Satellite artificial state estimation sensor and method and artificial satellite

Abstract

A sensor (100) installable on an artificial satellite, comprising: a first acquisition device (1) for acquiring a first digital image (IM1) representative of a first zone of the space around the satellite and comprising a first non-stellar celestial body (SB1); a second acquisition device (2) for acquiring a second digital image (IM2) representative of a second zone of the space around the artificial satellite, different from said first zone, and comprising a second non- stellar celestial body (SB2), different from said first non-stellar celestial body (SB1); a memory in which ephemeris data of stellar and non-stellar celestial bodies are stored. In addition, there is a processor (3) configured to: analyse (204, 205) the first (IM1) and second (IM2) images to determine current attitude data (ATT) of the artificial satellite; analyse (206) the first (IM1) and second (IM2) images to recognise the first (SB1) and second (SB2) non-stellar celestial bodies based on the ephemeris data in the memory; determining (206, 207) a first position (rpl1) of the first non-stellar celestial body (SB1) in the first image (IM1) and determining a second position (rpl2) of the second non-stellar celestial body in the second image (IM2), taking into account the current attitude data (ATT); estimating (210, 211) a current state (xc) of the artificial satellite based on the current attitude data (ATT) and said first (rpl1) and second position (rpl2).

Description

[0001] "SATELLITE ARTIFICIAL STATE ESTIMATION SENSOR AND METHOD AND ARTIFICIAL SATELLITE".

[0002] DESCRIPTION

[0003] THECNICAL FIELD

[0004] The present invention relates to techniques for estimating the state of an artificial satellite.

[0005] STATE OF THE ART

[0006] Steering an interplanetary satellite consists of determining its position and speed, planning its trajectory and controlling its movement.

[0007] Nowadays, these activities are performed through the two-way exchange of a radio signal between ground stations and spacecraft. Specifically, the distance of the satellite from the earth station is determined by measuring the time between sending the radio signal from the station and receiving it from the ground. While the speed along the station-satellite junction is determined by measuring the Doppler effect of the signal sent.

[0008] Although this approach to navigation, guidance and control is highly reliable and accurate, its adoption will become unsustainable in the near future. Indeed, steering a satellite from the ground presents several challenges. Control from the ground introduces critical delays and absorbs a substantial part of the cost of the space mission, more than 5 per cent of the total budget (about $25 million for a small satellite, €0.5 million for a CubeSat), which is mainly allocated to personnel involved in mission control.

[0009] Moreover, given the significant growth trend of the space economy (+176% from 2005 to 2020) and the consequent proliferation of satellites in deep space, the traditional navigation approach, which uses radio stations and channels present in limited numbers on the ground, cannot be used extensively to control all new users in deep space. To limit oneself to the adoption of the standard approach of navigation, guidance and control would be to hinder the momentum towards the privatisation of deep space and its subsequent democratisation.

[0010] The following documents are useful for understanding the description of this patent application.

[0011] 1. Raymond K. R, Mortari D. Interplanetary autonomous navigation using visible planets (2015) Journal of Guidance, Control, and

[0012] Dynamics, 38(6), 1151-6. https: / / doi.org / 10.2514 / 1. G000575

[0013] 2. Franzese V, Topputo F (2020). Optimal beacons selection for deep- space optical navigation. The Journal of the Astronautical Sciences, 67(4):1775-

[0014] 3. Franzese, V., Topputo, F., Ankersen, F., & Walker, R. (2021). Deepspace optical navigation for M-ARGO mission. The Journal of the

[0015] Astronautical Sciences, 68, 1034-1055, https: / / doi.org / 10.1007 / s40295-021-

[0016] 00286-9;

[0017] Franzese, V., Topputo, F. Deep-Space Optical Navigation Exploiting

[0018] Multiple Beacons (2022). J Astronaut Sci 69, 368-384. https: / / doi.Org / 10.1007 / s40295-022-00303-5;

[0019] Andreis, E., Franzese, V., & Topputo, F. (2022). Onboard Orbit

[0020] Determination for Deep Space CubeSats. Journal of guidance, control, and dynamics, 45:8, 1466 1480.

