RU2614041C1 - Method for generating image of the ground surface in radar station with antenna aperture synthesis - Google Patents

Abstract

FIELD: radar ranging.

SUBSTANCE: invention relates to radar engineering, in particular, to aerospace onboard radar stations with antenna aperture synthesis (ARS), forming radar image (RI) of the ground surface using antenna aperture synthesis (AAS) when scanning this surface with ARS antenna directional pattern. Said result is achieved due to the fact, that the method for generating image of the ground surface in radar station with antenna aperture synthesis involves merging of radar images of the ground surface partial sections, subject to radar scanning, obtained by means of radiation and receiving coherent pulses when these sections are radiated by ARS antenna, analogue-to-digital conversion of the received radar signals, formation of two-dimensional arrays of digitized received signals by their distribution by range channels and radiation periods and further digital processing of generated two-dimensional arrays, provided that exposure of the ground surface sections to ARS antenna radiation is implemented via discrete or sliding method, and summation of amplitudes of partial RI resolution elements, corresponding to generated two-dimensional arrays, is carried out after switching said arrays from coordinate system "range-Doppler frequency” into normal earth-fixed axis system (NEFAS).

EFFECT: achieved technical result – reduced distortions of generated RIs, arising due to change of Doppler shift of radar signal carrier frequency reflected by elements of the ground surface while moving ARS carrier.

1 cl, 5 dwg

Description

The invention relates to radar technology, in particular to aerospace airborne radar stations with synthesizing antenna apertures (SAR), forming radar images (RLI) of the earth's surface using the synthesis of antenna aperture (ATS) in the process of scanning this surface with the radiation pattern of the SAR antenna. The invention can be used in civil and military aviation, in space technology.

A known method of forming a radar image (RLI) of a surface using a synthesized aperture (VN Antipov et al. Radar stations with digital synthesis of the antenna aperture. M: Sov. Radio, 1988, p. 158), based on sequential formation in the system coordinates (SC) "the range-Doppler shift of the carrier frequency" radar signals and combining partial frames _nk N (CW) radar images N _to the partial areas into which the area of the earth's surface to be radioloka ion monitoring on board the aircraft (LA) - PCA carrier.

Each of the partial radar image frames is formed by emitting coherent pulsed probing signals by a SAR antenna in the direction of the corresponding partial portion of the earth's surface, receiving and analog-to-digital conversion of the reflected signals, the formation of two-dimensional arrays of digitized signals by their distribution over the distance channels and periods of radiation, digital data processing, contained in the formed data arrays.

In this method, PC radar images are formed in the form of two-dimensional digital arrays of values of the amplitudes of the resolution elements that are displayed in the form of gradations of pixel brightness of a two-dimensional video image. Partial radar images are represented in the range-azimuth coordinate system, while the azimuthal position of the observed elements of the earth's surface is characterized by the values of Doppler shifts of the carrier frequency (values of Doppler frequencies) of radar signals reflected by these elements, depending on the direction of sight of these elements relative to the direction of the carrier velocity vector PCA.

The constancy of the azimuth resolution (the magnitude of the discrete discrepancies of Doppler frequency shifts) is supported by a corresponding change in the lengths of the synthesis aperture synthesis intervals at different viewing angles. In order to eliminate the speckle effect of N _{n of the} received partial frames, the radar images of each plot of the earth's surface incoherently accumulate and are added element-wise, forming N _{to the} averaged (incoherently accumulated) partial frames (NPC) of the radar image. Moreover, the total number of partial radar frames: N _pc = N _to N _n .

To form the partial frames of the radar image of the partial areas of the earth's surface area to be radar-guided from the aircraft, scanning is carried out by the radiation pattern of the SAR antenna, which is sequentially (discretely) installed and fixed (positioned) in the direction to each partial area of the indicated area, subject to current sighting in the mode synthesizing aperture antenna.

Similarly, radar images of the earth's surface are formed using the synthesized aperture according to the method described in (M. Skolnik, Radar Handbook, th. Ed., McGrowHill, 2008, ch. 5.4, fig. 5.34).

