EPR has the dimensions of the area, but is not a geometric area, but is an energy characteristic, that is, it determines the magnitude of the power of the received signal.

The RCS of the target does not depend on the intensity of the emitted wave, nor on the distance between the station and the target. Any increase in ρ 1 leads to a proportional increase in ρ 2 and their ratio in the formula does not change. When changing the distance between the radar and the target, the ratio ρ 2 / ρ 1 changes inversely proportional to R and the EPR value remains unchanged.

EPR of common point targets

For most point targets, information about the EPR can be found in radar manuals.

convex surface

The field from the entire surface S is determined by the integral It is necessary to determine E 2 and the ratio at a given distance to the target ...

where k is the wave number.

1) If the object is small, then the distance and field of the incident wave can be considered unchanged. 2) The distance R can be considered as the sum of the distance to the target and the distance within the target:

,
,
,
,

flat plate

A flat surface is a special case of a curvilinear convex surface.

Corner reflector

The principle of operation of the corner reflector

Corner reflector consists of three perpendicular surfaces. Unlike a plate, a corner reflector gives good reflection over a wide range of angles.

Triangular

If a corner reflector with triangular faces is used, then the EPR

Application of corner reflectors

Corner reflectors are applied

as decoys
like radio contrast landmarks
when conducting experiments with strong directional radiation

chaff

Chaffs are used to create passive interference with the operation of the radar.

The value of the RCS of a dipole reflector generally depends on the observation angle, however, the RCS for all angles:

Chaffs are used to mask air targets and terrain, as well as passive radar beacons.

The reflection sector of the chaff is ~70°

EPR of complex targets

RCS of complex real objects are measured at special installations, or ranges, where the conditions of the far irradiation zone are achievable.

#	Target Type	σ c
1	Aviation
1.1	Fighter aircraft	3-12
1.2	stealth fighter	0,3-0,4
1.3	frontline bomber	7-10
1.4	Heavy bomber	13-20
1.4.1	B-52 bomber	100
1.4	Transport aircraft	40-70
2	ships
2.1	Submarine on the surface	30-150
2.2	Cutting a submarine on the surface	1-2
2.3	small craft	50-200
2.4	medium ships	²
2.5	big ships	> 10²
2.6	Cruiser	~12 000 14 000
3	Ground targets
3.1	Automobile	3-10
3.2	Tank T-90	29
4	Ammunition
4.1	ALSM cruise missile	0,07-0,8
4.2	The warhead of an operational-tactical missile	0,15-1,6
4.3	ballistic missile warhead	0,03-0,05
5	Other purposes
5.1	Human	0,8-1
6	Birds
6.1	Rook	0,0048
6.2	mute swan	0,0228
6.3	Cormorant	0,0092
6.4	red kite	0,0248
6.5	Mallard	0,0214
6.6	Grey goose	0,0225
6.7	Hoodie	0,0047
6.8	field sparrow	0,0008
6.9	common starling	0,0023
6.10	black-headed gull	0,0052
6.11	White stork	0,0287
6.12	Lapwing	0,0054
6.13	Turkey vulture	0,025
6.14	rock dove	0,01
6.15	house sparrow	0,0008

The simplest objects are those whose RCS can be calculated analytically rather simply. These include a flat sheet, a cylinder, a ball, a corner and a biconical reflector, a half-wave vibrator, a portion of a diffusely scattering surface, as well as some group and distributed targets. The determination of the RCS of such objects may be of independent interest, and it may also be necessary to calculate the RCS of objects of complex configuration, which can be represented by a set of the simplest objects.

To find the RCS of a section S of a well-conductive convex surface (Fig. 8.2), we use formula (8.4), in which the ratio can be obtained by summing the elementary fields created at the location of the radar by reflected signals from surface elements. If the distance from the radar antenna to the element under consideration is equal to D and the irradiation occurs at an angle to the normal with the field strength, then the field strength , at the location of the radar

where is the distance from the radar to the nearest surface point. Then

because the .

Substituting the value into formula (8.4), we find the expression for the EPR of the surface:

Let us use the resulting expression to calculate the effective scattering area of some simple objects.

EPR of a flat well-conducting plate. If a metal sheet, whose dimensions a and b are much larger, but much smaller than D, is located perpendicular to the direction of irradiation (Fig. 8.3), then expression (8.6) takes the form

because and due to the small size of the sheet compared to the range D and its location perpendicular to the direction of arrival of radio waves.

Thus, under normal irradiation, a perfectly conductive sheet mirrors all the incident energy in the direction of the radar, which provides a large RCS compared to the sheet area. At cm, the area of a sheet when irradiated along the normal is several times greater than the RCS of a large aircraft.

However, even with a slight deviation of the direction of irradiation from the normal, the RCS of a flat sheet drops sharply. Let us assume that the direction of irradiation deviates from the normal in the horizontal plane by an angle . Considering the sheet as a flat in-phase antenna with a radiation pattern described by the function, the expression for the EPR can be written as

The dependence of the RCS on the irradiation angle is called the target scatterplot.

