RU2166774C2 - Method and gear to form polarization portrait of ground or sea surface in double-frequency radar with synthesized aperture - Google Patents

Abstract

FIELD: radiolocation. SUBSTANCE: invention is intended for airborne radars with synthesized aperture. In correspondence with invention two arrays ( apertures ) are formed artificially on different frequencies of radiation of sounding pulses. Each aperture is formed from its own antenna with space centers separated along direction of flight of carrier of radar with synthesized aperture. Antennas have polarization orthogonal each other. Each aperture is formed with its own synthesizing interval whose value is calculated proceeding from condition of invariable resolution along azimuth. Signal reflected from under underlying surface goes to all antenna systems, each one being capable of reception of signals on both carrier frequencies. Thus reflected signals are separately received over four channels and moduli and phases of components of polarization matrix of scattering of section of surface which are polarization portrait are determined. Various procedures are used to form radar image of ground or sea surface from these components. EFFECT: improved quality of radar image of ground or sea surface formed in airborne radar. 2 cl, 2 dwg

Description

 The invention relates to the field of radar and can be used in airborne radars with a synthesized aperture antenna (PCA) when monitoring the land or sea surface.

 A known method for providing a radar image (RLI) of the earth's surface with fixed and moving targets in a three-frequency digital SAR (Patent N 2084920, MKI 5 G 01 S 13/52. Method for the selection of moving ground targets / N.A. Sazonov, V.N. Shcherbinin. Priority 01/26/94).

 This method of generating radar images consists in the fact that the signal sum module uses the module of the sum of signals of all frequency channels of the RSA.

 The disadvantage of this method is that the polarization structure of the received electromagnetic field (EMF) is not taken into account, which leads to losses in the energy sector and information losses during the formation of radar images, and information contained in the phase characteristics of the re-reflected EMF is not used.

 Closest to the claimed method is a method for detecting small ground targets in a polarizing radar (US Patent N 5334981, G 01 S 13/04. Priority 04/04/1992).

 The essence of this method is that the transmitting antenna - emits a signal on the main polarization. This signal propagates from the aircraft to the target and vice versa. The receivers are connected to the main and cross-polarized receiving antennas. The target is detected by processing the output signals of two receivers. Such a radar contains a transmitter, three antennas, two receivers and a processing processor.

 The disadvantages of this method are the low resolution in the azimuthal and long-range coordinates, as well as signal processing only at the main and cross polarizations. In this case, a complete polarization reception cannot be carried out.

 The technical result of the proposed method is to improve the quality of the generated radar image.

 The essence of the proposed method for the formation of a polarization portrait (PP) is that they form two synthesized apertures of the antenna at different carrier frequencies of the probe pulses, each aperture is formed with its own synthesis interval, the value of which is calculated based on the constant resolution in azimuth, the range resolution in each channel maintained constant, apertures form each with its own antenna array with phase centers spaced apart in space in the direction of flight of the radar carrier moreover, the polarizations of the antenna arrays are orthogonal to each other and determine two polarization channels, the passband of each of them allows you to receive signals of both carrier frequencies, the frequency spacing of the polarization channels is determined by the spectrum width of the probe signal and permissible changes in the characteristics of the re-reflection of the objects being located, receive the reflected signals separately by four channels (at each frequency two polarization channels) and determine the modules and phases of the components of the polarization matrix of the scattering of the surface area.

 The essence of the method is illustrated by the following reasoning.

 The SAR antenna system is a combination of two antenna arrays with phase centers spaced apart in space by a distance d in the direction of flight of the SAR carrier (see Fig. 1). The polarizations of the antennas are orthogonal to each other and define two polarization channels. Frequency diversity is used in the signal system [1]. The choice of the magnitude of the frequency spacing of the polarization channels is determined by the width of the spectrum of the probing signal and the permissible changes in the characteristics of the rereflection of the located objects.

