GB2323730A - Passive rangefinder - Google Patents

Abstract

A passive rangefinder apparatus for determining the range to an object emitting infra-red radiation over a 3-5Ám waveband wider than an atmospheric absorption band (3 fig 1 not shown) includes means to measure infra-red radiation received from narrow wavebands (10) on the flank of the absorption band and means to determine the range from these measurements. The rangefinder includes a scanning mirror 20 and an optical system including lens 22, reflecting mirror 73, "V" formation mirrors 74 for de-rotation, wedge prism 77 and lens 29 to form an image of the scene on a diffraction grating 24. A detector array 25 is placed to receive diffracted radiation in the narrow wavebands 10 on the flank of the red wing 5 of atmospheric absorption band 3. Other detectors in the array 25 are used to measure sun glint radiation (waveband 7) and also the blue spike radiation (4) using wavebands (8) and (9) for source identification. A rotatable optical chopper 75 modulates the radiation reaching the different detectors at differing frequencies.

Description

PASSIVE IDENTIFICATION AND RANGEFINDER APPARATUS This invention relates to passive identification and rangefinder apparatus for identifying and determining the range to a distant object such as a jet aircraft, rocket or missile from infra-red spectrophotometer measurements.

For the identification and rangefinding of missiles and aircraft in warfare, a passive equipment which operates by responding to radiation inherently associated with the flight of the missile or aircraft has obvious advantages.

The possibility of determining the range to an aircraft by measurements of its infra-red radiation has been disclosed by Ovrebo et al in US Patent Specification No. 3,103,586 and by Jenness et al in US Patent Specification No. 3,117,228. These specifications disclose arrangements in which the total infra-red radiation received from an aircraft is compared with the proportion of the received radiation which passes a filter which cuts out radiation within the absorption bands of atmospheric carbon dioxide and nitrogen oxide. If the aircraft is at a comparatively short range, of the order of 1 kilometre or less, there will be a significant difference in these measurements because some proportion of radiation emitted by the aircraft within the absorption bend will reach the observation point in spite of atmospheric absorption. Assuming that the radiation initially emitted had the spectral distribution of a Planck radiation curve for a black body at some specific temperature, these specifications disclose methods and apparatus for deducing a range measurement from the difference between the measurements. At longer ranges however, the atmospheric absorption becomes very similar to the absorption produced by the filter; the difference between the measurements becomes comparatively small and insensitive te further increases in range.

Where missiles, rockets or modern high-speed aircraft are concerned, it is clearly highly desirable to make identifications and range measurements at ranges very muc longer than 1 kilometre.

It is an object of the present invention to provide means for passive rangefinding with respect to missiles, rockets or aircraft which can give useful range indications at ranges very much greater than 1 kilometre. For such measurements, it is important to note that the radiation associated with and seen in any observation of a distant missile, rocket or high speed aircraft is the resultant of several contributions and therefore has a spectral distribution sigificantly different from any single Planck radiation curve. The inventors have found that with detailed considerations and due allowances for the complications of this application as hereinafter described, it is possible to lake useful range measurements on missiles, rockets and aircraft out to ranges many tines greater then 1 kilometre by passive radiometry comprising a plurality of measurements of specific selected narrow wavebands. The suaurlrenents may also serve to identify various types of missile, rocket or aircraft.

According to the present invention there is provided passive rangefinder apparatus for det the range to an object such as an aircraft, rocket or missile which emits infra-red radiation having blue spike and red wing features as hereinafter defined, including:- optical means for foxing a real image of a field of view; spectrophotometer means which includes an entrance aperture placed to receive at least a part of said real image, dispersion means for separating infre-red radiation of different wavelengths and detector means for measuring radiation received from distinct portions of the said entrance aperture and in several distinct narrow wavebands on the flank of an atmospheric infrared spectral absorption band; and data processing means for receiving radiation measurement signals from the detector means and identifying localised radiation from aircraft, rocketsor missiles and deriving range indicating signals from the said radiation measurement signals.

Preferably the spectrophotometer includes means for measuring gun glint radiation in a waveband of wavelengths less than 2.9 microns, and the data processing means includes means for subtracting sun glint corrections from the radiation measurement signals Preferably the sun glint radiation is measured in the second order of diffraction from the dispersion means from a waveband such that this second order diffraction occurs within the area of occurrence of the said atmospheric absorption band in the first order diffracted radiation.

Preferably the spectrophotometer includes means for measuring radiation in a waveband within the blue spike and radiation in a waveband adjacent to the blue spike and the data processing means includes means for deriving identification signals from these measurements.

