Aperture synthesis. Synthetic aperture radar systems

FOREIGN MILITARY REVIEW No. 2/2009, pp. 52-57

Captain M. VINOGRADOV,

Candidate of Technical Sciences

Modern radar equipment installed on aircraft and spacecraft currently represents one of the most rapidly developing segments of radio-electronic technology. The identity of the physical principles underlying the construction of these means makes it possible to consider them in one article. The main differences between space and aviation radars lie in the principles of radar signal processing associated with different aperture sizes, the characteristics of the propagation of radar signals in different layers of the atmosphere, the need to take into account the curvature of the earth's surface, etc. Despite these differences, the developers of synthetic aperture radars (RSA) are making every effort to achieve maximum similarity in the capabilities of these reconnaissance assets.»

Currently, on-board radars with synthetic aperture allow solving the problems of visual reconnaissance (shooting the earth's surface in various modes), selecting mobile and stationary targets, analyzing changes in the ground situation, shooting objects hidden in forests, and detecting buried and small-sized marine objects.

The main purpose of SAR is a detailed survey of the earth's surface.

By artificially increasing the aperture of the on-board antenna, the main principle of which is the coherent accumulation of reflected radar signals over the synthesis interval, it is possible to obtain high angular resolution. In modern systems, resolution can reach tens of centimeters when operating in the centimeter wavelength range. Similar range resolution values are achieved through the use of intrapulse modulation, for example, linear frequency modulation (chirp). The antenna aperture synthesis interval is directly proportional to the flight altitude of the SAR carrier, which ensures that the shooting resolution is independent of altitude.

Rice. 3. View of images at different levels of detail

Currently, there are three main modes of surveying the earth's surface: overview, scanning and detailed (Fig. 1). In the survey mode, surveying of the earth's surface is carried out continuously in the acquisition band, while lateral and front-lateral modes are separated (depending on the orientation of the main lobe of the antenna radiation pattern). The signal is accumulated over a period of time equal to the calculated interval for synthesizing the antenna aperture for the given flight conditions of the radar carrier. The scanning shooting mode differs from the survey mode in that shooting is carried out over the entire width of the viewing swath, in stripes equal to the width of the capture swath. This mode is used exclusively in space-based radars. When shooting in detailed mode, the signal is accumulated over an increased interval compared to the overview mode. The interval is increased by moving the main lobe of the antenna radiation pattern synchronously with the movement of the radar carrier so that the irradiated area is constantly in the shooting area. Modern systems make it possible to obtain images of the earth's surface and objects located on it with resolutions of the order of 1 m for overview and 0.3 m for detailed modes. The Sandia company announced the creation of an SAR for tactical UAVs, which has the ability to survey with a resolution of 0.1 m in a detailed mode. The resulting methods of digital processing of the received signal, an important component of which are adaptive algorithms for correcting trajectory distortions, have a significant impact on the resulting characteristics of SAR (in terms of surveying the earth's surface). It is the inability to maintain a rectilinear trajectory of the carrier for a long time that does not allow obtaining resolutions comparable to the detailed mode in continuous overview shooting mode, although there are no physical restrictions on resolution in overview mode.

The inverse aperture synthesis (ISA) mode allows the antenna aperture to be synthesized not due to the movement of the carrier, but due to the movement of the irradiated target. In this case, we may not be talking about forward motion, characteristic of ground-based objects, but about pendulum motion (in different planes), characteristic of floating equipment swaying on the waves. This capability determines the main purpose of IRSA - detection and identification of marine objects. The characteristics of modern IRSA make it possible to confidently detect even small-sized objects, such as submarine periscopes. All aircraft in service with the Armed Forces of the United States and other countries, whose missions include patrolling the coastal zone and water areas, are capable of filming in this mode. The characteristics of the images obtained as a result of shooting are similar to those obtained as a result of shooting with direct (non-inverse) aperture synthesis.

The interferometric survey mode (Interferometric SAR - IFSAR) allows you to obtain 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. To characterize image data, in addition to the usual resolution, an additional parameter is introduced, called height accuracy, or height resolution. Depending on the value of this parameter, several standard gradations of three-dimensional images (DTED - Digital Terrain Elevation Data) are determined:

DTEDO......................... 900m

DTED1.........................90m

DTED2........................ 30m

DTED3.........................10m

DTED4........................ Zm

DTED5........................ 1 m

The type of images of an urbanized area (model), corresponding to different levels of detail, is presented in Fig. 3.

Levels 3-5 are officially called "high-resolution data"(HRTe - High Resolution Terrain Elevation data). The location of ground objects in images of levels 0-2 is determined in the WGS 84 coordinate system, the height is measured relative to the zero mark. The coordinate system for high-resolution images is currently not standardized and is under discussion. In Fig. Figure 4 shows fragments of real areas of the earth's surface obtained as a result of stereo photography with different resolutions.

In 2000, the American Space Shuttle, as part of the SRTM (Shuttle Radar Topography Mission) project, the goal of which was to obtain large-scale cartographic information, carried out interferometric surveys of the equatorial part of the Earth in the band from 60° N. w. to 56° south sh., resulting in a three-dimensional model of the earth's surface in DTED2 format. Is the NGA HRTe project being developed in the USA to obtain detailed 3D data? within which images of levels 3-5 will be available.

In addition to radar surveying of open areas of the earth's surface, airborne radar has the ability to obtain images of scenes hidden from the eyes of the observer. In particular, it allows you to detect objects hidden in forests, as well as those located underground.

Penetration radar (GPR, Ground Penetrating Radar) is a remote sensing system, the operating principle of which is based on the processing of signals reflected from deformed or compositionally different areas located in a homogeneous (or relatively homogeneous) volume. The earth's surface probing system makes it possible to detect voids, cracks, and buried objects located at different depths, and to identify areas of different densities. In this case, the energy of the reflected signal strongly depends on the absorbing properties of the soil, the size and shape of the target, and the degree of heterogeneity of the boundary regions. Currently, GPR, in addition to military applications, has developed into a commercially viable technology.

Probing of the earth's surface occurs by irradiation with pulses with a frequency of 10 MHz - 1.5 GHz. The irradiating antenna can be located on the earth's surface or located on board an aircraft. Some of the radiation energy is reflected from changes in the subsurface structure of the earth, while most of it penetrates further into the depths. The reflected signal is received, processed, and the results of the processing are displayed on the display. As the antenna moves, a continuous image is generated that reflects the state of the subsurface soil layers. Since reflection actually occurs due to differences in dielectric constants of different substances (or different states of one substance), probing can detect a large number of natural and artificial defects in a homogeneous mass of subsurface layers. The depth of penetration depends on the condition of the soil at the irradiation site. The decrease in signal amplitude (absorption or scattering) largely depends on a number of soil properties, the main of which is its electrical conductivity. Thus, sandy soils are optimal for probing. Clayey and very moist soils are much less suitable for this. Probing of dry materials such as granite, limestone, and concrete shows good results.

Sensing resolution can be improved by increasing the frequency of the emitted waves. However, an increase in frequency has a negative effect on the radiation penetration depth. Thus, signals with a frequency of 500-900 MHz can penetrate to a depth of 1-3 m and provide a resolution of up to 10 cm, and with a frequency of 80-300 MHz they penetrate to a depth of 9-25 m, but the resolution is about 1.5 m.

The main military purpose of subsurface sensing radar is to detect mines. At the same time, a radar installed on board an aircraft, such as a helicopter, allows you to directly open maps of minefields. In Fig. Figure 5 shows images obtained using a radar installed on board a helicopter, reflecting the location of anti-personnel mines.

Airborne radar designed to detect and track objects hidden in forests (F.O.- PEN - Foliage PENetrating), allows you to detect small objects (moving and stationary) hidden by tree crowns. Shooting objects hidden in forests is carried out similarly to regular shooting in two modes: overview and detailed. On average, in survey mode, the acquisition bandwidth is 2 km, which makes it possible to obtain output images of areas of the earth's surface 2x7 km; in detailed mode, surveying is carried out in 3x3 km sections. The shooting resolution 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 methods of image analysis make it possible to detect and subsequently identify objects in the resulting radar image with a fairly high probability. In this case, detection is possible in images with both high (less than 1 m) and low (up to 10 m) resolution, while recognition requires images with a sufficiently high (about 0.5 m) resolution. And even in this case, we can talk for the most part only about recognition by indirect signs, since the geometric shape of the object is very distorted due to the presence of a signal reflected from the foliage, as well as due to the appearance of signals with a frequency shift due to the Doppler effect that occurs in as a result of leaves swaying in the wind.

