RLS with synthesized aperture. Aperture synthesis. What was before

The synthesis of antenna aperture is one of the most promising directions for the development of radar, which appeared in the late 50s and immediately attracted widespread attention. The main advantage of this area is multiple (in 1000 or more times) an increase in the angular resolution of the radar. This ensures the possibility of radio formation of radar objects and detecting small-sized objects, increasing the accuracy of the target designation and noise immunity of the radar. At the first stage of the development of this area, basic successes were achieved in a significant increase in the efficiency of air and cosmic intelligence. In the future, the aperture synthesis methods began to be used in reconnaissance and impact complexes, multifunctional radar of aircraft for detecting small-sized and group purposes and guidance of managed weapons, in the RLS planning review, radio engineering and navigation systems.

There were two stages in the development of the theory and practice from radar from radar.

At the first stage, thanks to the use of broadband (100 MHz or more), the probing signals were able to provide a high resolution by delaying signals and as a result of a high range permission (a unit of meters and better). The range permit is determined by the expression

The width of the spectrum of the probing signal.

- wavelength radar; C 1 - antenna size,

The formation of a real antenna. To explain the principle of the synthesis of aperture, first consider the formation of a real antenna pattern diagram, which determines the resolution of the corner coordinate of the usual radar.

Let there be a linear antenna aperture of the size D, which drops a flat electromagnetic wave at an angle 0 (Fig. 2.1), i.e. The antenna works at the reception.

Under the aperture (open) is understood as the part of the antenna, which is involved in radiation or receiving an electromagnetic wave. The wave front is the surface of equal phases. In this case, this is a plane. The phase of the electromagnetic wave along the aperture (X axis) is determined by the delay of the wave edge relative to the aperture center:

where r (x) is the distance from the front of the wave to the point x on the aperture.

The radiation diagram is formed as a result of the syphan summation of the electromagnetic wave falling on the aperture:

Electromagnetic wave tension.

The normalized orientation diagram in this case is equal

at the level of 0.7 or, that the same, 0.5 by power:

When operating one antenna, not only reception, but also on the transfer of DN is defined as

and the equivalent width of the day on the transfer and reception

In a more general case, the antenna aperture sets the volume of the analyzed space-time signal, which is the dependence of the intensity, phase and polarization of the electromagnetic field from spatial coordinates and time. Thus, aperture is characterized by the geometric sizes of the analyzed volume of the electromagnetic wave, the analysis time, polarization and frequency parameters. In this case, the resolution of the angular coordinate is determined by the change in the space-time signal in the antenna aperture, depending on the angular position of the source of the electromagnetic wave.

The known examples of such a space-time signal can serve as bulk holographic lenses and synthesized apertures.

Synthesizing aperture. The main difference between the synthesized (artificial) apertures from conventional (real) apertures of the antenna is that the synthesized aperture (CA) is formed sequentially in time. In each given moment, the reception of the electromagnetic wave is carried out by a real aperture, and the synthesized aperture is the result of a real aperture in the time of receiving an electromagnetic wave with a different position with its different position relative to the source of the electromagnetic wave. Consider the synthesis process on the example of the formation of the rectilinear Aperture of the RCA (Fig. 2.2).

Its focus on the reception is determined in the same way as the DN of the real aperture. Wave phase raid between two positions of the real antenna on the trajectory

twice more than a conventional aperture, which is due to the double passage of the electromagnetic wave of the distance r (when transmitting and when receiving). As a result, the width of the radiation pattern in the synthesized Aperture of the RA of this type is less than that of the real aperture of the same size:

The main result of the synthesis of the aperture is that the size of the aperture has increased in N times compared with the size of the real aperture.

synthesized aperture formed as a result

displacement of a real antenna. In this case, the effect is achieved by increasing the volume of the analyzed field in space and time.

The main properties of the synthesized aperture. Consider the main properties of the synthesized aperture.

for onboard aircraft and space systems. Typical values \u200b\u200bof the relative dimensions of the apertures of various systems are as follows:

Thanks to the large size of the Aperture of the RCA, it is possible to obtain a high linear permission through the corner coordinate on large ranges:

The synthesized aperture is formed as a result of the reception and processing of signals reflected from the target, i.e. The synthesized aperture determines the days only to receive. DN on the transmission during the synthesization of the aperture is determined by the DN of the real antenna. Polarization and frequency properties of Ca are also defined by a real antenna.

