WO2000055643A1

WO2000055643A1 - Passive range determination of object

Info

Publication number: WO2000055643A1
Application number: PCT/IL2000/000153
Authority: WO
Inventors: Joseph Wallace
Original assignee: Israel Aircraft Industries Ltd., Tamam Division
Priority date: 1999-03-16
Filing date: 2000-03-14
Publication date: 2000-09-21
Also published as: AU3188400A; IL129010A0

Abstract

A method and system are described for passive determination of the range to a cold object having the temperature of 200K-500K, which may be only slightly different from or larger than the ambient background temperature. By measuring the relative ratio of electrical currents generated by the electrooptical sensor at various ranges from a predefined object, a user builds a calibration function using the subtracted current ratios as a function of the range. By knowing this function, a user can passively determine the range to a target. A possibility of the method to work with relatively cold objects, having a temperature between 200K and 500K, is procured by subtracting the background current from each pixel defining the calibration object. An elimination of knowledge of the optical signature and/or temperature is achieved by the proper choice of radiation filters. The techniques described can also be applied for determining the range to an extended cold object whose size in one direction is less than the effective projected pixel-image-size at the object and to an object, which cannot be found by the sensor whose signal-to-clutter is very low.

Description

PASSIVE RANGE DETERMINATION OF OBJECT

FIELD OF THE INVENTION

The present invention relates to a method and system for the passive determination of the range to an object.

BACKGROUND OF THE INVENTION

Currently, the most popular methods and systems for determining the range to an object either use a laser range finder, global positioning satellite (GPS), active radar systems or inertial navigation systems mounted on an aircraft. However, there are several disadvantages in using these techniques. The accuracy of determining the range by means of the GPS is limited. The inertial navigation system also suffers from errors when viewing an object for a low elevation angle. Finally a major disadvantage of radar and laser systems is they are active system, as opposed to passive.

A part of the aforementioned drawbacks is overcome in methods and systems of the passive range determination. In the passive methods, the range is determined by using the relative attenuation of the object electromagnetic radiation among a plurality of continuous spectral lines of an object. The relative attenuation depends on a known way on range to the object and is used for the range determination.

The main disadvantages of the prior art methods of passive range determination is a limited sensitivity in the case when the temperature of the object is close to the ambient atmospheric temperature or the temperature of the object is cold. i.e. having the temperature of 20.0K-500K. For instance, passive ranging methods and techniques disclosed in U.S. Patent 4,694,172 by Powell and Spring, and in U.S. Patent 5,677.761 by Hasson. can be only used in a case of hot objects, such as fires, rocket plumes and incandescent filaments, etc. A method, which can be used for passive range determination in a case of relatively cold objects, is disclosed in U.S. Patent 5.282.013 by Gregoris. However, this method requires prior knowledge of the target temperature which is the stagnation of the airframe skin. The proposed technique of Patent 5.282.013 suffers from errors, which appear from the atmospheric transmittance estimation and the numerous simplifications of the mathematical equations used in the calculations. Hence, despite the prior art mentioned above, there is still a need in the art to provide an improved method and system for a passive range determination. In particular, there is a need to provide for a technique suitable for a cold object having a temperature around 200K - 500K and for which its temperature is may be only slightly different from the ambient background temperature.

There is an additional need in the art to provide a technique for determining the range to an extended cold object whose size in one dimension is less than a pixel image size, and to an object, which cannot be sensed by the sensor whose signal-to-clutter is very low.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method for range determination to preferably but not necessarily an object having a temperature only slightly different from or larger than the ambient background temperature by passive detector means, such as an imaging sensor, without the prior knowledge of the target's optical signature or temperature. According to the invention, the object that is subject to the range finding is cold with the temperature around 200K - 500K. By measuring the subtracted ratio of the integrated subtracted differences between the object and background electrical currents generated by the electrooptical sensor for at least two wavebands at various known ranges from a predefined object, it is possible to build a calibration function using the subtracted current ratios as a function of the range. The calibration function enables to passively determine the range to a target.