In Fig. 6 shows images (optical and radar) of the same area. Objects (a column of cars), invisible on an optical image, are clearly visible on a radar image, however, it is impossible to identify these objects, abstracting from external signs (movement on the road, distance between cars, etc.), since at this resolution information about the geometric structure of the object is completely missing.

The detail of the resulting radar images made it possible to implement a number of other features in practice, which, in turn, made it possible to solve a number of important practical problems. One of these tasks includes tracking changes that have occurred on a certain area of the earth's surface over a certain period of time - coherent detection. The length of the period is usually determined by the frequency of patrols in a given area. Tracking of changes is carried out based on the analysis of coordinate-wise combined images of a given area, obtained sequentially one after another. In this case, two levels of analysis detail are possible.

The first level involves the detection of significant changes and is based on the analysis of amplitude readings of the image, which carry basic visual information. Most often, this group includes changes that a person can see by simultaneously viewing two generated radar images. The second level is based on the analysis of phase readings and allows you to detect changes invisible to the human eye. These include the appearance of traces (of a car or a person) on the road, changes in the state of windows, doors (“open - closed”), etc.

Rice. 5. Maps of minefields in three-dimensional representation when shooting in different polarizations: model (right), an example of an image of a real area of the earth’s surface with a complex subsurface environment (left), obtained using a radar installed on board a helicopter

Another interesting SAR capability, also announced by Sandia, is radar video. In this mode, the discrete formation of the antenna aperture from section to section, characteristic of the continuous survey mode, is replaced by parallel multi-channel formation. That is, at each moment of time, not one, but several (the number depends on the tasks being solved) apertures are synthesized. A kind of analogue to the number of apertures formed is the frame rate in regular video shooting. This feature allows you to implement the selection of moving targets based on the analysis of received radar images, applying the principles of coherent detection, which is inherently an alternative to standard radars that select moving targets based on the analysis of Doppler frequencies in the received signal.

The effectiveness of implementing such moving target selectors is highly questionable due to significant hardware and software costs, so such modes will most likely remain nothing more than an elegant way to solve the selection problem, despite the emerging opportunities to select targets moving at very low speeds (less than 3 km/ h, which is not available to Doppler SDC). Direct video recording in the radar range is also not currently used, again due to high performance requirements, so there are no operating models of military equipment that implement this mode in practice.

A logical continuation of improving the technology of surveying the earth's surface in the radar range is the development of subsystems for analyzing the received information. In particular, the development of systems for automatic analysis of radar images that make it possible to detect, isolate and recognize ground objects within the survey area is becoming important. The difficulty 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 irradiating a target with a large effective scattering surface. In addition, the quality of the radar image is somewhat lower than the quality of a similar (in terms of resolution) optical image. All this leads to the fact that effective implementations of algorithms for recognizing objects in radar images do not currently exist, but the amount of work carried out in this area, certain successes achieved recently, suggest that in the near future it will be possible to talk about intelligent unmanned reconnaissance vehicles that have the ability to assess the ground situation based on the results of analyzing information received by their own on-board radar reconnaissance equipment.

Another direction of development is integration, that is, coordinated integration with subsequent joint processing of information from several sources. These can be radars that survey in various modes, or radars and other reconnaissance means (optical, IR, multispectral, etc.).

Thus, modern radars with synthetic antenna aperture make it possible to solve a wide range of problems associated with conducting radar surveys of the earth’s surface, regardless of the time of day and weather conditions, which makes them an important means of obtaining information about the state of the earth’s surface and the objects located on it.

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Owners of patent RU 2397509:

The invention relates to the field of radio engineering, in particular to the field of nonlinear radar technology, and can be used to search and detect objects with nonlinear electrical properties. The achieved technical result of the invention consists in implementing an algorithm for synthesizing the antenna aperture in a nonlinear radar station (radar) and achieving angular resolution close to potential. The essence of the invention is to measure the average speed of movement and random deviations of the nonlinear radar carrier from a given trajectory along the abscissa, ordinate, and applicate axes and implement in each of the echo signal processing channels of the nonlinear radar a known algorithm for synthesizing the antenna aperture, taking into account the measurement results. 3 ill.

SAR is known, consisting of a series-connected antenna device, a transceiver, phase detectors, analog-to-digital converters, a digital processing system, a display system processor, an indication system, as well as a recording system and a transmission system over a broadband channel, the operating principle of which is based on the formation of a synthesized aperture large-sized antennas using a real small-sized antenna. At the same time, to reduce the influence of random spatial deviations of the SAR carrier from a given trajectory (trajectory instabilities) on the results of its operation, a compensation system for trajectory instabilities is used, based on the integrated use of two inertial navigation systems - a standard inertial navigation system with correction from radio sensors (GLONASS, DISS or Radar in the mode of measuring speed and drift angle) and a broadband inertial navigation system with a system of accelerometers and angular velocity sensors (micro-navigation). However, SAR does not allow searching and detecting OENS, since the processing of echo signals from radar targets is carried out only at the carrier frequency of the probing signal (SS) ω 0.

The closest in technical essence (prototype to the proposed invention) is a nonlinear radar (NRLS), for example, consisting of a transmitter, a transmitting antenna and two identical signal processing channels at the frequencies of the second 2ω 0 and third 3ω 0 harmonics of the ES, each of which contains series-connected receiving antenna and receiver, as well as display devices. The operating principle of the navigation radar is based on receiving response signals from the OENS at frequencies 2ω 0 and 3ω 0, processing them and indicating levels. This is ensured by the fact that usually OENS with semiconductor components have a response signal level at the second harmonic that is 20-30 dB higher than at the third harmonic. For contact-type OENS, as a rule, the inverse relationship holds. The disadvantages of a nonlinear radar are the lack of consideration of the influence of trajectory instabilities on the process of its functioning and the unreliability of the comparison of the levels of response signals from the OENS at the second and third harmonics of the ES due to the strong dependence of the change in the scattered OENS power at the harmonics of the ES on the position of the OENS relative to the sounding direction and the harmonic number of the ES.

The problem to be solved by the proposed nonlinear radar with a synthetic aperture antenna is to increase the angular resolution of the nonlinear radar.

The technical result of the invention is expressed in the implementation of an algorithm for synthesizing the antenna aperture in a nonlinear radar and achieving angular resolution close to potential.

The technical result is achieved by the fact that in the well-known navigation radar, consisting of a transmitter, a transmitting antenna and two identical signal processing channels at the frequencies of the second 2ω 0 and third 3ω 0 harmonics of the Earth, each of which contains a receiving antenna and receiver connected in series, as well as display devices, additionally, a reference generator, a frequency synthesizer and a compensation unit for trajectory instabilities are introduced, designed to generate a corresponding mismatch correction signal based on the measured average speed of movement and random deviations of the nonlinear radar carrier from the given trajectory, and in each of the channels there is a phase shift device, the first and second phase detectors, first and second analog-to-digital converters, a first reference function calculator designed to form the sine component of the reference function, a second reference function calculator designed to form the cosine component of the reference function, a digital processing system designed to form a radar image of objects with nonlinear electrical properties , while the output of the reference oscillator is connected to the input of the frequency synthesizer and to the second inputs of the receivers of the first and second channels, the first output of the frequency synthesizer is connected to the input of the transmitter, the output of which is connected to the input of the transmitting antenna, the second output of the frequency synthesizer is connected in each channel to the second input of the first phase detector and the input of the phase shift device, the output of the phase shift device of each channel is connected to the second input of the second phase detector of the corresponding channel, the output of the receiver of each channel is connected to the first inputs of the first and second phase detectors of the corresponding channel, the outputs of which are connected respectively to the inputs of the first and second analog - digital converters of the corresponding channels, the outputs of which in each channel are connected, respectively, to the first and second inputs of the digital processing system of the corresponding channel, the inputs of the first and second reference function calculators of each channel are connected to the output of the trajectory instability compensation unit, the outputs of the first and second reference function calculators of each channels are connected respectively to the third and fourth inputs of the digital processing system of the corresponding channel, the outputs of the digital processing systems of the first and second channels are connected respectively to the first and second inputs of the display device, and the trajectory instability compensation unit contains a clock pulse generator, a scaling device, a device for determining the direction of movement along axes of a rectangular coordinate system based on the measured average speed of movement and random deviations of the nonlinear radar carrier from a given trajectory, a timer, a storage device, a key block consisting of three keys, a subtraction device, a summation block consisting of three summation devices, a storage device block consisting of three storage devices, a scaling unit consisting of three scaling devices, a code multiplying unit consisting of three code multipliers, an adder and a code converter, while a clock pulse generator and a device for determining the direction of movement along the axes of a rectangular coordinate system are connected in series, an adder, a converter codes, the scaling device and the storage device are connected in series, in addition, the first, second and third outputs of the device for determining the direction of movement along the axes of the rectangular coordinate system are connected to the first inputs of the corresponding keys of the key block, the second inputs of which are connected to the output of the timer, the first output of the direction determining device movement along the axes of the rectangular coordinate system is also connected to the second input of the subtraction device, the outputs of the first, second and third keys of the key block are connected to the first inputs of the corresponding summation devices of the summation block, the outputs of which are connected to the inputs of the corresponding storage devices of the storage device block, the outputs of which are connected to the second inputs of the corresponding summing devices of the adding block and with the inputs of the corresponding scaling devices of the scaling block, the output of each scaling device of the scaling block is connected to the first and second inputs of the corresponding code multipliers of the code multiplying block, the outputs of the first, second and third code multipliers of the code multiplying block are connected to the corresponding inputs of the adder , the output of the storage device is connected to the first input of the subtraction device, and the output of the subtraction device, the second and third outputs of the device for determining the direction of movement along the axes of the rectangular coordinate system, and the output of the code converter are, respectively, the first, second, third and fourth outputs of the trajectory instability compensation unit.

