(19)
(11)EP 3 525 170 A1

(12)EUROPEAN PATENT APPLICATION

(43)Date of publication:
14.08.2019 Bulletin 2019/33

(21)Application number: 19157018.3

(22)Date of filing:  13.02.2019
(51)International Patent Classification (IPC): 
G06T 7/70(2017.01)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(30)Priority: 13.02.2018 IT 201800002654

(71)Applicant: Thales Alenia Space Italia S.p.A. Con Unico Socio
00131 Roma (IT)

(72)Inventor:
  • CESARE, Stefano
    10146 Torino (IT)

(74)Representative: Bergadano, Mirko et al
Studio Torta S.p.A. Via Viotti, 9
10121 Torino
10121 Torino (IT)

  


(54)IMAGING-BASED DETECTION OF TARGETS UNIDENTIFIABLE WITH RESPECT TO THE BACKGROUND


(57) An imaging-based target detection system comprising an electromagnetic radiation emitting source (1) to be associated with a target (2) to be detected and pulsedly operable according to an operation pattern to emit electromagnetic radiation; an imaging sensor (3) sensitive to the electromagnetic radiation emitted by the electromagnetic radiation emitting source (1), and operable to capture digital images of the electromagnetic radiation emitting source (1); and an electronic processing unit (5) connected to the imaging sensor (3) and configured to operate the imaging sensor (3) synchronously with respect to the pulsed operation of the electromagnetic radiation emitting source (1) so as to cause the imaging sensor (3) to carry out a digital image capture cycle during which a plurality of digital images are successively captured and comprise a first digital image captured when the imaging sensor (3) is impinged by electromagnetic radiation emitted by the electromagnetic radiation emitting source (1), and at least a second image captured when the imaging sensor (3) is not impinged by any electromagnetic radiation emitted by the electromagnetic radiation emitting source (1); and process the captured digital images to detect the target (2). The electronic processing unit (5) is further configured to synchronise operation of the imaging sensor (3) with respect to the pulsed operation of the electromagnetic radiation emitting source (1) based on the digital images captured by imaging sensor (3).




Description

Cross-Reference to Related Patent Applications



[0001] This patent application claims priority from Italian patent application no. 102018000002654 filed on February 13, 2018.

Technical Field of the Invention



[0002] The present invention relates in general to imaging-based target detection, in particular targets not directly identifiable with respect to the background, and in particular to synchronization of operation of imaging sensors with respect to pulsed operation of electromagnetic radiation emitting sources associated with the targets to be detected.

[0003] The present invention finds advantageous, though not exclusive, application in the aerospace field to detect targets in the form of target flying vehicles, such a spacecrafts, for example satellites, or aircrafts, by detecting flying vehicles, to which the following description shall refer to without any loss of generality.

[0004] Nonetheless, the present invention could also find application in the terrestrial field to detect targets in the form of installations, equipment, or objects.

[0005] Furthermore, the present invention finds advantageous implementation by exploiting electromagnetic radiation emitting sources capable of emitting light radiations in the range of wavelengths where the imaging sensors are more sensitive, and which can belong indifferently to the spectrum visible to the human eye, or to the spectrum invisible to the human eye, such as infrared, ultraviolet or X-ray spectrum, to which the following description shall make reference without any loss of generality.

State of the Art



[0006] As is known, in the aerospace field, one of the aspects that covers a role of paramount importance is represented by the determination of the attitude and reciprocal positions of satellites forming part of a satellite constellation in which the satellites must assume a certain flight formation. Given the enormous distances that can exist between the satellites, determination of the attitude and reciprocal positions of the satellites is typically based on an imaging-based detection technology, a principle block diagram of which is shown by way of example in Figure 1.

[0007] Imaging-based detection technology basically envisages providing a light source 1 on a target satellite 2 and a light-sensitive imaging sensor 3 on a detector satellite 4. The light source 1 may, for example, be a laser source, also known as beacon, and is capable of emitting light radiation in the range of wavelengths where the imaging sensor 3 is most sensitive, typically around 600 nanometres for light sensor of common digital cameras, while the imaging sensor 3 may, for example, be a CCD (Charge-Coupled Device) camera, a CMOS (Complementary Metal-Oxide-Semiconductor) camera, also known as APS (Active Pixel Sensor) camera, or even a thermographic camera.

