DE19546873C1 - Determining distance of object flying through atmosphere and radiating energy - Google Patents

Abstract

The method involves detecting the energy radiated from the object using a passive imaging sensor, which detects sensor signals corresp. to the detected radiation power in two or more narrow wavelength regions of different atmospheric damping coefficient. The ratio of at least two sensor signals for different wavelength regions is formed and used to determine the distance of the object. The object distance is measured at two time pints and its speed derived from the time and distance differences.

Description

The invention relates to a method to determine the distance one by the Atmosphere of flying object and on a device for Execution of the procedure.

In particular, the invention is important when a Airplane threatened by a self-propelled missile becomes. Most short and medium missiles Usually have an infrared seeker that has range radiation from aircraft, mainly Radiation from the engines is sensitive and after a goal-seeking steering process works. Such Guided missiles are relatively easy to manufacture and can also from shoulder-carried weapons, so-called Pilot fists are shot down.

Since such missiles - in contrast to radar-guided - Passively heading for their destination, warning sensors are growing Importance to be on time and also at the right time Countermeasures against such flying missiles to be able to trigger.

In principle, two types of warning sensors are possible, namely active and passive warning sensors. Active Warning sensors traditionally work according to the Doppler Radar principle, with which data immediately within the scope of the resolution  about distance and / or speed of the approaching Missiles can be obtained. The threatening missiles, especially small surface-to-air missiles are shorter Range, but usually have only a very small radar cross-section, so that the range of such active warning sensors is limited. Another disadvantage is that active warning sensors are treacherous due to their radiation are.

Passive workers will therefore be preferred in the future Warning sensors are used as missiles with these sensors with a small radar cross-section, even at longer distances can be detected; they also have the potential a better angular resolution. A disadvantage compared to An active warning sensor is a passive warning sensor no direct determination of the distance and / or the Relative speed of the approaching missile allowed.

There would now be the possibility of two passive warning sensors to arrange and combine at the ends of a base, then through the removal of an object Triangulation could at least be roughly estimated. For this purpose, however, there are sensors with a very good one Angular resolution necessary. Imaging electro-optical Sensors that are of interest here Spectral ranges of the ultraviolet or the middle Work infrared and are in development today achieve an angular resolution of approximately 1 °. Then it can can be estimated that this angular resolution at Installation on aircraft - because of the short base length - just enough to distance the approaching Missile up to about 600 meters somewhat quantitative  specify. But it is precisely the area that is larger Distances of more than 1500 meters in which a knowledge the distance and / or the speed of the Approaching missile is important to countermeasures Defense in a timely and effective manner can.

Countermeasures are e.g. B. the deflection of the missile by exposing shining targets, such as infrared Exchange bodies etc., or ultimately the fault or Destruction of the approaching missile or its Seeker head with the help of directed countermeasures.

If you look at the response and delay times at the Defense of the approaching missile with exposed Considering pseudo targets, it turns out that Good results, especially with large aircraft can be achieved when the approaching missile already detected at a distance of more than 2.5 km becomes.

Likewise, the seeker's head must be destroyed Missile, e.g. B. with the help of laser radiation, in one such a distance that the approaching missile after the search head was destroyed, it actually did Airplane missed. The minimum distance at which the Missile must be detected in order to be a defense have depends on various parameters, such. Belly the relative angle between the missile and the aircraft, the Angle change and relative speed between aircraft and missile. It turns out that this Minimum distance for a sensor (destruction) even more  chosen larger than in the case of the suspension of false targets must become.

DE 37 38 222 C1 describes a method for Missile interference-free even against a background or to be able to locate exposed false targets. this will thereby achieved that on the missile to be located modulatable reference radiator is attached and that on the other hand, the detector only signals with predetermined Modulation frequencies reused. To determine the spatial angular position, a detector array is used, whereby it is possible to make an apparent target of the one to be located Separate missiles spatially. This allows one Resolution of the solid angle of the missile to be located, however not determining its distance. Be further noted that most of the missiles approaching none have modulated reference emitters, so that this Procedures for the relevant case practically not is applicable.