где q = (ω₁-ω₂)/Δω; Δω - ширина спектра зондирующего импульса, обеспечивающая требуемое разрешение по дальности (q≥1); ω₁ и ω₂ круговые несущие частоты сигналов каналов основных поляризаций; λ₂ - длина волны, соответствующая частоте ω₂; ; R₀ - дальность до картографируемой полоски дальности (ПД); δ_x - линейное разрешение по азимуту; W - скорость полета носителя РСА. Следует учесть тот факт, что при относительно небольшом разносе частот каналов (q=1...2), необходимо синхронизировать моменты излучения передатчиков, для того, чтобы бланкирующие импульсы не приводили к потере информации в соседнем частотном канале.In order for the azimuth resolution in the different frequency channels of the studied SAR to be the same, it is necessary to choose the length of the synthesis intervals (IS) in the frequency channels (CC) based on the known requirements [2]. So, if we conditionally assume that ω ₁ > ω ₂ , then, with a purely lateral view, signal accumulation at frequency ω ₁ will begin after the signal accumulation at frequency ω ₂ begins

where q = (ω ₁ -ω ₂ ) / Δω; Δω is the spectrum width of the probe pulse, providing the required range resolution (q≥1); ω ₁ and ω ₂ circular carrier frequencies of the signals of the channels of the main polarizations; λ ₂ is the wavelength corresponding to the frequency ω ₂ ; ; R ₀ - the distance to the mapped range strips (PD); δ _x - linear resolution in azimuth; W is the flight speed of the PCA carrier. It should be noted that with a relatively small spacing of the channel frequencies (q = 1 ... 2), it is necessary to synchronize the radiation moments of the transmitters so that blanking pulses do not lead to loss of information in the adjacent frequency channel.

(2)
где A_и - амплитуда зондирующих импульсов; N₁, N₂ - число накапливаемых импульсов на первом и втором ИС соответственно;

сигнал единичной мощности, описывающий законы амплитудной и фазовой модуляции; T_и - период повторения зондирующих импульсов; Φ₁ и Φ₂ начальные фазы сигналов каналов основных поляризаций.Thus, the signal emitted by the antenna system can be represented as

(2)
where A _and - the amplitude of the probe pulses; N ₁ , N ₂ - the number of accumulated pulses on the first and second IP, respectively;

a unit power signal describing the laws of amplitude and phase modulation; T _and - the repetition period of the probe pulses; Φ ₁ and Φ _{2 are the} initial phases of the signals of the channels of the main polarizations.

где

поляризационная матрица рассеивания (ПМР) 1-го элемента разрешения;

В этом случае сигнал, отраженный от рассматриваемой полоски дальности, на выходе антенной системы поляризационной РСА может быть представлен в виде

где R $\genfrac{}{}{0ex}{}{\text{ω}}{\text{1}}$ 2(t) - расстояние между 1-м элементом разрешения в полоске дальности (ПД) и фазовым центром антенны i-го частотного канала; с - скорость света.It is convenient to represent the radar elevation model (RLR) of the resolved range of range as a set of L elementary in the polarization sense reflectors located over a distance equal to the azimuth resolution δ _x at a range of R ₀

Where

polarization dispersion matrix (PMR) of the 1st resolution element;

In this case, the signal reflected from the considered range strip at the output of the polarization SAR antenna system can be represented as

where r $\genfrac{}{}{0ex}{}{\text{ω}}{}$$\genfrac{}{}{0ex}{}{}{\text{1}}$ 2 (t) is the distance between the 1st resolution element in the range bar (PD) and the phase center of the antenna of the i-th frequency channel; c is the speed of light.

[1].It is advisable to use the criterion of maximum likelihood functional as a criterion of optimality in the absence of a priori information about the nature of the distribution of the polarization function of the XRD. The development of algorithms for the formation of PP in SAR is reduced to the synthesis of algorithms for assessing PMR for each resolution element. It should be noted that in the case under consideration of a single-position SAR, the equality of the diagonal elements of the PMR is observed, i.e.

[1].

где N₀ - математическая спектральная плотность комплексного шума частотного канала; δ_ij- - символ Кронекера; δ(t₁-t₂) - дельта-функция Дирака; i, j - номера поляризационных каналов (ПК).The noise at the output of the SAR antenna system is approximated by a complex spatio-temporal white noise with the following characteristics

where N ₀ is the mathematical spectral density of the complex noise of the frequency channel; δ _ij - is the Kronecker symbol; δ (t ₁ -t ₂ ) is the Dirac delta function; i, j are the numbers of polarization channels (PC).