A flat sheet has a scatterplot described by a function of the form .

For large ratios of the sheet size to the wavelength (in the case considered), the scattering diagram will be very sharp, i.e., with an increase in a, the EPR value of the sheet changes sharply in accordance with the function, decreasing in some directions to zero.

For a number of applications, it is desirable to maintain a high RCS value over a wide range of irradiation angles. This is necessary, for example, when using reflectors as passive radio beacons. This property has a corner reflector.

EPR corner reflector. The corner reflector consists of three mutually perpendicular metal sheets, it has the property of reflecting radio waves towards the irradiating radar, which is explained by the triple reflection from the walls of the reflector (Fig. 8.4), which the wave experiences if the direction of irradiation is near the axis of symmetry (within the solid angle) corner reflector. From fig. 8.4 it can be seen that a triple reflection occurs if the incident beam passes within the hexagon inscribed in the outer contour of the reflector. Consequently, the RCS of a corner reflector is approximately equal to the RCS of a flat sheet in the form of such a hexagon, irradiated along the normal. Substituting the expression for the area of the hexagon in (8.7), we obtain the formula for calculating the RCS of a corner reflector:

(8.9)

At and see RCS of a corner reflector. Thus, the RCS of a corner reflector is somewhat smaller than the RCS of a flat plate with dimensions . However, the corner reflector retains a large value of the RCS in a fairly wide sector, while the RCS of the plate decreases sharply at slight deviations of the irradiation direction from the normal. It must be emphasized that the achievement of the theoretical value is possible only with high precision of its manufacture, especially when operating at waves shorter than 3 cm. To expand the active sector, corner reflectors are used, consisting of four corners.

As passive radar beacons at sea, biconical reflectors (Fig. 8.5), made up of two identical metal cones, are also used.

Rice. 8.4 Fig. 8.5

If the angle between the generatrices of the cones is equal to , then the beam, after double reflection from the surface of the cones, is directed towards the PLC, which ensures a large RCS value. The advantage of a biconical reflector is a uniform scattering pattern in a plane perpendicular to its axis.

EPR of the ball. To determine the RCS of a large (compared to) ball with a perfectly conducting smooth surface, formula (8.6) can be used. However, in this case, this is not necessary, since such a ball meets the requirements for a hypothetical target, the cross-sectional area of \u200b\u200bwhich is its RCS. Thus, the RCS of a ball, which also has a smooth ideally conducting surface, is equal to its cross-sectional area, regardless of the wavelength and direction of irradiation:

Due to this property, a large ball with a well-conducting surface is used as a reference for experimental measurement of the RCS of real objects by comparing the intensity of the reflected signals.

When the ratio of the radius of the ball to the wavelength decreases to the values of the function (Fig. 8.6), a number of resonant maxima and minima appear, i.e., the ball begins to behave like a vibrator. With a ball diameter close to , the EPR of the ball is four times its cross-sectional area. For a small ball with EPR, it is determined by the Rayleigh diffraction formula and is characterized by a strong dependence on the wavelength of the irradiating radio waves.

This case occurs, for example, when radio waves are reflected from raindrops and fog.

Taking into account the value of the dielectric constant of water () EPR of raindrops

where is the droplet diameter.

It is customary to distinguish between specular, diffuse and resonant reflections. If the linear dimensions of the reflecting surface are much larger than the wavelength, and the surface itself is smooth, then a specular reflection occurs. In this case, the angle of incidence of the radio beam is equal to the angle of reflection, and the secondary radiation wave does not return to the radar (except in the case of normal incidence).

If the linear dimensions of the surface of the object are large compared to the wavelength, and the surface itself is rough, then diffuse reflection takes place. In this case, due to the different orientation of the surface elements, electromagnetic waves are scattered in different directions, including in the direction of the radar. Resonant reflection is observed when the linear dimensions of the reflecting objects or their elements are equal to an odd number of half-waves. Unlike diffuse reflection, secondary resonant radiation usually has a high intensity and a pronounced directionality, depending on the design and orientation of the reflecting element.

In cases where the wavelength is large compared to the linear dimensions of the target, the incident wave goes around the target and the intensity of the reflected wave is negligible.

From the point of view of signal formation upon reflection, objects of radar observation are usually divided into small-sized and distributed in space or on the surface.

Small-sized objects include objects whose dimensions are much smaller than the dimensions of the radar resolution element in terms of range and angular coordinates. In some cases, small-sized objects have the simplest geometric configuration. Their reflective properties can be easily determined theoretically and predicted for each specific relative location of the target in question and the radar. In real conditions, goals of the simplest type are quite rare. More often you have to deal with objects of complex configuration, which consist of a number of rigidly interconnected simple reflective elements. Aircraft, ships, various structures, etc. can serve as examples of targets of complex configuration.

Other targets are a collection of individual objects distributed in a certain area of space, much larger than the resolution element of the radar. Depending on the nature of this distribution, volume-distributed (for example, a rain cloud) and surface-distributed (land surface, etc.) targets are distinguished. The signal reflected from such a target is the result of the interference of reflector signals distributed within the resolution bin.