The main disadvantages of the described method of forming radar images of the earth's surface in a radar station with the synthesis of the antenna aperture are:

- the need to position the radiation pattern of the SAR antenna (in the direction of each partial portion of the earth's surface to be monitored), while N _n synthesized partial frames of the radar image of this portion to be accumulated are being generated;

- the difficulties of combining the synthesized partial radar image frames initially formed in a “range-Doppler frequency” coordinate system that arises due to a change in the Doppler frequencies of the radar signals reflected by the elements of the earth’s surface from frame to frame, as well as a change in the position of the origin of the specified coordinate system, due to the displacement of the PCA carrier.

More universal and closest to the proposed method for the formation of radar data using ATS is the method described in RF patent No. 2511216 C1 for the invention “Method for surface imaging in a radar station with synthesizing antenna aperture”, IPC G01S 13/89, publ. 04/10/2014, This method is considered as a prototype.

The essence of this method of forming a radar image of the earth’s surface in a radar station with synthesizing an aperture of the antenna consists in combining radar images spaced in azimuth of partial frames obtained by emitting coherent pulsed sounding signals, irradiating a portion of the earth’s surface with an SAR antenna, and analog-to-digital conversion of the received signals, the formation of two-dimensional arrays of digitized received signals by their distribution over the channel m range and the emission periods and digitally processing the generated two-dimensional arrays consisting of:

a) N-fold execution:

- correction of the location in range and the dependence of the phase on the range;

- azimuthal presummation;

- range compression;

- recording the results of range compression in the buffer memory,

- azimuthal phase correction;

- the formation of azimuthal resolution elements through the fast Fourier transform (FFT);

- autofocus;

- amplitude detection and averaging;

b) overlapping separately obtained N RLI (N = N _n , see above) by

performing a rolling summation of the amplitudes of the azimuthal resolution elements in each range channel and the formation of averaged (incoherently accumulated) partial frames for the resulting radar image;

c) compression of the dynamic range of the amplitudes of the resolution elements obtained after applying the radar image.

The formation of the partial frames of the radar image in the prototype is carried out in the coordinate system "range-Doppler frequency" with a rolling scan of the earth's surface by the radiation pattern of the SAR antenna, which reduces the total time of formation of these frames.

As mentioned above, in real conditions, in the process of obtaining a complete radar image of the Earth’s surface area to be examined with scanning the bottom of the PCA antenna, partial radar frames of sections of this area are formed on different segments of the flight path of the PCA carrier (aircraft), as shown in FIG. 1 (for the case of discrete scanning) and in FIG. 2 (for the case of a sliding scan of the antenna bottom). During the formation of partial radar image frames, due to the movement of the aircraft (in the general case along a curved path, for example, while maneuvering simultaneously), the magnitude and direction of the aircraft’s flight speed vector, the angles of sight of the observed ground objects in both azimuthal (horizontal) and and in vertical planes.

Accordingly, the Doppler shifts of the carrier frequency of the received SAR radar signals reflected by observable ground objects also change.

In the anterolateral view, the Doppler shift F _d (ϕ _g (t), ε _y (t)) of the carrier frequency of the radar signals reflected by the observed small-sized (point) ground object

Where:

V _g (t) is the horizontal (track) component of the aircraft flight speed;

V _y (t) is the vertical component of the flight speed of the aircraft;

ϕ _g (t) is the deviation of the projection of the line of sight of the observed point object on the horizontal plane from the projection of the aircraft velocity vector on the same plane;

ε _y (t) is the angle of inclination of the line of sight of the observed ground-based point object;

λ is the wavelength of the probing SAR signals.

A change in the Doppler shifts of the carrier frequency of radar signals reflected by observable ground objects entails, among other things, a shift in the radar marks of these objects (from frame to frame) in the frequency discretes of the partial radar frames generated by the FFT:

a) when the values of the ground speed V _g (t) of the flight of the aircraft and the angles ϕ _g (t), ε _y (t) of the sight of the observed ground object in the horizontal and vertical planes change (the first term in the numerator (1)). The consequence is the erosion of these marks in radar images formed by the amplitude summation of partial frames;

b) in the presence of a vertical component of the speed V _y (t) of the aircraft motion and different values of the slope angles ε _y (t) of the line of sight of objects observed on the earth's surface. Objects located on the same azimuth bearing, but at different distances from the aircraft (the second term in the numerator (1)).