[0008] Imaging-based detection technology further envisages appropriately synchronizing the image capture with respect to the pulsed light emission, i.e., the operation of the imaging sensor 3 with respect to the pulsed operation of the light source 1, so as to cause the imaging sensor 3 to capture at least a pair of digital images of a portion of space where the possible presence of the target satellite 2 is wished to be detected. One of the digital images is captured when the imaging sensor 3 is impinged by a light ray emitted by the light source 1, so resulting in a luminous spot representative of the light ray emitted by the light source 1 being consequently shown in the digital image, and the other digital image is captured when the imaging sensor 3 is not impinged by any light ray emitted by the light source 1, so resulting in no luminous spot representative of the light ray emitted by the light source 1 being consequently shown in the digital image.

[0009] The captured digital images are then sent to an electronic processing unit 5, where they are processed to identify the light source 1 shown in one of the two digital images. In particular, the electronic processing unit 5 is programmed to receive the two digital images captured by the imaging sensor 3 and compute a digital difference image as the difference between the two captured digital images.

[0010] The difference operation removes the background in the two digital images, which is typically formed by luminous spots representative of light rays emitted by background light sources, such as stars, and/or deriving from straylight emitted, for example, by the target satellite 2, so resulting in the digital difference image showing only, at least in principle, the luminous spot representative of light ray emitted by the light source 1 on the target satellite 2 and that has impinged on the imaging sensor 3, and based on which the electronic processing unit 5 may then detect and determine the coordinates of the target satellite 2.

[0011] Capturing a sequence of pairs of digital images and processing the captured pairs of digital images in the above-described manner, it is also possible to determine the trajectory of the target satellite 2 for tracking and formation-keeping purposes.

[0012] In order for the removal of the background in the two digital images to be complete and, resultingly, for the luminous spot representative of the light ray emitted by the light source 1 to stand out in the digital difference image, it is required that the luminous intensity of the background in the two digital images from which the difference is computed be the same. In order to cause that to happen, it is therefore required that the capture period of a digital image, also known as exposure time of the imaging sensor 3, i.e., the period of time for which the imaging sensor 3 remains exposed to light to capture a digital image, be identical for both digital images.

[0013] In imaging-based detection technology, the synchronization of the digital image capture with respect to the pulsed light emission covers a paramount role for identifying the target satellite. An erroneous or imprecise synchronization could in fact result in the capture of pairs of digital images in both of which a luminous spot is shown with the same luminous intensity representative of the light emitted by the light source 1, so resulting in the luminous spot representative of the light ray emitted by the light source 1 being totally cancelled from the difference digital image, and, resultingly, in the target satellite 2 being failed to be identified.

[0014] Solutions are known in which the of digital image capture and the pulsed light emission are synchronised based on a common synchronization signal shared by the target and detector satellites 2 and 4 through inter-satellite relaying communications, such as a radio frequency or optical link, or is locally computed by the target and detector satellites 2 and 4 based on radio Signals in Space (SIS) transmitted by a GNSS (Global Navigation Satellite System) satellite constellation, as described, for example, in EP 1 989 681.

[0015] US 2009/324015 A1 instead discloses an emitter tracking system in which the emitter's presence is determined by a relatively low-power detection module before the images of the emitter and/or its surroundings are captured with a relatively high-power image capture module, and where the capture of images of the emitter can be synchronised with the flashes emitted by the emitter to increase the signal-noise ratio of the captured images.

[0016] WO 2014/078811 A1 lastly discloses synchronised infrared beacon/infrared detector system comprising an infrared beacon module configured to generate a time-varying encoded infrared signal, an infrared detector module configured to capture the encoded infrared signal generated by the infrared beacon module, a synchronizer configured to generate a synchronization signal that controls timing of the infrared beacon module and the infrared detector module, and a processor, in communication with the infrared detector module and configured to analyse the infrared signal captured by the infrared detector module. The infrared signal may be modulated at frequencies undetectable by human vision. The synchronizer signal may be produced independently of the capture of, and without input from, the infrared signal.

