RU2707556C1 - Method of determining terrain elevation height of a radar with synthesized antenna aperture - Google Patents

Abstract

FIELD: radar ranging and radio navigation.

SUBSTANCE: invention relates to radar ranging and can be used in radar with synthesized antenna aperture mounted onboard aircraft for rapid determination of terrain elevation. Method is based on the fact that the radar signal is radiated in the direction of the terrain section and the reflected signal is received, the received signal s(t) is coherently accumulated. After signal s(t) is accumulated, two signals s₁(t) and s₂(t) with overlapping spectra and central frequencies ƒ₁ and ƒ₂, wherein ƒ₂>ƒ₁. Further, the antenna aperture is synthesized according to s₁(t) and s₂(t) in form of a set of complex amplitudes of signals Ṡ_C1[i,k], Ṡ_C2[i,k] in coordinates i – azimuth, k – range, then performing phase correction of complex amplitudes of signals on range, then determining a set of phase differences Δϕ[i, k] between compensated complex amplitudes of signals and according to appropriate formula is determined elevation of terrain section relief.

EFFECT: high accuracy of measuring terrain elevation height from the phase difference of paired signals formed with frequency overlap from a single coherent accumulated signal for implementation in synthetic aperture radar.

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Description

The invention relates to the field of radar and can be used in a radar with a synthesized aperture antenna (PCA), installed on board the aircraft, to quickly determine the height of the terrain.

Known interferometric SAR mode designed to obtain three-dimensional radar data in the coordinates of the "range-azimuth-height" of each resolved element ["Radio. Radar Systems for Remote Sensing of the Earth ”, Textbook for High Schools, ed. G.S. Kondratenkova, M.: Radio engineering, 2005 p. 349-354]. The method consists in the formation of resolution elements in range and azimuth by a conventional SAR method. And to obtain information about the elevation of the terrain and objects, an additional channel is used to measure the elevation angle of each resolution element in range and azimuth. An SAR antenna system is used to measure the elevation angle, and the larger the size of the antenna in the elevation plane, the higher the accuracy of the height measurement. To simplify the design and reduce weight, an antenna system is used, consisting of two antennas spaced apart along the elevation angle — an interferometer. The phase of the interferometer signal carries information about the height of the relief. In the simplest interferometric mode, the SAR is applied to the X-ray lines of equal elevation angles, the distance between which is proportional to the change in the height of the relief.

The disadvantage of this method is that to achieve acceptable accuracy of measuring the height of the terrain, the placement of two antennas with a large interferometer base (distance between antennas) is required, which is often impossible on PCA carriers (aircraft, unmanned aerial vehicles, spacecraft).

A known method of generating SAR images for interferometric processing is described in the publication on the patent "Acquisition of SAR images for computing a height or a digital elevation model by interferometric processing" (PCA imaging for calculating heights or digital height models by interferometric processing) [US 9019144 , published on April 28, 2015, IPC G01S 13/00, G01S 13/90]. The method consists in forming a radar image of the same area with the geometry of formation with one on-board synthesized aperture (PCA) radar antenna so as to provide interferometric processing of said PCA images. The specified method is characterized by the geometry of the formation in which each SAR image of the region is obtained in the corresponding direction of formation, which determines the corresponding deviation angle of the maximum radiation pattern relative to the direction of flight, and in which the angles of deviation of the maximum radiation pattern are such that they form an average angle of deviation of the maximum radiation pattern different from zero; and SAR images are obtained with only one SAR sensor, which is located on the air / satellite platform, and uses only one antenna and receives SAR images in only one pass of the air / satellite platform.