As an example in FIG. 4 shows radar images of a square configuration of a flat test site shown in FIG. 3, consisting of 9 point reflectors, each of which is 500 meters from its nearest neighbors and is observed from the aircraft at an inclined distance of 15 km to the center of the site. The flight altitude is 7.5 km. Aircraft flight speed V _Aircraft = 300 m / s. The deviation of the vector of the ground speed of the aircraft from the direction to the center of the sighted area ϕ _g = -25 °.

The radar images shown in FIG. 4, obtained by mathematical modeling and correspond to: a) the absence and b) the presence of the vertical component of the flight speed of the aircraft. In the latter case, the angle of inclination of the aircraft velocity vector θ = -30 °.

The indicated displacements of the radar marks of the observed ground objects by the Doppler frequency (azimuthal coordinate of the partial radar frames in the range-Doppler frequency coordinate system) can lead to distortions of the formed (in parts - when scanning the antenna bottom) full radar surface of the earth. At the same time, these distortions can occur both during incoherent accumulation (averaging) of partial radar images of partial sections of the earth’s surface and during the formation of a complete radar image of this surface by combining incoherently accumulated (averaged) partial radar images into a complete radar image. In the latter case, it is possible both the overlapping of incoherently accumulated partial frames on each other, and the appearance of failures at their borders in full radar.

The reduction in the total time of formation of the partial radar image frames provided by the prototype allows to reduce the magnitude of the indicated displacements of the radar marks of the observed ground objects by the Doppler frequency in the generated partial radar image, but does not exclude their appearance, as well as their negative impact on the formation of the radar image.

The technical result of the proposed method for obtaining radar images of the earth’s surface by airborne radar stations forming these radar images using the synthesis of the antenna aperture during scanning of this surface by the radiation pattern of the SAR antenna is to reduce the distortion of radar images of the earth’s surface formed by obtaining and combining partial frames of radar images, namely reduction of distortions arising due to interframe change Doppler x shifts of the carrier frequency of the radar signals reflected by the elements of the observed earth's surface, as well as changes in the position of the origin close to the polar coordinate system "range-Doppler frequency", in which the partial radar frames are formed when moving the SAR carrier.

The essence of the proposed method for generating a radar image of the earth’s surface in a radar station with aperture synthesis when scanning with a radiation pattern of a radar antenna consists in combining partial frames of radar images obtained by emitting coherent pulses of sounding signals when an SAR antenna irradiates partial sections of the region of the earth’s surface to be examined, analog digital conversion of received signals, the formation of a two-dimensional array s of the digitized received signals by their distribution over the range channels and periods of radiation and digital processing of the generated two-dimensional arrays, consisting of:

a) N-fold execution:

- correction of the location in range and the dependence of the phase on the range;

- azimuthal presummation;

- range compression;

- recording the results of range compression in the buffer memory;

- azimuthal phase correction;

- the formation of azimuthal resolution elements through the fast Fourier transform (FFT);

- autofocus;

- amplitude detection and averaging;

- conversion of the generated two-dimensional arrays corresponding to the received radar data from the coordinate system “range-Doppler frequency” to the normal earth coordinate system (NSC);

b) overlapping (both with sliding and with discrete scanning) N partial radar frames formed separately in the NSCC by summing (incoherent accumulation) of the amplitudes of the corresponding resolution elements (pixels) of the converted two-dimensional arrays;

c) compression of the dynamic range of amplitudes of resolution elements averaged (incoherently accumulated) in the NSC partial radar data;

d) combining averaged (incoherently accumulated) partial radar images into a complete radar image of the region of the earth's surface to be surveyed.

The difference between the proposed method of forming radar images (RLI) of the earth’s surface using the synthesis of the antenna aperture during scanning of this surface by the directivity pattern of the SAR antenna from the prototype is that before the addition (incoherent accumulation) of the partial SARs of the earth’s surface formed during scanning by the SAR antenna , each of these images is transferred from the “range-Doppler frequency” coordinate system to the normal earth coordinate system (NSC), knitted with the area of the earth's surface to be radar survey. In this case, the horizontal plane of the NWCC coincides with the local horizontal plane corresponding to the region of the earth's surface to be radar survey using synthesis of the antenna aperture and scanning by the radiation pattern of the SAR antenna.

This translation can be carried out, for example, as follows.