Object and Summary of the Invention



[0017] The Applicant has experienced that those solutions in which the synchronization of digital image capture with the pulsed light emission is based on common synchronization signals are highly critical due to their sensitivity to the availability of inter-satellite relaying communications, via which the common synchronization signals are shared, or of a GNSS service that provides GNSS SIS that may be received and processed by both the target and detector satellites 2 and 4 to locally compute common synchronization signals.

[0018] The lack of availability of this telecommunications technology, a situation that may arise, for example, when one or both satellites are in an interplanetary orbit where no GNSS service is available, or are at a distance such that inter-satellite relaying communications are not practicable, the synchronization of digital image capture with pulsed light emission may not be guaranteed, so resulting in the identification of the target satellite being impracticable or, at best, more difficult or more imprecise.

[0019] The Applicant has further experienced that the solutions in which the synchronization of digital image capture with pulsed light emission is based on common synchronization signals are further critical due to the propagation delay of the light from the light source 1 to the imaging sensor 3, which makes identification of the target satellite 2 more complicated.

[0020] The object of the present invention is hence to provide a technology that allows the aforementioned drawbacks to be overcome, in particular a technology that allows the digital image capture to auto-synchronize with pulsed light emission, without requiring a common synchronization signal to be shared or to be locally computed, and that, at the same time, is also insensitive to the light propagation delay.

[0021] This object is achieved by the present invention, which relates to an imaging-based target detection system, as claimed in the appended claims.

Brief Description of the Drawings



[0022] 

Figure 1 shows a principle block diagram of the imaging-based detection technology exploitable by a detector satellite to detect a target satellite.

Figures 2, 3 and 4 show the time trends of physical quantities involved in the imaging-based detection technology shown in Figure 1.


Detailed Description of Preferred Embodiments of the Invention



[0023] The following description is provided to enable an expert in the art to embody and use the invention. Various modifications to the embodiments will be obvious to an expert in the art, without departing from the scope of the claimed invention. Therefore, the present invention is not intended to be limited to the embodiments set forth, but is to be accorded the widest scope consistent with the principles and features described and claimed in the appended claims.

[0024] Unless defined otherwise, all technical and scientific terms used in this description have the normal meaning for an expert in the art to which the described embodiments belong. In the event of conflict, this description shall prevail, including the definitions. Furthermore, the examples are solely illustrative and should not be considered limitative. In particular, the block diagrams shown in the figures and described herein should not be considered as representing structural characteristics, i.e. structural limitations, but should be considered as representing functional characteristics, i.e. intrinsic properties of devices defined by the effects achieved or by the functional limitations and which can be implemented with different structures, in this manner protecting their functionality (possibility to operate).

[0025] In order to facilitate the understanding of the described embodiments, reference shall be made to certain embodiments and a specific language will be used to describe them. The purpose of the terminology used herein is to describe only particular embodiments, and should not be considered as limiting the scope of the present description.

[0026] The idea underlying the present invention is to pulsedly operate the light source 1 according to an operation pattern that must be known to the detector satellite 4, and to operate the imaging sensor 3 synchronously with respect to the pulsed operation of the light source 1 based on the digital images captured by the imaging sensor 3. In this way, the synchronization of the operation of the imaging sensor 3 with respect to the pulsed operation of the light source 1 results in no telecommunications technology being required to allow the target and detector satellites 2 and 4 to share a common synchronization signal or to locally compute the latter based on GNSS SIS, and is furthermore totally insensitive to the light propagation delay.

[0027] The synchronization of the operation of the imaging sensor 3 with respect to the pulsed operation of the light source 1 based on the digital images captured by the imaging sensor 3 will be described below with reference to Figures 2, 3 and 4, in which the time trends of the physical quantities involved are shown.

[0028] In particular, Figure 2 shows the time trends of the amount of light (number of photons) emitted by the light source 1 (top chart) and captured by the imaging sensor 3 in a digital image capture cycle, of the amount of light (number of photons) captured by the imaging sensor 3 in two successive digital images (middle chart) captured during a digital image capture cycle, and of the amount of light present in a difference image (bottom chart) computed as the difference between the two successive digital images, where the amount of light in a digital image computable based on the brightness of the pixels in the digital image.

[0029] Furthermore, in Figure 2, only the amount of light emitted by the light source 1 is taken into consideration, leaving out the light emitted by other light sources, such as the stars or straylights, as this represents a common background noise that is removed during computation of the difference image.