Another principal option with a passive Sensor to roughly determine the distance of an object in US 3 103 586 indicating heat radiation Body described. The patent specification is the idea based, the distance is not active - as with radar - but to be determined passively by that of a body outgoing heat radiation in two spectral bands is measured, with one of the measuring bands in a Absorption band of atmospheric CO₂ or Water vapor, the other gang in a neighboring one Spectral range lies, its atmospheric transmission is not affected by this absorption band. By measuring those detected in the two bands  Radiation power with an IR sensitive detector can - with known concentration of the absorbing gas and known temperature of the radiating body - the Distance from the ratio of the detected Radiation powers are determined.

The disadvantage of this method is that the detection small objects with increasing distance more and more Background radiation falls on the detector, so that Signal-to-noise ratio with increasing distance always gets worse, making the distance no longer can be determined. In principle, it is possible with the help of magnifying optics the signal To improve noise ratio, however, this will Improvement with a reduction in the visual field bought, so that the actual task of a Warning sensor - d. H. the warning about a bigger one Field of vision - is missed.

Thus, with that described in the above patent Device the distance for smaller objects, such as approaching missiles, determined only in the close range and here are similar distance restrictions like the triangulation method with several on the plane installed sensor heads.

The invention has for its object a method and to provide a device with which data a approaching object faster and more accurately than before already made available from larger distances to be able to effectively take countermeasures against that to be able to grasp the flying object.  

This task is related to the procedure with the subject of claim 1, with respect to Device with the subject of Claim 8 solved.

Accordingly, a method and an apparatus with a imaging passive sensor proposed in at least two wavelength ranges in which the atmospheric damping coefficients differ are, the approaching object is detected.

By using an imaging sensor, in particular with a larger number of detector elements, is in Comparison to US 3 103 586 achieved that also in the signal-to-noise ratio even further away is acceptable and therefore a distance determination is made possible.

The detection of the object in several Wavelength ranges can e.g. B. done in that before the optical system of the passive warning sensor rotating filter, which consists of several Sectors exist, whose transmission ranges accordingly the selected wavelength ranges are different. The sensor signals corresponding to that from the sensor detected radiation power of the approaching object at the different wavelengths can now can be used to take into account the Difference in the atmospheric damping coefficient Distance and / or speed of the object, e.g. B. of a missile approaching an aircraft. In principle there is a spatial one for this determination No resolution of the object with an imaging sensor  necessary if only the entire of one detector element Object, or at least its radiant power emitting parts - without disturbing background radiation - be recorded. With large distances of the object can the signal from a single detector, d. H. a pixel suffice while at short distances the object is on several detector elements is mapped and thus several pixels in the sense of a spatial resolution of the Object must be evaluated.

The method and the device are not only for that Detect threatening missiles from a threatened one Plane, but more generally for recognizing flying objects from a moving or stationary Observation site, e.g. B. on the ground, if applicable Objects in flight radiation, especially heat and / or emit UV radiation. The ones to be recognized Missiles therefore do not have to constantly or at least be missiles powered by engines for a short time. Radiation can also be detected by the Friction of the missile during its flight through the Atmosphere is created. Active sensors are with the described procedure is not necessary.

For the following consideration, it is assumed that the warning sensor has a passive imaging sensor that works at two wavelengths λ₁ and λ₂, and that the object is a powered missile with one or more engines. The detected sensor or image signal P₁ or P₂ at the two wavelengths depends on the atmospheric damping coefficients A₁ or A₂ and the radiation power P _FK1 or P _FK2 emitted by the missile _{exhaust gas jet} and the distance R between the missile and the sensor as follows:

Here i = 1 or 2, Fi is the effective detector area at the wavelength λ _i , which is also determined by the filter, Δλ _{i is} the effective spectral width of the filter and η _{s is} an efficiency factor dependent on the wavelength. The ratio of the image signals at the two wavelengths is then:

Before installing the warning sensor at the observation site, e.g. B. in an airplane, the characteristic filter data, ie the effective spectral width Δλ _i and the effective detector area F _i determined by the filter, are determined in calibration measurements. Thus, as a provisionally still unknown variables, in addition to the relative missile / aircraft _distance , the intensities P _FK1 and P _FK2 emitted by the exhaust jet of the missile and the difference in the atmospheric damping coefficients A₁ and A₂ at the respective wavelengths λ₁ and λ₂ remain. The difference in atmospheric damping coefficients can be assumed to be known in most cases either by taking them from existing tables or by measuring them from the aircraft. In many cases, the ratio of the radiation _powers P _FK1 / P _FK2 emitted by the exhaust jet of an approaching missile can also be assumed to be known at the different wavelengths, e.g. B. if the wavelengths are close to each other and on the other hand if only a small number of missiles of different types are used, the characteristic data of which are usually known to the enemy.

If this is not the case, one can Process variant specified below applied will.

Assuming that the difference between the damping coefficients A _i and the ratio of the radiated intensities P _{FK1 are} known, the ratio of the detected powers according to equation (2) above depends only exponentially on the distance R between the missile and the aircraft, the change in this ratio with the distance R is decisively determined by the difference in the atmospheric damping coefficients A₁ and A₂.

To achieve sufficient accuracy, it is necessary to work at such wavelengths that give a difference in the atmospheric damping coefficients (A₁ - A₂) <1 / R _min , where R _{min is} the distance at which an approaching missile should be detected in any case . If R _{min is given} as 2.5 km, then the difference (A₁ - A₂) should be at least 0.4 km ^-1 . It is now advisable to work at wavelengths in the spectral vicinity of atmospheric absorption lines, preferably absorption lines which are caused by gases, the concentration of which fluctuates only slightly with the climatic conditions. Accordingly, absorption lines that are caused by water vapor, for example, should be avoided.

In the following, a UV warning sensor and an IR Warning sensor Examples of distance determination described.

A UV sensor normally works in the "solar blind" Range of the UV spectrum with an effective Working wavelength λ₁ <280 nm. This working wavelength is already in the long-wave flank of the Ozone absorption band at 255 nm. By adding a suitable filter, the choice of the second wavelength λ₂ allowed between 300 nm and 320 nm, the difference of atmospheric damping coefficient (A₁ - A₂) large enough be made to ensure sufficient accuracy in the Quotient determination according to equation (2) achieve. This method of distance determination works particularly well during a night operation. If the UV Sensor used during the day, so the larger one can Share of UV backlight is an additional expense in the Image processing to determine the image signal of the Missile become necessary.

The difference in atmospheric damping coefficients at the two wavelengths depends on such a UV Warning sensor primarily depends on the ozone concentration; this varies with the location and with the time of day or season.  

If the ozone concentration in the concerned Area of application of the aircraft is not known, it can also on board the aircraft z. B. in connection with a Laser rangefinders can be determined. Such a Range finders usually work on the base of a pulsed Nd: YAG laser. By using the higher Harmonics, especially four times the laser frequency a wavelength of approximately 265 nm, which is close to the Maximum of the ozone absorption band is from the mathematical evaluation of the backscattered light a measure for the ozone concentration near the warning sensor or of the aircraft.

If the emitted intensities P _FK1 and P _{FK2 of} the missile are very different at the selected wavelengths, it can be achieved by a suitable sectoring of the filter that the detected electrical signals averaged over time are approximately the same size. In this way, the signal-to-noise ratio of the overall signal P₁ / P₂ can be significantly improved.