For a fixed relative position of the radar and reflecting objects, the amplitude and phase of the reflected wave have a well-defined value. Therefore, in principle, the resulting total reflected signal can be determined for each specific case. However, during radar surveillance, the relative positions of the targets and the radar usually change, resulting in random fluctuations in the intensity and phase of the resulting echoes.

Effective target scattering area (ESR).

The calculation of the range of radar observation requires a quantitative characteristic of the intensity of the reflected wave. The power of the reflected signal at the input of the station receiver depends on a number of factors and, above all, on the reflecting properties of the target. Typically, radar targets are characterized by an effective scattering area. Under the effective scattering area of the target in the case when the radar antenna radiates and receives electromagnetic waves of the same polarization, it is understood the value σt, which satisfies the equation σtP1=4πK2P2, where P1 is the power flux density of the direct wave of this polarization at the target location; P2 is the power flux density of a wave of a given polarization reflected from the target at the radar antenna; R is the distance from the radar to the target. The RCS value can be directly calculated by the formula

σcP1=4πR2P2/ P1

As follows from the formula above, σц has the dimension of area. Therefore, it can be conditionally considered as a certain area equivalent to the target, normal to the radio beam, with area σц, which, isotropically scattering all the wave power incident on it from the radar, creates at the receiving point the same power flux density P2 as the real target.

If the RCS of the target is given, then with known values of P1 and R, it is possible to calculate the power flux density of the reflected wave P, and then, having determined the power of the received signal, estimate the range of the radar station.

The effective scattering area σc does not depend on the intensity of the emitted wave, nor on the distance between the station and the target. Indeed, any increase in P1 leads to a proportional increase in P2, and their ratio in the formula does not change. When changing the distance between the radar and the target, the ratio P2/P1 changes inversely proportional to R2 and the value of σc remains unchanged.

Complex and group goals

Consideration of the simplest reflectors does not cause difficulties. Most real radar targets are a complex combination of different types of reflectors. In the process of radar observation of such targets, one deals with a signal that is the result of the interference of several signals reflected from individual elements of the target.

When a complex object is irradiated (for example, an aircraft, a ship, a tank, etc.), the nature of the reflections from its individual elements strongly depends on their orientation. In some positions, certain parts of the aircraft or ship may produce very intense signals, and in other positions, the intensity of the reflected signals may drop to zero. In addition, when the position of the object relative to the radar changes, the phase relationships between the signals reflected from various elements change. This results in fluctuations in the resulting signal.

Other reasons for changes in the intensity of the reflected signals are also possible. Thus, there may be a change in conductivity between the individual elements of the aircraft, one of the causes of which are vibrations caused by the operation of the engine. When the conductivity changes, the distributions of the currents induced on the aircraft surface and the intensity of the reflected signals change. For propeller and turboprop aircraft, an additional source of change in the intensity of reflections is the rotation of the propeller.

Fig 2.1. Dependence of the RCS of the target on the angle.

In the process of radar observation, the mutual position of the aircraft (ship) and the radar is constantly changing. The result of this is the fluctuations of the reflected signals and the corresponding changes in the EPR. The laws of probability distribution of the effective scattering area of the target and the nature of changes in this value over time are usually determined experimentally. To do this, the intensity of the reflected signals is recorded and, after processing the record, the statistical characteristics of the signals and EPR are found.

As many studies have shown, the exponential distribution law is valid with sufficient accuracy for the fluctuation σc of aircraft

The invention relates to methods and techniques for measuring the scattering characteristics of radar targets, in particular to measuring the effective scattering area (ESR) of ground objects by side-scan aircraft radars with a synthetic aperture antenna (SAR). The technical result of the invention is to reduce the error in measuring the RCS of ground objects. A method for measuring the effective scattering area of ground-based SAR objects based on the absolute amplitude calibration of the SAR path includes the use of an external (ground) calibration system (SVC) in the form of sets of reference corner reflectors (CR) located on a homogeneous area of the earth's surface, aerial photography using SAR of this area of the earth surface at given values of the height and course of the flight of the carrier, obtaining radar images (RI) of a section of the earth's surface with reference CR, measuring the image parameters of each reference reflector on the received radar image, processing the measurement results and estimating the calibration parameters of the SAR end-to-end path and EPR of ground objects. 6 ill.

Drawings to the RF patent 2308050

Technical field

The level of technology.

At present, a large number of Earth remote sensing (ERS) complexes, which include SAR, have been created in the world. Thematic processing of sounding results obtained using SAR is effective only if they obtain data on the absolute value of the specific effective scattering surface (SES) 0 of the objects under study. Obtaining the specified data using aviation and space SAR is possible only when performing absolute calibration of the end-to-end SAR path and the radar images obtained by them.

SAR calibration is understood as the solution of the problem of adequate description of the mathematical model (TM) of the transfer function (TF) of the end-to-end SAR path based on the use of ground-based reference tools (artificial active repeaters, passive reflectors or surface-distributed objects of natural origin) for estimating the parameters of the MM and taking into account the results of the assessment MM PF during the formation of radar images.