The essence of the invention is to measure the average speed of movement and random deviations of the nonlinear radar carrier from a given trajectory along the abscissa, ordinate, and applicate axes and implement in each of the echo signal processing channels of the nonlinear radar a known algorithm for synthesizing the antenna aperture taking into account the measurement results, which allows achieving angular resolution close to potential.

The block diagram of the proposed nonlinear radar with a synthesized aperture antenna is shown in Fig. 1.

The proposed nonlinear radar with a synthesized aperture antenna consists of a transmitter 5, a transmitting antenna 1, receiving antennas of the first and second channels 2 and 4, receivers of the first and second channels 7 and 8, an indication device 26, a reference oscillator 3, a frequency synthesizer 6, a trajectory compensation unit instabilities 19, phase shift devices of the first and second channels 9 and 10, first and second phase detectors of the first channel 11 and 12, first and second phase detectors of the second channel 13 and 14, first and second analog-to-digital converters of the first channel 15 and 16, first and second analog-to-digital converters of the second channel 17 and 18, first and second reference function calculators of the first channel 20 and 21, first and second reference function calculators of the second channel 22 and 23, digital processing systems of the first and second channels 24 and 25, connected as shown in figure 1.

Transmitter 5 generates a probing signal at frequency ω 0 with specified parameters (power, type of modulation, etc.). Transmitting antenna 1 is designed to emit a probing signal at frequency ω 0 . The receiving antennas of the first and second channels 2 and 4 are used to receive echo signals from the OENS at frequencies 2ω 0 and 3ω 0, respectively. Receivers of the first and second channels 7 and 8 transfer signals received at frequencies 2ω 0 and 3ω 0 to the intermediate frequency ω pr and amplify them. The reference oscillator 3 produces a signal of a stable frequency ω og. Frequency synthesizer 6 generates carrier signals ω 0 and intermediate ω pr frequencies at its first and second outputs, respectively. Phase shifters of the first and second channels 9 and 10 shift the phase of the reference signal in each channel by π/2. The first phase detectors of the first and second channels 11 and 13 select sine components of the signals in the corresponding channels, and the second phase detectors of the first and second channels 12 and 14 - cosine ones. The first and second analog-to-digital converters of each channel 15, 16, 17 and 18 are designed to convert analog signals to digital ones. The trajectory instabilities compensation unit 19 monitors random deviations of the radar carrier from a given trajectory and generates a corresponding mismatch signal to correct the reference function. The first reference function calculators of the first and second channels 20 and 22 form the sine components of the support functions, the second reference function calculators of the first and second channels 21 and 23 - the cosine components of the support functions of the corresponding channels, taking into account the mismatch signals coming from the instability trajectories compensation unit 19. Digital systems processing of the first and second channels 24 and 25 are used to generate radar images of the OENS based on signals received at frequencies 2ω 0 and 3ω 0 . The display device 26 is necessary to display radar images with the required brightness, dynamic range and scale.

The claimed nonlinear radar with a synthetic aperture antenna operates as follows. During the antenna aperture synthesis time interval T s, rectilinear motion of the nonlinear radar carrier is ensured at a constant speed (the most important case for practice). To ensure coherence, the signal of the reference oscillator 3 at frequency ω og is fed to the second inputs of the receivers of the first and second channels 7 and 8, which are the inputs of the external reference oscillator, as well as to the input of the frequency synthesizer 6, which generates the carrier signals ω 0 and intermediate ω r frequencies. Based on the signal at frequency ω 0 coming from the first output of frequency synthesizer 6 to the input of transmitter 5, an ES with the required parameters at frequency ω 0 is formed. The signal thus generated is fed to the input of the transmitting antenna 1 and radiated into a given area of space. The signal at the intermediate frequency ω from the second output of the frequency synthesizer 6 is supplied to the second inputs of the first phase detectors of the first and second channels 11 and 13, as well as to the inputs of the phase shift devices of the first and second channels 9 and 10. In addition, the signal at the intermediate frequency ω pr also comes from the output of the receiver of each channel to the first input of the first phase detector of the corresponding channel. The output signal of the phase shift device of each channel 9 and 10 is fed to the second input of the second phase detector of the corresponding channel 12 and 14. Since the reference signals at the intermediate frequency ω pr at the second inputs of the first and second phase detectors of each channel 11 and 12, 13 and 14 have phase shift π/2, at the outputs of the first phase detectors of each channel 11 and 13, sine components of the signals coming from the receivers of the first and second channels 7 and 8 are formed, and at the outputs of the second phase detectors 12 and 14 - cosine components. The generated quadrature components are converted into digital form using the first and second analog-to-digital converters of each channel 15, 17 and 16, 18 and are fed, respectively, to the first and second inputs of the digital processing system of the corresponding channel 24 and 25. Mismatch signal generated by the trajectory instabilities compensation unit 19, is supplied in each channel to the inputs of the first and second reference function calculators 20, 22 and 21, 23. The first and second reference function calculators of each channel 20, 22 and 21, 23 generate, respectively, the sine and cosine components of the reference function, which are supplied respectively to the third and fourth inputs of the digital processing system of the corresponding channel 24 and 25. In the digital processing systems of the first and second channels 24 and 25, a well-known algorithm for synthesizing the antenna aperture is implemented and, as a result, radar images of the OENS are formed from signals received at frequencies 2ω 0 and 3ω 0, respectively. The radar images formed in this way are supplied from the outputs of the digital processing systems of the first and second channels 24 and 25 to the corresponding inputs of the display device 26, with the help of which the radar images are visually displayed.

The compensation unit for trajectory instabilities can be made, for example, in the form of a device, the block diagram of which is shown in Fig. 2.

The trajectory instability compensation unit includes a clock pulse generator 1, a scaling device 2, a device for determining the direction of movement along the axes of a rectangular coordinate system 3, a timer 4, a storage device 5, a key block 6, a subtraction device 7, a summation block 8, a storage device block 9, a block scaling 10, code multiplying unit 11, adder 12, code converter 13, connected as shown in Fig.2.

Clock pulse generator 1 is designed to generate a sequence of pulses of a given duration τ and with a period T i. Timer 4 serves to maintain the key block 6 in the open state for a given time interval T t . The device for determining the direction of movement along the axes of the rectangular coordinate system 3 generates at the first, second and third outputs signals corresponding to the movement of the radar carrier during time T and along the abscissa axes Δx i, ordinate Δy i and applicate Δz i, respectively, where Key Block 6 ensures the passage of signals from the first, second and third inputs of the device for determining the direction of movement along the axes of the rectangular coordinate system 3 to the output of the corresponding key of the key block 6. The summation block 8 is used to sum the signals available at the first and second inputs of each summation device of the summation block 8. Memory block 9 is necessary to store the result of the sum obtained in the summation block 8. The scaling block 10 averages the results of the summation of signals and generates signals at the first, second and third outputs corresponding to the average values of the movements of the radar carrier along the abscissa ordinate and applicate axes. The code multiplication block 11 is intended for constructing square of values and Adder 12 is used to implement a mathematical operation

Code converter 13 performs a mathematical operation of calculating the average speed of movement of the radar carrier

Scaling device 2 is necessary to calculate the reference value of the movement of the radar carrier along the x-axis The memory device 5 stores the obtained value Δx 0 . In the subtraction device 7, a mathematical operation is carried out to subtract the value of the current movement of the radar carrier along the abscissa axis of the rectangular coordinate system Δx i from the reference value Δx 0 .