1. On the horizontal plane of the selected NSC, a rectangular region is determined that includes the region of the earth’s surface to be radar-surveyed using ATS when scanning the radiation pattern of the SAR antenna, as shown in FIG. 5.

The sides of this region can be oriented, for example, along (the X axis of the region) and across (the Z axis of the region) of the azimuthal direction of sighting of its center from the point corresponding to the middle of the segment of the flight path of the aircraft, on which radar images should be formed with scanning the bottom of the SAR antenna (as shown in Fig. 5).

The specified region is divided into N _K disjoint partial sections to be reviewed with positional (discrete) or sliding scanning radiation pattern of the SAR antenna. The partial sections of the earth's surface defined in the NSCC correspond to the partial radar frames to be formed using the synthesis of the antenna aperture when scanning by the radiation pattern of the SAR antenna. Scanning by the radiation pattern of the SAR antenna can be carried out both in a discrete way and in a sliding one, including, as indicated in the prototype. It is assumed that scanning the antenna bottom can be carried out both in horizontal and vertical planes.

In the mode of sliding azimuthal scanning by the antenna radiation pattern, the linear azimuthal length L of the partial sections of the Earth's surface region defined in the NSCC and subject to radar observation (when this region is evenly divided into sections), the number N = N _{n of} incoherently accumulated (summed) partial frames corresponding to these areas, and the azimuthal angular width Θ of the scanning antenna beam are related by

,

,

Where:

DS is the value of the slant range to the nearest RLI range channel formed in the SC “range-Doppler frequency” (corresponding to the near boundary of the observed partial section).

) ПК РЛИ, подлежащего формированию в НЗСК, индексируются:Resolution Elements of Each (kth,

) PK RLI to be formed in the NZSK are indexed:

along the z axis:

i _k = (i1 _k , ..., i2 _k ), where i1 _k , i2 _k are the initial and final values of the indices, respectively;

along the x axis:

j _k = (j1 _k , ..., j2 _k ), where j1 _k , j2 _k are also the initial and final values of the indices.

2. At the end of the formation of each frame of the radar image of the earth’s surface in the coordinate system “range-Doppler frequency” (in the current position of the scanning antenna bottom of the SAR antenna) for each (i _k , j _k ) th element of the PC of the radar image corresponding to a partial portion of the earth’s surface, covered at the current time by scanning the radiation pattern of the antenna are carried out:

до элемента земной поверхности, соответствующего (i_k, j_k)-му элементу формируемого в НЗСК (n-го,

) парциального кадра РЛИ k-го участка земной поверхности2.1. Calculation of the current estimated slant range

to the element of the earth's surface corresponding to the (i _k , j _k ) th element formed in the NSCC (n-th,

) the partial frame of the radar image of the k-th plot of the earth’s surface

,

,

Where:

X a ⁽ⁿ⁾ , Y a ⁽ⁿ⁾ , Z a ⁽ⁿ⁾ are the calculated coordinates of the phase center of the SAR antenna (aircraft) at the beginning of the formation of the n-th partial radar frame of the k-th portion of the NWCC;

Y a ⁽ⁿ⁾ is the flight altitude of the aircraft in NZSK;

,

- расчетные координаты (i_k, j_k)-го элемента k-го ПК РЛИ, соответствующего k-му участку в выбранной НЗСК.

,

- the calculated coordinates of the (i _k , j _k ) th element of the k-th radar search system corresponding to the k-th section in the selected NSC.

канала дальности РЛИ,сформированного в СК «дальность-доплеровская частота», которому принадлежит (i_k, j_k)-й элемент ПК РЛИ, формируемого в НЗСК2.2. The number is determined

the radar range channel formed in the SC “range-Doppler frequency”, which belongs to the (i _k , j _k ) th element of the radar radar array formed in the NSC

,

,

Where:

DS ⁽ⁿ⁾ - the value of the slant range to the nearest channel of the radar range formed in the SC "range-Doppler frequency";

Δd is the size of the discrete along the range of the radar image generated in the SC "range-Doppler frequency".

Round - corresponds to the operation of rounding the number in brackets to an integer.

наклона линии визирования элементов земной поверхности, соответствующих (i_k, j_k)-м элементам n-го ПК РЛИ (k-го участка), формируемого в НЗСК.2.3. Angle values are calculated

the slope of the line of sight of the elements of the earth's surface corresponding to the (i _k , j _k ) th elements of the n-th radar detector (k-th section) formed in the NSC.