The detector means may include a row of detector elements placed to receive radiation from a part or parts of the red wing away from the flank of the absorption band, and several rows of detectors placed to receive radiation from narrow wavebands on the flank of the absorption band, each row comprising a plurality of detector elements placed to receive radiation from distinct portions of the entrance aperture. It may also include a row of detector elements placed to receive sun glint radiation of wavelength less than 2.9 microns, a rov of detector elements placed to receive radiation from within the blue spike and a row of detector elements placed to receive radiation from a waveband adjacent to the blue spike.

A missile, rocket or aircraft in high speed flight will emit infra-red radiation from surfaces which are aerodnamical1y heated, and this radiation will have a spectral distribution substantially acocrding to a Planck radiation curve for the temperature of the surface involved. It-wi also produce considerable infra-red radiation from the ezhaiist gases of its propulsion engines or rockets; much of this may be black-body radiation from gases or combination products at a different specific temperature, but particularly in the case of a missile or rocket there may also be considerable radiation emissions characteristic of partianlar products or chemical reactions occurring in the exhaust gases. Thus its spectrum is at least a combination of two or more black body curves and may well be more complex; it will be significantly different from any single Planck radiation curve. The observed spectrum will depend on the attitude of the missile relative to the observer's line of sight since in various attitudes parts of the missile body may partially screen the exhaust plume (or the exhaust plume may screen the missile body) from observation. In any observation of a very distant object the observed radiation is likely to include some illumination from the sky or other background in the vicinity of the object; it may also include a substantial contribution from sunlight reflected by the object into the direction of the observer. These two contributions when they are significant will have the spectrum of sunlight as modified by very long range transmission through the atmosphere; they will hereinafter be called sun glint radiation. The proportion of sun glint radiation observed will depend on the relative directions of the sun and the observer and on environmental conditions.

Carbon dioxide and nitrogen oxide in the atmosphere produce a strong absorption band in the region from about 4.1 microns to about 4.3 microns approximately. In most cases, the exhaust plume from a missile rocket or aircraft will have a core area containing hot carbon dioxide which radiates strongly in and adjacent to the region of this absorption band; however, the radiation within the absorption band is strongly absorbed by cooler carbon dioxide on the fringe of the exhaust plume and in the atmosphere between the aircraft and the observer. Hence the observed spectrum from a distant missile or high speed aircraft typically has a sharp peak, hereinafter called the blue spike, immediately on the short wavelength side of the aforesaid absorption band, and a much broader spectrum, hereinafter called the red wing, on the long wavelength side of the absorption band.

The inventors have found that with detailed considerations and allowances for all the complexities of the situation, it is possible to make useful range measurements on missiles, rockets and aircraft out to ranges considerably greater than one kilometre by passive infra-red spectrophotometry comprising a plurality of measurements of specific selected narrow wavebands, as hereinafter described. These measurements may also serve to identify various types of missile and to distinguish missiles from high speed aircraft.

Further details and embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings of which: Figure 1 is a graph showing typical spectra of sun glint radiation and observations of a distant missile; Figure 2 is a diagrammatic perspective drawing showing basic features of an embodiment of the invention; Figure 3 is a graph showing the transmission characteristic of a filter which may optionally be used in embodiments of the invention; Figure 4 is a plan view drawing of an array of infra-red detectors used in the embodiment of Figure 2; Figure 5 is a graph showing the absorption by water vapour in the atmosphere, and typical spectra observed at various ranges from a distant missile as a function of wavelength; Figure 6 is a graph showing typical responses received from measurements of the radiation from a distant missile at several specific wavelengths, plotted as a function of range; Figure 7 is a diagrammatic sectional elevation drawing of another embodiment of the invention; Figures 8, 9 and 10 are drawings showing further details of the embodiment of Figure 7.

In Figure 1 the broken curve 1 represents a typical spectrum of sun glint radiation showing two absorption bands 2 and 3. The solid curve shows a typical spectrum from a distant missile showing a blue spike 4 and a red wing 5. The bars on the line below the figure indicate narrow wavebands which are particularly used in the embodiments of the invention hereinafter described. The bar 7 indicates a waveband used for the measurement of sun glint radiation; bar 8 represents a waveband adjacent to and on the short wavelength side of the blue spike and bars 9 indicate wavebands used for the measurement of the blue spike; the bars 10 represent wavebands on the flank of the red wing adjacent to the absorption band 3; and the bar 11 indicates a waveband in the centre of the red wing. Symbols to to > 4 represent the wavelengths of the individual wavebands of the group 10, and > 1 represents the wavelength of the band 11.

11 Figure 2 is a diagrammatic perspective drawing showing basic features of a passive rangefinder according to the invention.