Sensors in the IR range preferably work in the middle Infrared, d. H. in the spectral range between about 3 µm and 5 µm. In this spectral range, radiation from the Exhaust jet of the approaching missile is emitted. Here it makes sense to use two wavelengths near the CO₂ absorption band, which is about 4.24 microns work, e.g. B. with the wavelength λ₁ on the short-wave Side of the absorption band at about 4 µm and with the Wavelength λ₂ on the long-wave side between 4.4 and 4.7 µm. Because the atmospheric damping in the last-mentioned spectral range with the water vapor content in  the atmosphere grows significantly and therefore also the Difference of the damping values (A₁ - A₂) accordingly with influenced, this difference can be determined by determining the Computed water vapor content of the surrounding atmosphere be adapted to the respective local value. In addition could also the damping coefficients in the Wavelengths λ₂ can be determined in situ when on board a pulsed laser with the aircraft in question Wavelengths are in the relevant spectral range, so that from the backscattered from the atmosphere Laser signal the damping can be determined.

Whether a warning sensor is used in the UV or IR range depends on several factors, including: a. the atmospheric Damping, sensor sensitivity, UV or IR Background radiation and the aircraft mission. Imaging IR sensors are currently only as scanning sensors with one Sensor line available, whereas UV sensors with "starring arrays" are available. With the latter sensors faster processing of the image signals is possible, so that at the same observation time of the approaching Missile more or less noise data than one Scanning sensor are available. So the usable Distance range larger and the expected rate of false alarms are lower. UV sensors can also be used then be of advantage if the missile takes the aircraft off Approaching aft: IR sensors could be Aircraft engine exhaust jets emitted IR Radiation are disturbed. Generally a UV sensor to be preferable when surface-to-air missiles with lower Range to be identified; at higher altitudes IR warning sensors will be preferred, especially if Air-to-air missiles are detected at greater distances  should be. This is particularly important for High-speed missile used as an air-to-air missile have only a briefly burning engine and then be controlled aerodynamically, but they due to the aerodynamic heating, especially the Missile tip, can emit infrared radiation.

Equation (2) above cannot be used if the type of missile approaching is not known or cannot be determined, ie if there is no data available on the ratio of the radiated power of the missile engine at the individual wavelengths. Then, however, the change in distance R can be determined by comparing the signal ratio at different distances R _i . For this purpose, the quotients of the above image signals P₁ and P₂ at two successive times t1 and t2, that is, as a ratio for two different distances R₁ and R₂. If now within the measuring time t = (t2 - t1) between the distances R₁ and R₂ the ratio of the radiation _powers P _FK1 / P _FK2 emitted by the object or missile hardly changes, which can be favored by choosing closely adjacent wavelengths, so it follows that the ratio of the signal quotients for different distances depends only on the difference in the atmospheric damping coefficients (A₁ - A₂) and on the distance difference (R₁ - R₂):

The distance difference and thus the relative speed V _R between the missile and the aircraft can thus be determined from the double quotient according to this equation (3) by signal evaluation:

V _R = (R₁ - R₂) / t (4).

Further refinements of the invention result from the Sub-claims emerge.

The invention is in exemplary embodiments based on the Drawing explained in more detail. In this represent:

Figure 1 schematically shows a warning sensor according to the invention with a single imaging passive detector and a filter for two different wavelengths.

2 schematically shows a filter having two sectors for the two different wavelengths.

Fig. 3 shows schematically a filter whose sensitivity is electrically switchable for the different wavelengths; and

Fig. 4 shows a warning sensor according to the invention, which works with two separate detectors with associated filters.

In Fig. 1, an optical system 2 and a sensor 3 is of a warning sensor 1 shown, as indicated schematically of an arrangement of a plurality of small individual detectors. A circular filter disk 4 is arranged in front of the optics and, as indicated by the arrow, rotates about the optical axis of the warning sensor. The filter disk 4 is shown schematically in FIG. 2. It has two sectors S1 and S2 and possibly two dark sectors D lying between these sectors. Sectors S1 and S2 each have a defined transmission for a wavelength λ1, λ2 or a wavelength range Δλ1, Δλ2; the sector size is adapted to the respective application in order, as mentioned above for the distance determination with the aid of a UV warning sensor, to achieve by means of this suitable sectoring that the detected image signals averaged over time are approximately the same size.