When synthesizing the aperture, it can work at the same time (emit, take only one antenna element (real antenna). In this case, there are no electrodynamic problems in the formation of the entire aperture, since there is no interaction of elements by an electromagnetic field. The task of the synthesis of the aperture and the formation of the radiation diagram is actually reduced to the development of algorithms and their execution of the processor for processing the trajectory signal. As for the real antenna, the DN of the synthesized aperture is the dependence of the signal at the output of the processor from the corner coordinate of the point source of radiation or re-energization (in the case of an active RCA).

DN can be single-pulse, multipath, monoimpulse, adaptive, etc.

pCA observation objects in most cases are in the intermediate zone (Fresnel zone) aperture, and not in the far zone, like most of the real antennas. When receiving the far zone, the front of the wave on the aperture is considered flat. With increasing aperture size (or a decrease in the distance to the object) the sphericity of the front of the wave can not be neglected. Usually the conditions of the far zone is written as

For a real antenna, the onboard radar border of the far zone of about 100 m, and during synthesizing it is calculated by thousands of kilometers. Therefore, in the RSA in the processing of the trajectory signal, it is necessary to take into account the spherical of the phase front of the electromagnetic wave. In the simplest Rs, when the size of Ca is small, the curvature of the front of the electromagnetic wave is not taken into account. Such a regime is called the Doppler lunch (DOL), and an increase in permission is small (10 ... 30) times.

Accounting for the sphericity of the wave front during the processing of the trajectory signal is called focusing, and aperture according to the focused aperture. In fig. 2.3 shows the distribution of the field of non-flowing (d) (a) and focused (b) apertures in the intermediate and long-distance zones of the day.

At a focus distance, i.e. The same as in the usual antenna in the far zone. It can be said that the focusing process transfers the properties of the aperture direction from the far zone to the intermediate.

Since the sphericness of the front of the wave depends on the distance to the object, a different focusing law is necessary for different ranges, i.e. To ensure focus Ca, you need multichannel algorithm for the processing of the trajectory signal.

Thanks to the focusing of the Ca provides permission in the intermediate zone not only in the corner, but also by the range even with the modulated signal. However, it is usually small, and the range resolution is ensured by modulating the probing signal.

The main sources of errors - the non-hotterness of the trajectory signal are the phase instability of transceiver modules, the trajectory instability of the radiation of the RAS carrier and the instability of the electromagnetic wave. So, a permissible error in the knowledge of the antenna movement trajectory is equal to several millimeters (in the centimeter range of the electromagnetic wave). This requires special compensation measures for these errors using micro navigation systems and autofocus algorithms.

The energy characteristics of the CA (signal / invention ratio of noise) are determined by the gain coefficient of the real antenna and the synthesis time, i.e. The time of coherent accumulation of Sig Nalov. Noise immunity from external active and passive interference is defined as a real antenna, as well as directed properties of Ca, i.e. Spatial interference selection.

Indeed, in each position of the antenna in the synthesizing aperture, the power of the received signal is determined by the radiation power and the antenna gain, and the syntheasic addition of these signals during synthesizing is equivalent to accumulating the signal energy during the synthesis time at a constant spectral power of the internal noise. In relation to sources of external interference, an angular selection is possible, the effectiveness of which depends on the DN of real and synthesized apertures.

The relative movement of the antenna and the object required for the formation of the Ca can be performed by various methods. Forming Ca as a result of antenna movement with fixed objects, those are called direct synthesis, and the formation of Ca when the objective of the object and the fixed antenna is reverse synthesis. In this case, the formation of Ca as a result of the rotation of the object, which is equivalent to the movement of the antenna around the object is equivalent.

Using the synthesis in the process is simultaneously not one, and many antennas makes it possible to synthesize not only linear, but also flat and volumetric Ca.

What is an exceptionally complex task for onboard TsMM. In the ground conditions, this task successfully solves the optical processor, which uses an entry of the trajectory signal to the film and analog signal processing using a coherent optical system.