The closest in technical essence is the "Method for measuring the surface topography of the Earth" [RU 2643790, published 02/06/2018, IPC G01C 11/00, G01S 13/89]. The method consists in the fact that one radar with a synthesized antenna aperture (SAR) is placed on board one carrier, with the help of which an interferometer base is artificially formed during successive observations of the Earth’s surface, while emitting sounding signals and receiving signals reflected from the earth’s surface with synthesis of a pair of radar images (RLI) and their interferometric processing. The base of the interferometer is formed along the flight path of the SAR carrier during its natural movement in space, successive observations of the Earth's surface are carried out at a constant altitude and speed of flight of the SAR carrier. The first observation session is carried out by emitting sounding signals and receiving signals reflected from the Earth’s surface with the use of X-ray radiation during azimuthal telescopic viewing over the synthesis interval. After moving the SAR carrier to the distance of the base of the interferometer, a second observation session is carried out for the same region of the Earth’s surface, also by emitting sounding signals and receiving signals reflected from the Earth’s surface with XRD synthesis during azimuthal telescopic viewing over the synthesis interval. After conducting a pair of observation sessions, interferometric processing of a pair of radar images is carried out with information on the relief of the underlying surface, while the presence of a perpendicular projection of the base of the interferometer that occurs when using a telescopic view with an azimuth angle α ₁ less than 90 ° provides a relationship that displays a unique relationship between the relief the Earth’s surface and observation parameters with an interferometric phase difference in the form -

where H is the constant altitude of the SAR carrier; R ₁ - the distance to the surface of the Earth; B is the distance of the base of the interferometer; α ₁ is the azimuthal angle; λ is the operating wavelength of the SAR; ϕ is the interferometric phase difference.

The disadvantage of these methods is the lack of accuracy in measuring the height of the terrain due to temporary decorrelation of several accumulated signals, which determine the height. Temporary decorrelation of signals reflected from the same area is due to the span of the SAR carrier between the signal accumulation intervals to form the base of the interferometer.

The technical result of the invention is to improve the accuracy of measuring the height of the terrain.

The technical problem solved by the invention is the creation of a high-precision method for measuring the height of the terrain for implementation in radars with a synthesized aperture.

The essence of the invention lies in the fact that they emit a radar signal in the direction of the site and receive the reflected signal, coherently accumulate the received signal s (t), synthesize the antenna aperture.

в координатах i - азимут, k - дальность. Далее осуществляют фазовую коррекцию комплексных амплитуд сигналов

по дальности, затем определяют совокупность разностей фаз Δϕ[i,k] между скомпенсированными комплексными амплитудами сигналов

и

Затем определяют высоту рельефа участка местности по соотношению:New in the claimed method is that after the accumulation of the signal s (t), two signals s ₁ (t) and s ₂ (t) are extracted from it with overlapping spectra and center frequencies ƒ ₁ and ƒ ₂ , and ƒ ₂ > ƒ ₁ , and the synthesis of the antenna aperture is carried out according to the signals s ₁ (t) and s ₂ (t) in the form of a set of complex signal amplitudes

in coordinates i - azimuth, k - range. Next, phase correction of the complex amplitudes of the signals is carried out.

range, then determine the set of phase differences Δϕ [i, k] between the compensated complex signal amplitudes

and

Then determine the height of the relief of the site by the ratio:

где Δƒ=ƒ₂-ƒ₁ - разность центральных частот сигналов s₁(ƒ) и s₂(ƒ), с - величина скорости распространения электромагнитных волн, θ - угол места луча диаграммы направленности антенны радиолокатора с синтезированной апертурой (РСА), α - азимут луча диаграммы направленности антенны РСА, R₀ - дальность от антенны РСА до участка местности, T_Н - время накопления сигнала s(t), V - скорость носителя РСА.

where Δƒ = ƒ ₂ -ƒ ₁ is the difference between the central frequencies of the signals s ₁ (ƒ) and s ₂ (ƒ), c is the propagation velocity of electromagnetic waves, θ is the elevation angle of the beam of the radiation pattern of the synthetic aperture radar (PCA), α is the azimuth of the beam of the radiation pattern of the SAR antenna, R ₀ is the distance from the SAR antenna to the terrain, T _N is the signal accumulation time s (t), V is the SAR carrier speed.

где Δƒ=ƒ₂-ƒ₁, а Δƒ_СП - ширина спектра излучаемого сигнала.The signals s ₁ (t) and s ₂ (t) with overlapping spectra are extracted by converting the accumulated signal s (t) into the frequency domain s (ƒ), by dividing the converted signal s (ƒ) in the frequency domain into two signals s ₁ (ƒ) and s ₂ (ƒ) with overlapping spectra and center frequencies ƒ ₁ and ƒ ₂ , converting the signals s ₁ (ƒ) and s ₂ (ƒ) into the time domain s ₁ (t) and s ₂ (t). The accumulated signal s (t) is converted into the frequency domain by the fast Fourier transform. Signals s ₁ (ƒ) and s ₂ (ƒ) are converted into the time domain by the inverse fast Fourier transform. The difference Δƒ of the center frequencies ƒ ₁ and ƒ _{2 of the} signals s ₁ (t) and s ₂ (t) is determined by the relation

where Δƒ = ƒ ₂ -ƒ ₁ , and Δƒ _SP is the width of the spectrum of the emitted signal.

