GB1604866A - Optical infra-red surveillance systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL INFRA-RED SURVEILLANCE SYSTEMS (71) We, THE MARCONI COMPANY LIMITED, a British Company, of Marconi House, New St., Chelmsford, Essex CM 1 1 PAL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to optical surveillance systems and in particular to infra-red optical surveillance systems and it is an object of this invention to provide improved such systems in which some discrimination is provided against stationary targets and/or spatially large targets.

According to this invention an optical surveillance system comprises means for viewing different regions of a field of view, means responsive to energy received from said different regions for producing target presence indicating signals (heinafter referred to as detections) each indicative of the presence of a target in a different one of said regions, a first store arranged to store detections produced at one time, a second store arranged to store detection produced at a later time, means for comparing detections stored in said first store with detections stored in said second store, means for applying detections from said first store which relate to regions to which no detections in said second store relate to a first unpaired detection store, means for applying detections from said second store which relate to regions to which no detections in said first store relate to a second unpaired detection store whereby a detection relating to a given region and appearing in both said first and second stores, and representing a target which is stationary between said two times, is omitted from said unpaired detection stores.

Preferably means are provided for deriving detections from said two unpaired detection stores and means are provided for separating out those derived detections which are provided in said first unpaired detection store but which have no counter part in said second unpaired detection store whereby to discriminate between, on the one hand, detections relating to targets which have moved between said times or have newly appeared and, on the other hand, detections relating to targets which have disappeared.

Preferably said different regions are comprised of different but adjacent cells in said field of view.

Preferably again said cells are rectangular.

In order to reduce the effects of quantisation error, means may be provided for comparing any detections stored in said first unpaired detection store and relating to cells surrounding a cell in relation to which a detection is stored in said second unpaired detection store and for eliminating any such detections stored in said first unpaired detection store where any detection is found in said second unpaired detection store which appears to relate to a target within one of said surrounding cells.

Said means responsive to energy received from said different regions preferably comprises an array of energy responsive devices arranged to extend linearly and means are provided for scanning said linear array in an orthogonal direction whereby said linear array is caused to scan a desired volume of space. In practice, usually said linear array will be arranged to extend in the horizontal and said scanning is arranged to be effective in the vertical. Said cells may be formed by the effects of scanning said linear array in said orthogonal direction, each cell being formed by the effects of an element in said array scanned through a predetermined angle in said orthogonal direction.

Preferably the means for scanning said array in an orthogonal direction comprises a rotatable many-sided drum having energy reflectors on its many sides so that the field of view may be swept repeatedly in said orthogonal direction over said array. Normally a focusing lens is provided to form an image of said field of view upon the sides of said rotatable drum. Preferably said many-sided drum is octagonal in section.

Preferably also said array and the means for scanning the same are rotatable in a plane orthogonal to the direction of scanning whereby a series of successive, preferably overlapping, sweeps in said orthogonal direction are effected.

Preferably said surveillance system is an infra-red surveillance system, said responsive means comprising infra-red responsive means responsive to infra-red energy received from said different regions.

The invention is illustrated in and further described with reference to the drawing accompanying the provisional specification which shows schematically one infra-red surveillance system in accordance with the present invention.

Referring to the drawing, a sensor 1 consists as shown inset, of a horizontal array 2 of twenty nine square infra-red detector elements which are scanned together vertically in space by a cylinder 3 of octagonal cross-section having plane mirrors on its eight sided face. Provided to focus energy from the field of view onto the rotating cylinder 3 is an infra-red lens 4 so that any infra-red energy incident upon the lens 4, and within the field of view, is reflected from one of the plane mirrors on the cylinder 3 onto the detector array. Thus the rotating cylinder 3 causes each of the detectors in the array 2 to sweep across the field of view in vertical sweeps. In this particular example the extent of a vertical sweep is 10'.The angular field of view of anyone of the detectors in array 2 is 1 milliradian in both horizontal and vertical extent, so that a single vertical sweep examines a swathe 29 milliradians in azimuth and 10' in elevation.

Twenty nine separate electrical outputs are provided which are connected via a trunk path 5 to a point source detector arrangement 6 containing twenty nine point source detectors, one for each of the twenty nine separate electrical outputs and each of which is as described in the specification accompanying our co-pending application 53049/77 (Serial No 1605048).