An adjustable scanning mirror 20 is placed to receive infra-red radiation 21 from a distant field of view and to direct it into an infra-red telescope 22. The telescope 22 forms a real image of the field of view in the plane of an entrance slit 23 of a spectrophotometer which includes a dispersive element in the form of a diffraction grating 24 and an array of infra-red detector elements 25.

Outputs from the elements 25 are connected to data processing apparatus 26. Naturally, the spectrophotometer apparatus is screened from any stray light by an enclosure 27. An optional but desirable addition to the apparatus is a filter 28 which is placed immediately above or below the slit 23. The spectrophotometer also includes an achromatic lens 29 which collimates light from the slit 23 and an achromatic lens 30 which focuses diffracted light from the grating 24 onto the plane of the array 25.

Figure 3 shows the transmission characteristic of the desirable but optional filter 28. It has a narrow pass band in the range between 2.1 and 2.2 microns, it obstructs radiation of wavelengths shorter than 2.1 microns and of wavelengths between 2.2 and 3.0 microns, and it has a uniform high pass characteristic between 4 microns and 5 microns.

Figure 4 shows an enlarged drawing of the array 25, which comprises infra-red detector elements in a rectangular array of rows and columns. The apparatus of Figure 2 is so arranged that the radiation which passes through the entrance slit 23 will be derived from a vertical strip in the field of view, and radiation ori & inating from different elevations will appear at different positions across the array in the direction of the arrow 50 in Figure 4. The dispersive effect of the diffraction grating 24 acts at right angle to this direction, so that radiation from different wavebands is spread down the array in the direction of the arrow 51 in Figure 4. The array includes a row of detectors 40 placed to receive radiation in the waveband indicated by the bar 8 in Figure 1. The array includes at least one row 41 of detectors placed to receive one of the wavebands 9; preferably it has at least twc such rows, and they should be narrow enough to ensure a satisfactory measurement of the blue spire peak.

A row of detectors 43 is placed to received second-order diffracted radiation in the waveband indicated by the bar 7 on Figure 1. The row 43 is flanked by rows 42 and 44 which should normally receive very little radiation they are placed where they should only be able to receive second order diffracted radiation from frequencies outside the narrow pass band of the filter 28 or firSt order diffracted radiation within the absorption band 3 but thermally radiated by constituents of the atmosphere or objects in the field of view at a ccotratirely short range of 1 kilometre or less, which may be called foreground radiation. A number of rows of detectors 45 to 49 are provided in place to receive signals in the red wing wavebands shown at 10 and 11 in Figure 1. The actual number of rows and columns of detectors is arbitrary and more rows may be used for ;the purposes herein described The responses of the detectors are measured at regular time intervals, converted into digital form, and stored by the data processing apparatus 26. Signals a ii the output from detectors in the rows 42 and 44 represent the foreground radiation with ang spurious electrical interference which is likely to affect all the signal channels equally; these signals are subtracted from all the other corresponding responses. In particular they are subtracted from the responses of corresponding detectors in the row 43 to derive signals representing the sun glint radiation in the waveband 7 of Figure 1. Taking these signals as a measure of the sun glint radiation and knowing its spectrum, and making due allowance for the effects of meteorological conditions upon the form of the sun glint spectrum, estimates can be made of the contribution of the sun glint radiation to the responses in all the measured wavebands. me measurement at the longest wavelength #11 is likely to have the best signal to noise ratio and may be preferentially used for the identification of objects of interest which may prove to be distant missiles or aircraft. Hence one looks in the responses from the detectors of row 49 for isolated high responses or small groups of higher than average responses adjacent in elevation or in time. Since the mirror 20 in Figure 2 will normally be scanning in aiimuth responses detected at different times will be coming from different directions in azimuth.

The widths of the individual detectors in the direction 50 in conjunction with the optical system and the elevation scanning adjustment of the mirror 20 determine increments of elevation, and the instance of measurement in conjunction with the azimuth scanning motion of the mirror 20 define increments of azimuth within which any distant object of interest may appear; but if the distant object happens to cross the boundaries of such increments between instants of measurement its radiation may be distributed among responses which are adjacent either in time or in position of detection in the array 25. Natural phenomena which produce or reflect radiation detectable by the array normally do so over a larger area producing longer sequences of adjacent high responses. Objects such as wave crests reflecting sunlight may be distinguished because they have the spectrum of sun glint radiation and show no greater response in the red wing channels than would be expected from a correction derived from the response of the sun glint channel detectors in row 43 at the same elevation.