It should be noted, that the term "slightly different from the ambient background temperature^" is determined in the description depending upon particular application. Thus, for example, in the case of a large object (say a car) and a relatively large range (say several kilometers) the temperature difference of IK is applicable for the technique proposed. By way of another non limiting example, in the case of a small object (say a wire) and a short range (of say several hundreds meters) the difference of 0.5K is applicable for the technique proposed. In accordance with the invention, by properly choosing radiation filters, it is no longer required to know in advance the optical signature or the temperature of the target. In particular, one estimates an atmospheric transmission as a function of range and wavelength by using available meteorological conditions. Then, one calculates the subtracted ratio for at least two varied object temperatures in the temperature region 200K-500K and one background temperature at a given range to a sensor. Finally, the selection of the wavebands is accomplished by requiring that the subtracted ratio is substantially independent of the object and background temperatures and optical signature. According to another embodiment of the present invention, there is provided a procedure for reducing systems errors in measurements. The error reduction is achieved by averaging the data for the calibration curve, constituting the ratio of the subtracted currents, over substantially all the pixels, which constitute the object, and preferably by frame averaging the measured data.

By another embodiment, a range is determined to an extended cold object whose width or dimension along one axis being less than the effective projected pixel-image-size in the object plane and whose length is at least two pixel image sizes. In this case, the radiation from a pixel, which includes the invisible object and background, produces the temperature difference between neighboring pixels sufficient for the object to appear on the screen as a lit up pixel. Thus, in the specific case of a wire, a string of pixels appears on the monitor and it is possible to determine the range to the wire by averaging the subtracted ratio for said string of pixels.

In accordance with still another embodiment of the present invention, the proposed technique of the invention can also be used in the case, when the object cannot be sensed by the sensor because of a low signal-to clutter ratio. In such a case, one may opt to use a multispectral method as disclosed in the co-pending Patent Application No. 122258 filed on November. 20 1997 owned by the Applicant. This method enables users to determine the pixels and to display the object. Taking into account these pixels and the nearest neighbor pixels, one can determine the subtracted ratio as a function of the range to the object using the techniques disclosed in the present invention.

In accordance with the invention, there is further provided a system for detecting an object and for the passive determination of the range to the object.

The proposed technique of the present invention differs from hitherto known approaches for the passive range determination inter alia in providing a possibility to be applied to the cold object whose temperature is only slightly different from or larger than the ambient background temperature. Further, essential distinction from the prior art methods is its applicability to the objects having less than a pixel size dimension. The last but not least advantage of the method to be provided by the invention is its applicability to the case when an object cannot be sensed by the sensor because of a low signal-to clutter ratio.

Accordingly, the present invention provides for a method for constructing a calibration function S_πyg of an averaged subtracted ratio versus range R. to a calibration object, the method comprising the steps of: (i) measuring a range from a measurement device to the calibration object; (ii) measuring a radiation received from the object and from its background in at least two wavelength bands, so as to constitute at least one image composed of pixels; (iii) for each one of said at least two wavelength bands, calculating a subtracted current intensity N_nh/(R)-N_h(R) for each of at least one of said pixels. wherein N,_)h, and N_h are the respective current intensities of an object pixel and its neighboring background pixel; (iv) calculating a subtracted ratio S_m, in respect of each one of said at least one pixel, the S_{r /} value depends upon the N_oh/(R)-N_h(R) obtained in respect of each one of said bands; (v) calculating the averaged subtracted ratio values S_avg over said at least one pixel and over at least one frame, whereby an increase of signal-to-noise ratio and decrease of error in the passive range determination is obtained; (vi) repeating the steps (ii) to (v) for at least additional two more ranges, whereby an additional at least two R values and corresponding at least two the S_avg values are obtained; and (vii) fitting the data obtained in steps (v) and (vi) to a fitting curve that constitutes said calibration function of S _vg versus range R.

The present invention further provides for a method for passive determination of the range to an object of interest, comprising:

(a) providing a calibration function of S _vg versus R;

(b) measuring a radiation received from the object and from its background in at least two wavelength bands, so as to constitute at least one image composed of pixels;

(c) calculating an averaged subtracted current ratio S_avg by performing: (i) for each one of said at least two wavelength bands, calculating a subtracted current intensity N_nh,(R)-N_h(R) for each of at least one of said pixels, wherein N„_/v and N_h are the respective current intensities of an object pixel and its neighboring background pixel: (ii) calculating a subtracted ratio S_r , in respect of each one of said at least one pixel, the S,._a, value depends upon the N_υbj(R)-N_h(R) obtained in respect of each one of said bands; (iii) calculating the averaged subtracted ratio S_rat value over said at least one pixel and over at least one frame; (d) calculating the range to the object by applying the S_avg value obtained in step (c) to said calibration function. Still further the invention provides for a system for the passive determination of a calibration function to an object having a temperature which is only slightly different from an ambient background temperature, said system comprising:

(i) an object and background spectral radiance measurement device capable o measuring, at each of the consecutive observation times and distances, a radiation emitted by the object and its background: (ii) a signal processor unit coupled to the spectral radiance measurement device capable of analyzing the spectrum of the radiation received by the device and producing an output dependent on the distance to the object and on the meteorological conditions The invention further provides for a system for the passive determination of the range to an object having a temperature which is only slightly different from an ambient background temperature, said system comprising:

(i) an object and background spectral radiance measurement device capable of measuring a radiation emitted by the object of interest and its background; (ii) a signal processor unit coupled to the spectral radiance measurement device capable of analyzing the spectrum of the radiation received by the device, and by using a calibration function dependent on the distance to a calibration object and on the meteorological conditions. obtaining the range to the object. Still further the invention provides for a method for passive range determination of a calibration function to an object having a temperature which is only slightly different from an ambient background temperature, said method comprising the steps of:

(i) measuring at each of the consecutive observation times and distances, a radiation emitted by the object and its background; (ii) analyzing the spectrum of the radiation received by the device and producing an output dependent on the distance to the object and on the meteorological conditions. The invention further provides for a method for passive range determination of the range to an object having a temperature which is only slightly different from an ambient background temperature, said method comprising the steps of:

(i) measuring a radiation emitted by the object and its background: (ii) analyzing the spectrum of the radiation received by the device, and by using a calibration function dependent on the distance to a calibration object and on the meteorological conditions, obtaining the range to the object.

BRIEF DESCRIPTION OF THE DRAWINGS The presented invention will be more fully understood hereinafter from the following detailed description of preferred embodiments when taken in conjunction with the following drawings in which Fig. 1 is a graphic representation of an example of ideal relative ratio calibration curve;

Fig. 2 is a graphic representation illustrating a typical relative ratio of the background subtracted current from the object current for two spectral bands as a function of object temperature for various selected ranges to the object:

Fig. 3 depicts schematically the pixels, which constitute an object and background used in accordance with the invention:

Fig. 4 is a block diagram of methods steps in determining the calibration function: Fig. 5 is a view of a wire using the multispectral method;

Fig. 6 is a diagram illustrating a use of the system for a range determination:

Fig. 7 is a graph illustrating an example of substantially the best calibration curve obtained after averaging over all the pixels and least squares fitting procedure: and

Fig. 8 is a graph illustrating a typical subtracted relative ratio function for a wire or object whose width or minimum dimension is less than the effective projected pixel size at the object.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Passive range determination in accordance with the present invention is based on measuring the transmitted radiation from an object and from the background radiation for two or more different narrow infrared wavelength bands. The bands are picked from the two main atmospheric windows which have relatively low attenuation for infrared electromagnetic radiation, such as 3-5 and 8-12 micrometer wavelength spectral intervals.

As will be apparent to those skilled in the art. the total radiation reaching the sensor in a waveband between λi and λ₂ reads as E = ){[I(λ _l )τ (λ. R))l_)hl +E_ulm (λ))dλ . ( 1)

wherein / is the monochromatic specific intensity of the energy radiated at wavelength λ per unit wavelength interval per unit time per unit solid angle into an infinitesimal cone from an element of the black-body surface that is of unit area in projection perpendicular to the cone^'s axis, which is given by the following Planck^'s formula.

2hc²

I(λ. T) = (2) A⁵[exp(/7c ^r) - l]

where h is the Planck constant, c is the speed of light, k is the Boltzmann constant, and T is the thermodynamic temperature of the black body. The terms E_aw, in Eq. ( 1 ) are the contribution of the atmospheric radiation between sensor and object. The effective temperature in Eq. ( 1 ) is defined in a special manner in order to take into account the actual temperature of the object that is a nonblackbody object and the emissivity as well as atmospheric and ground scatter radiation of the object. The parameter r in Eq. ( 1 ) is the atmospheric transmission, which depends on the range R between the object and sensor. In order to display the object on the monitor the currents of electrons are produced in the sensor and represented as pixels that constitute an image (or succession of images, e.g. succession of video frames). The total object current produced in the sensor is given by the following equation:

N, = (λ)dλ . (3)

where C is a geometrical constant of the sensor, η is the quantum efficiency, and η_op, is the optic transmission of the lens, sensor window and filters located before the sensor. The total object current N_t, produced by the sensor is a result not only the direct object radiation but. in addition, it also includes the atmospheric radiation. The total background current N_t,_/, has similar terms (see Eq. (4)). which stem from contributions of ground with effective temperature T_h- defined by taking into account the background emissivity, plus atmospheric radiation, to wit:

[I(λ. T_h._i._ll )r (λ. R))]_h +E_tll λ )} (4)

The current resulting from the background radiation plus atmosphere and skv is subtracted from the total electric signal in order to reduce or eliminate the effect of atmospheric components, whose dependence on the range is different from that of the pure object. As a result of this subtraction, the atmosphere produced currents, which are common for both the object and background, are removed and thus the remaining terms have the same range dependence for the object and background. Hence, the subtracted number of electrons produced can be calculated by the following approximation formula

Λ'„ η(λ)_ηι opt (λ)dλ . (5)

Such subtraction of the background currents should be performed from substantially each pixel defining the calibration object on the monitor by subtracting from each object pixel current a background pixel current

(preferably a neighboring background pixel and the most preferably the closest neighboring pixel). For convenience of explanation, reference is made in the description below to the term "neighboring pixel^", but it should be understood that this term encompasses also pixel, which reside in a close neighborhood or stand apart. It should be further noted when referring to a value corresponding to an object pixel or background pixels, the value should be construed as either the actual value of the pixel (e.g. gray level or color value) or the value of a function calculated on the basis of the values of the pixel and its neighboring pixel or pixels. One has to bear in mind, that Eq. (5) establishes a relationship between the subtracted number of electrons and the parameter τ depending on the range to the object R.

In the practice of the invention, the current intensity ratio function of two spectral wavebands at a given temperature is considered. According to preferable embodimentthe invention, this ratio function is defined as

S . = ( N c'Kllh ) />.„„ / I (6)

S ' * ι- //> λnwi./ l ^""^" ' * . Kii llwiull

and it establishes a relationship for determination the distance between the source and receiving apparatus. An illustration of a typical relationship between the ratio function and the range is shown in Fig. 1.

It is relevant to bear in mind here, that the ratio function depends on the meteorological conditions due to the the atmospheric transmission. The transmission can be estimated by using standard meteorological conditions. A user can perform the measurements in advance at different days (having respective different climatic prevailing conditions). He/she creates a library of such ratio function calibration curves and picks the relevant curve on the occasion when he needs to determine a range to the actual target. The extent of correspondence between the climatic conditions of the model and those that prevail when the range determination is actually performed may vary, depending upon the particular application. For example, for some applications. where high accuracy is required, the calibration should be performed a few hours or even less before the actual range determination. For applications where reduced accuracy may be tolerated, a model with a less degree of climatic condition correspondence can be used.

According to another embodiment of the present invention, an optimization of S_{r /} defined by Eq. (6). can be performed by means of appropriate selection of wavebands 1 and 2. A possible criterion for choosing the wavelength for filters is. for example, selecting wavebands 1 and 2 such that the slope of the ratio function, defined by Eq. (6). as a function of temperature should be nearly zero over the temperature range 200K-500K and for distances to the object of interest. For such selection, on the first step, one has to estimate the atmospheric transmission as a function of range and wavelength by using available meteorological conditions. Then, by using these data, a user calculates the ratio of the subtracted currents at two temperatures in the interval 200K-500K. and determines the ratio of the S_ra, values calculated for these temperatures. The user repeats this procedure several times, each time selecting the wavebands 1 and 2. until he/she obtains the ratio value to be close to unity over the temperature region, which corresponds to an approximate independence of S_ra, of temperature. As a result of such optimization, the subtracted ratio function that is dependent on the range to the target, and substantially independent of the target and background effective temperature can be obtained. A typical but not exclusive relative ratio of the background subtracted current from the object current for two spectral bands as a function of object temperature for various selected ranges to the object is illustrated in Fig. 2. As shown, a temperature independence is clearly displayed for T_eJ in the interval 300K- 330K (can be higher or lower) and T_h._cjj is different from T_eff by at least IK.