где c₀ - постоянный коэффициент, (T_с) - область интегрирования, равная наибольшему интервалу синтезирования; Z(t) - вектор-столбец сигналов на выходе антенной системы РСА; ^Э - операция эрмитова сопряжения.Given that the input signal of the polarization SAR (SAR) is an additive mixture of the useful signal (4) and noise with characteristics (5), the likelihood functional of the estimated parameters of the polarization function of the RLR (3) can be represented as

where c ₀ is a constant coefficient, (T _c ) is the region of integration equal to the largest synthesis interval; Z (t) is the column vector of the signals at the output of the SAR antenna system; ^E - Hermitian conjugation operation.

где n = 0, 1, 2...i=1, 2; - сигнал на выходе фильтра, настроенного на частоту ω_j, приемника i-го ПК после стробирования по дальности, гетеродинирования и сжатия по дальности;

- суть k-й импульс опорной функции i-го ЧК;

Структурная схема РСА в режиме формирования ПП представлена на фиг. 2. Она содержит антенные устройства 1 АУ1 и 2 АУ2, соединенные с антенными переключателями 3 АП1 и АП2. Вторые входы антенных переключателей соединены с выходами передатчиков 4 ПРД1 и ПРД2, входы которых соединены с выходами генераторов опорного сигнала 5 ГОС1 и ГОС2, а входы ГОС1 и ГОС2 соединены с выходами синхронизатора 6 СИНХ. Выходы антенных переключателей 3 АП1 и АП2 соединены со входами широкополосных усилителей радиочастоты 7 УРЧ1 и УРЧ2. Выход УРЧ1 соединен со входами полосовых фильтров 8Ф_ω1 и 9Ф_ω2, и выход УРЧ2 соединен со входами полосовых фильтров 9Ф_ω2 и 8Ф_ω1. Выходы полосовых фильтров соединены со входами приемников 10 ПРМ₁₁, ПРМ₂₁, ПРМ₁₂ и ПРМ22, вторые входы которых соединены с выходами генераторов опорного сигнала 5 ГОС1 и ГОС2. Выходы приемников соединены с входами каналов цифровой обработки 11 КЦ01, КЦ02, КЦ03 и КЦ04, вторые входы которых одновременно являются выходами и соединены с запоминающим устройством 12 ЗУ. Другие выходы каналов цифровой обработки соединены со входами 13 ЭВМ отображения информации. Выход ЭВМ соединен со входом индикатора 14 ИНД.Estimates of the phases and modules of the PMR components of the 1st element of the HRD by the criterion of the maximum likelihood functional are determined according to the following equalities

where n = 0, 1, 2 ... i = 1, 2; - the signal at the output of the filter tuned to the frequency ω _j , the receiver of the i-th PC after range gating, heterodyning and range compression;

- the essence of the k-th impulse of the support function of the i-th ChK;

The block diagram of the SAR in the mode of formation of the PP is shown in FIG. 2. It contains antenna devices 1 AU1 and 2 AU2, connected to antenna switches 3 AP1 and AP2. The second inputs of the antenna switches are connected to the outputs of the transmitters 4 PRD1 and PRD2, the inputs of which are connected to the outputs of the reference signal generators 5 GOS1 and GOS2, and the inputs of the GOS1 and GOS2 are connected to the outputs of the sync 6 synch. The outputs of the antenna switches 3 AP1 and AP2 are connected to the inputs of the broadband amplifiers of the radio frequency 7 URC1 and URC2. The output of the URC1 is connected to the inputs of the bandpass filters 8F _ω1 and 9F _ω2 , and the output of the URC2 is connected to the inputs of the bandpass filters 9F _ω2 and 8F _ω1 . The outputs of the bandpass filters are connected to the inputs of the receivers 10 of the PRM ₁₁ , the PRM ₂₁ , the PRM ₁₂ and the PRM22, the second inputs of which are connected to the outputs of the reference signal generators 5 GOS1 and GOS2. The outputs of the receivers are connected to the inputs of the digital processing channels 11 KTs01, KTs02, KTs03 and KTs04, the second inputs of which are simultaneously outputs and are connected to the storage device 12 of the memory. Other outputs of the digital processing channels are connected to the inputs 13 of the computer display information. The output of the computer is connected to the input of the indicator 14 IND.