The whole assembly of lens 4, rotating cylinder 3 and detector array 2, i.e. sensor 1, is rotated about a vertical axis with a one second period. Vertical swathes of twenty nine elements are repeated by reflections from successive mirror surface of the rotating cylinder 3 and successive swathes are arranged to cover adjacent portions of the horizontal field of view by means of the horizontal scan, with an overlap of two detector elements, i.e. 2 milliradians. Thus the spatial repeat interval for successive swathes is 27 milliradians giving 232 swathes in one complete horizontal revolution of the scanner 1. Each swathe period is thus 4.3 milliseconds. In this period only 66% (2.83 milliseconds) of the time is occupied scanning the field of view.A noise-limiting lowpass electronic filter will usually be included in each of the 29 vertical scanning channels, which has the effect of producing resolution broadening, typically of the order of 37%.

Therefore it is assumed that the resolution in the vertical direction is 1.37 milliradians, which leads to a total of 128 resolution points in the 10e vertical field of view.

In the arrangement illustrated in the drawing each vertical swathe is dealt with separately for point source detection, followed by MTI processing. The point source assessment is performed in real time during the 2.84.

millisecond sweep and the MTI processing for the swathe is carried out immediately afterwards, during the 1.46 millisecond interval before the next swathe commences.

The twenty nine signals from the twenty nine elements in array 2, fed to the point source detector arrangement 6 via trunk path 5 are applied to respective ones of the twenty nine point source detectors within arrangement 6. Each point source detector is such as to pass only those signals which are of the order of resolution width in both horizontal and vertical directions. The addresses of the three thousand seven hundred and twelve (the product of the number of vertical resolution points and the number of detector elements in the array) detection cells are generated in parallel with the signals from encoders (not shown) geared to both the azimuth and elevation rotation axis.A 24 bit word identifies each cell, this being made up of 8 bits applied to path 7 identifying the particular vertical swathe by number and a 16 bit address applied to path 8, which consists of nine bits to denote the elevation angle 1.37 =0.34 milliradians accuracy) 4 and seven bits to identify the azimuth cell (i milliradians accuracy).

Thus in both elevation and azimuth angles are measured to one quarter of the resolution cell size.

Detections are stored in one of two stores 9 and 10. By means of change over switches 11 and 12, the detection information from the output 13 of point source detector arrangement 6 and the sixteen bit addresses on path 8 is passed on alternate azimuth revolutions to the stores 9 and 10. Therefore the store for current detection information alternates between stores 9 and 10 at a I second rate. To store a detection, the 16 bit address within the swathe is repeatedly loaded into the store by means of the data input 14 or 15, as the case may be, the swathe number applied to the stores 9 and 10 via data inputs 16 or 17, as the case may be, from the path 7 having been already selected at the start of the azimuth revolution cycle.

If a maximum number of 1024 detections per revolution is assumed, this gives an average of less than five detections per swathe and in calculating the required capacity of the stores 9 and 10, a maximum of 100 detections for any one swathe is believed to be sufficient.

Assuming store 9 is for the time being the current azimuth revolution store, at the end of a swathe sweep the detections just stored in store 9 are compared with detections in the corresponding swathe of the previous azimuth revolution cycle stored in store 10. An identical address removal circuit 18 operates to remove any identical addresses appearing in both stores. Any remaining unpaired sixteen bit addresses are fed to an unpaired address store 19 if these latter are addresses of new detections from the current azimuth revolution (i.e. detections stored in store 9) or to an unpaired address store 20 if these latter are addresses of vanished detections which are not longer present (but are stored in store 10). This is achieved by means of changeover switches 11' and 12'. Switches 11, 12, 11' and 12' are ganged together as conventionally represented.

At this stage of stationary point sources which have crossed threshold on two successive azimuth revolutions have been eliminated leaving (a) New point-source targets, having just appeared.

(b) Point-source targets showing significant transverse movement.

(c) Stationary point-source targets with apparent movement to an adjacent resolution cell due to quantisation error.

(d) Signals of marginal amplitude, which just miss crossing threshold on one scan, but just cross on the next--due to noise fluctuations.

In order to eliminate signals of type (c) above, the eight addresses adjacent to each of the stored detections in previous scan unpaired address store 20 are generated and compared with the current scan unpaired addresses in store 19 in an adjacent address removal circuit 21. If any identical addresses are found in this manner, their generating addresses are removed so that only signals of type (a), (b) and (d) above are applied to a target processor 22. The path for signals of type (a), (b) and (d) above is represented at 23. If desired vanished detections may also be derived from previous scan unpaired address store 20 via the path 24 represented in dotted outline.