[0030] Furthermore, in Figure 2, the amount of light which is emitted by the light source 1 in the time period during which it is operated (is on), and which reaches the imaging sensor 3 is designated by A and is equal to A = h x Δt, where h represents the number of photons emitted by the light source 1 per unit time and that reach the imaging sensor 3, and Δt represents the time interval during which the light source 1 is operated; the amounts of light emitted by the light source 1 and captured by the imaging sensor 3 in a digital image capture cycle during which a pair of successive digital images are captured are designated by A1 and A2, while their difference is designated by ΔA = A2 - A1.

[0031] Furthermore, in Figure 2, an embodiment is shown in which the light source 1 is pulsedly operated according to an on-off operation pattern with a 50% duty-cycle, i.e. it is switched on for a switched-on time period Δt, during which it consequently emits a light ray that conveys a certain amount of light, and is then switched off for a switched-off time period Δt equal to the switched-on time period.

[0032] The imaging sensor 3 is consequently operated synchronously with respect to the pulsed operation of the light source 1 so as to cause the imaging sensor 3 to carry out a digital image capture cycle during which a plurality of digital images are successively captured, and comprise a first digital image containing the representation of the light ray emitted by the light source 1, and at least a second digital image not containing any representation of the light ray emitted by the light source 1, wherein the first and the second digital images are captured with a capture time distance that, in the embodiment considered and shown in Figure 2, is equal to the switched-on time period Δt of the light source 1 and which, hence, represents the operation time period of the imaging sensor 3 for carrying out a digital image capture cycle.

[0033] Furthermore, in the embodiment considered and shown in Figure 2, during a digital image capture cycle each of the digital images is captured using a capture time period lower than, or equal to, the switched-on time period Δt of the light source 1, depending on the time required to download the captured digital image from the imaging sensor 3 before starting the capture of the next digital image.

[0034] In other embodiments, the light source 1 may be pulsedly operated according to operation patterns other than the above-indicated one, for example on-off operation patterns with different duty-cycles, or pseudo-random on-off operation patterns, and the imaging sensor 3 is consequently operated with an operation period such as to allow at least two digital images to be successively captured in each digital image capture cycle, one of which contains the representation of the light ray emitted by the light source 1, and in the second one of which does not contain any representation of the light ray emitted by the light source 1.

[0035] The operation pattern of the light source 1 may be selectable, in the calibration phase of the imaging-based detection system, among a plurality of different available operation patterns, and information representative of the selected operation pattern, such as the type of the operation pattern, the switched-on time period Δt, and the switched-off time period Δt, or the total time period of the operation pattern (switch on and switch off) and the duty-cycle, is stored in the electronic processing unit 5, as the knowledge of this information, as mentioned above, is required in order for the electronic processing unit 5 to synchronize the digital image capture with respect to the pulsed light emission.

[0036] Furthermore, in a different embodiment, more than two digital images could be successively captured during a digital image capture cycle, and based on which the electronic processing unit 5 may then compute, by implementing opportune image processing techniques, the digital image in which only the luminous spot representative of the light ray emitted by the light source 1 on the target satellite 2 and that has impinged on the imaging sensor 3 is shown.

[0037] Moreover, in the embodiment considered and shown in Figure 2, operation of the imaging sensor 3 is shown to be offset by a time offset Δt0 ≠ 0 with respect to the pulsed operation of the light source 1, i.e., where the time origin O at which operation of the light source 1 starts does not coincide with the time origin O' at which operation of the imaging sensor 3 starts.

[0038] As it is possible to appreciate in Figure 2, the quantities A, A1 and A2 are always positive, as they represent the number of photons that impinge on the imaging sensor 3, whereas the quantity ΔA can be positive, negative or null, depending on whether operation of the imaging sensor 3 is time offset or not with respect to the pulsed operation of the light source 1, and its value and sign are indicative of whether operation of the imaging sensor 3 is time offset or not with respect to the pulsed operation of the light source 1, and, in the case where it is, of the time offset between the operation of the imaging sensor 3 and the pulsed operation of the light source 1.