The trajectory instability compensation unit operates as follows. First, the average speed of the navigation radar carrier is measured.

The speed measurement mode is activated manually by turning on timer 4, at the end of which it is automatically turned off, i.e. the duration of the value measurement mode is determined by the time T t . In the mode of measuring the average speed, clock pulses of duration τ and with a period T and, generated by the clock pulse generator 1, are supplied to the input of the device for determining the direction of movement along the axes of the rectangular coordinate system 3, which, when the radar carrier moves, generates a value at its first, second and third outputs movements along the abscissa axes Δx i, ordinate axes Δу i and applicate Δz i, respectively. During the time T t, the signal from the output of the timer 4 maintains the key block 6 in the open state, as a result of which the signals from the first, second and third outputs of the device for determining the direction of movement along the axes of the rectangular coordinate system 3, arriving at the first inputs of the corresponding keys of the key block 6, are supplied to the first inputs of the corresponding summing devices of the summing block 8. The summing block 8, together with the block of storage devices 9, sums up the digital codes of movements along the abscissa, ordinate and applicate axes, which are then received from the outputs of the first, second and third storage devices of the second block of storage devices 9, respectively to the inputs of the corresponding scaling devices of the scaling block 10, in which the received signals are multiplied by a digital value code and, as a result, the average values of displacements for the time interval T and along the abscissa axes of the ordinate and applicate are obtained. The signals thus obtained are then sent to the code multiplication block 11 and the adder 12 in order to obtain the sum of squares of the indicated signals which enters the code converter 13, where, in accordance with (1), it is converted into an average speed value. The resulting value is supplied to the input of the scaling device 2, where by multiplying it by the value T, the reference value of the movement of the radar carrier along the abscissa is formed The signal Δx 0 from the output of the scaling device 2 is supplied to the input of the memory device 5, where it is remembered and stored until the next determination of the average speed. Upon completion of the measurement, when the nonlinear radar with a synthesized aperture antenna is operating, the signal Δx 0 from the output of the memory device 5 is supplied to the first input of the device subtraction 7, the second input of which receives a signal from the first output of the device for determining the direction of movement along the axes of the rectangular coordinate system 3. In the subtraction device 7, mathematical operations are carried out to generate signals proportional to the deviation of the movement parameters of the radar carrier along the abscissa axis of the rectangular coordinate system from the specified parameters of the reference trajectory δx i =Δx 0 -Δx i .

The potential improvement K in the angular resolution of a navigation radar by synthesizing the antenna aperture has been theoretically studied according to Eq.

where Δl p and Δl are, respectively, the angular resolutions of the navigation radar without and with the use of an antenna aperture synthesis algorithm; λ GS - wavelength of GS; R - distance between navigation radar and OENS; d is the size of the actual receiving antenna; - harmonic number of the AP; - speed of the navigation radar carrier; θ n - observation angle of the OENS. Calculations carried out for the case of using the method of synthesizing the antenna aperture in the nonlinear locator "Lux" with the dimensions of real receiving antennas d = 0.25 m for the lateral viewing mode of space (θ n = π/2), as well as with T s =2 s, R=3 m, λ GS =0.3 m, indicate an improvement in the angular resolution at the second and third harmonics of the GS by 32 and 48 times, respectively.

The effectiveness of the operation of the trajectory instabilities compensation unit can be assessed using the evaluation of the OENS radar image distortions in the absence of trajectory instabilities compensation for the case of rectilinear uniform motion of the carrier along the x coordinate for fixed coordinates y=y 0, z=z 0. For these purposes, we will calculate the impulse responses of a nonlinear radar with a synthetic aperture antenna (RAS OENS) for the cases of the absence and presence of random deviations of the radar carrier from a given trajectory

where U(t+τ) is the trajectory signal; T s - time interval of the SA antenna; τ - time shift; h(t) - support function.

As a reference h(t), a weighted function is selected that is complexly conjugate to the signal reflected from the nonlinear target

where H(t) is the real weight function; - change in the current distance between the navigation radar and the OENS.

Assuming that in the case of compensation for trajectory instabilities δx 1 =0, and in the case of its absence - and given, for example, the values H(t)=1, T s =2 s, R=3 m, λ ZS =0.3 m, n=2, x=1 m, x 0 =0 m, we obtain, in accordance with (3), the impulse responses J 1 (r) and presented after normalization by the corresponding graphical dependencies 1 and 2 in Fig.3. As calculations show, the width of the main lobe of the impulse response is 1.15 times greater than J 1 (τ). This means that the compensation unit for trajectory instabilities, made in the form of a device, the block diagram of which is shown in Fig. 2, under given conditions, makes it possible to improve the resolution of a nonlinear radar with a synthesized antenna aperture along the angular coordinate by 15%.

Thus, in the proposed nonlinear radar with a synthetic aperture antenna, the angular resolution is increased due to the formation of a large antenna aperture on a given trajectory of the radar carrier, and a compensation unit for trajectory instabilities, made in the form of a device, the block diagram of which is shown in Fig. 2, provides potentially achievable angular resolution (its potential improvement in accordance with expression (2)) by reducing radar image distortion caused by the expansion of the main lobe of the impulse response (3).

The proposed technical solution is new, since a nonlinear radar with a synthetic aperture antenna is unknown from publicly available information, which differs from the known navigation radar, consisting of a transmitter, a transmitting antenna and two identical signal processing channels at the frequencies of the second 2ω 0 and third 3ω 0 harmonics of the Earth, each of which contains a receiving antenna and receiver connected in series, as well as display devices, in that a reference oscillator, a frequency synthesizer and a compensation unit for trajectory instabilities are additionally introduced, designed to generate an appropriate mismatch correction signal based on the measured average speed of movement and random deviations of the nonlinear radar carrier from the specified one trajectory, and in each of the channels - a phase shift device, the first and second phase detectors, the first and second analog-to-digital converters, the first reference function calculator designed to form the sine component of the reference function, the second reference function calculator designed to form the cosine component of the reference function functions, digital processing system, while the output of the reference oscillator is connected to the input of the frequency synthesizer and to the second inputs of the receivers of the first and second channels, the first output of the frequency synthesizer is connected to the input of the transmitter, the output of which is connected to the input of the transmitting antenna, the second output of the frequency synthesizer is connected in each channel to the second input of the first phase detector and the input of the phase shift device, the output of the phase shift device of each channel is connected to the second input of the second phase detector of the corresponding channel, the output of the receiver of each channel is connected to the first inputs of the first and second phase detectors of the corresponding channel, the outputs of which are connected respectively to inputs of the first and second analog-to-digital converters of the corresponding channels, the outputs of which in each of the channels are connected, respectively, to the first and second inputs of the digital processing system of the corresponding channel, the inputs of the first and second reference function calculators of each channel are connected to the output of the trajectory instability compensation unit, the outputs of the first and the second calculator of the reference function of each channel is connected, respectively, to the third and fourth inputs of the digital processing system of the corresponding channel, the outputs of the digital processing systems of the first and second channels are connected, respectively, to the first and second inputs of the display device, and the compensation unit for trajectory instabilities contains a clock pulse generator, a scaling device, a device for determining the direction of movement along the axes of a rectangular coordinate system based on the measured average speed of movement and random deviations of the nonlinear radar carrier from a given trajectory, a timer, a storage device, a key block consisting of three keys, a subtraction device, a summation block consisting of three summation devices, a storage unit consisting of three storage devices, a scaling unit consisting of three scaling devices, a code multiplying unit consisting of three code multipliers, an adder and a code converter, wherein the clock pulse generator and the device for determining the direction of movement along the axes of the rectangular coordinate system are connected in series, the adder, code converter, scaling device and storage device are connected in series, in addition, the first, second and third outputs of the device for determining the direction of movement along the axes of the rectangular coordinate system are connected to the first inputs of the corresponding keys of the key block, the second inputs of which are connected to the output of the timer, the first output of the device for determining the direction of movement along the axes of the rectangular coordinate system is also connected to the second input of the subtraction device, the outputs of the first, second and third keys of the key block are connected to the first inputs of the corresponding summation devices of the summation block, the outputs of which are connected to the inputs of the corresponding storage devices of the storage device block, the outputs of which are connected to the second inputs of the corresponding summing devices of the summing block and to the inputs of the corresponding scaling devices of the scaling block, the output of each scaling device of the scaling block is connected to the first and second inputs of the corresponding code multipliers of the code multiplying block, the outputs of the first, second and third code multipliers of the code multiplying block are connected to the corresponding inputs of the adder, the output of the storage device is connected to the first input of the subtraction device, and the output of the subtraction device, the second and third outputs of the device for determining the direction of movement along the axes of the rectangular coordinate system, the output of the code converter are, respectively, the first, second, third and fourth outputs of the compensation block trajectory instabilities.

