GB1580437A - Arrangement for producing a stabilised picture - Google Patents

Description

(54) AN ARRANGEMENT FOR PRODUCING A STABILIZED PICTURE (71) We, EMI LIMITED, a British company of Blyth Road, Hayes Middlesex, 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 stabilization arrangements for reducing degradation, caused by vibration of an air vehicle, of a picture derived from signals from a sensor mounted on the vehicle. The invention is particularly related to arrangements for surveillance from air vehicles, such as helicopters, using sensors examining an external scene which may include a feature of interest.

In certain applications electro-optical sensors used for such surveillance may have a resolution better than 0.1 mrad and will therefore require some form of stabilization to hold a picture, derived from the sensor, steady to a factor significantly less than that value.

In practice, the stabilization is considered to be for three frequency bands. Low frequency perturbation of the sensor can be removed by the normal steering mechanism of the sensor. At the other extreme, high frequency mechanical vibration can be damped sufficiently to prevent significant internal motion of optical components. For the purpose of stabilizing the middle band vibration, it has been known to improve the steering response of the system and to reference this to an inertial system or to insert a gyro-stabilized mirror in the optical path.

However, both of these alternatives have disadvantages. It is therefore an object of this invention to provide an alternative stabilization system.

According to the invention there is provided an arrangement for producing a stabilized picture from a viewing system which includes a sensor, mounted on an air vehicle to provide a picture of a region to be surveyed from the vehicle, the arrangement including a first store arranged to hold, at respective locations, data, from said sensor, for picture points of the said picture; a second store arranged to hold new data, from said sensor, for some of said picture points; and means for comparing new data for a selected group of said picture points with the data in the first store for the corresponding and some surrounding picture points to determine displacement of the said group of picture points, and means arranged to utilize the determinations of displacement to reduce the effects on said picture of vibration of said sensor.

In order that the invention may be clearly understood and readily carried into effect, an example thereof will now be described with reference to the single figure of the drawings accompanying the provisional specification which shows in block diagrammatic form one circuit in accordance with the invention.

Known tracking or guidance arrangements compare at least two successive frames of a picture derived from a sensor, such as an infra-red sensor, to determine deviation between the frames and hence derive a guidance signal. The comparison can be by correlation techniques which are well established. It is proposed herein to use similar techniques to compare at least similar regions of such successive pictures to derive information about deviations therebetween caused by the undesired vibration.

The system will be considered in terms of an infra-red system such as the high performance surveillance systems known as forward looking infra-red (FLIR) systems.

In a typical arrangement an array of detectors aligned in the vertical or elevational direction is scanned in azimuth across a region to be viewed. A simplified example will be considered in which there is no interlace or vertical scanning. The effect of vertical scanning is provided by the vertical dimension of the detector array.

In order to achieve a bright non-flickering display, it is current practice to interpose a digital scan converter in the predisplay circuits.

Thus, at any instant the scan converter storage receives data from the detectors representing a limited vertical slice of the region as determined by the extent of the array. In the course of the horizontal (azimuthal) scan an area of the region is viewed.

The scanning of this area may be considered to be the passage of a "window" comprising a group of picture points for the vertical slice which is nominally being viewed at any instant and one or more consecutive vertical slices ahead of and behind that slice. The window thus gives a sequence of detector array positions for which the data are to be compared.

In the arrangement of the invention the scan converter is arranged so that it does not discard information for a particular position of the array until that position has next been viewed by a succeeding passage of the window. For this succeeding passage the data value for the picture points of the window are compared, using known correlation techniques, with data for the same and the surrounding picture points for the earlier passage as obtained from the scan converter store. Typically the data are compared with those for positions of the window displaced by one element ahead and behind in azimuth, by one above and below in elevation and for four window positions displaced by one point both in azimuth and elevation. Thus, including the nominal position of the window, each detector output is correlated with a picture point value from the scan converter store for each of nine displacements. Of course, the window may be correlated for a displacement through more picture points if vibration of greater amplitude is expected. The results for the nine comparisons are used to determine the position for best correlation so that the picture point data values for the succeeding passage of the window can be entered into the store at consistant locations, despite vibration. The earlier information may then be discarded.

There is shown in the Figure in block diagrammatic form a circuit for implementing the invention. The data for each successive scan position of an IR sensor 1 is entered into a buffer store 2. Simultaneously the sensor scanning arrangement, not shown since it may take many known forms, provides information indicating the progress of the scan to an address selector 3. Address selector 3 co-operates with a scan converter store 4 which may be a random access memory (RAM) store. Store 4 is caused to provide data previously stored for nine, say, picture points, surrounding each point in the array currently examined, to a correlator circuit 5 where the data for the corresponding old array are compared by known correlation techniques to the new picture point values for this current array. The position of best correlation is then provided as a displacement value to address selector 3 which correspondingly places the new values for the window at the most suitable locations in store 4, either replacing the corresponding old data or being added to a fraction of the old data. Preferably, this placing of the data for each picture point is delayed until it is certain that the previous value will not be required for any further correlation, in the current passage of the window, to prevent flattening of the correlator response. As desired, and according to the operation of scan conversion, the data from store 4 is output at 6 to a display.

Such surveillance arrangements are generally used on helicopters for identification purposes but they can also be used for tracking purposes by FLIR sensors on fast moving aircraft or in conjunction with missiles. In such alternative applications it will be understood that placing each picture point datum in the scan converter store by correlation in this way will tend to cancel motion due to tracking error as well as that due to vibration. However, tracking error will tend to build up over an entire frame, whereas vibration errors should, in the long term, cancel. To utilize this effect, a tracking error store 7 may be included. This store collates over a long period, the displacements determined by correlator 5. Cumulative error over this period is then attributed to tracking error and is provided to address selector 5 to ensure that the display or guidance circuits properly indicate deviation of the vehicle from its desired path.