In the MM PF of the end-to-end path during SAR calibration, they include: a signal propagation path, an antenna system, a receiving-transmitting channel, a data recording system, a radar image reconstruction processor from a radio hologram (synthesis), as well as a method for measuring the parameters of ground-based reference facilities and objects of observation on radar images in the interests of assessment of their RCS.

Known approaches to solving the problem of measuring the RCS of ground objects using calibrated SAR (see D.M. Bychkov, A.S. Gavrilenko, E.M. Ganapolsky, et al. "Combined calibration of side-scan radars with real and synthetic aperture". Advances in modern radio electronics, 2005, No. 6; Belokurov A. A., Glybovsky S. I. "Methods and tools for calibrating radar systems for remote observation of the earth's surface." Foreign radio electronics, 1990, No. 2) show that the problem of absolute calibration of aviation and space SAR has not been solved completely, and the methods used for its solution, reference tools and estimation algorithms have a number of drawbacks that limit the achievable values of the error in the calibration and estimation of the RCS of objects.

One of the shortcomings of these approaches is that they do not fully take into account the features of the formation of radar images in SAR. When performing the calibration procedure in these works, the relationship between the power of the signal at the input of the SAR receiver reflected from the object under study and its RCS () is used in the form

where P pr - signal power at the input of the SAR receiver;

P izl - the average power of the emitted signal;

G() - directivity pattern of the physical SAR antenna in terms of power in the vertical plane with the width of the pattern in elevation 0 ;

Wavelength of the emitted SAR signal;

R n - slant range to the object under study;

To ppo - the transmission coefficient of the path for receiving, converting and processing SAR;

Effective scattering area of the object under study.

The equation is valid for circular (sector) coverage radars, in which the exposure time of an object is practically independent of the distance to the object (it is determined by the ratio of the antenna beamwidth in azimuth to the angular scanning velocity of the antenna). Calibration of the end-to-end path of the SAR based on this equation leads to incomplete consideration of the dependence of its PF on the slant range R n to the calibrated (estimated) object and additional errors in the estimates of the RCS of the measured objects.

In side-scan radars (including SAR), the exposure time of an object increases in proportion to the slant range to it, while the transfer function of the path for receiving, converting and processing SAR (see G.S. Kondratenkov, V.A. Potekhin, A.P. Reutov, Yu.A. Feoktistov "Side-scan radar stations". Soviet radio, 1985) and the relation equation between the signal power at the input of the SAR receiver and the EPR of the object is converted to the form

0 - beamwidth of the physical SAR antenna in azimuth,

P is the flight speed of the SAR carrier.

In this equation, the signal power at the input of the SAR receiver is inversely proportional not to the fourth, but to the third power of the slant range to the object.

When passive reflectors are used for SAR calibration, these works do not explicitly take into account the dependence of their reflection indicatrices on the viewing angles in azimuth and elevation, assuming the const value in the ranges of working calibration angles. Experimental measurements of the reflection indicatrices of a large group of corner reflectors with trihedral and square faces (see Sazonov N.I. et al. "System of SAR Ground Calibration", LII named after M.M. Gromov, Operation Manual, 2005) technology, has shown that their reflection indicatrices have a significant (up to 1.5...2 dB) spread from sample to sample in the range of operating angles of ±15° from the maximum. To reduce the influence of the specified spread of the RCS values of passive reference reflectors on the SAR calibration error, it is necessary to take into account the actual dependences of their reflection indicatrices on the viewing angles = ( , ) in each calibration session in the calibration methodology. In this case, the main sections of the reflection indicatrices of the CR should be measured in bench conditions (preferably in anechoic chambers) with an error of no more than 0.5 ... 1.0 dB.

It is important to note that the RCS of a passive reflector installed on the ground can differ significantly from the value measured on a stand in an anechoic chamber due to the influence of the interference factor due to the influence of reflections from the earth's surface in the range of operating SAR viewing angles in elevation. The methods proposed in the above works for minimizing these reflections based on covering the corresponding areas of the earth's surface in the vicinity of the reflectors with radio-absorbing material are expensive and time-consuming.

It is known that the output signal of the SAR significantly depends on the trajectory instabilities of the carrier flight, and the signal processing methods used in modern SAR do not provide full compensation for their influence. Modern SAR calibration methods do not take into account changes in the amplitude of the RI envelope of reference reflectors due to incomplete compensation of the influence of these instabilities, which leads to additional errors in estimating the amplitude of the RI of reference reflectors and the corresponding component of the calibration error.

When solving the problem of calibrating digital SAR in estimating the amplitude of the reflector radar image envelope, which is used as a reference parameter in the procedure for amplitude calibration of the SAR path, the discrete structure of the radar image is not taken into account, which leads to an unaccounted calibration error of up to 1.5 dB.