If signals of type (d) are found to be significantly numerous, it may be necessary to record the amplitude of point source detections in the initial processing stage. This also offers the possibility of oberving any large fluctuations in signal strength at one second intervals, thus assisting decisions on target priority for tracking requirements in the target processor.

It will be noted that whereas the point source detector 6 and the stores 9 and 10 are active during signal generation swathes, the identical address removal circuit 18 and the following elements up to the target processor 22 are active only in between swathes.

The design parameters of the particular example described above may be summarised as follows:- Scan period for 1 revolution 1 second Resolution angle for 1 detector in azimuth 1 m.rad.

Number of detector resolution points in 1 revolution 6283 Number of detectors 29 Repeat-number of detectors (2 overlap) 27 Number of swathes (vertical sweeps) in 1 rev. 232 Repeat period of swathes 4.3 m.sec.

Time for one swathe-sweep (scanner efficiency 66%) 2.84 m.sec.

Time between swathes 1.46 m.sec.

Vertical field 10' = 175 m. rad.

Number of resolution points in vertical 128 Resolution angle in vertical 1.37 m.rad.

Time to cover one vertical resolution point 22.2 psec.

WHAT WE CLAIM IS: 1. An optical surveillance system comprising means for viewing different regions of a field of view, means responsive to energy received from said different regions for producing target presence indicating signals (hereinafter referred to as detections) each indicative of the presence of a target in a different one of said regions, a first store arranged to store detections produced at one time, a second store arranged to store detections produced at a later time, means for comparing detections stored in said first store with detections stored in said second store, means for applying detections from said first store which relate to regions to which no detections in said second store relate to a first unpaired detection store, means for applying detections from said second store which relate to regions to which no detections in said first store relate to a second unpaired detection store whereby a detection relating to a given region and appearing in both said first and second stores, and representing a target which is stationary between said two times, is omitted from said unpaired detection stores.

2. A system as claimed in claim 1 and wherein means are provided for deriving detections from said two unpaired detection

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   average of less than five detections per swathe and in calculating the required capacity of the stores 9 and 10, a maximum of 100 detections for any one swathe is believed to be sufficient.

   Assuming store 9 is for the time being the current azimuth revolution store, at the end of a swathe sweep the detections just stored in store 9 are compared with detections in the corresponding swathe of the previous azimuth revolution cycle stored in store 10. An identical address removal circuit 18 operates to remove any identical addresses appearing in both stores. Any remaining unpaired sixteen bit addresses are fed to an unpaired address store 19 if these latter are addresses of new detections from the current azimuth revolution (i.e. detections stored in store 9) or to an unpaired address store 20 if these latter are addresses of vanished detections which are not longer present (but are stored in store

   10). This is achieved by means of changeover switches 11' and 12'. Switches 11, 12, 11' and 12' are ganged together as conventionally represented.

   At this stage of stationary point sources which have crossed threshold on two successive azimuth revolutions have been eliminated leaving (a) New point-source targets, having just appeared.

   (b) Point-source targets showing significant transverse movement.

   (c) Stationary point-source targets with apparent movement to an adjacent resolution cell due to quantisation error.

   (d) Signals of marginal amplitude, which just miss crossing threshold on one scan, but just cross on the next--due to noise fluctuations.

   In order to eliminate signals of type (c) above, the eight addresses adjacent to each of the stored detections in previous scan unpaired address store 20 are generated and compared with the current scan unpaired addresses in store 19 in an adjacent address removal circuit 21. If any identical addresses are found in this manner, their generating addresses are removed so that only signals of type (a), (b) and (d) above are applied to a target processor 22. The path for signals of type (a), (b) and (d) above is represented at 23. If desired vanished detections may also be derived from previous scan unpaired address store 20 via the path 24 represented in dotted outline.

   If signals of type (d) are found to be significantly numerous, it may be necessary to record the amplitude of point source detections in the initial processing stage. This also offers the possibility of oberving any large fluctuations in signal strength at one second intervals, thus assisting decisions on target priority for tracking requirements in the target processor.