[0021] SUMMARY OF THE INVENTION

[0022] The present invention addresses the problem of proposing a state estimation sensor for an artificial satellite that can operate autonomously, i.e. without relying on signal exchanges with ground stations. According to one aspect, the present invention relates to a sensor as described by claim 1 and preferred embodiments thereof as defined by claims 2-9.

[0023] Also covered by the present invention is a satellite as described by claim 10 and a particular form of its implementation as defined by claim 11.

[0024] According to another object, the invention relates to a method for estimating the state of an artificial satellite as defined by claim 12.

[0025] BRIEF DESCRIPTION OF THE DESIGNS

[0026] The present invention is hereinafter described in detail, by way of example and not limitation, with reference to the accompanying drawings, in which

[0027] - Figure 1 schematically shows an example of a sensor that can be installed on an artificial satellite;

[0028] - Figure 2 shows an example of such a sensor in a possible practical realisation;

[0029] - Figure 3 shows by means of a flow chart a method of operation of said sensor for the estimation of the state of the artificial satellite and the estimation of the attitude of said satellite.

[0030] DETAILED DESCRIPTION

[0031] Figure 1 schematically represents a sensor 100 installed on an artificial satellite (not shown). For the purposes of this description, an artificial satellite is defined as an interplanetary artificial satellite (also referred to as an interplanetary probe) or an artificial satellite intended to orbit the Earth or other celestial bodies. Depending on particular applications, the sensor 100 may be suitable for installation on miniaturised satellites for navigation operations beyond Earth orbit or on standard-sized satellites for navigation operations beyond Earth orbit. As will be described in more detail below, the sensor 100 is configured to estimate the state (i.e., position and velocity) assumed by the satellite and evaluated in a predetermined inertial reference system. In addition, sensor 100 is configured to estimate the satellite's attitude, i.e., its orientation in space (relative to the same inertial reference system). In addition to the attitude, other parameters can be estimated, for example: a mounting error of sensor 100, the error of the on-board clock and the residual accelerations acting on the satellite.

[0032] According to the example in Figure 1, the sensor 100, housed in a container 10, comprises a first digital image capture device 1, a second digital image capture device 2 and an estimation processor 3 (EST-PRC) configured to receive digital images from the capture devices 1 and 2 and make estimates of the status and, advantageously, also the attitude of the satellite.

[0033] The first digital imaging device 1 and the second digital imaging device 2 may be implemented with cameras or video cameras of known types. For example, the first digital image capture device 1 has a first lens 4 (including optical lenses), a first sensor 5 (e.g., a photodiode array) configured to convert visible radiation into electrical signals, and a first image processor 6 (PRC). The first image processor 6 is configured to receive electrical signals from the first sensor 5 and convert them into digital signals suitable for further processing.

[0034] Similarly, the second imaging device 2 comprises a second lens 7, a second sensor 8 and a second image processor 9.

[0035] The (e.g., miniaturised) estimation processor 3 is connected to the first image processor 6 and the second image processor 9 to receive digital signals corresponding to the images taken. Note that the image processor 6 and / or the second image processor 9 may be at least partially integrated into the estimation processor 3. Advantageously, the estimation processor 3 is suitable for use in spatial conditions and is equipped with memories (not shown). It is also possible to envisage that the estimation processor 3 is external to the container 10 or that part of the processing described below is carried out external to the container 10.

[0036] In a memory of the Estimation Processor 3, ephemerides EFF of celestial bodies are stored, which are known to contain calculated values, over a particular time interval, of various variable astronomical quantities related to the celestial bodies. In particular, these stored ephemerides EFF include information on the position, velocity and acceleration of the celestial bodies in the inertial system.

[0037] In addition, the estimation processor 3 is such that it controls the first and second imaging devices 1 and 2, for example, to trigger the taking of images.

[0038] The 100 sensor can be realised in small dimensions, with miniaturised components, making its adoption possible on different classes of satellites, from nanosatellites to standard satellites

[0039] For example, sensor 100 is designed to occupy 1 to 4 U of the satellite, with a footprint ranging from 1 dm3to 4 dm3with a maximum weight of 3 kg.