Thus certain responses may be selected by various algorithms as indicating possible objects of interest warranting further consideration of the corresponding responses in other wavebands. For each object of interest the response received at the peak of the blue spike (in waveband 9 measured by detectors 41) and the responses in the adjacent waveband 8 (measured by detectors 40) are each corrected by subtracting foreground radiation and estimated sub glint radiation contributions and their ratio is found. If the object appears to be producing a single isolated response this is taken for the measurements; but if the radiation from the object of interest appears to be spread over n adjacent responses, then these responses may be added together and n times the sun glint correction subtracted from the total, in the measurements for each waveband. The aforesaid ratio of the corrected channel 9 and channel 8 signals forms a measure of the height of the blue spike, and it has been found to form a useful object identification sigaal which has different values for different types of missile or aircraft.

When the object has been thus identified as a missile or aircraft of known type the red wing signals measured by the detectors in rows 45 to 49 inclusive may be further examined to determine its range. In the range of wavelengths of interest (from s1 to X ) the radiance 1 11 of an object may be represented by S = A1 [1 + A2 (# - #11)] (1) where A1 and A2 are constants appropriate to the particular type of object concerned.

A transmittance factor representing the proportion of radiation transmitted over a range R subject to absorption by atmospheric carbondioxide and nitrous oxide is approximately given by t = a exp(-bRc) (2) where a, b, and c are parameters dependent on wavelength, atmospheric pressure, and atmospheric conditions, also varying slowly with range.

Water vapour in the atmospherehas relatively small and spectrally insensitive ffects which asy be allowed for by a further transmission factor, t'= (1-wR) exp(-wOR) (3) where wO is a constant but w varies slightly with wavelength as shown by the broken curve 55 on Figure 5.

The observable response at any wavelength # will depend upon the product of these three factors Stt',and if we consider the ratio of the response at one wavelength #x divided by the response at another wavelength #y we should expect it to be

where the suffixes x and y are used to indicate values appropriate to the wavelengths #x and #y. On Figure 5 the cross markings on the curve 55 shots that it is possible to choose particular wavelengths and 6 and > 4 for which the parameter w has the same value; it is certainly possible to choose many pairs of wavelengths for which Wx = Wy; thus the wavelengths may be chosen so that the term due to water vapour absorption can be neglected. The parameter A2 depends upon the type of object being observed and an apDropriate value may be chosen according to the identification of the object from the aforesaid ratio of the corrected blue spike response signals. Appropriate values for the parameters ax, ay, by, bx, Cx, cy are chosen according to the prevailing meteorological conditions and may be adaptively adjusted after a first estimate of range has been obtained. Hence theoretical values for the ratios of various responses may be calculated for various ranges, and these may be compared with the ratios of the actual experimental observations after correction for any significant effects of sun glint radiation; by well known curve-fitting techniques, a value of the range R may be found which gives the best correlation between the theoretical and experimental results, for each object of interest. The data processing apparatus 26 does this.

Instead of using equation 1 as written, the apparatus 26 may use sets of stored values of the function 1 + A2 (#- > 11) for various wavelengths > and types of object. The set which gives the best correlation may then be chosen and may be taken as a confirmation or a more specific indication of the type of object being observed.

The nature of the effects of range on the responses at various wavelengths is shown by the solid curves in Figure 5 and 6. In Figure 5 the solid curves show the variation in the responses from a typical missile at various wavelengths when the missile is at a comparatively short range (curve 56), at a medium range (curve 57), and at a longer range (curve 58). In Figure 6 the curves 60, 61. 62 and 63 respectively show the responses received from a typical missile at the four frequencies 1' 2' and > 4 of the wavebands 10 illustrated in Figure 1, plotted against range using logarithmic scales. From these figures it is clear that when an object is first observed at comraratively long range its radiation at the wavelengths 1 and #@ will probably be insignificant or undetectable but the ratio of the response at wavelength > 4 divided by the response at #3 Kill be a sensitive and useful indication of range. As the object approaches the response at wavelength > 2 rises and the ratio of the response at wavelength > 3 divided by the response at wavelength # 2 then becomes a sensitive and useful indication of range. When the object approaches still nearer the response at wavelength > 1 increases and the ratio of the ratio of the response at wavelenghth #2 divided by the response at wavelength #1 then becomes a sensitive indication of the range. This suggests an alternative and simpler arrangement of the data processing apparatus 26. After subtracting the corrections for foreground radia- tion, electrical interference and sun glint radiation, the apparatus 2 may be programmed to check how any of the corrected red ving responses are significant, then select and calculate the appropriate ratio as indicated above, and then to derive an estimate of the range to the object by comparison of the selected ratio with corresponding stored empirical results of observations on an object of similar type.