The proposed technique of the invention may employ procedure(s) for reducing error. Thus for example, according to one embodiment of the present invention, there is provided a procedure for reducing systems errors in determination of the target range. Since the range is nearly independent of the temperature of object and background, the subtracted currents N _u0 for each waveband are obtained by subtracting from substantially each object pixel current the neighboring background pixel current. The ratio of the subtracted currents S_ra, is obtained substantially for all the pixels constructing the object, and then it is averaged over the number of the pixels that were subject to the subtraction (see Fig. 3). By using this procedure, one increases the signal-to-noise ratio and decreases the error in the passive range by the square root of the number of pixels used in the averaging. In addition, one can further reduce the signal to noise ratio by frame averaging over all frames of the sensor video output displaying the object for the given range to the object and thus further reduce the error in the range estimation.

The first step in the method of passive range determination is to create a subtracted ratio calibration function for a calibration object. Fig. 4 illustrates a logic diagram of steps performed in obtaining the calibration function for a calibration object. In order to perform such a calibration, one needs preferably at the day of measurements to pick some random calibration object located, say, several kilometers from the proposed target whose range is of interest (Block 1 ). Then, by using one or several of the techniques, such as: a military GPS and an electronic map; an inertial navigation system (INS) and an electronic map; and/or a laser range finder (LRF). there is defined the location of the random object for which the calibration curve will be created (Block 2). It should be noted, that such measurements can be performed within the accuracy of say 10 - 30 meters. Further, for the given measured range, the user experimentally determines the ratio S,. _l for substantially each pixel of the object by using at least two wavebands (Block 3). A reduction of the measurement errors (see Block 4) is achieved by averaging the result over substantially all pixels constructing the target. In addition, one can use frame averaging over video output obtained from and constituting the object and background when the range to the target during a frame time changes little to the target range. As a result of such averaging, the averaged subtracted ratio S_aγg is obtained. In the next step of constructing the calibration curve, the user moves away from the calibration object and measures the distance to the calibration object and the subtracted ratio S_ml for the given range in the process of the movement (Block 5). In the process of the movement the user averages the results for each measurement of S_rM over substantially all the pixels of the calibration object and applies the frame averaging in tha manner specified above. Thus the user obtains the corresponding averaged subtracted ratio S _rg. The procedure described with reference to blocks 1 to 5 bring about three or more pairs each of which including a value o£S _vg and a range value. Finally, a fitting procedure is applied to the data of the averaged subtracted ratio S _vg versus the range obtained in steps 1 through 5. Thus, for example, the least-squared fitting procedure is applied to the data specified so as to give rise to substantially the best analytical fitting (Block 6). The data obtained as a result of steps 1 to 5 is fitted to some analytical monotonically increasing or decreasing function, for example power polynomial, whereby the calibration function providing a dependence of subtracted ratio versus range is produced. In particular, this calibration function can be plotted as a curve on the graph of S_avg versus R.

Once a user has the calibration curve, he/she can determine the range to the targets of interest. For this purpose, according to the method of the invention. the user performs the following. He/she measures the averaged subtracted ratio and calculates the range by using either analytical function obtained as a result of the fitting procedure or the plot of the calibration curve. A use of the calibration function or plot of the calibration curve of the present invention provides a rather high accuracy in the range determination, normally within less than 2%.

According to another aspect of the present invention, the methods described above can be also applied for determining the range to an object whose size in one dimension is less than the effective projected pixel-image-size at the object in that direction. In this case, the combined radiation from a pixel, which includes the object and background, produces the temperature difference between neighboring pixels sufficient for the object to appear on the screen as a lit up pixel. Thus application of the method disclosed in the invention to a wire, provides a string of pixels that appears on the monitor and it is possible to determine the range to the wire by averaging the subtracted ratio for this string of pixels.

