GB2300082A

GB2300082A - Distance measuring apparatus

Info

Publication number: GB2300082A
Application number: GB9508026A
Authority: GB
Inventors: Michael Francis Armour
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1995-04-21
Filing date: 1995-04-21
Publication date: 1996-10-23
Anticipated expiration: 2015-04-21
Also published as: GB2300082B; GB9508026D0

Abstract

Distance measuring apparatus for determining the height or range of an object from a viewing point, said apparatus comprising imaging means for forming an image of the scene viewed along an optical axis, and means for determining the size of the image of said object within the image of the viewed scene, thereby to determine the distance of the said object from said imaging means, based upon the actual size of said object. Two measurements in different directions can give object attitude (fig 2).

Description

Distance Measurina Apparatus This invention relates to distance measuring apparatus for measuring the height or range of an object from a viewing point, and in particular, though not exclusively, to an altitude measurement system for measuring the height of an object above a viewing point.

In many applications it is important to be able to measure the height of an aircraft or other flying body from outside, without using an altimeter or other height measuring apparatus on board the aircraft. In the past this has been done on trial aircraft for example by using optical tracking facilities provided by two kine-theodolites located at each end of an airfield runway which determine the height of the aircraft using triangulation techniques. However kine-theodolites are expensive to purchase, maintain and operate. Thus a need exists for a means of accurately measuring the height of a flying body from the ground which measures the height independently of any height indicating apparatus on the flying body and without the need for specialist precision optical equipment such as kinetheodolites.

Accordingly, in one aspect of this invention there is provided distance measuring apparatus for determining the height or range of an object from a viewing point, said apparatus comprising imaging means for forming an image of the scene viewed along an optical axis, and means for determining the relative size of the image of said object within the image of the viewed scene, thereby to determine the distance of the said object from said imaging means, based upon the actual size of said object.

In another aspect, there is provided a method of determining the height or range of an object from a viewing point, which comprises forming an image of a viewed scene containing said object, determining the actual size of said object, determining the relative size of the image of said object within the image of the viewed scene, and thereby deducing the distance of the object from said viewing point.

Whilst the invention has been described above it extends to any invention combination of the features set out above or in the following description.

The invention may be performed in various ways and an embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings, in which:- Figure 1 is a schematic view showing an arrangement for calculating the height of an aircraft using an upwardlooking video camera, and Figure 2 is a schematic view of a typical image of an aircraft as seen by the video camera in the arrangement of Figure 1.

Referring to Figure 1, a CCD (Charge Coupled Device) video camera 10 is located on flat, level ground pointing vertically upwards towards the aircraft 12. The height of the camera film plane used in the expression below is hc.,,..

the camera field of view is , and the height of the aircraft 12 above ground is shown as In use, the aircraft 12 is flown over the camera 10 so that the camera sees at least one T.V. frame with the aircraft fully within the frame, and this frame is analysed, either by an image processor (not shown) which receives data from the video camera, or manually. The height is determined by noting the relative size of the aircraft within the scene viewed by the camera, and knowing the actual size of the aircraft. The image is analysed by noting the addresses of the pixels in the CCD array of the camera which correspond to well-defined points on the aircraft, in this example the noses of the wing-tip pods 14 (see Figure 2), whose actual separation is known. From the addresses, the X and Y separations, AXVISTA and AYVISTA are determined.These values are fed into the following expression to determine the height of the aircraft from the ground, taking into account the actual separation of the wing tip pods, the bank angle of the aircraft, and the field of view of the camera:

-h S. cos+.512 ha/c=camera 2. S.cos.512 2. tana,,. JXVI;)Z+(L\y,,)2 where = - the height of the aircraft above ground camera = the height of the camera plane off the ground S = the span between the noses of the wing tip pods m = aircraft bank angle = = camera field of view AXVISTA = X difference on CCD between wing tip pods AYVISTA = Y difference on CCD between wing tip pods and the CCD array is 512 x 512 pixels.

The height as determined may be referred to a particular datum position on the aircraft by appropriate res6l- ution, knowing the attitude of the aircraft. This may be required, for example if the height is also being measured for test purposes using a static nose boom.

In practice we have found that it is beneficial to determine the field of view (g) ) empirically as manufacturer's estimates may vary by up to 10% or so, and this will degrade the accuracy of this system.

In the above system, the focal length or field of view of the camera is preferably selected with regard to the expected height of the aircraft, such that the image of the aircraft fits comfortably within the field of view. Thus, where the height of an aircraft of given size is to be measured at higher fly past altitudes, the lens of the camera is preferably changed for one of greater focal lengths.

The aircraft speed may be determined by monitoring movement of the aircraft, by comparing the images from two separate frames at a known time interval, monitoring the distance travelled within the viewed scene of a distinctive part of the aircraft, converting this to distance, given knowledge of the effective scale within the image (e.g. the wing tip pod separation) and then converting to speed.

Again this can be compensated for the attitude of the aircraft by revolving about the body axes, as was effected in the above expression, for the aircraft bank angle. The camera data may be stored for later analysis or it may be processed automatically by an image processor.

Where the output of the video camera is stored for subsequent processing and analysis, and comparison with data gathered on board the aircraft, the data collected on the ground and that on board the aircraft may require precise time correlation. This may be achieved by making both the ground station and the aircraft use the same time base.

Claims

1. Distance measuring apparatus for determining the height or range of an object from a viewing'point, said apparatus comprising imaging means for forming an image of the scene viewed along an optical axis, and means for determining the relative size of the image of said object within the image of the viewed scene, thereby to determine the distance of the said object from said imaging means, based upon the actual size of said object.

2. Distance measuring apparatus according to Claim 1, wherein said imaging means is relatively fixed, and said apparatus further includes means for determining relative movement with time of the image of said object within the image of the viewed scene, thereby to determine the velocity of said object.

3. Distance measuring apparatus according to Claim 1 or 2, wherein said means for determining the relative size includes means for determining the respective co-ordinates of two reference points on the object.

4. Distance measuring apparatus according to Claim 3, wherein said imaging means comprises an image-forming lens of predetermined focal length, and a pixellated image sensor, and said means for determining the respective coordinates includes means for determining the pixel or pixels containing each of said reference points.

5. Distance measuring apparatus according to any preceding claim, wherein said imaging means is oriented with its optical axis pointing vertically, whereby the height of said object above said imaging means may be determined.

6. A method of determining the height or range of an object from a viewing point, which comprises forming an image of a viewed scene containing said object, determining the actual size of said object, determining the relative size of the image of said object within the image of the viewed scene, and thereby deducing the distance of the object from said viewing point.

7. A method according to Claim 6, wherein the relative size of the image is determined by identifying the relative positions of two reference points on the object.