pointing; description of the algorithms under investigation; statistical evaluation and performance that can be achieved with inertial pointing for different star configurations; study in association with satellite dynamics for characterizing the time signatures of errors and impact on the operation of the algorithms proposed. The simulation tool used is suitable only for simple dynamics at constant speed. This suffices to simulate the dynamics that are encountered with geocentric pointing and to evaluate the performance that can be achieved with a moving star field; and an analysis of the robustness of the process. The dimensions of the sensor were based on the data described above so as to use a sensor close to the characteristics that are typically desired. field of view 28°×28°; focal length 30 cm; number of stars tracked: 12; frequency: 4 hertz (Hz); measurement errors at the level of one star: noise: 37″ at 3σ; distortion residue: 9″ at 3σ. Other faults were also proposed: fluctuation in focal length: 1.5″/K at the edge of the field with temperature environment ±2° C.; chromatic aberrations: 3″ at 3σ; and catalog errors: 0.3″ at 3σ. Concerning white noise, this is measurement noise having variance modeled as being constant over time. For simplification purposes, it contains interpolation errors which can be considered as being white noise. White noise is taken as being equal to 37″ at 3σ along each measurement axis X and Y. FIG. 17 plots the time-varying outputs in attitude as calculated by means of conventional filtering on the basis of noisy measurements. The noise obtained is practically centered (the mean depends on the number of shots, i.e. on the time spent on simulation and on measurement frequency) with a standard deviation on the cross axes of about 2.5″. If the following multiplying factors are applied that correspond respectively: to the number of stars: {square root}12; and to combining two cross axes: {square root}2; then, on going from 1σ to 3σ, the value of 37″ is found for the measurement noise injected at each star. Distortion is an aberration that is a function of the position of the object point in the field. It does not affect the quality of the image of a point, but only its position, as shown in FIG. 18. The rays coming from a star at an angle α converge at a point M′ in the focal plane instead of at the point M as would be the case in an instrument without distortion. This aberration is due to the quality of the shape of the lens. In the above example, distortion is an increasing function of focal plane, i.e.: OoptM′>OoptM. Nevertheless, it should be mentioned that distortion can equally well be a decreasing function. Although the distortion-patterns can sometimes be complex in shape, with distortion being non-monotonic in the field, distortion is modeled in application of a radial relationship (distance from the point on the optical axis), as follows: The following two magnitudes are defined: r=OoptM=f0 tan α r′=OoptM′ as determined by ray tracing (optical code). A relationship linking r to r′ is interpolated in the following polynomial form: r′=r(1+D3r2+D4r3+D5r4+D6r5+D7r6 . . . )=p(r) The degree of the polynomial p(r) is a function of the desired accuracy. Knowledge of the polynomial coefficients (D3, D4, D5, D6, D7, . . . ) makes it possible at all points of the field to calculate the contribution of the error due to distortion, and to correct it should that be necessary. In general, for optical systems that are circularly symmetrical, symmetry considerations make it possible to assume that the even coefficients (D4, D6, . . . ) are zero. In the description below, we make this assumption, which does not make the investigation any less general, i.e.: r′=r(1+D3r2+D5r4+D7r6 . . . )=p(r) On star sensors, calibration processes are implemented for the purpose of reducing the impact of distortion. Treatment makes it possible to take account of the coefficient D3 and to reduce the errors associated with this defect. We assume that distortion is calibrated to third order. In which case, the radial errors due to distortion residue can be modeled as follows: r′=r(1+D5r4+D7r6) Nevertheless, if necessary, it is possible also to take account of error in estimating the coefficient D3. For the present description, only one coefficient has been taken into account D5=6 m−4, estimated as follows in order to be representative of the performance of a typical star sensor. In order to obtain values for fluctuation in focal length at the edge of the field of the kind defined in [1], the “apparent focal length” f′ is determined at the edge of the field: This gives: tan ⁢ ⁢ α = r f nominally. If distortion is considered to be a variation in focal length, the following relationship
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