The synthesis of aperture requires a certain time, which leads to a delay in information in the RSA. The minimum delay of information is determined by the synthesis time, i.e. The time of the formation of sa. It usually makes tenths - units of seconds. The maximum delay is determined by taking into account the time of performing the synthesis algorithm to the corresponding processor for processing traverter. Ground optical processors have the greatest delay. It consists of an airplane flight time in the RSA area, the return time to the base, the time of delivery of the film with the entry of trajectory signals to the laboratory, the time of photochemical processing of the film, optical processing and writing the image to the secondary film and, finally, photochemical processing of the secondary film. This time can reach a few hours.

Captain M. Vinogradov,
Candidate of Technical Sciences

Modern radar means, installed on aircraft and spacecraft, are currently representing one of the most intensively developing segments of radio-electronic technology. The identity of the physical principles underlying the construction of these funds makes it possible to consider them within the framework of one article. The main differences between the cosmic and aviation radar are in the principles of processing the radar signal associated with a different aperture size, the features of the propagation of radar signals in various layers of the atmosphere, the need to account for the curvature of the earth's surface, etc. Despite this kind of difference, radar developers with aperture synthesizing (RCA) make every effort to achieve maximum similarity of reconnaissance data capabilities.

Currently, on-board radars with aperture synthesizing make it possible to solve the tasks of species intelligence (carry out the survey of the earth's surface in various modes), selection of mobile and stationary purposes, analyzing the changes in the ground environment, to shoot objects hidden in forestry arrays, detecting beugoned and small-sized marine objects.

The main purpose of the RC is the detailed shot of the earth's surface.


Fig. 1. Modes of shooting of modern RSA (A - detailed, b - panorable, in - scanning)	Fig. 2. Examples of real radar images with permissions 0.3 m (at the top) and 0.1 m (below)

Fig. 3. Picture of images at different levels of detail

Fig. 4. Examples of fragments of real sections of the earth's surface obtained at DTED2 detail levels (left) and DTED4 (right)

Due to the artificial increase in the aperture of the onboard antenna, the basic principle of which is the coherent accumulation of reflected radar signals on the synthesis interval, it is possible to obtain high-resolution over the corner. In modern systems, permission can reach tens of centimeters when working in a centimeter wavelength range. Similar values \u200b\u200bof the range permissions are achieved through the use of intapulse modulation, for example, linear frequency modulation (LFM). The antenna aperture synthesis interval is directly proportional to the height of the Rs carrier flight, which ensures the independence of the shooting resolution from height.

Currently, there are three main shooting modes of the earth's surface: a review, scanning and detailed (Fig. 1). In the survey mode, the removal of the earth's surface is carried out continuously in the capture strip, while separating the side and front-wind mode (depending on the orientation of the main petal of the antenna orientation diagram). The signal accumulation is carried out for a time equal to the estimated antenna aperture synthesis interval for these flight conditions of the radar carrier. The scanning shooting mode is different from the review that the shooting is conducted on the entire width of the bandwidth, strips of equal strip width. This mode is used exclusively in the radar of space based. When shooting in a detailed mode, the signal accumulation is carried out on an increased interval compared to the overview mode. The increase in the interval is carried out by synchronous with the movement of the radar carrier of the movement of the main petal of the antenna orientation diagram in such a way that the irradiated area is constantly in the shooting zone. Modern systems allow you to obtain the earth's samples and objects located on it with permissions of about 1 m for the review and 0.3 m for detailed modes. Sandy chanda announced the creation of a RS for tactical blah, having the ability to shoot with a resolution of 0.1 m in a detailed mode. The applied methods of digital processing of the received signal, an important component of which adapt algorithms for the correction of trajectory distortions are essential (in terms of shooting the earth's surface). It is the inability to withstand for a long time a straight line path of the carrier movement does not allow you to get permission comparable to the detailed mode in continuous overview mode, although there are no physical resolution restrictions in the overview mode.

The inverse synthesis mode of aperture (IRSA) allows the synthesis of the antenna aperture is not due to the movement of the carrier, but by the movement of the operated target. In this case, we can not go about the translational motion characteristic of land objects, but about the pendulum movement (in different planes), which is characteristic of floating funds swinging on the waves. This feature determines the main purpose of IRSA - the detection and identification of marine objects. The characteristics of modern IRSs allow you to confidently detect even small-sized objects, such as the periscopes of submarines. To shoot in this mode have the opportunity for all aircraft consisting of the US Armed Forces and other states, whose tasks include patrolling the coastal zone and water management. The images obtained as a result of photography in their characteristics are similar to the images obtained by shooting with direct (non-intersion) aperture synthesis.