If the filter 28 is omitted, sun glint corrections will have to allow for the effects of second order diffracted rays in the range from 2.0 to 2.5 microns, which may make the corrections larger and less certain. Some detectors may be masked to prevent them receiving any radiation so that their outputs will indicate any spurious electrical interference. If the filter 28 is used it will be preferable to put it in a part of the optical system where the rays are parallel, (for instance below the lens 29 in Figure 2), since its transmission characteristics may be appreciably affected by the angle of incidence of the radiation, but this may require a larger area of filter.

The number of detector elements required in the array may be reduced by providing for a full elevation coverage only in the row 49 or other rows which are used for the identification of objects of interest. The other rows may be reduced to provide measurements over a limited range of elevation if there is an automatic system for controlling the elevating adjustment of the scanning mirror 20 so as to bring the image of any object of interest to the centre-line of the array 25. There may be some advantage in retaining a moderate range of elevation cover in the rows used to measure the blue spike and the longest range measurement wavelength #4 so that object identification signals and long range measurements may be derived before the elevation adjustment is optimised. The other rows of the array may be considerably reduced on the assumption that the elevation adJustment will be optimised before they are needed. On the other hand the full array will have a greater capacity for observations of objects which appear concurrently indifferent parts of the field of view.

As previously noted the entrance slit 23 should be arranged to receive radiation from a vertical strip in the field of view.

With the simple basic arrangement as shown in Fig 2, this would require that the whole apparatus should rotate with the azimuthal scanning motion of the mirror 20. To avoid this it is highly desirable to incorporate an image de-rotation device in the optical system ahead of the spectrophotometer apparatus. Some advantages can be gained by using spectrophotometer with two or more entrance slits distinctively modulated by a chopper knees; then at least sane of the rows of the array of detectors can be placed to do double or triple duty, receiving different wavebands froo radiation coming through different entrance slits, their corresponding reap cones being distinguisned by their different modulation patterns. If one particular extrancffl slit is nsed for the measurements of the sun glint in waveband 7, it can have a filter passing only the wavelengths between 2.1 and 2.2 microns and the other slits can have a filter passing radiation in the range from 4.0 to 5.0 microns instead of using the special filter with the characteristic shown in Figure 3.

Figure 7 is a diagrammatic sectional elevation showing a rangefinder apparatus with these modifications. In this drawing the parts numbered from 20 to 30 correspond to the similarly- numbered parts in Figure 2. The scanning mirror 20 is shown mounted on a horizontal pivot 70 which is mounted in a housing 71 which can be rotated an a bearing 72 for azimuth scL=ing purposes.

It should be appreciated that while the telescope 22 and the lenses 29 and 30 are shown diagrammatically as single lenses, in practice known @@ lens arrsegements may be used. aays from the telescope 22 are reflected by an inclined mirror 73 into a de-rotation device 74. The device 74 two has mirror surfaces in a vee formation on a rotatable mount which is controlled to rotate with half the rotation of any adjustment of the housing 71 on the bearing 72; it is preferably controlled by an imae-stabilisin servo-system (not shown) which is also responsive to motions of the entire apparatus when it is installed on a mobile platform such as a ship or aircraft. After reflection from the de-rotation device 74 the rays pass through an aperture in the inclined mirror 73 and form a real image in the plane of the rear surface of a chopper wheel 75 which is rotated by a motor 76.

Figure 8 shows a portion of the chopper wheel 75 as viewed from the direction of the de-rotation device 74 through the aperture in the mirror 73. The shaded areas in Figure re a are actually opaque. The chopper wheel 75 has three completely translucent tracks 80, 81, and 82, and a number of modulating tracks 83 to 88 inclusive each having a distinctive pattern of opaque and translucent sections. As shown, each of these tracks has a pattern of equal arcs alternately translucent and opaque so that each track will pass radiation modulated with a distinctive chopping frequency. The rotation of the device 74 is so arranged with respect to the rotation of the housing 71 that each of the tracks on the chopping wheel 75 will receive and pass radiation from a substantially vertical slightly curved strip in the field of view. If the horizon lies in the field of view being scanned, it will appear in the image formed by the optical system as a line transverse to the chopping tracks as shown by 89 in Figure 8. As all the tracks are circular, they are slightly curved but with radii nuch greater than the width of the image. elevation scanning adjustments which tilt the mirror 20 about the pivot 70 will move the image of the field of view across the aperture of the spectrophotometer as indicated by the arrow 18 in Figure 8; azimuth 1 scanning adjustments by rotation of the housing 71 will move the image across the aperture in the direction shown by the arrow 19 in Figure 8.

