It is well known that in the course of large-scale military operations anti-tank mines are usually installed on the ground surface, which is related to high efficiency of the given method of installation. Besides that, the remote-acting mine installation facilities (aviation, artillery, salvo missile systems, etc.), having the highest capacity, install mines on the ground surface alone.

The usage of passive millimeter-wave radiometric sensor is one of the possible means for detection of mines with metallic cases installed by random throwing on the ground surface. The radiometric sensor registers the own and reflected radiation of mines in the HF range.

Radiometry is in many respects similar to radiolocation. Just as classic radiolocation, radiometry is intended for determination of coordinates of remote objects. The basic distinction between the methods lies in that radiolocation uses the radiation generated by radar proper in the capacity of the radiation source illuminating the target. The detection of objects by the radiometer uses the natural radiation of the objects themselves a nd the sky as the illumination source. In this respect radiometry is similar to operation of the passive infrared detectors but employs radio frequency in the capacity of the working range of the wavelengths.

equipment carrier; radiometer antenna; antenna of illumination noise generator; scanning device; observation object (mine) on ground surface; observation sector of main lobe of radiometer antenna on ground surface; sector of ground surface radiated by antenna of noise generator; sky radiation. Diagram of reception of radiometric signals of various radiation sources from the land carrier

This figure also has a diagram of arrangement of the radiometric sensors on a land carrier that includes a noise illumination generator the radiation pattern of the main lobe of the antenna of which is wider than that of the radiometer antenna. The noise generator and the radiometer are uniaxially anchored on the scanning device to ensure the survey of a terrain sector in front of movement of the land carrier along an azimuth. A line-by-line scanning is provided thanks to movement of the carrier. Such a selection of the parameters and positioning of the devices assure the condition of uniform illumination by the noise generator of a section of the ground surface observed by the radiometer antenna.

An experimental radiometer complex of the mm wave range was developed in cooperation with NPO "Vzleyt" in mid' 80s to check the possibility of detection of mines with metal cases. The selection of the wave range was related to the fact that high contrast of anti-tank mines installed on the ground surface can be provided in this range with an acceptable aperture of a radiometer antenna. The employment of a passive-active mm wave radiometer was proposed to improve the detection parameters.

General view of experimental installation

The complex included an 8-mm wave range radiometer, a noise generator of the same range and a TV camera. All these devices were coaxially secured at a mechanical appliance, which provides for scanning in two planes: according to the elevation angle and to the azimuth.

By doing so, the possibility of survey in a terrain sector from a fixed carrier was ensured. A main lobe of the radiometer antenna was 1.0 deg and of the noise generator antenna - about 10.0 deg. The relative positions of the generator and radiometer antennas were selected in such a way that a minimum level of the direct signal from the noise generator to the radiometer was achieved. The TV camera allowed an operator to compare images obtained in the HF and visual ranges.

It should be noted that the usage of the noise generator with the spectrum filling the entire reception band of the radiometer in the capacity of a terrain illumination source is a must. Otherwise the image obtained as a result of scanning is random in character and cannot be identified with the observed objects if a monochromatic generator is used. It is because of coherent summation of the signals reflected from the observation object and the direct transmission of the signal from the illumination noise generator to the radiometer's receiving antenna.

The operation of the equipment provides for simultaneous generation of two images: one in the passive mode when the noise generator was switched off and the other one in the active mode when the illuminator was switched on.

The picture of a proving ground is seen in figure. Nine Russian metallic anti-tank mines of TM-62M type were laid in three staggered rows on the ground surface in the foreground. The distance from the nearest and most distant mines to the radiometric complex was 10.0 and 22.0 m, respectively. The mines in the right row were installed on supports and inclined towards the radiometric complex to enhance contrast.

The row of metallic plates can be seen in the left bottom corner of the picture. At the background of the picture a metal tower (in the center) and a flat metal roof (in the left corner) are visible.

General view of proving ground with mines

The results of survey of the terrain in the passive mode and in the mode with illumination are illustrated in figures, respectively. Gradations of brightness in these figures are chosen so that darker sections of image correspond to objects with lower radiometric temperature. Since metal objects have the reflection factor equal to 1 in the HF range, they look darker on the radiometric image, reflecting the sky radiation the radio brightness temperature, which is lower than the temperature of the ground cover.

Passive radiometric image of proving ground

Passive image clearly demonstrates the mine nearest to the radiometer and the mines of the right row the contrast of which is higher. The row of metallic plates is seen to the left of the minefield, and the metal roof of a building and the outline of a tower are clearly visible in the background. The produced image shows that the contrast of mines laid in parallel to the ground surface is sufficient for their detection at a distance up to 10.0 m only with the specified parameters of the radiometric sensor in the passive mode. At greater distances their contrast lies at the level of the natural variations of radio brightness temperature of the underlying surface.

When the noise generator is switched on, the image changes qualitatively. The contrast of metal objects with respect to the background depends not only on the type of the surface but also on the shape of objects observed. Thus, flat objects, which reflect the radiation of the noise generator with a high brightness temperature like a mirror, can be viewed in the image as objects having low brightness temperature. It can be explained by the fact that the sky radiation alone is reflected in the direction of the radiometer antenna. The metal roof of the building and the metal reference mark are such objects in the image.

Active radiometric image of proving ground with noise generator being switched on

The objects of complex shapes, being sets of "brilliant points", reflect the illuminator radiation in the direction of the radiometer antenna as well. Such objects change the contrast relative to the background when the noise generator is switched on.

Mines and the tower change their contrast but the row of plates and the flat roof keep contrast in this picture. We can see various reflections from local objects on the ground.

For the selection of complex-shaped objects like mines an algorithm was proposed which operates in accordance with a scheme of coincidences and separates only those objects in the images which change their contrast relative to the background when the illuminator is switched on. Figure below demonstrates effectiveness of the proposed algorithm. In this picture we can see only six mines and the tower. All the other objects have vanished from the image.

Results of co-processing passive and active radiometric images

In conclusion it is necessary to note that the proposed method of detection of metal objects against the background of the underlying surface can also be used for other types of the military equipment, such as tanks, artillery guns, etc. For this purpose the illuminator should operate as a stroboscope. Thus, when the noise generator is switched off and the radiometer antennas are in fixed positions, an element of the passive radiometric image will be registered, but when the mentioned generator is switched on, an element of the active radiometric image will be recorded. With proper selection of the switching frequency of the noise generator, a complete matching of both images can be obtained in the course of their subsequent processing.