Interferometric SAR mode (Interferometric SAR - IFSAR) allows you to get three-dimensional images of the earth's surface. At the same time, modern systems have the ability to conduct single-point shooting (that is, use one antenna) to obtain three-dimensional images. In addition to the normal resolution, an additional parameter is entered to characterize these images, called the accuracy of the height definition, or permission in height. Depending on the value of this parameter, several standard gradations of three-dimensional images are defined (DTED - Digital Terrain Elevation Data):
Dtedo .............................. 900 m
Dted1 .............................. 90m.
Dted2 ............................ 30m.
Dted3 .............................. 10m.
Dted4 ............................ ZM.
Dted5 .............................. 1m.

The type of image of the urbanized territory (model) corresponding to different levels of detail is shown in Fig. 3.

Levels 3-5 received the official name "High Resolution Data" (HRTE-High Resolution Terrain Elevation Data). Determining the location of ground objects on the images of level 0-2 is conducted in the WGS 84 coordinate system, the height countdown is carried out relative to the zero mark. The coordinate system of high-resolution images is currently not standardized and is at the discussion stage. In fig. 4 shows fragments of real sections of the earth's surface obtained as a result of a stereon with different resolution.

In 2000, American SRTTL ICCC in the framework of the SRTM project (Shuttle Radar Topography Mission), the purpose of which was to obtain large scale cartographic information, performed the interferometric shooting of the equatorial part of the Earth in the strip from 60 ° C. sh. up to 56 ° Sh., Having obtained at the exit, the three-dimensional model of the earth's surface in DTed2 format. To obtain detailed three-dimensional data in the US, the NGA HRTE project is developed? As part of which images of levels 3-5 will be available.
In addition to the radar shooting of open areas of the earth's surface, the onboard radar has the ability to obtain images of scenes hidden from the eye of the observer. In particular, it allows you to detect objects hidden in forest arrays, as well as underground.

Penetrating radar (GPR, Ground Penetrating Radar) is a remote sensing system, the principle of operation is based on the processing of signals reflected from areas deformed or differing in its composition located in a homogeneous (or relatively homogeneous) volume. The system sensing system of the earth's surface allows you to detect those in different depths of emptiness, cracks, swallowed objects, identify areas of different density. At the same time, the energy of the reflected signal strongly depends on the absorbing properties of the soil, the size and shape of the target, the degree of heterogeneity of the boundary regions. Currently, GPR, in addition to the military-applied orientation, has developed in commercially favorable technology.

The sensing of the earth's surface occurs by irradiating with pulses with a frequency of 10 MHz - 1.5 GHz. The irradiating antenna may be on the earth's surface or is located on board the aircraft. A part of the irradiation energy is reflected from changes in the subsurface structure of the Earth, the largest part penetrates further into depth. The reflected signal is accepted, processed, and the processing results are displayed on the display. When the antenna moves, a continuous image is generated, reflecting the state of the subsurface layers of the soil. Since actually reflection occurs due to the difference in the di-electrical permeable-hairs of various substances (or different states of a substance), then the testing can be detected a large amount of natural and artificial defects in the homogeneous mass of the subsurface layers. The penetration depth depends on the state of the soil at the place of irradiation. Reducing the amplitude of the signal (absorption or scattering) significantly depends on a number of soil properties, the main of which is its electrical conductivity. So, the optimal for probing are sandy soils. Much less suitable for this clay and very wet soils. Good results show sensing dry materials such as granite, limestone, concrete.

Resolution is improved by increasing the frequency of radiated waves. However, the increase in frequency adversely affects the depth of the penetration of radiation. Thus, the signals with a frequency of 500-900 MHz may penetrate to a depth of 1-3 m and provide a resolution to 10 cm, and with a frequency of 80-300 MHz penetrate into a depth of 9-25 m, but the resolution is about 1.5 m.

The main military assignment of the subsurface sensing radar is the detection of the mines. In this case, the radar installed on board the aircraft, such as a helicopter, allows you to directly open the maps of minefields. In fig. 5 shows the images obtained by radar mounted on board the helicopter reflecting the arrangement of anti-personnel mines.