The radiation passing through the translucent portions of the chopping disc 75 is re-directed by a prism 77 onto the achromatic collinating lens 29. The lens 29 directs the radiation onto the reflection grating24 and the diffracted rays are focussed by the lens 30 onto the plane of the array of detector elements 25. An itfr red filter 78 which passes radiation in the range from 4.0 to 5.0 microns is either formed by deposition cr mounted on the front face of the prism 77 very close to the tracks on the chopping disc 75 and is placed to intercept radiation from the tracks 80 to 87 inclusive but not the radiation from the track 88.

Figure 9 shows the electrical connections of typical detector elements 90 within the array 25. Each detector 90 is connected through a separate amplifier 91 to a plurality of filter units 92 which are tuned to respond to signals having the various chopping frequencies fl to f6 produced by the rotation of the tracks 83 to 88 inclusive of the chopping wheel 75. Outputs from the filters 92 are converted into digital form, stored and used to derive object identification signals and range signals as hereinbefore described in the data processing apparatus 26. As the different tracks 83 to 88 will pass radiation from different strips in the image of the field of view, their responses from any distant object will appear in the output derived from different tracks with various relative time delays dependent on the separation of the tracks and the asi=ithal scanning motion of the housing 71. To ensure that the responses from any particular distant object are satisfactorily correlated the detector signals may be retired with appropriate time delays or alternatively they may be stored in locations which correspond to the location of the parts of the field of view from which they are derived. For this purpose the data processing apparatus 26 may be arranged to receive transducer signals on lines 93 and 94 from transducers not shown which are provided to indicate the azimethal and elevation adjustments of the scanning mirror 20, and signals on lines 95, 96 representing adjustments of the image derotation and imagestabilising device 74. In allocating the responses to storage locatior the apparatus uses the transducer signals with offsets allowing for the relative displacement of the tracks through which the radiation has been received. Thus if the azimuth transducer signal represents the azimuthal direction of the part of the field of view which is imaged on the path of the track 83 and this track applies a chopping frequency fl, the responses measured from the outputs of filters tuned to the frequency f1 will be stored at locations depending on the azimuth transducer signal with zero offset; but if f6 represents the chopping frequency applied by the track 88 the response received from filters tuned to the frequency f6 will be stored in locations selected with an offset representing the relative azimuthal displacement of the area in the field of view image on the track 88.

The fact that different tracks will see different portions of the field of view is a considerable complication and a disadvantage associated with the use of plural tracks or entrance slits; it means for instance that when the apparatus is used in a staring or tracking mode to examine a particular object of interest a residual scanning motion of the image in the azimuthal direction must be maintained to ensure that the image of the selected object will be scanned across all the tracks which are being used to get responses to measure its range.

Figure 10 shows the arrangement of detectors actually used in the array 25 in the embodiment of Figure 7. The array is so placed that radiation from different increments of elevation in the field of view is spread out across the array in the direction indicated by the arrow 100; radiation of different wavelengths is dispersed across the array in the direction of the arrow 101. The directions 100 and 101 in Figure 10 correspond to the directions 50 and 51 respectively in Figure 4. The detectors shown are arranged in eight rows referenced 110 to 117 respectively in Figure 10. The longest row 110 has > central region of twenty detectors 118 flanked by two end sections each comprising five elongated detectors 119. This row of detectors 110 is placed to receive first order diffracted radiation from the three continuously translucent tracks 80, 81 and 82 of the chopping disc 75 of three different wavelengths within the red wing area 5 of a typical missile spectrum. They are used when the device is operated in a surveillance node in which the housing 71 and mirror 70 are continuously rotated at a comparatively fast rate. This continuous scanning motion will sweep the image of any distant object across the three tracks 80, 81 and 82 in quick succession. If the object is a distant missile its radiation will tend to produce three successive peaks in the response of one of the detectors or of two adjacent detectors of the row 110. dny natural sources of red wing radiation which will tend to be less localised, will tend to produce a single extended peak rather than three successive peaks. Where a missile or aircraft is present this distinctive three-peaked response should be repeated in every revolution of the housing n; the detectors of the row 110 are connected to digital circuits which are particularly arranged to detect and respond to these repeated triple peska. Clearly two, four or five such tracks could be used instead of three. The separation of the tracks and/or the uimthal scanning speed can be adjusted so that the period of the multiple response produced equals one of the chopping frequencies fl to f6 allowing a corresponding one of the detector circuits to be used to assist the detection of the responses.