[0039] In fact:
  • if the time offset Δt0 is null:

  • if the time offset Δt0 is equal to half of the switched-on time period Δt of the light source 1:

  • if the time offset Δt0 is less than half of the switched-on time period Δt of the light source 1:

  • if the time offset Δt0 is between half and the entire switched-on time period Δt of the light source 1:



[0040] The knowledge of the time offset Δt0 between the time origins O and O' therefore allows whether operation of the imaging sensor 3 is time offset or not with respect to the pulsed operation of the light source 1 to be determined and, in the case where it is not, operation of the imaging sensor 3 to be temporally aligned with respect to the pulsed operation of the light source 1.

[0041] The time offset Δt0 between the time origins O and O' is computable by the electronic processing unit 5 in the manner described below, with reference to Figure 3.

[0042] To this end, the electronic processing unit 5 is programmed to:
  • cause the imaging sensor 3 to carry out a first digital image capture cycle during which a first pair of digital images are captured with a capture time distance that, in the embodiment considered, is equal to the operation time period Δt of the light source 1,
  • compute, based on the first pair of digital images captured during the first digital image capture cycle, a first quantity ΔA indicative of the difference between the amounts of light that impinged on the imaging sensor 3 when the digital images of the first pair of digital images were captured,
  • after a reoperation time period τ, which is a function of the operation time period Δt of the light source 1 and, in the embodiment considered, is equal to x = Δt/2, has elapsed from the end of the capture of the second image of the first pair of images, cause the imaging sensor 3 to carry out a second digital image capture cycle during which a second pair of digital images is captured with a capture time distance that, in the embodiment considered, is equal to the operation time period Δt of the light source 1,
  • compute, based on the second pair of digital images captured during the second digital image capture cycle, a second quantity ΔA' indicative of the difference between the amounts of light that impinged on the imaging sensor 3 when the digital images of the second pair of digital images were captured, and
  • compute the time offset Δt0 between the time origins O and O', based on Δt, ΔA and ΔA', expediently according to the following formula:



[0043] Once the time offset Δt0 is known, the electronic processing unit 5 is programmed to synchronize operation of the imaging sensor 3 with respect to the pulsed operation of the light source 1 in the following manner:
  • computing firstly an alignment time period τ', based on Δt, ΔA, ΔA', and Δ(t0), expediently according to the following formula:

  • reoperating the imaging sensor 3 after the alignment time period τ' has elapsed from the end of the capture of the last digital image in the second digital image capture cycle.


[0044] As shown in Figure 4, the alignment time period τ' causes realignment of the operation of the imaging sensor 3 with respect to the pulsed operation of the light source 1, restoring the above-indicated condition of perfect alignment, namely:



[0045] Based on the foregoing, it is possible appreciate that the present invention allows the intended objects to be achieved, namely synchronizing the operation of the imaging sensor with respect to the pulsed operation of the light source, without requiring any common synchronization signal to be shared or locally computed, and also causing the auto-synchronization to be insensitive to the light propagation delay.


Claims

1. An imaging-based target detection system comprising:

▪ an electromagnetic radiation emitting source (1) to be associated with a target (2) to be detected and pulsedly operable according to an operation pattern to emit electromagnetic radiation,

▪ an imaging sensor (3) sensitive to the electromagnetic radiation emitted by the electromagnetic radiation emitting source (1), and operable to capture digital images of the electromagnetic radiation emitting source (1), and

▪ an electronic processing unit (5) connected to the imaging sensor (3) and configured to:

- store information representative of the operation pattern of the electromagnetic radiation emitting source (1),

- operate the imaging sensor (3) synchronously with respect to the pulsed operation of the electromagnetic radiation emitting source (1) based on the stored information representative of the operation pattern of the electromagnetic radiation emitting source (1), so as to cause the imaging sensor (3) to carry out a digital image capture cycle during which a plurality of digital images are successively captured and comprise a first digital image captured when the imaging sensor (3) is impinged by electromagnetic radiation emitted by the electromagnetic radiation emitting source (1), and at least a second image captured when the imaging sensor (3) is not impinged by any electromagnetic radiation emitted by the electromagnetic radiation emitting source (1), and

- process the captured digital images to detect the target (2);

the imaging-based detection system is characterised in that the electronic processing unit (5) is further configured to synchronise operation of the imaging sensor (3) with respect to the pulsed operation of the electromagnetic radiation emitting source (1) based on digital images captured by the imaging sensor (3).
 