The proposed technical solution has an inventive step, since it does not clearly follow from published scientific data and known technical solutions that a nonlinear radar with a synthetic aperture antenna allows one to achieve an angular resolution close to the potential one.

The proposed technical solution is industrially applicable, since standard radio engineering components and devices used in SAR, as well as microwave equipment and materials of widespread technology, can be used for its implementation.

The compensation block for trajectory instabilities can be made using standard pulse and digital devices.

Thus, a device for determining the direction of movement along the axes of a rectangular coordinate system can be made, for example, on the basis of an optical manipulator of the “mouse” type, provided that the coordinate y=y 0 =h 0 is fixed, where h 0 is the height of the flat surface for moving the optical manipulator of the type “mouse” above floor level in a room where a nonlinear radar with a synthetic aperture antenna is used. The clock generator can be built as a transistor blocking oscillator or as an integrated circuit blocking oscillator. To implement a block of keys, transistor switches can be chosen. The timer is executed as a single-cycle. The basis of the storage device and storage unit can be semiconductor random access or read only memory devices. The adder and summing unit can be constructed using a parallel adder circuit. The scaling unit, scaling device and code converter can be made according to a known code converter circuit. The subtraction device is supposed to be built on the basis of adders that perform subtraction. The code multiplication unit is based on known devices for code multiplication.

Information sources

1. Antipov V.N., Goryainov V.T., Kulin A.N. and others. Radar stations with digital synthesis of the antenna aperture. / Ed. V.T.Goryainova. - M.: Radio and communication, 1988.

2. Kondratenkov G.S., Frolov A.Yu. Radiovision. Radar systems for remote sensing of the Earth. - M.: Radio engineering, 2005.

3. Nonlinear locator "Lux". Technical description and operating instructions. - M.: Novokom, 2005.

4. Gorbachev A.A., Koldanov A.P., Lartsov S.V., Tarakankov S.P., Chigin E.P. Signs of recognition of nonlinear scatterers of electromagnetic waves // Nonlinear radar. Digest of articles. Part 1. / Sub. Ed. Gorbacheva A.A., Koldanova A.P., Potapova A.A., Chigina E.P. - M.: Radio engineering, 2005. - P.15-23.

5. Semenov D.V., Tkachev D.V. Nonlinear radar: NR concept // Special technology. / Research Institute of Special Equipment of the Ministry of Internal Affairs of Russia, 1999, No. 1-2. - P.17-22.

6. Kondratenkov G.S., Potekhin V.A., Reutov A.P., Feoktistov Yu.A. Earth survey radar stations. / Ed. G.S. Kondratenkova. - M.: Radio and communication, 1983.

7. Goldenberg L.M. Pulse and digital devices: Textbook for communication institutes. - M.: Communication, 1973.

8. Lebedev O.N., Sidorov A.M. Pulse and digital devices: Digital nodes and their design on microcircuits. - L.: VAS, 1980.

9. Handbook on radar. / Ed. M. Skolnik, New York, 1970: Trans. from English (in four volumes). / Under the general editorship. K.N. Trofimova; Volume 2. Radar antenna devices. - M.: Sov. radio, 1979.

10. Dulin V.N. Electronic and quantum microwave devices: A textbook for students of higher technical educational institutions. 2nd edition, revised. - M.: Energy, 1972.

11. From the point of view of optical mice...//URL:http://www.iXBT.com.

12. Simonovich S.V. and others. The Big Book of a Personal Computer. - M.: OLMA Media Group, 2007.

13. Brammer Yu.A. Pulse and digital devices: Textbook. for students of electrical and radio instrument-making environments. specialist. textbook establishments. / Yu.A. Brammer, I.N. Pashchuk. - 6th ed., revised. and additional - M.: Higher School, 2002.

A nonlinear radar station (radar) with a synthetic aperture antenna, consisting of a transmitter, a transmitting antenna and two identical signal processing channels at the frequencies of the second 2ω 0 and third 3ω 0 harmonics of the sounding signal (SA), each of which contains a series-connected receiving antenna and receiver, as well as an indication device, characterized in that a reference oscillator, a frequency synthesizer and a trajectory instability compensation unit are additionally introduced, designed to generate an appropriate mismatch correction signal based on the measured average speed of movement and random deviations of the nonlinear radar carrier from a given trajectory, and into each of the channels - a phase shift device, first and second phase detectors, first and second analog-to-digital converters, a first reference function calculator designed to form the sine component of the reference function, a second reference function calculator designed to form the cosine component of the reference function, a digital processing system designed to form a radar image of an object with nonlinear electrical properties, while the output of the reference generator is connected to the input of the frequency synthesizer and to the second inputs of the receivers of the first and second channels, the first output of the frequency synthesizer is connected to the input of the transmitter, the output of which is connected to the input of the transmitting antenna, the second output of the synthesizer frequencies are connected in each channel to the second input of the first phase detector and the input of the phase shift device, the output of the phase shift device of each channel is connected to the second input of the second phase detector of the corresponding channel, the receiver output of each channel is connected to the first inputs of the first and second phase detectors of the corresponding channel, outputs which are connected, respectively, to the inputs of the first and second analog-to-digital converters of the corresponding channels, the outputs of which in each of the channels are connected, respectively, to the first and second inputs of the digital processing system of the corresponding channel, the inputs of the first and second reference function calculators of each channel are connected to the output of the trajectory instabilities compensation unit , the outputs of the first and second reference function calculators of each channel are connected, respectively, to the third and fourth inputs of the digital processing system of the corresponding channel, the outputs of the digital processing systems of the first and second channels are connected, respectively, to the first and second inputs of the display device, and the trajectory instability compensation unit contains a clock pulse generator , scaling device, device for determining the direction of movement along the axes of a rectangular coordinate system based on measurements of the average speed of movement and random deviations of the nonlinear radar carrier from a given trajectory, timer, storage device, key block consisting of three keys, subtraction device, summation block consisting of three summing devices, a storage unit consisting of three storage devices, a scaling unit consisting of three scaling devices, a code multiplying unit consisting of three code multipliers, an adder and a code converter, with a clock pulse generator and a device for determining the direction of movement along the axes rectangular coordinate system are connected in series, the adder, code converter, scaling device and storage device are connected in series, in addition, the first, second and third outputs of the device for determining the direction of movement along the axes of the rectangular coordinate system are connected to the first inputs of the corresponding keys of the key block, the second inputs of which are connected with a timer output, the first output of the device for determining the direction of movement along the axes of the rectangular coordinate system is also connected to the second input of the subtraction device, the outputs of the first, second and third keys of the key block are connected to the first inputs of the corresponding summation devices of the summation block, the outputs of which are connected to the inputs of the corresponding storage devices a block of storage devices, the outputs of which are connected to the second inputs of the corresponding summing devices of the summing block and to the inputs of the corresponding scaling devices of the scaling block, the output of each scaling device of the scaling block is connected to the first and second inputs of the corresponding code multipliers of the code multiplication block, the outputs of the first, second and third multipliers codes of the code multiplication block are connected to the corresponding inputs of the adder, the output of the storage device is connected to the first input of the subtraction device, and the output of the subtraction device, the second and third outputs of the device for determining the direction of movement along the axes of the rectangular coordinate system, the output of the code converter are respectively the first, second, third and the fourth outputs of the trajectory instability compensation block.

Synthetic Aperture Radar (SAR)- this is a method that allows you to obtain radar images of the earth's surface and objects located on it, regardless of meteorological conditions and the level of natural illumination of the area, with detail comparable to aerial photographs.