The onboard radar, designed to detect and tracking objects hidden in forest arrays (Fo-Pen - Foliage Penetrating), allows you to detect small-sized objects (moving and stationary) hidden by crowns of trees. The shooting of objects hidden in forest arrays is carried out similarly to the usual shooting in two modes: overview and detailed. On average, the width of the capture bandwidth is 2 km, which allows obtaining a 2x7 km on the output of the image sections; In the detailed mode, shooting is carried out by sections of 3x3 km. The resolution of the shooting depends on the frequency and varies from 10 m at a frequency of 20-50 MHz to 1 m at a frequency of 200-500 MHz.

Modern image analysis methods allow you to detect with a fairly high probability and make the subsequent identification of objects on the resulting radar image. In this case, the detection is possible in the pictures of both high (less than 1 m) and low (up to 10 m) permission, while the recognition requires images with a sufficiently high (about 0.5 m) resolution. And even in this case, it is possible to speak mostly only about recognizing on indirect features, since the geometric shape of the object is very distorted due to the presence of a signal reflected from the deciduous cover, as well as due to the appearance of signals with frequency displacement due to the Doppler effect arising in As a result of foliage fluctuations in the wind.

In fig. 6 are presented iso-breeds (optical and radar) of the same area of \u200b\u200bthe terrain. Objects (columns of machines), invisible on the optical image, are clearly visible on the radar, however, to identify these objects, abstracting from external signs (movement along the road, the distance between the machines, etc.), it is impossible, since with this permission information about The geometric structure of the object is completely absent.

The detail of the resulting radar images made it possible to implement another number of features in practice, which, in turn, made it possible to solve a number of important practical tasks. To one of these tasks belongs to track changes that occurred on a certain section of the earth's surface for a certain period of time - coherent detection. The duration of the period is usually determined by the frequency of patrolling the specified area. Tracking changes is carried out on the basis of the analysis of the coordinately combined images of a given area obtained consistently after each other. At the same time, two levels of the detail of the analysis are possible.


Figure 5. Maps of minefields in a three-dimensional presentation when shooting in various polarizations: model (right), an example of an image of a real section of the earth's surface with a complex subsurface environment (left) obtained by radar installed on board the helicopter

Fig. 6. Optical (at the top) and radar (bottom) image of the area of \u200b\u200bthe area with moving along the forest road of the column car

The first level involves the detection of significant changes and is based on an analysis of the amplitude samples of the image carrying the main visual information. Most often, this group includes changes that a person can see, browsing at the same time two formed radar images. The second level is based on the analysis of phase samples and allows you to identify changes, invisible to the human eye. These include the appearance of traces (machine or hum) on the road, changing the condition of windows, doors ("openly closed"), etc.

Another interesting possibility of Rs, also an announced Sandondy company is a radar video shooting. In this mode, the discrete formation of an antenna aperture from the portion to the section characteristic of a continuous overview mode is replaced by parallel multichannel formation. That is, at each moment of time, not one is synthesized, and several (quantity depends on the tasks) apertures. A kind of analogue of the number of formed apertures is the frequency of frames in the usual video. This feature allows you to implement the selection of moving goals based on the analysis of the resulting radar images, applying the principles of coherent detection, which is essentially an alternative to standard radar, which selection of moving targets based on the analysis of up-plerian frequencies in the accepted signal. The effectiveness of the implementation of such selectors of moving goals is very doubtful due to significant hardware and software costs, therefore such modes are highly likely to remain no more than an elegant way to solve the problem of selection, despite the opening opportunities to select targets moving at very low speeds (less than 3 km / h, which is not available to the Doppler IDC). Directly video recording in the radar range at present also did not find use, again due to the high requirements for speed, therefore the active samples of military equipment that implement this mode in practice is not.

A logical continuation of improving the technique of shooting the earth's surface in the radar range is the development of subsystems for analyzing the information received. In particular, the development of automatic radar images of radar images will be important, allowing to detect and recognize ground objects that fall into the shooting zone. The complexity of creating such systems is associated with the coherent nature of radar images, the phenomena of interference and diffraction in which lead to the appearance of artifacts - artificial glare, similar to those that appear when the target is irradiated with a large efficient scattering surface. In addition, the quality of the radar image is slightly lower than the quality of the same (by permission) of the optical image. All this leads to the fact that the effective implementation of object recognition algorithms on radar images currently does not exist, but the number of works carried out in this area, certain successes achieved recently, suggest that in the near future it will be possible Intelligent unmanned intelligence devices that have the ability to assess the ground environment based on the results of the analysis of information obtained by its own onboard means of radar intelligence.