[0040] With regard to the optical components used (e.g., those included in lens 4), it should be noted that a miniaturised optical component may have limited performance when viewing dimly-lit celestial bodies, such as asteroids. On the other hand, a higher-performance, less miniaturised optical component, as useful in the case of application on standard satellites, allows sensor 100 to observe even less luminous objects.

[0041] According to a form of realisation involving the use of miniaturised components (e.g. usable when sensor 100 is installed on board nanosatellites), camera 1 and camera 2 advantageously have full fields of view (FOV) with values between 5° and 30°, preferably between 10° and 20°.

[0042] Note that the field of view associated with sensor 100 can also be extended by having more than two cameras.

[0043] Regarding the relative orientation of the optical components (i.e., of the first lens 4 and the second lens 7), consider the angle between the optical axes of these components, schematically represented in figure 1 as first optical axis Al and second optical axis A2. According to a particular form of realisation, the angle between the first optical axis Al and the second optical axis A2 is chosen between the values 20° and 160° and preferably, between 40° and 140°, for example, this angle is 90°. It should be noted that the appropriate values of this angle depend on the type of mission, i.e. the particular trajectory the satellite is to take, e.g. Earth-Mars or Jupiter-Saturn, or other. Therefore, the value of that angle is chosen on a case-by-case basis. The Applicant noted that smaller angles are needed when covering internal trajectories, i.e. from Earth to the inner planets of the Solar System, while larger angles are needed when covering trajectories to outer planets.

[0044] It should be noted that the optimal positioning and orientation of optics 4 and 7 is chosen so as to optimise, throughout the mission, the detection of celestial bodies whose relative angle is closest to 90°. In this situation, the celestial triangulation problem is well constrained and the highest accuracy in the navigation solution can be achieved.

[0045] Therefore, this angle can be chosen at a different value depending on the type of mission to be performed (time of year and choice of celestial bodies to be observed). According to the theoretical approach, the optimal relative angle between the two optical axes is 90°, as this ensures the detection of celestial bodies that, if present, lead to the most accurate navigation solution.

[0046] Figure 2 shows a possible realisation of the optical sensor 100.

[0047] Figure 3 shows, by means of a flow chart, an example of an operation method 200 of the sensor 100, mounted on board a satellite, during a mission. Some of the steps of the operation method 100 are implemented, for example, by the estimation processor 3, at which a corresponding computer program may reside.

[0048] At an initial time (first stage 201), processor 3 has the state xc-i of the satellite estimated at a time prior to the initial time. The state of the satellite is at least the position and velocity values expressed in a chosen inertial reference system (such as a system centred at the centre of gravity of the solar system).

[0049] At the initial time, the first camera 1 (second stage 202) acquires a first digital IM1 image which is supplied to the estimation processor 3. Such first digital image IM1 captures a first area around the satellite. For example, such first digital image IM1 captures at least one first non-stellar celestial body SB1. Furthermore, such first digital image IM1 may also include a first star STI.

[0050] A non-stellar celestial body is defined as all astronomical objects other than stars, for example: planets, asteroids, comets, dwarf planets, moons, natural and non-natural satellites, space debris, etc.

[0051] The second camera 2 acquires (third stage 203) a second digital IM2 image which is supplied to the estimation processor 3. This second IM2 image captures a second zone around the satellite that is different (i.e. not coincident) with the first zone, due to the different mutual orientation of cameras 1 and 2.

[0052] For example, the second IM2 digital image captures at least a second non- stellar celestial body SB2 (different from the first celestial body SB1). This second IM2 digital image may also contain a second star ST2 (different from the first star STI)

[0053] In particular, according to a possible attitude estimation algorithm (which is known in itself), there must be at least three stars STj in total, which can be included in only one of the two images IM1 and IM2, or they can be distributed between these images. Note that the more stars in the images, the more accurate the attitude estimation will be.

[0054] Advantageously, the second camera 2 acquires the second digital IM2 image synchronously with the acquisition of the first image IM1.

[0055] Note that stars are considered fixed points, i.e. point-like bodies that do not change their position in time with respect to the satellite mounting sensor 100.