Radar works as follows. In the direction of the underlying surface, two orthogonally polarized probing signals are emitted simultaneously at frequencies ω ₁ and ω ₂ formed, respectively, in transmitters PRD1 and PRD2. Signals reflected from targets and the background of the terrain are received by antenna devices 1 and 2, with each antenna device receiving the main polarizing component at one frequency, and the cross-over component at the other, and fed to the inputs of broadband RF amplifiers URCH1 and URC2. Thus, the selection of reflected signals by polarization is carried out in antenna devices. Filtering the reflected signals using band-pass filters f _ω1 and f _ω2 allows you to separate the differently polarized components of the received signal. From the outputs of the filters, the signals are fed to the receivers of the PFP ₁₁ , PFP ₂₁ , PFP ₁₂ and PFP ₂₂ , where the range gating, heterodyning and range compression of the received components occur. The signals from the outputs of the receivers are fed to the first inputs of the digital processing channels KTs01, KTs02, KTs03 and KTs04, the second inputs of which are fed reference function samples from the memory. In KTs01-KTs04 according to expressions (7) - (10), the formation of synthesized antennas takes place in space. The intermediate results of the calculations come from KTs01-KTs04 in the memory and at the next step of the calculations again into the channels of digital processing. The final results of calculations by expressions (7) - (10) from the digital processing channels are sent to a computer for displaying radar information, where a radar image is formed in one of the possible ways. The generated image is displayed on the IND indicator.

 Thus, this radar can improve the quality of the radar image, since it takes into account the full polarization structure of the processed electromagnetic field.

Sources of information
1. Sarychev V.A. - Radio engineering, 1996, N 10.

 2. Sazonov N.A., Scherbinin V.N. - Radio engineering, 1995, N 11.

Claims (2)

1. The method of forming a polarization portrait of the earth or sea surface, which consists in the fact that two synthesized apertures of the antenna are formed at different carrier frequencies of the probe pulses, each aperture is formed with its own synthesis interval, the value of which is calculated on the basis of a constant resolution in azimuth, the range resolution in each channel is kept constant, characterized in that the apertures form each with its own antenna array with phase centers spaced apart in space along the flight of the radar carrier, and the polarization of the antenna arrays is orthogonal to each other and is determined by two polarization channels, the bandwidth of each of them allows you to receive signals of both carrier frequencies, the frequency spacing of the polarization channels is determined by the spectral width of the probe signal and allowable changes in the characteristics of the re-reflection of the located objects, take the reflected signals separately for four channels, at each frequency two polarization channels, and determine the modules and phase components of the field surface scattering matrix

Where

a signal at the output of a filter tuned to the frequency ω _{i of the} receiver of the i-th polarization channel after range gating, heterodyning, and range compression;
λ _i is the wavelength corresponding to the frequency ω _i ;

the essence of the K-th pulse of the reference function of the i-th frequency channel;

the difference in distances between the l-th resolution element and the phase center of the antenna of the i-th frequency channel and the slant range to the site of the earth's surface, the polarizing portrait of which is formed,
n = 0, 1, 2, ...;
j is the imaginary unit;
N _i is the number of sensing pulses in the i-th frequency channel.  2. The device for implementing the method according to claim 1, containing the first and second antenna devices connected, respectively, with the first and second antenna switches, the second inputs of which are connected, respectively, with the outputs of the first and second transmitters, the inputs of which are respectively connected to the outputs the first and second reference signal generators, the inputs of which are connected to the outputs of the synchronizer, the outputs of the first and second antenna switches are connected, respectively, to the inputs of the first and second broadband amplifier radio frequency, characterized in that the polarization of the antenna devices is orthogonal to each other, each device receives signals of both carrier frequencies, the output of the first broadband radio frequency amplifier is connected to the inputs of the first and second bandpass filters, and the output of the second wideband radio frequency amplifier is connected to the inputs of the third and fourth bandpass filters, the outputs of the bandpass filters are connected, respectively, with the inputs of the first, second, third and fourth receivers, the second inputs of which are connected to the outputs the first and second reference signal generators, the outputs of the receivers are connected to the inputs of the first, second, third and fourth channels of digital processing, the second inputs of which are simultaneously their outputs and connected to a storage device, the other outputs of the channels of digital processing are connected to the inputs of a computer designed to display information .

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