Another direction of development is the complexation, that is, a coherent association with subsequent joint processing, information from several sources. These may be radar, leading shooting in various modes, or radar and other means of reconnaissance (optical, IR, multi-spectral, etc.).

Thus, modern radars with antenna aperture synthesizing make it possible to solve a wide range of tasks related to the maintenance of radar shooting of the earth's surface, regardless of the time of day and weather conditions, which makes them an important means of extracting information about the state of the earth's surface and objects on it.

Foreign Military Review No. 2 2009 p.52-56

Aperture synthesis, method of obtaining high angular resolution using the synthesis of measurement results performed by a radio interferometer consisting of two small apertures moving within a large aperture, and a correlation (multiplayer) receiver. The measurement result by the method of aperture synthesis is similar to measurements with an antenna large aperture. With aperture synthesis, a large amount of measurements are performed at different positions of the elements and the results are summed with certain weights and phases.

The aperture synthesis method was proposed in 1952 by M. Rail, who studied the Radio Building of Galaxies with it. In 1974, Rail together with E. Hewish was awarded the Nobel Prize "For Innovative Studies in Radio Savorphysics". The most spread is an aperture synthesis received in radio astronomy and radar. In radio astronomy, an aperture synthesis is used due to the objectives of the study of the angular distribution of the radio source radiation intensity with a fine structure from angular minutes to a fraction of seconds. For such studies, antennas are needed with the ratio D / λ (D - the linear size of aperture, λ is the wavelength) of about 10 3 -10 6, therefore, for a centimeter radio range D, there should be about hundreds of meters and more. Naturally, conventional antennas with such aperture cannot be created, therefore the aperture is "synthesized", measuring in separate points located inside this synthesized aperture, and performing appropriate measurement processing. As a result, a high angular resolution is achieved.

When using the aperture synthesis method, a large antenna is divided into N elements. Falling waves, reflected from each element, fall into the focus of the antenna in the phase. Therefore, the high-frequency voltage V (T) in the focus can be recorded as a sum of the components of ΔV I (T) from the individual elements:

Power P at the output of a high antenna receiver is proportional to the average voltage square value:

From formula (2) it can be seen that the measurement result contains terms depending on the signals obtained only from the parameters of the elements. Each term can be measured with two small antennas with a size equal to an aperture element in positions I and K, and a correlation (multiple) receiver. If the observed section of the sky does not contain variable sources, then such an interferometer can be used to consistently measure the members of the row (2).

The cut of the East-West line on the surface of the Earth, visible from the side of the remote source, is rotated 180 ° in 12 hours. If all the elements of the antenna grid on this segment are followed by the source, then in 12 hours it is possible to synthesize a round aperture in the plane perpendicular to the axis of the earth rotation, with a diameter equal to the length of the segment. The width of the synthesized diagram in any direction is inversely proportional to the projection of the aperture to this direction. The deterioration of the resolution in directions close to the equator plane is eliminated when using a T-shaped antenna grid with segments oriented in the directions of East-West and North-South (Fig.).

Modern systems of aperture synthesis consist of a large number of full-time antennas and at the same time existing independent correlations interferometers, which significantly reduces the observation time. Rounding together with Earth, each interferometer measures a large number of categories (2). For multi-element interferometers, the aperture synthesis method allows to synthesize the beam with such a width that can be obtained with aperture having dimensions comparable to the size of an antenna grid.

For a more complete extraction of information from measurement results, a priori information about the brightness of the sky is used. Such a priori information allows the use of far-separated antennas systems, as well as to build the sky maps using only amplitude measurements when the phase information is not ordinary or missing.