In FIG. 1 presents a functional diagram of a radar station implementing the inventive method.

In FIG. 2 shows a geometric diagram for determining the height of the measurement of the terrain.

In FIG. 3 shows the spectra of the received and extracted pair signals.

In FIG. 4 presents the radar image, the image of the terrain and the altitude profile obtained by the claimed method.

The method for determining the height of the terrain can be implemented in a radar with a synthesized aperture of the antenna mounted on an airplane and constructed, for example, according to the following scheme. A PCA consists of an antenna (1), a transmitter (2), a receiver (3), a control processor (4), a signal processor (5), an indicator (6). The output of the control processor (4) is connected to the first input of the antenna (1), the output of the transmitter (2) is connected to the second input of the antenna (1). The output of the antenna (1) is connected to the input of the receiver (3). The output of the receiver (3) is connected to the input of the signal processor (5). The output of the signal processor (5) is connected to the indicator input (6).

The method of determining the height of the terrain by a radar with a synthesized aperture of the antenna is as follows.

The mode for determining the height of the terrain is launched by the pilot with the appropriate command from the control processor (4). The control processor (4) sets the control parameters of the antenna (1) to view the corresponding field of view. The beam of the antenna pattern (BOTTOM) is set by the antenna (1) to the desired position in azimuth α and elevation angle θ (Fig. 2). In this case, the shape and beam width of the bottom beam is determined from the necessary overlap of the viewing area - pencil beam, cosec ² , etc. The antenna (1) emits a coherent pulsed radar signal generated by the transmitter (2) (simple radio pulses, phase-shift keyed (PCM) or linearly frequency-modulated (LFM) pulses with a repetition period that provides overlap of the Doppler range of frequencies falling into the field of view, and unambiguous overlap of the zone in range .

The signal reflected from the earth's surface is received by the antenna (1). From the output of the antenna (1), the signal enters the receiver (3), where analog signal processing is performed, for example, amplification, filtering, etc. After completing the analog processing, an analog-to-digital conversion of the received signal is performed. Then, the received signal is coherently accumulated in the signal processor (5). After the accumulation of the received signal in the signal processor (5) is completed, its processing is started.

Processing of the accumulated signal s (t) begins by extracting from the accumulated signal s (t) the paired signals s ₁ (t) and s ₂ (t) with overlapping spectra and center frequencies ƒ ₁ and ƒ ₂ . For this, the accumulated signal s (t) is converted into the frequency domain. For this, the signal s (t) in the signal processor (5) is subjected to fast Fourier transform (FFT).

s ^mk (ƒ) = FFT (s ^nk (t))

where t is time, ƒ is the frequency, n is the number of the emitted pulse, k is the number of the range resolution element, m is the number of the frequency filter, s ^mk (ƒ) is the signal in the frequency domain.

и

с перекрывающимися спектрами (Фиг. 3), например, таким образом, что центральные частоты ƒ₁ и ƒ₂ отстают слева и справа от центральной частоты ƒ₀ сигнала s^mk(ƒ) на одинаковую величину

при этом нижняя граница спектра сигнала

совпадает с нижней границей сигнала s^mk(ƒ), а верхняя граница спектра сигнала

совпадает с верхней границей сигнала s^mk(ƒ). Такое выделение сигналов

и

осуществляется выборкой соответствующей части отсчетов сигнала s^mk(ƒ).Then, in the signal processor (5), two signals are extracted from the signal s ^mk (ƒ)

and

with overlapping spectra (Fig. 3), for example, in such a way that the center frequencies ƒ ₁ and ƒ ₂ are behind the left and right of the center frequency s _{0 of the} signal s ^mk (одинаков) by the same amount

while the lower limit of the signal spectrum

coincides with the lower boundary of the signal s ^mk (ƒ), and the upper boundary of the signal spectrum

coincides with the upper boundary of the signal s ^mk (ƒ). This selection of signals

and

is carried out by sampling the corresponding part of the signal samples s ^mk (ƒ).