In a case, when an object cannot be sensed by the sensor due to large amount of clutter, one may use a multispectral method as disclosed in the co-pending Israeli patent application No. 122258 filed on November. 20 1997 owned by the applicant. This multispectral method enables the user to determine the pixels and to display the object (see Fig. 5). Taking into account these pixels and subtracting from them the nearest neighbor pixels in accordance with Eq. (6). one can determine the relative ratio S_ral as a function of the range R to the object using the techniques described above. Turning now to Fig. 6 there is shown schematically a non-limiting form of a system for passive range determination. By this embodiment the system includes a collecting telescope 16, which views electromagnetic radiation, at least two spectral line filters 18, a detector of the electromagnetic radiation 20, and a signal processor unit 22. The electromagnetic radiation emitted by the target 12 and by the background 10 propagates through the atmosphere 14 to be collected by the telescope 16. The filters 18 provide different specific wavebands to transmit selectively only a specific portion of the spectra of the target and background electromagnetic radiation. The filters are chosen so that the subtracted current ratio S_m, is nearly independent of the temperature within the temperature interval of 200K-500K and distance to target. The filters may. for example, be those employed by forward looking infrared (FLIR) operating in the 2-5 and 8- 12 micrometers wavebands. The detector converts the electromagnetic energy into the electric signal, which is monitored as a set of pixels depicting the target and the background. The detector may. for example be a FLIR or spectrometer. Detector 20 is coupled with the signal processor unit 22. which converts the object and background signals into analog form and subtracts the digital background signal from the digital signal of each pixel defining the target. After such subtraction, the processor unit computes the subtracted ratio S_ral. performs the averaging procedure described above for each distance from the target and provides S_llvg. Further, the processor unit performs a best fitting of S _vg measured as a function of R to the analytical function, and plots the calibration curve. Finally, it calculates the range to the target of interests. The processing unit 22 may take any suitable form. For example, it may be in the form of a microprocessor controlled by suitable software calculating the ratio as follows:

(N_nhl(R) - N_h(R))_hι„ s,JR) = (7)

(N_ohl(R) - N_h(R))_hl,_llt + (N_nlv(R) - N_l,(R))_hι,_l,_{ιl 2} This ratio is derived from Eqs. 5 and 6. wherein N_oh/ and N_Λ are the digital signals received from the object and the background radiation, respectively, measured in two wavelength bands.

There follows two non-limiting examples which illustrate the use of the technique of the invention for different applications.

EXAMPLE 1

This typical example illustrates how the method and the system of the present invention can be applied for passive determination of the range to a large vehicle. Two FLIR type filters operating in the 7.6-8.2 and 8.3-9.4 micrometers wave band, respectively, were used. The system for passive range determination described above is located at an altitude of 3000ft. The meteorological conditions were the following: tropical atmosphere with ground temperature of 27 C. relative humidity of 62%. atmospheric pressure of 1013mb. and visibility of 5km. The object was located in rural environment. Fig. 7 plots the calibration curve corresponding to best fit of the ratio function obtained for S_avr data measured after averaging the results over 16 pixels for each measurement.

In particular, this curve was used for determination of the range to the vehicle located at the distance of 5000m. The average value of the subtracted ratio was found to be 0.9413. By using the calibration curve, a range of 4860m was find. The error in the range determination is less than 3% or 140 meters in the range. By using two frames of data one has a measured passive range value of 5023 meters or less than 0.5% error.

EXAMPLE 2

This example illustrates how the method and the system of the present invention can be applied for passive determination of the range to a wire. Two FLIR type filters operating in the 4.38-4.53 and 4.75-4.95 micrometers waveband, respectively, were used. The system for passive range determination described above is located at an altitude of 300ft. The meteorological conditions were the following: atmosphere with ground temperature of 21 C, relative humidity of 76%, atmospheric pressure of 1013mb, and visibility of better than 23km. The object was located in rural environment. The number 30 pixels defined the wire image for this example. Fig. 8 plots the calibration curve corresponding to the best fit of the ratio function obtained for S_avr data measured after averaging the results over 30 pixels for each measurement. In particular, this curve was used for determination of the range to the wire located at the distance of 500m. The average value of the subtracted ratio was found to be 0.2705. By using the calibration curve, one can find a range of 464m. The resultant error in the range determination is less than 8% or 36 meters in the range. The present invention has been described with a certain degree of particularity, but those versed in the art will readily appreciate that various alternations and modifications may be carried out without departing from the scope of the following claims:

Claims

CLAIMS:

1. A method for constructing a calibration function S_avg of an averaged subtracted ratio versus Range R. to a calibration object, the method comprising the steps of: (i) measuring a range from a measurement device to the calibration object; (ii) measuring a radiation received from the object and from its background in at least two wavelength bands, so as to constitute at least one image composed of pixels: (iii) for each one of said at least two wavelength bands, calculating a subtracted current intensity N_oh/(R)-N_h(R) for each of at least one of said pixels, wherein N _hl and N_h are the respective current intensities of an object pixel and its neighboring background pixel:

(iv) calculating a subtracted ratio S_r , in respect of each one of said at least one pixel, the S_m, value depends upon the N_oh/(R)-N_h(R) obtained in respect of each one of said bands; (v) calculating the averaged subtracted ratio values S_avg over said at least one pixel and over at least one frame, whereby an increase of signal-to-noise ratio and decrease of error in the passive range determination is obtained; (vi) repeating the steps (ii) to (v) for at least additional two more ranges, whereby an additional at least two R values and corresponding at least two the S _vg values are obtained: and (vii) fitting the data obtained in steps (v) and (vi) to a fitting curve that constitutes said calibration function of S_avg versus range R.

2. The method of Claim 1. wherein said calibration object being a cold object having a temperature which is only slightly different from an ambient background temperature.

3. The method of Claims 1 or 2. wherein the subtracted ratio S_ra, is defined as

(N„_hl (R) - N_h (R)) iii j I s„ΛR⁾ (N _h, (R) - N_h (R))_hιlllll , + (N„_A, (R) - Λ^' _Λ (R)) ₂

4. A method according to claim 1. wherein selection of the predetermined wavebands comprising the following steps: (i) estimating an atmospheric transmission as a function of range and wavelength by using available meteorological conditions: (ii) calculating the subtracted ratio S_mt for at least two object temperatures and at least one object background; (iii) selecting at least two wavebands so that S_rat is nearly independent of the object and background temperatures. - 11 -

5. The method according to any one of the preceding claims, wherein the temperature of the object is between 200K and 500K.

6. The method according to any one of the preceding claims, wherein said averaged subtracted calibration function is a monotonically decreasing or increasing analytical function.

7. The method according to any one of the preceding claims, wherein said analytical function is a power polynomial function.

8. A method for passive determination of the range to an object of interest, comprising:

(a) providing a calibration function oϊS_aγg versus R; (b) measuring a radiation received from the object and from its background in at least two wavelength bands, so as to constitute at least one image composed of pixels: (c) calculating an averaged subtracted current ratio S _vg by performing:

(i) for each one of said at least two wavelength bands, calculating a subtracted current intensity

N_obl(R)-N_h(R) for each of at least one of said pixels, wherein N_oh/ and N_h are the respective current intensities of an object pixel and its neighboring background pixel; (ii) calculating a subtracted ratio S_ra, in respect of each one of said at least one pixel, the S_ra, value depends upon the N_ohl(R)-N_h(R) obtained in respect of each one of said bands; (iii) calculating the averaged subtracted ratio S,._α, value over said at least one pixel and over at least one frame;

(d) calculating the range to the object by applying the S_arg value obtained in step (c) to said calibration function.

9. The method of Claim 8. wherein said object having a temperature which is only slightly different from an ambient background temperature.

10. A method according to Claim 8. wherein the temperature of the object is between 200K and 500K.

1 1. A method according to to any one of Claims 8 to 10. wherein said object has one dimension less than the effective projected pixel-image size of the object and another dimension at least two sizes of pixel-image.

12. A method according to any one of Claims 8 to 1 1, wherein the object for range determination is a wire.

13. A method according to Claims 8. wherein the object cannot be sensed by a sensor because of a low signal-to clutter ratio.

14. A system for the passive determination of a calibration function to an object having a temperature which is only slightly different from an ambient background temperature, said system comprising:

(i) an object and background spectral radiance measurement device capable of measuring, at each of the consecutive observation times and distances, a radiation emitted by the object and its background; (ii) a signal processor unit coupled to the spectral radiance measurement device capable of analyzing the spectrum of the radiation received by the device and producing an output dependent on the distance to the object and on the meteorological conditions.