The closest to the proposed method for measuring the effective scattering area of ground objects by a synthetic aperture radar is the technical solution described in the article by Belokurov A.A., Glybovsky S.I. "Methods and tools for calibrating radar systems for remote observation of the earth's surface". Foreign radio electronics, 1990, No. 2, which is taken as a prototype.

The present invention is aimed at achieving a technical result, which consists in reducing the error in measuring the RSA of ground-based SAR objects based on the absolute calibration of the SAR end-to-end path when using a set of passive corner reflectors (CR) placed in a special way on the earth's surface as an absolute calibration system by clarifying the MM PF end-to-end path of the SAR, as well as procedures for identifying the parameters of the MM and the calibration system.

The task is achieved by the fact that in the method for measuring the effective scattering area of ground objects by a synthetic aperture radar (SAR) based on the absolute amplitude calibration of the SAR path, including the use of an external (ground) calibration system (SVC) in the form of sets of reference CRs located on a homogeneous area of the earth's surface, aerial photography using SAR of this area of the earth's surface at given values of the height and course of the flight of the carrier, obtaining a radar image of a section of the earth's surface with reference CR, measuring the image parameters of each reference reflector on the received radar image, processing the measurement results and estimating the calibration parameters of the SAR end-to-end path and EPR of ground objects, as a set of reference reflectors, two lines of passive trihedral CRs are used, while the first line with the same calculated values of the EPR of reflectors is placed with a uniform step along the slant range (across the carrier flight direction) within the SAR swath, and the second, with different calculated values of the EPR, are placed along the line passing through the average UO of the first line orthogonally to it (in azimuth).

The actual values of the RCS of each reference CR included in the SCS are determined by preliminary measurement in an anechoic chamber of the main sections of the reflection indicatrices of the CR along the azimuth ind ( and reference values of the RCS of each i-th reflector in each calibration session in accordance with the formula

The maximum amplitudes of the reflectors on the radar image are determined from the maximum amplitudes of the envelopes of the RO images, reconstructed by two-dimensional interpolation of square sets of digital samples (pixels) in the vicinity of each reflector n × n y in size, using an interpolation algorithm based on a two-dimensional Fourier transform, modified to reduce the error interpolation. To do this, the maximum amplitude of the interpolated RI envelope A i max is measured, then, to reduce the effect of mismatches in the processing system, the measured amplitude A i max is brought to its value under test conditions, taking into account the property (volume constancy) of the SAR signal uncertainty function. according to expression

and its cross-sectional area in the presence of S i and the absence of mismatches S o is determined at the level of 0.5A i max by the values of the product of the width of the envelopes in two orthogonal sections (along the line of the actual path - X and orthogonal to it - Y).

To minimize the influence of the interference factor of the earth, the calibration coefficient K kal of the SAR end-to-end path is determined as the average value of the estimates of the calibration coefficients K kal (i), calculated for all calibration CRs in the range line

at the same time, estimates of the calibration coefficients K kal (i) for each CR are determined by the ratio of the amplitudes of the interpolated envelopes of the i-th CR to the corresponding reference values of their RCS iind ( , ), with normalization of these ratios to the gain value of the physical SAR antenna G( i - A) and slant range value according to the equation

The RCS values of point objects on an arbitrary (calibration and measuring) radar imager are determined by the equation

K kal - calibration coefficient of the SAR end-to-end path;

G( izm - A) is the relative gain of the SAR antenna at the viewing angle of the DO in the vertical plane izm and the SAR antenna installation angle in elevation A ;

The RCS values of spatially distributed objects on an arbitrary (calibration and measurement) radar image are determined by the equation

where is the average value of the pixel amplitude of the measuring radar image, measured over the field of a square fragment with a size of n f ×n f pixels, selected within a homogeneous area of the texture of a spatially distributed object,

Izm and R izmn are the values of the viewing angle and slant range corresponding to the center of a square fragment of a spatially distributed object;

S 0 is the area of the resolution element of the measuring radar image (taken equal to its value obtained during the calibration procedure).

The proposed method provides a reduction in the error in measuring the RCS of ground objects due to the absolute calibration of the SAR end-to-end path based on the use of a set of passive corner reflectors (CR) placed in a special way on the earth's surface, refinement of the MM PF SAR and can be used to significantly increase the efficiency of using SAR in aviation earth surface monitoring systems.

The invention is illustrated by drawings, in which:

Figure 1 shows a diagram of the installation on the earth's surface of the SVK from two linear sets of reference passive UO, placed orthogonally in a uniform area across and along the direction of flight of the carrier within the SAR swath (1 - line UO in range, 2 - line UO along the path ; 3 - carrier aircraft; 4 - SAR swath).

Figure 2 shows geometric relationships illustrating the procedure for calibrating the SAR in the side view mode when shooting a set of reference passive CRs in the horizontal plane (1 - CR ruler in range, 2 - CR ruler along the track; 3 - carrier aircraft; 4 - band SAR survey; LZP - the line of the given path; LFP - the line of the actual path; Drift - the drift angle of the carrier aircraft).