The image signals are fed to an evaluation circuit 5 which carries out an evaluation using the quotient equations given above. The sensor-characteristic data, such as transmission range and effective detector area, are entered into the evaluation circuit beforehand via an input unit 6 . Furthermore, the measured or otherwise predetermined atmospheric damping coefficients A _i and the ratio of the radiation intensities P _FK1 / P _FK2 characterizing the exhaust gas jet of the approaching missile are input to the evaluation circuit 5 via an input unit 7 . The results of the evaluation circuit 5 are fed to a computer 8 , in which countermeasures against the approaching missile are prepared, and also a display 9 , on which relevant results are shown to the operating personnel of the attacked aircraft.

Instead of a rotating filter, a filter 4 a according to FIG. 3 can be used. With this filter, the transmission band is shifted with an alternating electrical signal. In this way it is also easier to reduce a largely spectrally homogeneous background radiation by forming differences in their effect on the image signal.

In Fig. 4 an arrangement with two passive imaging detectors 3 ₁ and 3₂ is shown, which in turn are each constructed as an arrangement of small individual detectors. The radiation falling through the input optics 2 is directed with a beam splitter 11 in two ways: in the first way, a filter 4 ₁ which is sensitive to a first wavelength range and the first sensor 3 , which emits the first image signal P 1; in the second way there is a filter 4 ₂ and the second sensor 3 ₂ which is sensitive to the second wavelength range and which emits the second image signal P₂. As described for FIG. 1, the image signals are further processed in an evaluation circuit. This embodiment requires more volume than that with only one sensor; it is also necessary to harmonize the signals from the two individual sensors.

Claims (13)

1. Method for determining the distance one by the Flying atmosphere, emitting radiation energy Object, in which the radiation emitted by the object with a passive imaging sensor is detected, the Sensor signals corresponding to those detected by the sensor Radiation power of the object at least two different narrow wavelength ranges at which the atmospheric damping coefficients differ are determined, the ratio of two Sensor signals at different wavelength ranges is formed, and from this the distance to the object is determined. 2. The method according to claim 1, in which at one Engines powered this at least in the object Drive phase is detected. 3. The method of claim 1, wherein the measurements at two different narrow wavelength ranges be performed. 4. The method according to any one of the preceding claims, at the distances of the object to two different ones Times are determined and from the Distance difference and the time difference of the two Times the speed of the object is determined.   5. The method according to any one of the preceding claims, at which the different narrow wavelength ranges in close to atmospheric absorption lines, preferably in the vicinity of such absorption lines that are caused by gases, the concentration of which is very low the climatic conditions fluctuates. 6. The method according to claim 5, characterized in that the different narrow wavelength ranges a wavelength λ <350 nm on the long-wave side of the Ozone absorption line, the focus of which is about 255 nm is in the ultraviolet spectral range. 7. The method according to claim 5, wherein the different narrow wavelength ranges in the middle infrared on the short-wave and long-wave side of the CO₂ Absorption line are around 4.25 µm. 8. Device for carrying out the method according to one of claims 1 to 7, with:
an input optic ( 2 ) with a filter arrangement ( 4 , 4 a, 4 ₁, 4 ₂) for several different wavelength ranges in which the atmospheric attenuation coefficients are different;
a passive imaging sensor ( 3 ; 3 ₁, 3 ₂) for detecting the radiation of the object at the different wavelength ranges;
an evaluation circuit ( 5 ) for calculating the ratio of the sensor signals (P _i ) at the two different wavelength ranges and for calculating the distance of the object therefrom. 9. The device according to claim 8, wherein the filter arrangement comprises a filter ( 4 , 4 a) which is transparent to the two different wavelength ranges. 10. The apparatus of claim 9, wherein the filter rotating filter with several, for different Wavelength ranges is transparent sectors (S1, S2). 11. The device according to claim 9, wherein the filter ( 4 a) consists of a material whose spectral passband can be electrically switched between different wavelength ranges. 12. The apparatus of claim 8, in which a separate optical path with a filter arrangement ( 4 ₁, 4 ₁) is provided for each wavelength range. 13. The apparatus of claim 12, wherein the optical paths are separated from one another by a beam splitter ( 11 ).