15. A system according to claim 14. wherein said device and processor unit are capable of constructing a calibration function of an averaged subtracted ratio S _vg versus range R to a calibration object by performing the following: (i) measuring a range from a measurement device to the calibration object; (ii) measuring a radiation received from the object and from its background in at least two wavelength bands, so as to constitute at least one image composed of pixels; (iii) for each one of said at least two wavelength bands, calculating a subtracted current intensity

N_0bj(R)-N_h(R) for each of at least one of said pixels, wherein N_υh/ and N_h are the respective current intensities of an object pixel and its neighboring background pixel;

(iv) calculating a subtracted ratio S_r , in respect of each one of said at least one pixel, the S_ral value depends upon the N_ohj(R)-N_h(R) obtained in respect of each one of said bands:

(v) calculating the averaged subtracted ratio values S_avg over said at least one pixel and over at least one frame, whereby an increase of signal-to-noise ratio and decrease of error in the passive range determination is obtained:

(vi) repeating the steps (ii) to (v) for at least additional two more ranges, whereby an additional at least two R values and corresponding at least two the S_avg values are obtained: and

(vii) fitting the data obtained in steps (v) and (vi) to a fitting curve that constitutes said calibration function of S _vg versus range R.

16. A system according to claim 14. wherein the object and its background spectral radiance measurement device comprising: (i) a collecting telescope for receiving a radiation propagating from the object and its background via atmosphere to said collecting telescope; (ii) at least two radiation filters for transmitting selectively only a specific portion of the spectra of the object and background electromagnetic radiation; and (iii) a detector of the electromagnetic radiation to convert an electromagnetic energy of a transmitted signal into an electric signal monitored as a set of pixels depicting the object and its background.

17. A system according to any one of claims 14 to 16, wherein said filters are chosen so that the subtracted current ratio function were substantially independent of the target and background effective temperature within the temperature intervals of 200K-500K.

18. A system according to any one of claims 14 to 16, wherein the filters are FLIR type filters operating in the 1-5 and 8- 12 micrometers.

19. A system according to anyone of claims 14 and 18. wherein said signal processor unit is in the form of a microprocessor controlled by a suitable software for calculation the subtracted current ratio function S,_t,_ι according to the relationship:

(N , (R) ~ N_h(R)) hand , R⁾ =

(N_ohl(R) - K_h(R)\_m , + (N . (R) - N_h(R))„

wherein N _bj and N_h are the respective current intensities of an object and a background, measured in two wavelength bands.

20. A system for the passive determination of the range to an object having a temperature which is only slightly different from an ambient background temperature, said system comprising: (i) an object and background spectral radiance measurement device capable of measuring a radiation emitted by the object of interest and its background;

(ii) a signal processor unit coupled to the spectral radiance measurement device capable of analyzing the spectrum of the radiation received by the device, and by using a calibration function dependent on the distance to a calibration object and on the meteorological conditions, obtaining the range to the object.

21. A system according to claims 20. wherein spectral radiance measurement device said signal processor unit are capable of:

(a) providing a calibration function fS_avg versus R;

(b) measuring a radiation received from the object and from its background in at least two wavelength bands, so as to constitute at least one image composed of pixels:

(c) calculating an averaged subtracted current ratio S _vg by performing:

N_obl(R)-N_b(R) for each of at least one of said pixels, wherein N_ob/ and N_b are the respective current intensities of an object pixel and its neighboring background pixel; (ii) calculating a subtracted ratio S_rar in respect of each one of said at least one pixel, the S_ra, value depends upon the N_obl(R)-N_b(R) obtained in respect of each one of said bands: (iii) calculating the averaged subtracted ratio S_ra, value over said at least one pixel and over at least one frame: (d) calculating the range to the object by applying the S _vg value obtained in step (c) to said calibration function.

22. A system according to any one of claims 14 through 21. wherein said object being a cold object having a temperature which is only slightly different from an ambient background temperature.

23. A method for passive range determination of a calibration function to an object having a temperature which is only slightly different from an ambient background temperature, said method comprising the steps of: (i) measuring at each of the consecutive observation times and distances, a radiation emitted by the object and its background: (ii) analyzing the spectrum of the radiation received by the device and producing an output dependent on the distance to the object and on the meteorological conditions.

24. A method for passive range determination of the range to an object having a temperature which is only slightly different from an ambient background temperature, said method comprising the steps of:

(i) measuring a radiation emitted by the object and its background; (ii) analyzing the spectrum of the radiation received by the device. and by using a calibration function dependent on the distance to a calibration object and on the meteorological conditions, obtaining the range to the object.