   It will be noted that whereas the point source detector 6 and the stores 9 and 10 are active during signal generation swathes, the identical address removal circuit 18 and the following elements up to the target processor 22 are active only in between swathes.

   The design parameters of the particular example described above may be summarised as follows:- Scan period for 1 revolution 1 second Resolution angle for 1 detector in azimuth 1 m.rad.

   Number of detector resolution points in 1 revolution 6283 Number of detectors 29 Repeat-number of detectors (2 overlap) 27 Number of swathes (vertical sweeps) in 1 rev. 232 Repeat period of swathes 4.3 m.sec.

   Time for one swathe-sweep (scanner efficiency 66%) 2.84 m.sec.

   Time between swathes 1.46 m.sec.

   Vertical field 10' = 175 m. rad.

   Number of resolution points in vertical 128 Resolution angle in vertical 1.37 m.rad.

   Time to cover one vertical resolution point 22.2 psec.

   WHAT WE CLAIM IS: 1. An optical surveillance system comprising means for viewing different regions of a field of view, means responsive to energy received from said different regions for producing target presence indicating signals (hereinafter referred to as detections) each indicative of the presence of a target in a different one of said regions, a first store arranged to store detections produced at one time, a second store arranged to store detections produced at a later time, means for comparing detections stored in said first store with detections stored in said second store, means for applying detections from said first store which relate to regions to which no detections in said second store relate to a first unpaired detection store, means for applying detections from said second store which relate to regions to which no detections in said first store relate to a second unpaired detection store whereby a detection relating to a given region and appearing in both said first and second stores, and representing a target which is stationary between said two times, is omitted from said unpaired detection stores.
2. 2. A system as claimed in claim 1 and wherein means are provided for deriving detections from said two unpaired detection

   stores and means are provided for separating out those derived detections which are provided in said first unpaired detection store but which have no counter part in said second unpaired detection store whereby to discriminate between, on the one hand, detections relating to targets which have moved between said times or have newly appeared and, on the other hand, detections relating to targets which have disappeared.
3. 3. A system as claimed in claim 1 or 2 and wherein said different regions are comprised of different but adjacent cells in said field of view.
4. 4. A system as claimed in claim 3 and wherein said cells are rectangular.
5. 5. A system as claimed in claim 3 or 4 and wherein means are provided for comparing any detections stored in said first unpaired detection store and relating to cells surrounding a cell in relation to which a detection is stored in said second unpaired detection store and for eliminating any such detections stored in said first unpaired detection store where any detection is found in said second unpaired detection store which appears to relate to a target within one of said surrounding cells.
6. 6. A system as claimed in any of the above claims and wherein said means responsive to energy received from said different regions comprises an array of energy responsive devices arranged to extend linearly and means are provided for scanning said linear array in an orthogonal direction whereby said linear array is caused to scan a desired volume of space.
7. 7. A system as claimed in claim 6 and wherein said linear array is arranged to extend in the horizontal and said scanning is arranged to be effective in the vertical.
8. 8. A system as claimed in claim 3 or in any of claims 3 to 7 as dependent upon claim 3 and wherein said cells are formed by the effects of scanning said linear array in said orthogonal direction, each cell being formed by the effects of an element in said array scanned through a predetermined angle in said orthogonal direction.
9. 9. A system as claimed in claim 7 or in claim 8 as dependent upon claim 7 and wherein the means for scanning said array in an orthogonal direction comprises a rotatable many-sided drum having energy reflectors on its many sides so that the field of view may be swept repeatedly in said orthogonal direction over said array.
10. 10. A system as claimed in claim 9 and wherein a focusing lens is provided to form an image of said field of view upon the sides of said rotatable drum.
11. 11. A system as claimed in claim 9 or 10 wherein said many-sided drum is octagonal in section.
12. 12. A system as claimed in claim 6 or in any of claims 7 to 11 as dependent on claim 6 and wherein said array and the means for scanning the same are rotatable in a plane orthogonal to the direction of scanning whereby a series of successive sweeps in said orthogonal direction are effected.
13. 13. A system as claimed in claim 12 and wherein said successive sweeps are overlapping.
14. 14. A system as claimed in any of the.

    above claims and embodied as an infra-red surveillance system, said responsive means comprising infra-red responsive means responsive to infra-red energy received from said different regions.
15. 15. An infra-red surveillance system substantially as herein described with reference to the drawing accompanying the provisional specification.