[0056] In a fourth step 204 (AT-DET), the estimation processor 3 determines the attitude ATT (e.g., in the form of an orientation-defining angle matrix) of the satellite, based on the first image IM1 and the second image IM2. As already mentioned, images of at least three stars may be required for attitude determination.

[0057] In particular, in this fourth step 204 the first image IM1 and the second image IM2 are analysed in order to recognise the stars contained in them, such as the first, second and third stars STI, ST2 and ST3 (different from each other), according to the example described here. Note that more than three stars can be advantageously used.

[0058] Next, the attitude ATT of the satellite is determined, i.e. its orientation with respect to the inertial reference system.

[0059] To determine the attitude ATT, processor 3 displays the geometry with which the three or more stars STI, ST2 and ST3 are arranged in the relevant images and compares this geometry with a database of geometries (also called the 'star catalogue') stored in the processor's memory and including, for example, angle values describing the distance between one star and another. In other words, the 'constellations' in the images are recognised and compared with a database of 'constellations' stored in the processor's memory.

[0060] For example, this can be determined using the well-known 'pyramid algorithm' for star pattern recognition. The algorithm is able to discriminate stellar objects from non-stellar celestial bodies, such as planets, asteroids, comets, dwarf planets, moons, natural and non-natural satellites, space debris, etc.

[0061] Once the three or more stars STI, ST2 and ST3 are recognised in both the images IM1 and IM2, each star is associated with its position in camera reference system 1 or 2 (extracted directly from the image) and its position in the inertial reference system (obtained by recognising the star within the onboard geometry database).

[0062] Advantageously, the information contained in the two images IM1 and IM2 concerning the position of the stars is used to achieve greater accuracy in the determination of the satellite attitude. According to an example, two nx3 matrices are created, where n is the total number of stars in the two images IM1 and IM2. In one of the two matrices, information about the position of the stars in the inertial system is stored, and in the other matrix, information about the position of the stars in the reference system of the respective camera 1 or 2 is stored. These two matrices are used as input for the attitude determination algorithm which, advantageously, can be the pyramidal algorithm.

[0063] Preferably, the determination of the ATT also takes into account additional effects (such as, for example, effects due to light aberration) present in interplanetary space that cause a displacement of stars in the image and corrects these effects in order to define corrected positions of stars in the image.

[0064] It should be noted that the use of two images IM1 and IM2 obtained by the different acquisition devices 1 and 2 with optics 4 and 7 oriented as discussed above, reduces the error in the attitude determination because the two images IM1 and IM2 present, most likely, a greater number of stars that can be used for the attitude calculation than the number of stars present in a single image. The use of the two acquisition devices 1 and 2 with optics 4 and 7 also allows different areas of space to be observed, thus providing more information on the entire celestial sphere. Consider also that with only one camera, one is more precise in the directions in the image plane and less precise in the rotation around the optical axis. Note that by using two cameras 1 and 2 with perpendicular optical axes, this effect is mitigated as each camera compensates for the shortcomings of the other. The attitude ATT thus determined is made available for further processing, as represented by a fifth stage 205 (OUT- ATT).

[0065] In a sixth phase 206 (AO-REC), the estimation processor 3 analyses the first and second images IM1 and IM2 and performs the recognition of non-stellar celestial bodies, distinguishing them from the stellar bodies within these images, such as the first and second non-stellar celestial bodies SB1 and SB2.

[0066] The recognition of non-stellar celestial bodies is based on an algorithm that takes into account the attitude ATT of the satellite, as determined in the previous step, and particular parameter values of the celestial bodies provided by the ephemerides EFF.

[0067] It should be noted that, according to one possible mode, to identify a non- stellar celestial body in the images IM1 and IM2, the expected position of the non-stellar celestial body (for each of bodies SB1 and SB2) in the images is estimated through the knowledge of the satellite's position xc-i at the previous time (as found in the first phase 201) and the ephemerides EFF for the celestial body of interest, and also considering statistical quantities (covariance) that express the precision of these two estimated positions.

[0068] Thus, referring to the first non-stellar celestial body SB1, the area in the image IM1 within which the non-stellar celestial body SB1 has a certain probability of appearing (e.g. 99.7 %) is defined.