It has been assumed that the vibrational motion is one of translation. There may, however, be an element of rotational vibration. In that case the analysis may be performed independently, first for one form of vibration, and after that has been corrected, then for the other form. After both corrections have been determined, the data can be added to its appropriate position in the scan converter store.

As an alternative the selected group to be correlated with corresponding data in the scan converter store may comprise all of the data in the buffer store, rather than data for a limited number of those points.

It should be noted that if, as in this embodiment, the data is to be electronically compensated after read-out, it is necessary that data be obtained from a sensor which does not integrate the signal. This includes scanning infra-red systems and other systems, including some forms of television such as gated low light TV (LLTV) used in conjunction with pulsed illumination. The form of "window" used is dictated by the form of sensor so that for LLTV systems it could be a few horizontal TV lines or perhaps the complete picture.

It is not necessary that the whole of the window be used for this purpose. One or more small zones in suitable locations may suffice.

It should also be noted that the output from correlator 5 can be used to drive servos directing the sensor array to compensate vibration as in known systems.

WHAT WE CLAIM IS:- 1. An arrangement for producing a stabilized picture from a viewing system which includes a sensor, mounted on an air vehicle to provide a picture of a region to be surveyed from the vehicle, the arrangement including a first store arranged to hold, at respective locations, data, from said sensor, for picture points of the said picture; a second store arranged to hold new data.

from said sensor, for some of said picture points; and means for comparing new data for a selected group of said picture points with the data in the first store for the corresponding and some surrounding picture points to determine displacement of the said group of picture points, and means arranged to utilize the determinations of displacement to reduce the effects on said picture of vibration of said sensor.

2. An arrangement according to Claim 1, in which means for comparing is arranged to compare the new data for each picture point of the group with data in the first store for the corresponding picture point and the eight adjacent picture points to determine displacement of the group.

3. An arrangement according to either of the preceding claims, wherein the means arranged to utilize the determinations of displacement includes means for entering the said new data into the first store at an appropriate location to compensate for said vibration.

4. An arrangement according to Claim 3 in which the means for entering is arranged to enter the new data into the first store to replace previous data for the same picture points.

5. An arrangement according to Claim 3 in which the means for entering is arranged to enter the new data into the first store in addition to a proportion of the previous data for the same picture points.

6. An arrangement according to any of Claims 3 to 5 in which the means for entering is arranged to enter the new data into the first store only after the corresponding previous data has been used for all possible comparisons with the new data.

7. An arrangement according to any of the preceding claims including means for accumulating errors over a period long compared with the period of the vibration, for use on tracking or guidance information.

8. An arrangement according to either of Claims 1 or 2 wherein the means arranged to utilize the determinations of displacement includes servo means arranged to move the said sensor and control means arranged to drive the said servo means in response to said determinations of displacement to compensate for said vibration.

9. An arrangement according to any one of the preceding claims in which the means for comparing is arranged to perform independent comparisons for rotation and translational displacement.

10. An arrangement for producing a stabilized picture, the arrangement being substantially as herein described with reference to the drawing filed with the Provisional Specification.

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

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. of "window" used is dictated by the form of sensor so that for LLTV systems it could be a few horizontal TV lines or perhaps the complete picture. It is not necessary that the whole of the window be used for this purpose. One or more small zones in suitable locations may suffice. It should also be noted that the output from correlator 5 can be used to drive servos directing the sensor array to compensate vibration as in known systems. WHAT WE CLAIM IS:-

1. An arrangement for producing a stabilized picture from a viewing system which includes a sensor, mounted on an air vehicle to provide a picture of a region to be surveyed from the vehicle, the arrangement including a first store arranged to hold, at respective locations, data, from said sensor, for picture points of the said picture; a second store arranged to hold new data.

from said sensor, for some of said picture points; and means for comparing new data for a selected group of said picture points with the data in the first store for the corresponding and some surrounding picture points to determine displacement of the said group of picture points, and means arranged to utilize the determinations of displacement to reduce the effects on said picture of vibration of said sensor.

2. An arrangement according to Claim 1, in which means for comparing is arranged to compare the new data for each picture point of the group with data in the first store for the corresponding picture point and the eight adjacent picture points to determine displacement of the group.

3. An arrangement according to either of the preceding claims, wherein the means arranged to utilize the determinations of displacement includes means for entering the said new data into the first store at an appropriate location to compensate for said vibration.

4. An arrangement according to Claim 3 in which the means for entering is arranged to enter the new data into the first store to replace previous data for the same picture points.

5. An arrangement according to Claim 3 in which the means for entering is arranged to enter the new data into the first store in addition to a proportion of the previous data for the same picture points.

6. An arrangement according to any of Claims 3 to 5 in which the means for entering is arranged to enter the new data into the first store only after the corresponding previous data has been used for all possible comparisons with the new data.

7. An arrangement according to any of the preceding claims including means for accumulating errors over a period long compared with the period of the vibration, for use on tracking or guidance information.

8. An arrangement according to either of Claims 1 or 2 wherein the means arranged to utilize the determinations of displacement includes servo means arranged to move the said sensor and control means arranged to drive the said servo means in response to said determinations of displacement to compensate for said vibration.

9. An arrangement according to any one of the preceding claims in which the means for comparing is arranged to perform independent comparisons for rotation and translational displacement.

10. An arrangement for producing a stabilized picture, the arrangement being substantially as herein described with reference to the drawing filed with the Provisional Specification.