и

Коэффициент корреляции определяется из соотношения [Бабокин М.И., «Алгоритмы оценки относительного рельефа местности в многопозиционных комплексах РСА», журнал «Радиотехника», №7 2009 г., стр. 53.]:The magnitude of the overlap of the spectra is determined based on the requirement of correlation of pair signals

and

The correlation coefficient is determined from the relationship [MI Babokin, “Algorithms for assessing the relative terrain in multi-position complexes of SAR,” Journal of Radio Engineering, No. 7, 2009, p. 53.]:

- коэффициент корреляции, Δƒ=ƒ₂-ƒ₁, с - абсолютная величина скорости распространения электромагнитных волн,

- разрешающая способность по дальности определяемая перекрытием спектров сигналов, δr - разрешающая способность по дальности,

- ширина спектра зондирующего сигнала.Where

is the correlation coefficient, Δƒ = ƒ ₂ -ƒ ₁ , s is the absolute value of the propagation velocity of electromagnetic waves,

- range resolution determined by overlapping signal spectra, δr - range resolution,

- the width of the spectrum of the sounding signal.

на допустимое частотное смещение накладывается ограничение

So, for example, with a correlation coefficient of signals

permissible frequency offset is limited

и

во временную область. Для этого в процессоре сигналов (5) осуществляют обратное быстрое преобразование Фурье (ОБПФ)

и

Next, the signal conversion

and

to the time domain. To do this, in the signal processor (5) carry out the inverse fast Fourier transform (IFFT)

and

и

В результате синтезирования апертуры антенны формируют два двумерных массива комплексных амплитуд сигналов

где i - номер азимутальной позиции, k - номер элемента разрешения по дальности. Основными операциями синтезирования апертуры антенны являются обработка сигнала согласованным фильтром, весовая обработка сигнала, компенсация квадратичного и линейного фазового набегов, быстрое преобразование Фурье. Подробное описание алгоритмов синтезирования апертуры антенны приведено, например, в монографиях [«Многофункциональные радиолокационные системы» под ред. Б.Г. Татарского, М.: Дрофа, 2007 г. стр. 181-190, рис. 7.9, 7.10] или [«Радиовидение. Радиолокационные системы дистанционного зондирования Земли» Учебное пособие для ВУЗов под ред. Г.С. Кондратенкова, М.: Радиотехника, 2005 г. стр. 174-195, рис. 6.11].Then carry out the synthesis of the antenna aperture from the signals

and

As a result of the synthesis of the antenna aperture, two two-dimensional arrays of complex signal amplitudes are formed

where i is the number of the azimuthal position, k is the number of the range resolution element. The main operations for synthesizing the antenna aperture are signal processing with a matched filter, weighted signal processing, quadratic and linear phase shift compensation, fast Fourier transform. A detailed description of the antenna aperture synthesis algorithms is given, for example, in the monographs [“Multifunctional Radar Systems”, ed. B.G. Tatarsky, Moscow: Drofa, 2007, pp. 181-190, Fig. 7.9, 7.10] or [“Radio-vision. Radar Systems for Remote Sensing of the Earth »Textbook for High Schools, ed. G.S. Kondratenkova, M .: Radio engineering, 2005, pp. 174-195, Fig. 6.11].

,

для компенсации разности фаз возникшей из-за разности центральных частот сигналов Δƒ=ƒ₂-ƒ₁. Компенсацию можно осуществить введением фазового сдвига для каждого элемента дальности k сигналов

после их комплексного сопряжения:Next, phase correction of signals by range

,

to compensate for the phase difference arising due to the difference in the center frequencies of the signals Δƒ = ƒ ₂ -ƒ ₁ . Compensation can be achieved by introducing a phase shift for each element of the range of k signals

after their complex pairing:

i is the number of the azimuthal position, k is the number of the current range resolution element, K is the total number of range resolution elements.