[0069] This area is defined by an uncertainty ellipse centred on the estimated position of the celestial body SB1. If a body is present within the uncertainty ellipse that is recognised as a non-star, for example, by the pyramid algorithm, then this is identified as the celestial body itself. If several non-star objects are present within the uncertainty ellipse, then the body whose position is closest to the centre of the uncertainty ellipse will be recognised as the body to be searched.

[0070] This recognition can be successful for all or only some of the celestial bodies in the images IM1 and IM2.

[0071] Having also recognised the first and second non-stellar celestial bodies SB1 and SB2 (distinct astronomical objects), this calculation algorithm also provides the (two-dimensional) coordinates rpu and rpi2 within the first and second images IM1 and IM2 of the non-stellar celestial bodies SB1 and SB2, respectively.

[0072] In a seventh phase 207 these coordinates are made available for further processing.

[0073] Note that method 200 also includes a prediction step (eighth step 208), according to which, starting from the state xc-i of the satellite determined in a previous processing step (first step 201), a prediction of the satellite state at the next step is made: the predicted satellite state is shown in the figure as a vector Xp.

[0074] In addition, the value of a statistical variable, indicative of the error of the prediction, is also provided at this prediction 208 stage. For example, the error covariance matrix Pp is provided. The predicted state xpand the error covariance matrix Pp are made available in a ninth step 209.

[0075] Returning to image processing, method 200 comprises a tenth step 210 (ST- EST), in which the estimation processor 3 on the basis of the position of the non-stellar celestial bodies rpn and rpi2 in the images IM1 and IM2, the attitude ATT of the satellite, the predicted state xpand the error covariance matrix Pp calculates the estimated state at the current step xcand the error covariance matrix at the current step Pc.

[0076] In more detail, in step 210 the predicted values xpand Pp are corrected according to the attitude of the ATT satellite and taking into account the position of the non-stellar celestial bodies rpi± in the images IM1 and IM2. This correction is carried out, for example, through the use of a dimensionless extended Kalman filter. The scaling of the filter is applied to make the computational cost and numerical stability of the filter optimal for implementation on a miniaturised platform.

[0077] Altogether, the method 200 outputs the estimated state at the current step xc(eleventh step 211) and the attitude ATT at the current time (twelfth step 212). Furthermore, advantageously, the matrix of error covariances at the current step Pc is also provided.

[0078] It should be noted that sensor 100 can thus effectively operate as an autonomous navigation sensor for the artificial satellite. For example, sensor 100 can be realised in plug-and-play form where the autonomous navigation software for interplanetary cruising, implemented on the estimate processor 3, is integrated with the hardware, and requires no support from external utilities. In other words, the sensor 100 can be ready-to-use for satellites and can also replace attitude sensors, thus decreasing total costs.

[0079] Regarding the choice of the two non-stellar celestial bodies SB1 and SB2 used in the method 200 described above, it should be noted that, as already mentioned, the optimal angle, with vertex in the satellite, formed by the first celestial body SB1 from the second celestial body is 90°. With non-stellar celestial bodies SB1 and SB2 arranged at 90°, the most accurate estimate of the satellite's state can be obtained.

[0080] However, it is possible that one or both of the celestial bodies of the optimal pair are not visible from the satellite because they are too faint to be observed or because they are too close to the sun. This situation is handled by processor 3, which selects the first non-stellar celestial body SB1 and the second non-stellar celestial body SB2 from other non-stellar celestial bodies in the first image IM1 and second IM2 so that they form an angle with vertex in the artificial satellite as close to 90° as possible. Therefore, if the optimal pair is not available, the suboptimal pair of non-stellar celestial bodies is chosen.

[0081] It is clear that by having two cameras, positioned as described above, it is more likely to be able to determine an optimal pair or at least one that ensures good accuracy than in the situation where a single camera is used.

[0082] The sensor 100 described above offers numerous advantages over the known art.

[0083] Due to the presence of two image acquisition devices 1 and 2, with their optics differently oriented (in short, "double-headed optics"), the sensor is able to estimate the navigation solution more accurately than the one processed by state-of-the-art stand-alone optical navigation algorithms.