Figure 3 shows the geometric relationships illustrating the procedure for calibrating the SAR in the side view mode when shooting a set of reference passive UOs in the vertical plane (1 - line of UOs in range, 3 - carrier aircraft; 4 - SAR swath; 5 - measured UO) .

Figure 4 illustrates a characteristic view of the indicatrices of reflection of the UO in the horizontal and vertical planes and the results of approximation by polynomials of the 9th degree (9, 11 - graphs for measuring and approximating the main section of the indicatrix of the reflection of the UO in azimuth; 10, 12 - graphs for measuring and approximating the main section reflection indicatrix of the ER in elevation).

Figure 5 shows an experimental fragment of the original RI with reference CR (5 - a selected rectangular fragment of the original RI of the measured CR with a size of n × n pixels; 6 - numbers of the 1st column and 1st row of the selected rectangular fragment in the coordinate system of the measuring radar; 13 , 14 - envelopes of the main sections of the original radar image of the reflector in range and azimuth, respectively).

Figure 6 shows a fragment of the original radar image with reference CR after interpolation restoration of the envelope using a modified two-dimensional FFT procedure (5 - selected rectangular fragment of the original radar image of the measured CR with a size of n × n pixels, 6 - numbers of the 1st column and 1st row of the selected rectangular fragment in the system of coordinate RI; interpolated RI of the measured VR with a size of n × n pixels; 7 - interpolated fragment 5 of the RI of the measured VR with a size of n × n pixels; 15, 16 - envelopes of the main sections of the RI of the reflector in range and azimuth).

The proposed method is carried out as follows.

In the method for measuring the RCS of objects, including (figure 1, 2) the use of SVK from two linear sets of passive UO, placed orthogonally on a homogeneous area of the earth's surface along 1 and across 2 of the direction of flight of the carrier SAR 3, aerial photography of the earth's surface area with UO in the swath 4 of the calibrated SAR at given values of the range, altitude and course of flight of the carrier, obtaining a radar image of this area of the earth's surface, as well as a digital automated processing system in which the assessment of the parameters of the radar image of each UO of the external calibration system and the identification of the parameters of the MM PF of the calibrated SAR are performed in accordance with the following procedures.

1. Calibration procedure:

All the calibration procedures considered below use the dependence of the amplitude A max of the SAR output signal (RI amplitude) on the square root of the EPR of the measured objects of the form

where K kal is the transfer coefficient of the calibrated SAR;

G() - normalized radiation pattern of the physical SAR antenna in terms of power in the vertical plane;

R n - slant range to the object under study, which for measuring digital SAR is linear in a wide dynamic range of EPR change;

On the radar image obtained for calibration, rectangular fragments 6 with a size of n x ×n y pixels are sequentially isolated (Fig. 5) with the image of the UO in the center of the fragment 5 and the coordinates of the selected fragment Y f, X f in the radar coordinate system ("slant range (Y 0) - the line of the actual path (X 0) of the PCA carrier ");

Perform (figure 6) the procedure of two-dimensional interpolation in K times (K=2 n , n=1, 2, ...) for each selected fragment 6 of the image of the UO using the interpolation algorithm based on the two-dimensional Fourier transform, modified to reduce the error two-dimensional interpolation, and get the interpolated image 7 of the fragment 6;

Measure (figure 6) the parameters of the main sections of the envelope 15, 16 interpolated RLI 7 of each UO, the rectangular coordinates of the UO in the coordinate system of the selected fragment (dX, dY), the maximum amplitude A imax of the envelope, as well as the values of its width in two orthogonal sections (in direction coinciding with the line of the actual path - Х i and orthogonal to it - Y i) at the level of 0.5, which determine the area S i = Х i · Y i , the base of the parallelepiped, the volume of which is equal to the volume of the uncertainty function of the SAR signal of the corresponding CR;

The distortions of the maximum amplitude of the envelope A imax are corrected due to the influence of mismatches in the processing system according to the expression

where S i \u003d X i Y i is the area of the SAR resolution element, equal to the area of the base of the parallelepiped, the volume of which is equal to the volume of the uncertainty function of the SAR signal of the i-th UO (S 0 - in the absence of mismatches);

Estimating the actual angular parameters of the sighting of each UO in azimuth and elevation by the values of the coordinates of the UO on the radar image and the altitude of the flight of the SAR carrier using an algorithm that takes into account the special geometry of the placement of the reference reflectors of the SVK on the ground;

The reference values of the RCS of each CR included in the SVK are determined by preliminary measurement in an anechoic chamber of the main sections of the reflection indicatrices of the CR in azimuth 9 ind () and elevation 10 ind () in the ranges of working viewing angles of objects and (± 25 ° relative to the maximum), approximation of the measured values by orthogonal polynomials 11 and 12 , the degree n of which is selected from the condition of implementing an approximation error of not more than 0.5 dB (Fig.4) and calculating the reference values of the RCS of each i-th reflector in each calibration session in accordance with the formula

The values and determined by the differences in the angles of sighting visas, visas when shooting calibrated SAR orthogonal linear sets of reference UO and the angles of their orientation on the ground yo, yo in the coordinate system of the generated radar image (figure 3);

The calibration coefficient of the SAR end-to-end path to exclude (minimize) the influence of the interference factor of the earth is determined as the average value of the estimates of the calibration coefficients for all the EVs in the line by the range of the calibration radar image

at the same time, estimates of the calibration coefficients K kal (i) for each CR are determined by the ratio of the amplitudes of the interpolated envelopes of the i-th CR to the corresponding reference values of their RCS iind ( , ), with these ratios reduced to the maximum value of the SAR physical antenna gain G( i - A) and the value of the slant range according to the equation

A - SAR antenna installation angle in elevation.