Features of obtaining a radar image

The simplest way to obtain a radar image (RL) of an area is to use the real beam mode, when a radar installed on a carrier aircraft surveys the earth's surface by scanning the antenna in a horizontal plane, for example, in a sector of ±90° relative to the velocity vector carrier. In this case, the image of the terrain in the viewing area is observed in the form of a sector measuring ±90° with a maximum radius equal to the radar range. The main disadvantage of this mode is the low azimuth resolution, which during incoherent processing is determined by the width of the radiation pattern (RP) of the real antenna in the horizontal plane. DN width $(\Theta)_(az)$ depends on the horizontal size of the antenna $d$ (aperture) and wavelength of electromagnetic oscillations emitted by the radar: $(\Theta)_(az)=\lambda / d$ . At the same time, the linear azimuth resolution increases in proportion to the slant range. For example, at wavelength $\lambda=3$ cm and antenna size 150 cm beam width $(\Theta)_(az)=1.15$ ° and at a range of 120 km the linear resolution will be about 2.5 km. Such a low resolution leads to the fact that only marks from large objects (bridges, settlements, ships) are observed in the image.

Obtaining high azimuth resolution requires the use of an antenna with a large aperture size. Placing large antennas on an aircraft is impossible, therefore, to ensure an azimuth resolution significantly better than that determined by the beam width of a real antenna, coherent operating modes are used, which make it possible to form a synthesized aperture of a larger (1000 or more times) size.

The essence of SAR

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Literature

Radar systems of multifunctional aircraft. T.1. Radar is the information basis for combat operations of multifunctional aircraft. Systems and algorithms for primary processing of radar signals / Ed. A. I. Kanashchenkova and V. I. Merkulova. - M.: Radio engineering, 2006. - 656 p. - ISBN 5-88070-094-1.
Kondratenkov, G. S. Earth survey radars / G. S. Kondratenkov, V. S. Potekhin [etc.]. - M.: Radio and communication, 1983. - 272 p.
Antipov, V. N. Radar stations with digital synthesis of the antenna aperture / V. N. Antipov, V. T. Goryainov [etc.]. - M.: Radio and Communications, 1988. - 304 p. - ISBN 5-256-00019-5.
Dudnik, P. I. Multifunctional radar systems: textbook. manual for universities / P. I. Dudnik, A. R. Ilchuk [etc.]. - M.: Bustard, 2007. - 283 p. - ISBN 978-5-358-00196-1.
- 2010

Bakhrakh L.D. Methods for measuring parameters of radiating systems in the near zone / Bakhrakh L.D. - Leningrad: Nauka, 1985. - 272 p.

Safronov G.S. Introduction to radio holography. - M.: Sov. radio, 1973. - 288 p.

An excerpt characterizing Radar Aperture Synthesis

Napoleon spent the entire day of August 25, as his historians say, on horseback, inspecting the area, discussing the plans presented to him by his marshals, and personally giving orders to his generals.
The original line of Russian troops along Kolocha was broken, and part of this line, namely the Russian left flank, was driven back as a result of the capture of the Shevardinsky redoubt on the 24th. This part of the line was not fortified, no longer protected by the river, and in front of it there was only a more open and level place. It was obvious to every military and non-military person that the French were supposed to attack this part of the line. It seemed that this did not require many considerations, there was no need for such care and troubles of the emperor and his marshals, and there was no need at all for that special highest ability called genius, which they so like to attribute to Napoleon; but the historians who subsequently described this event, and the people then surrounding Napoleon, and he himself, thought differently.
Napoleon drove across the field, thoughtfully peered at the area, shook his head with himself in approval or disbelief, and, without informing the generals around him of the thoughtful move that guided his decisions, conveyed to them only final conclusions in the form of orders. After listening to Davout's proposal, called the Duke of Ecmul, to bypass the Russian left flank, Napoleon said that this did not need to be done, without explaining why it was not necessary. To the proposal of General Compan (who was supposed to attack the flushes) to lead his division through the forest, Napoleon expressed his consent, despite the fact that the so-called Duke of Elchingen, that is, Ney, allowed himself to note that movement through the forest was dangerous and could upset the division .
Having examined the area opposite the Shevardinsky redoubt, Napoleon thought for a while in silence and pointed to the places where two batteries were to be set up by tomorrow to operate against the Russian fortifications, and the places where field artillery was to be lined up next to them.
Having given these and other orders, he returned to his headquarters, and the disposition of the battle was written under his dictation.
This disposition, about which French historians speak with delight and other historians with deep respect, was as follows:
“At dawn, two new batteries, built in the night, on the plain occupied by the Prince of Eckmuhl, will open fire on the two opposing enemy batteries.
At the same time, the chief of artillery of the 1st Corps, General Pernetti, with 30 guns of the Compan division and all the howitzers of the Dessay and Friant divisions, will move forward, open fire and bombard the enemy battery with grenades, against which they will act!
24 guards artillery guns,
30 guns of the Compan division
and 8 guns of the Friant and Dessay divisions,
Total - 62 guns.
The chief of artillery of the 3rd Corps, General Fouche, will place all the howitzers of the 3rd and 8th Corps, 16 in total, on the flanks of the battery, which is assigned to bombard the left fortification, which will total 40 guns against it.
General Sorbier must be ready, at the first order, to march with all the howitzers of the Guards artillery against one or another fortification.
Continuing the cannonade, Prince Poniatowski will head towards the village, into the forest and bypass the enemy position.
General Compan will move through the forest to take possession of the first fortification.
Upon entering the battle in this way, orders will be given according to the actions of the enemy.
The cannonade on the left flank will begin as soon as the cannonade of the right wing is heard. The riflemen of Moran's division and the Viceroy's division would open heavy fire when they saw the beginning of the attack of the right wing.
The Viceroy will take possession of the village [of Borodin] and cross his three bridges, following at the same height with the divisions of Morand and Gerard, which, under his leadership, will head to the redoubt and enter the line with the rest of the army.

Aperture synthesis is a technical technique that allows you to significantly increase the resolution of a radar in a direction transverse to the direction of flight and obtain a detailed image of a radar map of the area over which the aircraft is flying. The mode of generating such a map is called mapping and is used, for example, in survey and comparative navigation systems, to obtain maps of the area, and in other situations. In terms of quality and detail, such maps are comparable to aerial photographs, but unlike the latter, they can be obtained in the absence of optical visibility of the earth's surface (during flight, above clouds). The detail of a radar image depends on the linear resolution of the radar. In the radial direction relative to the radar, the linear resolution, i.e., the range resolution dR, is determined by the sounding signal, and in the transverse direction (tangential resolution) dl - by the width of the radar bottom and the distance to the target (Figure 2.1). The smaller the dR and dl, the higher the detail of the radar image of the area.

Figure 2.1 Parameters characterizing the detail of the radar image

Figure 2.2 Side-scan radar patterns

The problem of reducing HR is solved by using probing signals with short pulse durations or by switching to complex signals (frequency modulated or phase-shift keyed). However, reducing dl is not so easy to achieve. since dl is proportional to the range R to the target and the width of the bottom, and in the horizontal plane, where l is the wavelength, and ba is the longitudinal size (length). The main ways to increase tangential resolution are the use of radars along the fuselage antennas and the synthesis of the antenna aperture when the aircraft is moving.

The first path led to the development of so-called side-scan radars (Figure 2.2). In such radars, the larger the longitudinal dimension df of the aircraft fuselage, the higher the tangential resolution. Since lf is larger than the fuselage diameter df, on which the antenna size da usually depends, the image detail in radars with along-fuselage antennas improves, although the dependence on range remains.

The second, more radical path leads to RSA during the forward motion of the aircraft.

The principle of aperture synthesis. Let a linear phased array of size (aperture) L (Figure 2.3, a) consist of N+1 emitters. By summing up the signals received by the irradiators, it is possible to obtain a phased array diagram with a width of . If it is required to provide a given target, then it is possible to synthesize a phased array by sequentially moving one emitter along this aperture at a certain speed V, receiving signals reflected from the target, storing them, and then processing them together (Figure 3.6). In this case, an aperture of a linear antenna with an effective size L and a beam pattern width cs = l/L is synthesized; however, the time required for synthesis tc = L/V increases and the radar equipment becomes more complicated.

Figure 2.3 Phased array antenna (a) and aperture synthesis circuit when moving the feed (b)

Let the aircraft move at a certain height with a constant speed V rectilinearly and parallel to the earth's surface (Figure 2.4).

Figure 2.4 The relative position of the target and the aircraft during aperture synthesis.