визирования элементов земной поверхности, соответствующих (i_k, j_k)-м элементам n-го ПК РЛИ в горизонтальной плоскости НЗСК относительно оси X.2.4. Angle values are calculated

sighting of the elements of the earth’s surface corresponding to the (i _k , j _k ) th elements of the nth radar detector in the horizontal plane of the NWCC relative to the X axis.

визирования элементов земной поверхности, соответствующих (i_k, j_k)-м элементам n-го ПК РЛИ в горизонтальной плоскости НЗСК относительно направления вектора путевой скорости2.5. Angle values are calculated

sighting of elements of the earth’s surface corresponding to (i _k , j _k ) th elements of the nth radar detector in the horizontal plane of the NSC relative to the direction of the ground speed vector

,

,

Where:

- угол пути ЛА (носителя РСА);

- the angle of the path of the aircraft (PCA carrier);

Vx ⁽ⁿ⁾ is the projection of the aircraft flight velocity vector on the X axis of the selected NSC;

Vz ⁽ⁿ⁾ is the projection of the flight speed vector of the aircraft on the Z axis of the selected NSC.

доплеровских сдвигов несущей частоты сигналов, отраженных элементами земной поверхности, соответствующих (i_k, j_k)-м элементам n-го ПК РЛИ, формируемого в НЗСК2.6. The values are calculated

Doppler shifts of the carrier frequency of the signals reflected by the elements of the earth's surface, corresponding to (i _k , j _k ) th elements of the n-th radar detector formed in the NSC

Where:

- значение горизонтальной составляющей вектора скорости носителя РСА;

- the value of the horizontal component of the velocity vector of the SAR carrier;

Vy ⁽ⁿ⁾ is the value of the vertical component of the aircraft velocity vector.

несущей частоты радиолокационных сигналов, отражаемых элементами земной поверхности, соответствующими (i_k, j_k)-м элементам формируемого n-го ПК РЛИ k-го участка. Указанные значения рассчитываются относительно нижней границы

частотного окна, в котором должен осуществляться анализ доплеровских сдвигов частот при формировании n-го кадра РЛИ в СК «дальность-доплеровская частота».2.7. Doppler shifts are calculated

the carrier frequency of the radar signals reflected by the elements of the earth's surface, corresponding to the (i _k , j _k ) th elements of the generated n-th radar radar detector of the k-th section. The indicated values are calculated relative to the lower limit.

the frequency window in which the analysis of Doppler frequency shifts should be carried out during the formation of the nth frame of the radar image in the SC "range-Doppler frequency".

Where:

- нижняя граница окна анализа доплеровских сдвигов частот в частотной области;

- the lower border of the window for the analysis of Doppler frequency shifts in the frequency domain;

floor - corresponds to the operation of selecting the integer part, the number enclosed in brackets;

- расчетное значение доплеровского сдвига несущей частоты радиолокационных сигналов, отраженных от точки земной поверхности, относительно которой осуществляется фокусировка траекторного сигнала РСА при формировании n-го парциального кадра РЛИ k-го участка;

- the estimated value of the Doppler shift of the carrier frequency of the radar signals reflected from a point on the earth’s surface, relative to which the PCA trajectory signal is focused during the formation of the nth partial radar frame of the kth section;

T _p - the repetition period of the probe pulses of the SAR.

несущей частоты сигналов, отраженных от элементов земной поверхности, соответствующих (i_k, j_k)-м элементам n-го ПК РЛИ k-го участка, относительно значения

доплеровского сдвига несущей частоты для соответствующей точки фокусировки РЛИ2.8. Doppler shifts are calculated

the carrier frequency of the signals reflected from the elements of the earth's surface, corresponding to the (i _k , j _k ) th elements of the n-th radar detector of the k-th section, relative to the value

Doppler shift of the carrier frequency for the corresponding focus point

при

at

при

at

Where:

2.9. The numbers of Doppler filters corresponding to the partial frame of the radar image of the earth's surface, formed in the coordinate system "range-Doppler frequency", (i _k , j _k ) -th elements of the n-th partial frame of the radar image (k-th section) formed in the NSCC are determined .