[0084] Such a double optical head not only increases the accuracy of satellite attitude determination but also the accuracy of state determination, with an error on satellite position of less than 1000 km. Note that the accuracy in the determination of the state increases with the accuracy in the determination of the attitude and the latter is optimal when the two optics are arranged at 90°.

[0085] In addition, the synchronous observation of two celestial bodies (stellar or non-stellar) avoids the need to use manoeuvres for satellite resetting, during which no celestial body is observed and thus the uncertainty in the satellite's state estimation is further increased. By avoiding these manoeuvres, the time used for resetting the satellite can be invested in a longer-lasting observation of celestial bodies, which then leads to a more accurate navigation solution.

[0086] The dual optical head increases the sensor's field of view compared to using a single imaging device. By expanding the field of view, the sensor 100 is able to achieve a more accurate navigation solution than would be achieved with a single imaging device.

[0087] Although the above description refers to the case where at least two different planets (at least one for each separate camera) are displayed, sensor 100 is able to provide the satellite's state and attitude even if these nominal conditions are not met. In fact, the necessary condition for a navigation solution is that at least one planet is visible in one of the two cameras. This includes cases where only one planet is visible in the image provided by camera 1 and none in that provided by camera 2 (or vice versa).

[0088] List of symbols of figure components - sensor 100

[0089] - first digital image capture device 1

[0090] - second digital image capture device 2

[0091] - estimation processor 3

[0092] - first objective 4

[0093] - first sensor 5

[0094] - first image processor 6

[0095] - a second objective 7

[0096] - second sensor 8

[0097] - second image processor 9

[0098] - container 10

[0099] - EFF ephemerides

[0100] - first optical axis Al

[0101] - second optical axis A2

[0102] - method of operation 200

[0103] - state at the previous time xc-i

[0104] - first IM1 digital image

[0105] - first star STI

[0106] - first non-stellar celestial body SB1

[0107] - second digital image IM2

[0108] - second star ST2

[0109] - second non-stellar celestial body SB2

[0110] - a third star ST3

[0111] - third non-stellar celestial body SB3

[0112] - satellite attitude ATT

[0113] - prediction of satellite state xp - predicted matrix of error covariances Pp

[0114] - position in images of non-stellar celestial bodies rpn and rpi2

[0115] - satellite state at current step x

[0116] - the error covariance matrix at the current step Pc

Claims

CLAIMS1. A sensor (100) installable on an artificial satellite, comprising: a first acquisition device (1) configured to acquire a first digital image (IM1) representative of a first zone of space around the satellite and comprising a first non-stellar celestial body (SB1) and a first stellar celestial body (STI); a second acquisition device (2) configured to acquire a second digital image (IM2) representative of a second zone of the space around the artificial satellite, different from said first zone, and comprising a second non-stellar celestial body (SB2), different from said first non-stellar celestial body (SB1), and a second stellar celestial body (ST2); wherein: said first digital image (IM1) and / or said second digital image (IM2) includes at least a third stellar celestial body (ST3); a memory in which ephemeris data of stellar and non-stellar celestial bodies are stored; a processor (3) in which resides an estimation software configured to: analyse (204, 205) the first and second digital images and recognize (204) the first stellar celestial body (STI), the second stellar celestial body (ST2) and the third stellar celestial body (ST3) based on said stellar celestial body data and non-stellar celestial body data; determine (204, 205) current attitude data (ATT) of the artificial satellite basing on the first stellar celestial body (STI), the second stellar celestial body (ST2) and the third stellar celestial body (ST3); analyse (206) the first (IM1) and second (IM2) images to recognise the first (SB1) and second (SB2) non-stellar celestial bodies based on the ephemeris data stored in the memory;determine (206, 207) a first position (rpu) of the first non-stellar celestial body (SB1) in the first image (IM1) and determine a second position (rpi2) of the second non-stellar celestial body in the second image (IM2), taking into account the current attitude data (ATT); estimate (210, 211) a current state (xc) of the artificial satellite based on the current attitude data (ATT) and said first (rpn) and second position (rpi2).