и

Для этого формируют массив разностей фаз Δϕ[i,k] по массиву комплексно-сопряженных скомпенсированных сигналов

например по следующему соотношению:In the signal processor (5), the phase difference Δϕ between the compensated samples of the complex signals is calculated

and

For this, an array of phase differences Δϕ [i, k] is formed from the array of complex conjugate compensated signals

for example, in the following ratio:

Next, determine the height of the object with coordinates [i, k], which corresponds to the reference phase difference Δϕ [i, k]. To do this, multiply the phase difference Δϕ [i, k] by the coefficient

where Δƒ = ƒ ₂ -ƒ ₁ , s is the magnitude of the speed of propagation of electromagnetic waves, θ is the angle of the beam of the beam, α is the azimuth of the beam of the beam, R ₀ is the distance from the SAR antenna to the terrain, T _N is the signal accumulation time s (t ), V is the speed of the PCA carrier.

Thus, an array of elevations of the relief of the area is formed, with high accuracy by determining the phase difference from two signals extracted from one coherently accumulated signal.

Then, using the numerical values of the heights, either an image of the terrain or a height profile is formed and displayed on the indicator (6) for display to the pilot. The figure 4 shows the radar image of the site with the image of Mount Bogdo (Astrakhan region), the relief of this site (the brightness is proportional to the height, the brighter the point - the higher the height) and the altitude profile of the site in azimuth obtained by the claimed method.

Then, the next site is surveyed due to the span of the SAR carrier, or the pilot manually changes the field of view by switching the beam of the bottom beam to another position in azimuth and elevation and determines the height of the relief of the new specified site.

Thus, by determining the height of the relief from the phase difference of the pair of signals formed with overlapping frequency from a single coherently accumulated signal, the accuracy of assessing the topography of the terrain increases.

Claims (6)

1. The method of determining the height of the terrain by a radar with a synthesized aperture of the antenna, which consists in emitting a radar signal in the direction of the terrain and receiving the reflected signal, coherently accumulating the received signal s (t), synthesizing the antenna aperture, characterized in that after signal accumulation s (t) extract from it two signals s ₁ (t) and s ₂ (t) with overlapping spectra and center frequencies ƒ ₁ and ƒ ₂ , with ƒ ₂ > ƒ ₁ , and the antenna aperture is synthesized using signals s _l (t ) and s ₂ (t) as a set and complex signal amplitudes

in coordinates i - azimuth, k - range, then phase correction of complex signal amplitudes is carried out

range, then determine the set of phase differences Δϕ [i, k] between the compensated complex signal amplitudes

and

and the elevation of the terrain is determined as

where Δƒ = ƒ ₂ -ƒ ₁ is the difference of the central frequencies of the signals s ₁ (ƒ) and s ₂ (ƒ), c is the magnitude of the propagation velocity of electromagnetic waves, θ is the elevation angle of the beam of the radiation pattern of the synthetic aperture radar (PCA), α is the azimuth of the beam of the radiation pattern of the SAR antenna, R ₀ is the distance from the SAR antenna to the terrain, T _N is the signal accumulation time s (t), V is the SAR carrier speed. 2. The method of determining the height of the terrain by a radar with a synthesized antenna aperture according to claim 1, characterized in that the signals s ₁ (t) and s ₂ (t) with overlapping spectra are extracted by converting the accumulated signal s (t) into the frequency domain s ( ƒ), by dividing the converted signal s (ƒ) in the frequency domain into two signals s ₁ (ƒ) and s ₂ (ƒ) with overlapping spectra and center frequencies ƒ ₁ and ƒ ₂ , by converting the signals s ₁ (ƒ) and s ₂ ( ƒ) to the time domain s ₁ (t) and s ₂ (t). 3. The method of determining the height of the terrain by a radar with a synthesized antenna aperture according to claim 2, characterized in that the accumulated signal s (t) is converted into the frequency domain by a fast Fourier transform. 4. The method of determining the height of the terrain by a radar with a synthesized aperture of the antenna according to claim 2, characterized in that the signals s ₁ (ƒ) and s ₂ (ƒ) are converted into the time domain by the inverse fast Fourier transform. 5. The method for determining the height of the terrain by a radar with a synthesized aperture of the antenna according to claim 1, characterized in that the difference Δƒ of the center frequencies ƒ ₁ and ƒ _{2 of the} signals s ₁ (t) and s ₂ (t) is determined by the ratio

where Δƒ = ƒ ₂ -ƒ ₁ , and Δƒ _SP is the width of the spectrum of the emitted signal.

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