2. The procedure for evaluating the RCS of point objects:

The effective scattering area of point ground objects on an arbitrary (calibration and measurement) radar image is determined by the equation

where is the amplitude of the interpolated envelope of the i-th CO in the measuring RI;

R izmn - slant range to a point object on the measuring radar;

G( izm - A) is the relative gain of the SAR antenna at the angle of sight of the DO in the vertical plane izm and the angle of installation of the SAR antenna in elevation A ;

the ratio of the gain coefficients of the SAR end-to-end path in terms of amplitude in the measurement and calibration modes

3. The procedure for evaluating the EPR of spatially distributed objects:

The effective scattering area of spatially distributed ground objects on an arbitrary (calibration and measurement) radar image is determined by the equation

where is the average value of the radar image pixel amplitude, measured over the field of a square fragment with a size of n f ×n f pixels, selected within a homogeneous area of the texture of a spatially distributed object,

K kal - coefficient of transfer (calibration) of SAR;

Izm and R izmn - viewing angle and slant range values corresponding to the fragment center,

S izm is the area of the resolution element of the measuring radar image (assumed to be equal to its estimate during the calibration procedure).

An example of the application of the proposed method

The proposed method for measuring the effective scattering area of ground-based SAR objects was tested at the Federal State Unitary Enterprise "LII named after M.M. Gromov" in the course of research and development work (R&D) on the development and creation of an "Aviation Complex (AC) for environmental monitoring and research natural resources of the earth".

When conducting research, the main methodological procedures were implemented to ensure that the result of calibration and measurements was obtained with an error corresponding to the potential capabilities of the proposed method.

In the course of the experiments, radar images of the ground-based SAR calibration system were obtained in the centimeter range. The results of measurements and processing are presented in table 1.

To determine the position of the SAR physical antenna rigidly fixed on the fuselage of the aircraft in terms of elevation, synchronous measurements of the angular positions of the antenna and the aircraft were carried out in the experiments.

The NSC was used in the experiments, which included a ruler composed of 9 RHs with calculated RCS values of 3000 m2, installed with a uniform step of 500 m, and 4 RHs installed orthogonally to the ruler's reflectors on a homogeneous underlying surface of the "meadow-summer" type.

For all CRs of the calibration system in the anechoic chamber, the main cross sections of their reflection indicatrices were measured at the operating wavelength of the SAR transmitter.

To test the operability and accuracy of the proposed method for measuring RCS, three radar image fragments were selected, including images of the VR of the calibration system.

For all selected radar images, a calibration procedure was performed by processing the images of the reference CRs in accordance with the previously described procedures.

Then, for each of the selected radar images, the procedure for measuring the RCS of the reference CRs for all three radar images was performed.

The results presented in Table 1 show that the procedure for measuring the calibration coefficient for any of the three fragments gives stable estimates, the maximum difference of which did not exceed 5%.

When measuring the RCS of reference CRs on radar image fragments using the calibration results of the current fragment, the average measurement error does not exceed 10%.

Estimates obtained when using any of these fragments for SAR path calibration and measuring the RCS of the CR on two other (measuring) radar image fragments showed that the average values of errors in the RCS estimates on the measuring radar images also did not exceed 10%.

Thus, the experimental data obtained confirmed the high efficiency of the proposed method for estimating the RCS of ground-based objects based on the solution of the problem of absolute calibration of the SAR path and the radar images generated by them using ground-based reference EPRs of a passive type with a significant reduction in the error of RCS estimates compared to known methods.

CLAIM

A method for measuring the effective scattering area of ground objects by a synthetic aperture radar (SAR) based on the absolute amplitude calibration of the SAR path, including an external calibration system (ECS) in the form of sets of reference corner reflectors (CR) placed on a homogeneous area of the earth's surface, aerial photography using SAR of this area of the earth's surface at given values of the height and course of the flight of the carrier, obtaining a radar image (RI) of the area of the earth's surface with reference CR, measuring the image parameters of each reference reflector on the received RI, processing the measurement results and evaluating the calibration parameters of the through path of the SAR and EPR of ground objects , characterized in that as a set of reference reflectors, two lines of passive trihedral UOs are used, while the first line with the same calculated values of the RCS of the reflectors is placed with a uniform step along the slant range (across the direction of flight of the carrier) within the SAR swath, and the second along line passing through the average SV of the first line orthogonally to it (along the azimuth), the actual values of the RCS of each reference SV are determined by preliminary measurement in the anechoic chamber of the main sections of the reflection indicatrices along the azimuth ind ( and ), the degree of which is chosen from the condition for the implementation of the approximation error not more than 0.5 dB and calculation of the reference values of the RCS of each i-th reflector in each calibration session in accordance with the formula