First work using for aperture synthesis of small mobile antennas were performed in Cambridge (United Kingdom) in 1954. In Sydney (Australia) in 1956, the Earth's rotation was used for the first time for the synthesis of a two-dimensional grid with a linear. The most famous system of aperture synthesis is an antenna grinning VLA (Very Large Array) in New Mexico (USA), completed in 1981. It consists of 27 full-time paraboloids with a diameter of 25 m each, which can move along three-21-kilometer railway tracks, laid in the form of the letter Y. The angular resolution of this system at a wavelength of 1.3 cm is 0.05 ".

The aperture synthesis method is also used in the interferometers formed by antennas, spread by hundreds and thousands of kilometers (radio interfernteters with super-long bases). This allows you to synthesize the apertures comparable to the dimensions of the Earth, and getting an angular resolution of about 0.001 ", much superior to the achieved in optical astronomy. In the future - the creation of an aperture of land-space, some of the elements of which will be placed on spacecraft (Radiastron project, Russia) .

Lit.: Kraus J.D. Radio Astronomy. 2nd ed. Powell, 1986; Christian S., Hyogb I. Radioteleeskopa. M., 1988.

Technical task

Develop RTS :

Type of RTS ............... .... aircraft;

Purpose. ............... RLS side view with synthesized aperture;

Tactical and technical characteristics of the developed RTS:

1 Analysis of the technical task

In aircraft radar, there are hard restrictions on the dimensions of the antennas, which prevents the achievement of resolution of azimuth.

To overcome this obstacle, one of two methods implemented in the lateral review radar. In the first case, the antenna is located along the fuselage, which allows you to significantly increase its size and improve this resolution due to this. In the second method, an artificial increase in the size of the antenna due to the so-called aperture synthesis is used.

The technical assignment requires the development of an aircraft radar of the side view with a synthesized aperture. In such radars, large size antenna is installed motionless along the fuselage of the aircraft. The beam of the antenna system is directed perpendicular to the axis of the aircraft. Typically, two antennas are installed, the rays of which are directed to the right and left from the direction of flight. Viewing the specified part of the earth's surface occurs due to the movement of the aircraft itself during the flight (Figure 1).

Figure 1 - The principle of view of space in the direction perpendicular to the axis of the aircraft.

The principle of the operation of the radar with the synthesis of aperture (RSA) is based on the creation of equivalent apertures with an increased efficient length, which is achieved using special methods of processing signals, and not an increase in the physical dimensions of the aperture of the real antenna. In RSA, only one emitting antenna element (real antenna) is used, which consistently occupies the position along the trajectory of the flight. In each of these positions, signals are emitted and accepted (Figure 2).

Reflected from the targets of the signals as amplitude and the phase of received signals. Set in the memory device,

Figure 2 - Principle of formation of artificial (synthesized) opening.

After the resulting movement of the emitting element by the signal, the signals in the storage device become very similar to the signals that were taken by the elements of the real linear lattice. If the signals are in the memory to be processed by the same algorithm as in the formation of a real linear lattice, we obtain the effect of receiving signals to the large-sized antenna (the method of "aperture synthesis").

In addition, in the RS, the signals in the memory can be selected by the range and, if necessary, signals from different ranges can be processed in various ways (focus).

When turning the plane begins to roll, resulting in an error of measuring height. To eliminate the error, it is necessary to fix the antenna on the balancing device, as a result of which the main petal of the antenna pattern of the antenna is directed perpendicular to the earth's surface.

Usually in the RTS side view, a signal with pulse modulation is used.

The antenna has a sovereign orientation chart.

In order not to worsen the aerodynamic properties of the aircraft, the antenna is placed under a special fairing, which does not interfere with the passage of the radio signal. In the calculations it is necessary to consider that the aircraft is over different types of ground surfaces that have different reflective properties.

2 Features of the construction of some radar blocks with a synthesized aperture.

Antenna

The horizontal size of the aperture of the RSL antenna determines the linear resolution of azimuth, practically achievable in the radar with the synthesizing aperture. When processing signals, it is assumed that the CBD of the real antenna when the aircraft is span remain constant. Therefore, it is necessary to have the stabilization of the antenna, so that the residual fluctuations of the beam were significantly less than the width of the day. In most cases, the antenna is installed in the lateral direction.

Transceiver

In the radar with synthesizing aperture should provide high coherence of signals. Therefore, more strict requirements for the stability of the frequency of generators and parameters of the elements are presented. The output signal of the coherent radar is the voltage at the output of the synchronous detector. The output signal is a bipolar video signal in which the reference bias level corresponds to a zero signal offset.