2. The imaging-based target detection system of claim 1, wherein the electromagnetic radiation emitting source (1) is pulsedly operated to emit electromagnetic radiation for an emission time period (Δt); and wherein the electronic processing unit (5) is further configured to synchronise operation of the imaging sensor (3) with respect to the pulsed operation of the electromagnetic radiation emitting source (1) based on the digital images captured by the imaging sensor (3) by:

- causing the imaging sensor (3) to carry out a first digital image capture cycle during which a first digital image and at least a second digital image are captured,

- computing, based on the digital images captured during the first digital image capture cycle, a first quantity (ΔA) indicative of a difference between the amounts of electromagnetic radiation that impinged on the imaging sensor (3) when the digital images were captured,

- after a reoperation time period (τ), which is a function of the electromagnetic radiation emission time period (Δt), has elapsed from the capture of the last digital image during the first digital image capture cycle, causing the imaging sensor (3) to carry out a second digital image capture cycle during which a first digital image and at least a second digital image are captured again,

- computing, based on the digital images captured during the second digital image capture cycle, a second quantity (ΔA') indicative of a difference between the amounts of electromagnetic radiations that impinged on the imaging sensor (3) when the digital images were captured,

- computing a time offset (Δt0) between the operation of the electromagnetic radiation emitting source (1) and the operation of the imaging sensor (3) based on the emission time period (Δt) and on the computed first and second quantities (ΔA, ΔA'), and

- synchronising the operation of the imaging sensor (3) with respect to the pulsed operation of the electromagnetic radiation emitting source (1) based on the emission time period (Δt), the computed first and second quantities (ΔA, ΔA'), and the computed time offset (Δt0).


 
3. The imaging-based target detection system of claim 2, wherein the electronic processing unit (5) is further configured to synchronise the operation of the imaging sensor (3) with respect to the pulsed operation of the electromagnetic radiation emitting source (1) based on the emission time period (Δt), the computed first and second quantities (ΔA, ΔA'), and the computed time offset (Δt0) by:

- computing an alignment time period (τ') based on the electromagnetic radiation emission time period (Δt), the computed first and second quantities (ΔA, ΔA'), and the computed time offset (Δt0), and

- causing the imaging sensor (3) to carry out a further digital image capture cycle after the alignment time period (τ') has elapsed from the capture of the last digital image in the second digital image capture cycle.


 
4. The imaging-based target detection system of claim 2 or 3, wherein the reoperation time period (τ) is equal to half the electromagnetic radiation emission time period (Δt).
 
5. The imaging-based detection system according of any one of the claims 2 to 4, wherein the electronic processing unit (5) is further configured to compute the time offset (Δt0) between the operation of the electromagnetic radiation emitting source (1) and the operation of the imaging sensor (3) according to the following formula:

where:

- Δt0 is the time offset between the operation of the electromagnetic radiation emitting source (1) and the operation of the imaging sensor (3),

- Δt is the electromagnetic radiation emission time period, and

- ΔA and ΔA' are the computed first and second quantities, respectively.


 
6. The imaging-based target detection system of claim 3, wherein the electronic processing unit (5) is further configured to compute the alignment time period (τ') according to the following formula:

where:

- τ' is the alignment time period,

- Δt is the electromagnetic radiation emission time period,

- Δt0 is the time offset between the operation of the electromagnetic radiation emitting source (1) and the operation of the imaging sensor (3), and

- ΔA and ΔA' are the computed first and second quantities, respectively.


 
7. The imaging-based detection system of any one of the preceding claims, wherein the information representative of the operation pattern of the electromagnetic radiation emitting source (1) is indicative of a an electromagnetic radiation emission time period (Δt) and of an electromagnetic radiation non-emission time period (Δt).
 
8. An electronic processing unit (5) configured as claimed in any one of the preceding claims, for the imaging-based target detection system of any one of the preceding claims.
 
9. A software loadable in an electronic processing unit (5) of the imaging-based target detection system of any one of the preceding claims, and designed to cause, when executed, the electronic processing unit (5) to become configured as claimed in any one of the preceding claims.
 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description