Thus any objects of interest which may be missilesor aircraft are identified and the equipment is automatically switched into a checking mode, in which the scazzvAlg motion is switched to a slower speed as it approaches and scans past any position where an object of interest has been identified. In this node the interrupted tracks 83 to 88 of the chopper wheel are used in conjunction with the other detectors of the array 25 shown in Figure 10. There is another row 111 comprising twenty detectors placed to receive various red wing wavelengths through the tracks 83, 84 and 85 and to receive second order diffracted radiation of wavelength 2.15 microns through the track 88. There are also four rows each comprising a pair of detectors 112, 113, 114 and 115, and two rows 116 and 117 of detectors placed to receive blue spike radiation from the tracks 86 and 87 shown in Figure 8. The tracks 86 and 87 and the detectors in the rows 116 and 117 are narrower than the rest of the tracks and the other detectors in order to have a final spectral resolution so as to measure the blue spike satisfactorily. As shown there are eight detectors in row 116 and ten detectors in row 117, but clearly the number of detectors used in any row is arbitrary. It should be noted that Figures 8 and 10 are not drawn to scale.

Outputs from the detectors of the rows 111 to 117 inclusive representing radiation received at various wavelengths are distinguished by circuits shown in Figure 9 and these signals are used as hereinbefore described with reference to Figures 4 to 6 inclusive to produce identification signals representing the type of object identified, and range estimates.

While the modulated tracks of the chopper wheel 75 are shown as having equal arcs of alternately transparent and opaque sections, they could alternatively have distinctive sequences or code patterns of transparent and opaque sections in which case the filter circuits 92 of Figure 9 would be replaced by correlation circuits. Instead of opaque sections it is possible to use sections which radiate or reflect a steady emission eg black-body radiation of the temperature of the apparatus. Instead of lenses, focussing mirror arrangements or lens/mirror combinations may be used.

Claims (14)

1. CLAIMS t. A passive rangefinder for determining the range to an object such as an aircraft, rocket or missile which emits infra-red radiation having blue spike and red wing features as hereinbefore defined, including: optical means for forming a real image of a field of view; spectrophotometer means which includes an entrance aperture placed to receive at least a part of said real images dispersion means for separating infra-red radiation of different wavelengths and detector means for measuring radiation received from distinct portions of the said entrance aperture and in several distinct narrow wavebands on the flnnk of an atmospheric infra-red spectral absorption band; and data processing means for receiving radiation measurement signals from the detector means and cntifying localised radiation from an aircraft, rocket or missile and deriving range indicating signals from the said radiation measurement signals.
2. 2. A passive rangefinder as claimed in Claim t and wherein the spectrophotometer means includes means for measuring sun glint radiation as hereinbefore defined in a waveband of wavelengths less than 2.9 microns1 and wherein the data processing means includes means for subtracting sun glint corrections from the said radiation measurement signals.
3. 3. A passive rangefinder as claimed in Claim 2 and wherein the sun glint measuring means comprises means for measuring sun glint radiation in the second order of diffraction from the dispersion means from a waveband such that this second order diffraction occurs within the area of occurrence of the said atmospheric absorption band in the first order diffracted radiation.
4. 4. A passive rangefinder as claimed in Claim 1 or Claim 2 and wnerein the spectrophotometer means includes means for measuring radiation in a waveband within the blue spike and radiation in e waveband adjacent to the blue spike and the data processing means includes means for deriving identification signals from these measurements.
5. 5. A passive rangefinder as claimed in Claim 1 and wherein the detector means includes an array of detector elements each element being positioned to receive radiation from a distinct portio of the entrance aperture and within a particular narrow waveband.
6. 6. A passive rangefinder as claimed in Claim 5 and wherein the array of detector elements comprises a row of detector elements placed to receive radiation from a part or parts of the red wing away from the flank of the absorption band, and several rows of detectors placed to receive radiation from narrow wavebands on the flank of the absorption band, each row comprising a plurality of detector elements placed to receive radiation from distinct portions of the entrance aperture.
7. 7. A passive rangefinder as claimed in Claim 5 or 6 and wherein the array includes a row of detector elements placed to receive sun glint radiation of wavelength less than 2.9 microns.
8. 8. A passive rangefinder as claimed in Claim 5 or Claim 6 and wherein the array includes a row of detector elements placed to receive radiation from within the blue spike and a row of detector elements placed to receive radiation from a waveband adjacent to the blue spike.
9. 9. A passive rangefinder as claimed in Claim 1 and wherein the spectrophotometer means includes a plurality of entrance apertures in the form of translucent or partially translucent tracks on a rotatable modulating wheel.
10. 10. A passive rangefinder as claimed in Claim 9 wherein the optical means includes rotatable means for scanning the field of view and scanning the image thereof across the entrance aperture or entrance apertures of the spectrophotometer means.
11. 11. A passive rangefinder as claimed in Claim 10 and wherein the modulating wheel has a plurality of completely translucent tracks and the data processing means includes circuits particularly constructed to distinguish the responses produced when a localised source of radiation is scanned across these tracks.
12. 12. A passive rangefinder as claimed in Claim 9 and wherein the modulating wheel carries a plurality of tracks each having a distinctive pattern of translucent and opaque portions, and wherein at least some of the detector elements are connected to circuits particularly responsive to modulations produced by the said patterns thereby forming distinct radiation measurement signals representing amounts of radiation received by the said elements through the said tracks.
13. 13. A passive rangefinder as claimed in Claim 10 and having means for reducing the scanning speed of the optical means whenever the area of a suspected object is being scanned across the entrance aperture or entrance apertures of the spectrophotometer means.
14. 14. A passive rangefinder substantially as hereinbefore described with reference to any combination of the accompanying drawings.