the maximum amplitudes of the reflectors on the radar image are determined by the maximum amplitudes of the envelopes of the original images of the RO, restored by two-dimensional interpolation of square sets of digital samples (pixels) in the vicinity of each reflector of size n x ×n y using an interpolation algorithm based on a two-dimensional Fourier transform, modified to reduce the interpolation error , then to reduce the influence of mismatches in the SAR processing system, the measured amplitude of the interpolated radar image envelope A i max is brought to its value under test conditions, taking into account the property (volume constancy) of the SAR signal uncertainty function according to the expression

in which the parameters S i and S o are determined by the values of the product of the width of the main sections of the two-dimensional envelope of the radar image of the reflector at the level of 0.5 A imax (along the line of the actual path - X and orthogonal to it - Y) in real and test conditions, the calibration coefficient K kal through of the SAR path is defined as the average value of the estimates of the calibration coefficients K kal (i), calculated for all N calibration UOs in the range line

A - SAR antenna installation angle in elevation,

RCS values of point objects on an arbitrary (calibration and measurement) radar image are determined by the equation

where is the amplitude of the interpolated envelope of the i-th CO in the measuring RI;

K kal - calibration coefficient of the SAR end-to-end path;

R izmn - slant range to a point object on the measuring radar;

The ratio of the gain coefficients of the SAR end-to-end path in terms of amplitude in the measurement and calibration modes, izmn - the values of the viewing angle and slant range corresponding to the center of a square fragment of a spatially distributed object;

S 0 is the area of the resolution element of the measuring radar image (taken equal to its value obtained during the calibration).

Effective area of dispersion of the target (EPR)

The calculation of the range of radar observation requires a quantitative characteristic of the intensity of the reflected wave. The power of the reflected signal at the input of the station receiver depends on a number of factors and, above all, on the reflecting properties of the target. Typically, radar targets are characterized by an effective scattering area. Under the effective scattering area of the target in the case when the radar antenna radiates and receives electromagnetic waves of the same polarization, is understood the value of y c, satisfying the equality y c P 1 \u003d 4pK 2 P 2, where P 1 is the power flux density of the direct wave of this polarization at the location of the target; P 2 - power flux density reflected from the target wave of a given polarization at the radar antenna; R is the distance from the radar to the target. The RCS value can be directly calculated by the formula

at c P 1 \u003d 4pR 2 P 2 / P 1

As follows from the formula above, u has the dimension of area. Therefore, it can be conditionally considered as some area equivalent to the target, normal to the radio beam, with an area of \u200b\u200bc, which, isotropically scattering all the wave power incident on it from the radar, creates at the receiving point the same power flux density P 2 as the real target.

If the RCS of the target is given, then with known values of P 1 and R, it is possible to calculate the power flux density of the reflected wave P, and then, having determined the power of the received signal, estimate the range of the radar station.

The effective scattering area y u does not depend on the intensity of the emitted wave, nor on the distance between the station and the target. Indeed, any increase in P 1 leads to a proportional increase in P 2 and their ratio in the formula does not change. When changing the distance between the radar and the target, the ratio P 2 /P 1 changes inversely proportional to R 2 and the value of y c remains unchanged.

Complex and group goals

Fig 2.1.

As many studies have shown, the exponential distribution law

W (y c) = (1/<у ц >) exp (-- y c /<у ц >).

Where<у ц >- the average value of the RCS.

The return radiation diagrams of ships have a finer lobe structure than aircraft diagrams, which is explained by the significantly larger size and complex design of the ships. The reflective elements of a ship are numerous and varied, so the ship can also be considered as a group of elements whose reflections have random phases.

Experimental studies show that fluctuations in the EPR of a ship are also approximately described by an exponential distribution law.

Data on the laws of distribution of signal amplitudes or EPR are necessary to calculate the range of the radar and substantiate the method of signal processing. Information about the correlation function and fluctuation spectrum is also important in determining the accuracy of coordinate measurements.

In a practical assessment of the range of a radar station, first of all, the average RCS value is usually used<у ц >This value can be obtained by averaging the values<у ц >for different directions of incidence of the irradiating wave. The table shows the average values of the RCS of various real targets, obtained as a result of summarizing a large number of measurements at centimeter wavelengths. Using these values, it is possible to calculate the average values of the detection range of various targets.

EPR of common point targets

convex surface

flat plate

Corner reflector

Triangular

Application of corner reflectors

chaff

EPR of complex targets

Effective target scattering area (ESR).

Complex and group goals

Drawings to the RF patent 2308050

CLAIM

Effective area of ​​dispersion of the target (EPR)

Complex and group goals

Effective area of dispersion of the target (EPR)