Antenna having a bottom width tsa and rotated 90° to the track line, sequentially passes a series of positions i = --N/2; ...; --2; --1; 0; +1; +2; . . . +N/2, in which it receives signals reflected from a target located at point M on the earth’s surface. At different antenna positions (at different i), signals from the same point travel different distances, which leads to a change in the phase shifts of these signals caused by the signal path difference?R. Because the signal goes through R twice; in the direction of the target and away from it, then two signals received at adjacent antenna positions differ in phase by:

Depending on whether the phase shifts Dc on segments DRi are compensated or not when summing the signals, focused and unfocused SAR are distinguished. In the first case, processing comes down to moving antennas, storing signals, compensating for phase shifts and summing signals (see Figure 2.3, b), and in the second - to the same operations, but without compensating for phase shifts.

Structural diagram of SAR. The basis of SAR is coherent-pulse radars, built according to a scheme with internal coherence (Figure 2.5). A coherent CG generator at a frequency fp.ch serves to generate a probing signal with a frequency fо+fp.ch in a single-sideband modulator. The source of oscillations with frequency fо is the frequency distribution. The probing signal is modulated by a pulse sequence from the modulator M. The PA power amplifier is the final stage of the transmitter. Signal processing (memorization, phase compensation, summation) is usually performed at low frequency. Therefore, the circuit provides quadrature channels, each of which begins with a corresponding phase detector. The reference voltage source for phase detectors is a coherent local oscillator KG. The signals from the quadrature channels (which store phase information) are fed either to an analogue US recording device or to a real-time processing device of the USS.

Figure 2.5 Block diagram of a synthetic aperture radar

Principles of signal processing in SAR. For any type of processing, frame-by-frame storage of information about targets is necessary. The frame dimensions are set in azimuth by the effective value of the synthesized aperture LEf and the viewing range Rmin. . . Rmax (Figure 2.6, a). Since the signals received at each antenna position arrive at the receiver input from the viewing distance sequentially in time, they are recorded sequentially in each of the N+1 azimuth channels, which is conventionally shown by arrows in Figure 2.6, b. In this case, an image frame with dimensions xk and Rx is formed corresponding to the terrain area. It is possible to obtain information about the angular position of the target, i.e., about the x coordinate, when synthesizing the aperture only by analyzing the signals reflected from this target, recorded during the synthesis interval LEph. Therefore, information from the recording device is read sequentially in each of the n range channels (Figure 2.6, c).

Figure 2.6 Memorized terrain frame (a), diagrams of recording (b) and reading (c) signals

INFORMATION PROCESSING AND CONTROL X

UDC 621.396.96

DIRECTIONS OF DEVELOPMENT OF SPACE-BASED SYNTHESIS APERTURE RADAR

O. L. Polonchik,

Ph.D. tech. Sciences, Associate Professor

Northern (Arctic) Federal University named after. M. V. Lomonosova, Arkhangelsk

The main directions of development of space-based radar systems for monitoring the earth's surface are analyzed. The subject area of using radar technical means has been defined, including for solving applied problems of economic development in the northern and Arctic regions of Russia. A comparative assessment of existing methods for viewing the earth's surface has been carried out. A new method for constructing on-board radar systems based on rotation-stabilized spacecraft is proposed. Ways to improve the technical characteristics of an airborne radar are considered.

Key words - side-view radar, radiation pattern, mechanical scanning, aperture synthesis.

Introduction

Modern airborne radar equipment represents one of the most rapidly developing areas of radio-electronic technology. A special place among them is occupied by airborne synthetic aperture radars. These technical means perform sounding of the earth's surface at any time of the day, season and year, do not depend on climatic conditions and the presence of clouds, which is especially important for areas with a small number of sunny days a year. In the Russian Federation, these include vast areas in the north of the country and in the Arctic, constituting almost a third of the territory of our state, very rich in a variety of minerals, oil and gas.

The solution of the most important national economic problems, such as high-precision assessment of the terrain, the formation of three-dimensional images of the earth's surface, and the study of dynamic processes on the earth's and sea surfaces, is entrusted to promising means of remote sensing of the Earth.

Particularly relevant for solving the problems of sustainable development of the northern and Arctic regions is the acquisition of radar survey materials with high measuring properties, ensuring the creation and updating of state topographic maps,

plans and cartographic basis of the state real estate cadastre.

Obtaining information about the condition of these areas is a task of exceptional importance and will help minimize material losses.

History of the development of radar remote sensing of the Earth

The development of airborne radar stations (radars) led to the creation of all-round radar systems, the main disadvantage of which was low resolution. Further research to improve the earth's surface survey radar was aimed at overcoming the main limitation in increasing the resolution associated with the size of the antenna devices.

The detail of the radar image depends on the linear resolution (range resolution) of the radar, which in the radial direction is determined by the sounding signal, in the transverse direction (tangential resolution) - by the width of the radiation pattern (DP) and the distance to the target.

The problem of increasing range resolution is solved by using sounding signals with short pulse durations.

Aircraft

pulses or transition to complex signals - frequency-modulated or phase-shift keyed.

An increase in tangential resolution is achieved by using an antenna in the on-board radar located along the fuselage of the aircraft, or by synthesizing the antenna aperture while the aircraft is moving.

The first path led to the development of side-scan radars. The implementation diagram of the method is shown in Fig. 1. In such radars, the larger the longitudinal size of the aircraft fuselage, the higher the tangential resolution, although the dependence on range remains.

The resolution of this type of radar was increased by approximately 10 times compared to panoramic all-round radars. And yet, in terms of their capabilities, these stations are still significantly inferior to optical devices.

The second, more radical way is to create synthetic aperture radars (SAR) during the forward motion of the aircraft.

A huge contribution to the development of the theory of SAR was made by famous domestic scientists A. P. Reutov, G. S. Kondratenkov, P. I. Dudnik, Yu. L. Feoktistov, N. I. Burenin, Yu. A. Melnik, V. A. Potekhin et al.

Synthetic aperture radars

The essence of the method is the emission of a radar installed on a mobile carrier (aircraft, spacecraft (SC) or unmanned aerial vehicle), coherent sounding signals, reception of the corresponding reflected signals along the rectilinear flight path of the carrier, their storage and addition. As a result of adding the accepted

signals, the antenna beam is compressed and the resolution of the radar along the carrier path line is significantly increased.

Depending on whether phase shifts are compensated or not when summing signals, focused and unfocused SARs are distinguished. In the first case, processing comes down to moving the antenna, storing signals, compensating for phase shifts and summing signals, in the second - to the same operations, but without compensating for phase shifts.

The potential resolution of such stations approaches the characteristics of optical surveillance equipment. These radars make it possible to realize high linear resolution, independent of the observation range and wavelength of the probing signal.

Currently, there are three main modes of surveying the earth's surface (Fig. 2): route, survey and searchlight (detailed).

Modern systems make it possible to obtain images of the earth's surface and objects located on it with resolutions of about 1 m for survey modes and 0.3 m for spotlight modes. The applied methods of digital processing of the received signal have a significant impact on the resulting SAR characteristics.

In route mode, the earth's surface is photographed continuously in the acquisition zone. The signal is accumulated over a period of time equal to the calculated interval for synthesizing the antenna aperture for the given flight conditions of the radar carrier.

The overview shooting mode differs from the route shooting mode in that shooting is continuously carried out over the entire width of the swath in stripes equal to the width of the capture swath. Six beams are sequentially switched by elevation to view the entire swath (Figure 3).

The lateral and anterolateral modes are divided depending on the orientation of the main lobe

Searchlight

Antenna pattern. The signal is accumulated over a period of time equal to the calculated interval for synthesizing the antenna aperture for the given flight conditions of the radar carrier.

When shooting in spotlight mode, signal accumulation occurs at an increased interval compared to overview mode. Expansion of the interval is achieved by moving the main lobe of the antenna pattern, and the irradiated area is constantly located in the shooting area. This movement is synchronized with the movement of the radar carrier.

To keep the pattern spot on the same surface area, four beams are sequentially switched in azimuth (Fig. 4).

Thus, an analysis of the main modes of surveying the earth’s surface using the SAR method shows that:

1) with the side view method, the maximum width of the strip of the underlying surface being viewed is similar to the viewing width;

2) an increase in linear resolution in the spotlight mode is achieved by increasing the aperture, while the viewed band narrows;

3) an increase in linear resolution in survey mode is carried out by using a set of highly targeted patterns.

The minimum linear azimuth resolution 8хш1п for antennas with an unfocused artificial aperture is determined by the relation

The linear azimuth resolution of a radar with a focused artificial aperture is determined by the expression

5х - ©Р0 - ^,

where ya is the size of the antenna opening in a given plane.

A radar with a focused artificial aperture makes it possible, in contrast to an unfocused one, to obtain a linear resolution in azimuth, independent of the range and wavelength of the probing signal. The resolution of such radars increases as the size of the actual antenna decreases. This is a significant advantage of SAR compared to other methods of sensing the earth's surface.