,

,

Where:

T _with - the synthesis time of the antenna aperture during the formation of one partial frame of the radar image.

, соответствующего значению этого уровня в элементе разрешения РЛИ, сформированного в СК «дальность-доплеровская частота»

, в

канале дальности и в

доплеровском фильтре.2.10. The (i _k , j _k ) -th element is being assigned to the k-th section of the signal of the signal level (amplitude) formed in the NSCS of the n-th radar detector

corresponding to the value of this level in the radar resolution element formed in the SC "range-Doppler frequency"

, at

range channel and in

Doppler filter.

This assignment can also be carried out taking into account the values of signal levels in neighboring resolution elements of the radar image generated in the SC “range-Doppler frequency” using window filtering.

Next, incoherent summation (overlapping) of N partial radar image frames obtained separately in the NSCC is carried out by summing the amplitudes of the corresponding resolution elements (pixels) of the radar data source. The totality of all (N _k ) formed averaged (incoherently accumulated) RS radar images, obviously, forms the complete (resulting) radar radar of the Earth's surface survey area in the NSC.

As follows from the above description, the application of the proposed method in comparison with the prototype due to:

- translation of the partial frames of the radar data generated by the SAR during positional or sliding scanning of the antenna bottom from the coordinate system "range-Doppler frequency" to the normal earth coordinate system associated with the region of the earth's surface subject to radar observation;

- incoherent summation (overlapping) of N partial radar frames (corresponding to the same partial plot of the earth's surface) after their transfer to the NSC;

- combining partial (summed in NZSC) N _to non-overlapping frames of partial radar images directly adjacent to each other of the partial terrain into a complete (resulting) radar image of the earth's surface subject to radar observation, eliminates the appearance of distortion in the resulting full radar images, which may arise due to:

a) summation (incoherent accumulation) in the coordinate system "range-Doppler frequency" of the partial radar frames in which the Doppler shifts of the carrier frequency of the radar signals reflected by the same elements of the earth's surface are different;

b) combining the partial radar image frames accumulated in the “range-Doppler frequency” coordinate system with a different position of the origin of this coordinate system into a complete radar image of the earth's surface.

Moreover, the application of the proposed method is possible both with sliding and with discrete scanning of the bottom of the SAR antenna with an increase in the speed of the survey of the earth's surface.

Information sources

1. V.N. Antipov et al. Radar stations with digital synthesis of the antenna aperture. M .: Sov. Radio 1988, p. 158.

2. M. Skolnik, Radar Handbook, th. ed., McGrowHill, 2008, ch. 5.4, fig. 5.34.

3. RF patent No. 2511216 C1, IPC G01S 13/89, publ. 04/10/2014.

Claims (2)

1. A method of forming an image of the earth’s surface in a radar station with synthesizing antenna aperture (SAR), based on the combination of partial frames of radar images (azimuth and distance), each obtained by emitting coherent pulsed sounding signals, irradiating a scanning SAR antenna partial sections of the earth's surface area to be radar survey, analog-to-digital conversion of the received reflected signals, the formation of two-dimensional of digital signals and digital processing of the data contained in the data arrays by N-fold correcting the location in range and phase dependence on range, azimuthal summing, range compression, recording the results of range compression in the buffer memory, azimuthal phase correction, formation of azimuthal resolution elements by fast Fourier transform, autofocus, amplitude detection and averaging, then superimposing N partial radar images by summing the amplitudes of the corresponding resolution elements, compressing the dynamic range of the amplitudes of the resolution elements obtained after applying the partial radar image, characterized in that the overlapping (summing the amplitudes of the resolution elements) separately received N partial frames of the radar image of the Earth’s surface, compressing the dynamic range of the amplitudes of the resolution elements obtained after applying the partial radar image , and the combination of averaged (incoherently accumulated) partial radar images into a complete radar image of the Earth's surface area Subject Review, performed after converting the generated two-dimensional array corresponding to the obtained partial RLI earth's surface from the coordinate system "range - Doppler frequency" in normal terrestrial coordinate system (NZSK). 2. The method of imaging the earth's surface in a radar station with the synthesis of the antenna aperture according to claim 1, characterized in that the superposition of N partial radar frames obtained separately in the NSCS can be performed both with a rolling and discrete scanning of the earth's surface by the antenna radiation pattern PCA.

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