Recording signals and memorization.

A characteristic feature of the RC is the need to memorize the received signals, since the synthesized DN signals needed to enter the input is not simultaneously, but throughout a certain time interval. Processing stored signals and allows you to get a high resolution. The same signal is used to generate output signals for a large number of points of the radar image. Requirements for the capacity of memory devices are very high. In the RLS with high resolution requires a large amount of memory, so they usually use a photographic storage device.

Foreign Military Review No. 2/2009, p. 52-57

Captain M. Vinogradov,

candidate of Technical Sciences

The main purpose of the RC is the detailed shot of the earth's surface.

Fig. 3. Picture of images at different levels of detail

Dtedo .............................. 900m

Dted1 .............................. 90m.

Dted2 ............................ 30m.

Dted3 .............................. 10m.

Dted4 ............................ ZM.

Dted5 .............................. 1 m

The type of image of the urbanized territory (model) corresponding to different levels of detail is shown in Fig. 3.

Levels 3-5 received the official name "High Resolution Data"(HRTE - High Resolution Terrain Elevation. data.). Determining the location of ground objects on the images of level 0-2 is conducted in the WGS 84 coordinate system, the height countdown is carried out relative to the zero mark. The coordinate system of high-resolution images is currently not standardized and is at the discussion stage. In fig. 4 shows fragments of real sections of the earth's surface obtained as a result of a stereon with different resolution.

In addition to the radar shooting of open areas of the earth's surface, the onboard radar has the ability to obtain images of scenes hidden from the eye of the observer. In particular, it allows you to detect objects hidden in forest arrays, as well as underground.

Penetrating radar (GPR, Ground Penetrating Radar) - a remote sensing system, the principle of operation of which is based on the processing of signals reflected from areas deformed or differing in its composition located in a homogeneous (or relatively homogeneous) volume. The system sensing system of the earth's surface allows you to detect those in different depths of emptiness, cracks, swallowed objects, identify areas of different density. At the same time, the energy of the reflected signal strongly depends on the absorbing properties of the soil, the size and shape of the target, the degree of heterogeneity of the boundary regions. Currently, GPR, in addition to the military-applied orientation, has developed in commercially favorable technology.

Solving permission can be improved by increasing the frequency of radiated waves. However, the increase in frequency adversely affects the depth of the penetration of radiation. Thus, the signals with a frequency of 500-900 MHz may penetrate to a depth of 1-3 m and provide a resolution to 10 cm, and with a frequency of 80-300 MHz penetrate into a depth of 9-25 m, but the resolution is about 1.5 m.

Raps, designed to detect and tracking objects hidden in forest arrays (FO.- Pen. - Foliage. Penetrating), allows you to detect small-sized objects (moving and stationary), hidden by crowns of trees. The shooting of objects hidden in forest arrays is carried out similarly to the usual shooting in two modes: overview and detailed. On average, the width of the capture bandwidth is 2 km, which allows obtaining a 2x7 km on the output of the image sections; In the detailed mode, shooting is carried out by sections of 3x3 km. The resolution of the shooting depends on the frequency and varies from 10 m at a frequency of 20-50 MHz to 1 m at a frequency of 200-500 MHz.

In fig. 6 are presented iso-breeds (optical and radar) of the same area of \u200b\u200bthe terrain. Objects (columns of machines), invisible on the optical image, are clearly visible on the radar, but it is impossible to accomplish the identification of these objects, abstracting from external signs (move along the road, the distance between the machines, etc.), it is impossible, because at this resolution Information about the geometric structure of the object is completely absent.

Fig. 5. Maps of minefields in a three-dimensional presentation when shooting in various polarizations: model (right), an example of an image of a real section of the earth's surface with a complex subsurface environment (left) obtained by radar installed on board the helicopter

The effectiveness of the implementation of such selectors of moving goals is very doubtful due to significant hardware and software costs, therefore such modes are highly likely to remain no more than an elegant way to solve the problem of selection, despite the opening opportunities to select targets moving at very low speeds (less than 3 km / h, which is not available to the Doppler IDC). Directly video recording in the radar range at present also did not find use, again due to the high requirements for speed, therefore the active samples of military equipment that implement this mode in practice is not.

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