Side-scanning radars. Basic relationships

Determining the location of the target during a side view is carried out in the coordinate system: track range x, slant range R.

When viewed from the side, the antenna pattern is perpendicular to the carrier ground speed vector. Determination of the position of targets on the ground is carried out in a rectangular coordinate system xY. The viewing area is a strip parallel to the flight path of the carriers (Fig. 5, a). The bandwidth is determined by the range of the radar.

It is possible to orient the antenna pattern at an angle to the ground speed vector different from l/2.

■ Fig. 4. Spotlight mode

■ Fig. 5. Diagram of a side view in a rectangular (a) and oblique (b) coordinate system

At the same time, the field of view narrows, targets can be detected proactively (Fig. 5, b). In this case, the terrain is surveyed in an oblique coordinate system.

It is known that the resolution of a radar for viewing the earth's surface over a horizontal range directly under the carrier deteriorates in comparison with the limit determined by the duration of the probing pulse. Therefore, the carrier's flight altitude is usually taken as the closest boundary of the swath, where the range resolution deteriorates insignificantly.

The method is described by the following characteristics:

Irradiation time;

Radar detection range;

Resolution.

Irradiation time

Ttyo _ Ш ’

where © is the angular width of the radar antenna pattern in the horizontal plane; W - projection of speed along the direction of the path.

A characteristic feature of the side-view method is the one-time irradiation of targets. When the observation direction is perpendicular to the ground speed vector, the image is formed only abeam the flight path.

The second feature is an increase in the target irradiation time in proportion to the range. This leads to the fact that the energy of signals reflected from targets increases with increasing target range.

Let us determine the radar detection range for the case of lateral scanning.

It is known that the detection range of a target (terrain background) D0 with an effective reflective surface st when using one transceiver antenna has the form

64l k0kGots

where E is the target irradiation energy; b - antenna directivity coefficient; X is the wavelength of the radar transmitter; £ш - noise figure of the receiving device; £ - Boltzmann constant; T0 - absolute temperature (usually 280 K); "L = Es tt/^sh is the required value of the discernibility coefficient of the radar receiving device. Here Es t1n is the threshold value of the energy of the received reflected signal, characterizing the sensitivity of the radar receiving device; Ysh is the spectral noise density at the receiver input: Ysh = £sh £ T0.

The irradiation energy of a target (terrain element) is determined by the relation

V - £Pe^tayo>

where Рср is the average power of the emitted signal.

Taking into account the relationship for the target irradiation energy, we obtain a formula for the range in the side view method

Rpa©0С2stХ2

64l 1Ak0k7O"p

Analysis of the expression shows that it is possible to increase the operating range of the considered method compared to all-round visibility.

All-round radar with synthetic aperture based on a spacecraft with rotation stabilization. Basic relationships

To implement this method of viewing the earth's surface, a spacecraft with rotation stabilization and a radar with a parabolic antenna are needed. The antenna pattern has an inclination angle relative to the local vertical.

The radar antenna, due to the circular rotation of the spacecraft body to which it is rigidly attached, scans the underlying earth's surface. The projection of the antenna pattern in the azimuthal and elevation planes onto the earth's surface is shown in Fig. 6 and 7.

The energy of the radar in the method is better compared to SAR, since a narrower beam pattern of the biased antenna is used. It is determined by choosing the minimum and maximum elevation angle of the antenna pattern.

Let's consider the position of the radar antenna at different times (Fig. 8). Antenna at

Antenna projection

■ Fig. 6. Type of projections of the radar antenna pattern onto the earth's surface in the azimuthal plane: Oa is the angular velocity of rotation of the spacecraft radar antenna in the azimuthal plane; Yatah - maximum distance to the target Ts^ V - speed of the spacecraft

■ Fig. 7. Viewing the swath of the spacecraft radar antenna

■ Fig. 8. Positions of the spacecraft radar antenna in the plane of rotation at different times, taking into account translational motion and rotation: I - the distance that the spacecraft flies during half a rotation period

rotation around the local vertical, taking into account the ground speed, sequentially occupies these positions (points 1, 2, 3, etc.). The radius of rotation of the antenna is insignificant (on the order of several meters). The spacecraft moves at the first escape velocity, and the antenna motion curve turns almost into a straight line in a time interval equal to half the rotation period.

At each point on this curve the electrical axis of the antenna will be perpendicular to it. It becomes possible to synthesize an artificial aperture.

The location is defined in a polar coordinate system. The range R and azimuth ß are measured. The flight altitude H and elevation angle y are determined. The target azimuth is measured from the direction of movement (see Fig. 6).

Radar surveillance is carried out in a certain area of space, which is called the working area, or radar viewing area. The dimensions of the working area are determined by the viewing intervals in terms of range Rmax - Rmin, azimuth "max - amin, elevation angle ßmax - ßmin and radial speed Vr max - Vr min. The length of each specified interval is determined by the number of radar resolution elements it contains along the corresponding coordinate.

Information about the presence of targets in various elements of the work area resolution is obtained during the review (viewing) of these elements. The order and time of viewing various elements, as well as the intensity of the signals emitted by the radar when viewing each element, are determined by the method (program) used for viewing the working area.

The review of elements of the work area can be carried out sequentially in time or simultaneously.

With a sequential review, the required rate of obtaining information about the presence and coordinates of targets in the viewing area cannot always be ensured. This is due to the fact that the target irradiation time T must exceed the maximum signal delay time tmax:

T> "^check 2^check / s

where Yatah is the maximum range of the radar; c is the speed of light.

The time for a single review of the entire zone T0 must satisfy the condition

T0 - T^a, p > (2^Shax / c)^a, p,

where Na p is the number of directional resolution elements.

In all-round viewing with synthetic aperture, a certain ratio must be met

T - 2l/Oa.

The number of pulses reflected by the target during this time will be

where is the pulse repetition rate in the burst.

The period of review of the working area determines the rate of receipt of information about the presence of a target in the area and cannot exceed a certain permissible value T0 max. If this value is given, then

Oa - 2l / ^Oshakh.

This ratio determines the minimum angular velocity of rotation of the radar antenna pattern during all-round viewing with synthetic aperture.

By selecting the rotation speed, one can view the earth's surface without gaps.

Main characteristics of the circular viewing method with synthetic aperture:

Target irradiation time;

Review period and number of review cycles per goal.

Comparison of the circular viewing method with aperture synthesis with other methods allows us to draw the following conclusions.

1. Scanning the receiving antenna pattern ensures viewing of the entire underlying earth

surface without gaps. In this case, the angular resolution of the resulting image will be comparable to the resolution of SAR in searchlight mode.

2. The irradiation time is practically independent of the target range.

3. Viewing of the underlying earth's surface during one rotation period occurs twice and depends on the angular velocity, which determines the number of cycles.

4. The energy of radar is significantly higher compared to the SAR method, since a narrower pattern is used. The target (terrain element) is located in the observation direction perpendicular to the angular velocity vector.

5. By choosing the antenna tilt angle, a horizontal view of the earth's surface is excluded

1. Sollogub A.V. et al. Assessing the effectiveness of a cluster of small spacecraft for remote sensing of the Earth in terms of efficiency and reliability of performing functional tasks // Information and control systems. 2012. No. 5(60). pp. 24-28.

2. Verba V. S., Neronsky L. B., Osipov I. G., Turuk V. E. Space-based land survey radar systems / ed. V. S. Verba. - M.: Radio engineering, 2010. - 680 p.

3. Vinogradov M. Capabilities of modern radars with antenna aperture synthesis // Foreign Military Review. 2009. No. 2. P. 52-56.

range directly under the carrier, where the resolution of the radar is extremely low.

Conclusion

This paper examines the main directions of development of space-based radar systems for monitoring the earth's surface and the history of the creation of these means. Existing methods are analyzed and a comparative assessment of the main technical characteristics is performed. A method for synthesizing an aperture is proposed based on the circular movement of the receiving antenna using a rotation-stabilized spacecraft. Ways have been identified to improve the technical characteristics of an onboard radar to solve applied problems.

4. Sesin A. E., Shepeta D. A. Mathematical model of echo signals of the sea surface observed by on-board locators of aircraft // Information and control systems. 2010. No. 2. P. 21-25.

5. Zakharova L.N. et al. Joint analysis of data from optical and radar sensors: capabilities, limitations and prospects // Radio engineering and electronics. 2011. T. 56. No. 1. P. 5-19.