2. Sensor (100) according to claim 1, wherein: the first acquisition device (1) comprises first optical components (4) associated with a first optical axis (Al); the second acquisition device (2) comprises second optical components (7) associated with a second optical axis (Al); the first optical axis (Al) and the second optical axis (A2) form an angle between 20° and 160°.

3. Sensor (100) according to claim 1, wherein the first acquisition device (1) and the second acquisition device (2) each have a relative total field of view comprised between 5° and 30°.

4. Sensor (100) according to claim 1, wherein said estimation software (3) residing in the processor is configured to: select the first non-stellar celestial body (SB1) and the second non-stellar celestial body (SB2) from among other non-stellar celestial bodies in the first (IM1) and second (IM2) images so that they form an angle with vertex in the artificial satellite closest to 90°.

5. Sensor (100) according to claim 1, wherein the processor is configured to: recognize the first stellar celestial body (STI), the second stellar celestial body (ST2) and the third stellar celestial body (ST3) by: generating geometry data describing how the first stellar celestial body(STI), the second stellar celestial body (ST2) and the third stellar celestial body (ST3) are placed in the first (IM1) and second (IM2) images; comparing said geometry data with a stored constellation geometry database.

6. Sensor (100) according to claim 1, wherein the estimation software is further configured to: have (201) a state of the satellite calculated at a previous time; making a prediction (208) of the state of the satellite taking into account said state at the previous time and providing a predicted state (xp); provide (208) a predicted statistical variable (Pp) representative of a previous state estimation error.

7. Sensor (100) according to claim 1, wherein estimate (210, 211) the current state (xc) comprises: correcting the predicted state (xp) based on the current attitude data (ATT) and said first (rpn) and second positions (rpi2); providing a current statistical variable (Pp) representative of a current state estimation error.

8. Sensor (100) according to claim 1, wherein the processor (3) is further such as to control the first acquisition device (1) and the second acquisition device (2) such that the first image (IM1) and the second image (M2) are taken synchronously.

9. Sensor (100) according to claim 1, wherein said sensor occupies from 1 U to 4 U of the satellite and has a footprint of from 1 dm3to 4 dm3, with a maximum weight of 3 kg.

10. Artificial satellite comprising: an autonomous navigation system including a sensor (100) made inaccordance with at least one of the preceding claims.

11. Artificial satellite according to claim 10, wherein said satellite is made according to one of the following types: a satellite for navigation operations beyond earth orbit, a satellite intended to orbit a celestial body, a nanosatellite, a miniaturized satellite, a standard satellite.

12. Estimation method (200), comprising: acquiring (202) a first digital image (IM1) representative of a first region of space around the satellite and comprising a first non-stellar celestial body (SB1) and a first stellar celestial body (STI); acquiring (203) a second digital image (IM2) representative of a second zone of space around the artificial satellite, different from said first zone, and comprising a second non-stellar celestial body (SB2), different from said first non-stellar celestial body (SB1), and a second stellar celestial body (ST2); wherein: said first digital image (IM1) and / or said second digital image (IM2) includes at least a third stellar celestial body (ST3); storing ephemeris data of stellar celestial bodies and non-stellar celestial bodies in a data memory; carrying out the following processing using a processoranalysing (204, 205) the first and second digital images and recognizing (204) the first stellar celestial body (STI), the second stellar celestial body (ST2) and the third stellar celestial body (ST3) based on said stellar celestial body data and non-stellar celestial body data determining current attitude data (ATT) of the artificial satellite based on the first stellar celestial body (STI), the second stellar celestial body (ST2) and the third stellar celestial body (ST3); analysing (206) the first (IM1) and second image (IM2) to recognise thefirst (SB1) and second (SB2) non-stellar celestial body based on the ephemeris data in the memory; determining (206, 207) a first position (rpn) of the first non-stellar celestial body (SB1) in the first image (IM1) and determining a second position (rpl2) of the second non-stellar celestial body in the second image (IM2), taking into account the current attitude data (ATT); estimating (210, 211) a current state (xc) of the artificial satellite based on the current attitude data (ATT) and said first (rpn) and second position (rp12).

Images