    14. A passive rangefinder substantially as hereinbefore described with reference to any combination of the accompanying drawings.

    Amendments to the claims have been filed as foflows 1. A passive rangefinder for determing the range to an object such as an aircraft, rocket or missile which emits infra-red radiation, the radiation received having a blue spike and red wing feature as hereinbefore defined, including: optical means for forming a real image of a field of view; spectrophotometer means which includes an entrance aperture placed to receive at least a part of said real image, dispersion means for separating infra-red radiation of different wavelengths and detector means for measuring radiation received from distinct portions of the said entrance aperture and in several distinct narrow 1savebands on the flank of an atmospheric infra-red spectral absorption band; and data processing means for receiving radiation measurement simxls from the detector means and identify localised radiation from an aircraft, rocket or missile and deriving range indicating signals from the said radiation measurement signals, 2. A passive rangefinder as claimed in Claim 1 and wherein the spectrophotometer means includes means for measuring sun glint radiation as hereinbefore defined in a waveband of wavelengths less than 2.9 microns, and wherein the data processing means includes means for subtracting sun glint corrections from the said radiation measurement signals.

    3. h passive rangefinder as claimed in Claim 2 and wherein the sun glint measuring means comprises means for measuring sun glint radiation in the second order of diffraction from the dispersion means from a uaveband such that this second order diffraction occurs within the area of occurrence of the said atmospheric absorption band in the first order diffracted radiation.

    4. çl passive rangefinder as claimed in Claim 1 or Claim 2 and wherein the spectrophotometer means includes means for measuring radiation in a ;aveband within the blue spike and radiation in z waveband adjacent to the blue spike rind the data processing means includes means for deriving identifiaztion signals from these measurements.

    5. A passive rangefinder as claimed in Claim 1 and wherein the detector means includes an array of detector elements each element being positioned to receive radiation from a distinct portioi of the entrance aperture and within a particular narrow waveband.

    6. A passive rangefinder as claimed in Claim 5 and wherein the array of detector elements comprises a row of detector elements placed to receive radiation from a part or parts of the red wing away from the flank of the absorption band, and several rows of detectors placed to receive radiation from narrow wavebands on the flank of the absorption band, each row comprising a plurality of detector elements placed to receive radiation from distinct portions of the entrance aperture.

    7. A passive rangefinder as claimed in Claim 5 or 6 and wherein the array includes a row of detector elements placed to receive sun glint radiation of wavelength less than 2.9 microns.

    8. A passive rangefinder as claimed in Claim 5 or Claim 6 and wherein the array includes a row of detector elements placed to receive radiation from within the blue spike and a row of detector elements placed to receive radiation from a waveband adjacent to the blue spike.

    9. A passive rangefinder as claimed in Claim 1 and wherein the spectrophotometer means includes a plurality of entrance apertures in the form of translucent or partially translucent tracks on a rotatable modulating wheel.

    10. A passive rangefinder as claimed in Claim 9 wherein the optical means includes rotatable means for scanning the field of view and scanning the image thereof across the entrance aperture or entrance apertures of the spectrophotometer means.

    11. A passive rangefinder as claimed in Claim 10 and wherein the modulating wheel has a plurality of completely translucent tracks and the data processing means includes circuits particularly constructed to distinguish the responses produced when a localised source of radiation is scanned across these tracks.

    12. A passive rangef,nder as claimed in Claim 9 and wherein the modulating wheel carries a plurality of tracks each having a distinctive pattern of translucent and opaque portions, and wherein at least some of the detector elements are connected to crits particularly responsive to modulations produced by the said patterns thereby forming distinct radiation measurement signals representing amounts of radiation received by the said elements through the said tracks.

    13. A passive rangefinder as claimed in Claim 10 and having means for reducing the scanning speed of the optical means whenever the area of a suspected object is being scanned across the entrance aperture or entrance apertures of the spectrophotometer means.