RU2597148C1 - Method of measuring vector field speed of ocean and river streams in space sar - Google Patents

Abstract

FIELD: radar.

SUBSTANCE: method of measuring vector field speed of extended surface relates to radar location of Earth's surface from spacecrafts and can be used for simultaneous formation of brightness and vector-speed portraits of river and ocean currents with required spatial resolution and binding to area coordinates. Method is suitable for use in two known versions of radar speed measurement - interference and Doppler, that is, in normal SAR and ISAR with longitudinal antenna base.

EFFECT: technical result is simultaneous use of two beams, symmetrically deflected at angle of ±β from traverse, that enables using projections of tangential and radial velocity components of reflector on both beams, as well as aperture synthesis algorithms properties, to calculate both speed components for each of allowable sites in wide area by range.

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Description

The invention relates to radar surface of the Earth from aircraft and can be used to form high-speed portraits of an extended surface - ocean and river currents. Various algorithms for processing the signal received by a synthetic aperture radar antenna (SAR) provide not only a high spatial resolution r _x along the longitudinal axis (axis of movement of the antenna), but also the possibility of separate formation of brightness and high-speed radar images. At the same time, the properties of the synthesizing algorithm should not impede the formation of a “four-dimensional” radar image of the area, where the image of moving objects is superimposed on the usual (brightness) picture showing the position of stationary (natural and artificial) objects. The direction of movement is usually indicated by an arrow oriented to the countries of the world, and the speed module by color. A similar problem also relates to the formation in SAR of images of sea and river currents against the background of the coastline and stationary objects on land.

In [1, 2], processing methods are proposed that make it possible to determine the velocity vector of a local reflector from an airplane, but only at high speeds sufficient to shift the reflector by dozens of longitudinal and transverse resolution elements during the synthesis. There are also known methods of signal processing in SAR, which make it possible to measure low speeds of objects on the Earth's surface. Including a method that allows to restore the radial velocity of marine mesoscale currents from the displacement of the median of the Doppler spectrum [3, 4]. In this case, the measured speed is (1-100) cm / s, and for its measurement it is necessary to accumulate signal samples on kilometer-long platforms, including several thousand independent resolution elements. The TerraSAR-X spacecraft was developed and put into operation, using an interferometer with a longitudinal antenna base [5] to form high-speed portraits at low speeds of objects with better spatial resolution - but only for the radial component of the velocity. In [6], various aspects and features of the interference method were considered. There are also patents [7, 8], where the radial component of the velocity is measured in the interference SAR (IRSA) with a longitudinal base using the traditional (differential-phase) synthesis algorithm. The difference-frequency synthesis algorithm for IRSA was proposed in a recently obtained patent [9], where a certain optimization of the processing algorithm is performed when forming a high-speed portrait of the surface, but again for the radial component of the velocity.

This application proposes a two-beam method that allows you to measure both components of the speed when using various signal processing algorithms. The multibeam method of vector radar measurements is itself known - for example, it has long been used in space microwave scatterometers when measuring the wind velocity vector above the sea surface using spatial anisotropy of sea waves [10].

The prototype of the proposed method can serve as a patent [8] for IRSA.

In [6], it was shown that the azimuthal response of the IRSA velocity channel, to a first approximation and without using optimization [9], has the form:

where U ₀ is the signal amplitude at a real (non-synthesized) aperture; N = L _x / W _x T _r - the number of coherently accumulated pulses at the size of the synthesized aperture L _x , the apparatus velocity W _x and the pulse repetition period T _r ; r _x = λR _n / L _x is the longitudinal (azimuthal) resolution at the wavelength of the signal λ and the slant range R _{n of the} sighted area; x _V = V _y R _n (sinγ _n ) / W _x - spatial shift due to the radial velocity V _{y of the} site, sighted at an angle γ _n ; ψ _n = arctan (2πf _dy T _r ) is the detected phase shift inside the amplitude peak, where f _dy = 2V _y sinγ _n / λ is the radial Doppler shift. When measuring low velocities from a spacecraft, it is permissible to take sinψ _n : ψ _n = 2πf _dy T _r . It can be seen that the measured velocity V _y in this case affects both the phase of the synthesized signal and its spatial (azimuthal) shift. At typical values of T _r = (0,1-0,3) · 10 ^-3 s for space and small Irsay measured speeds V _y <3 m / s the Doppler shift is less than 100 Hz, and the phase does not exceed 10 degrees. As for the spatial shift, under these conditions it is significant, amounting to ~ 200 m at a speed of V _y = 3 m / s. This means that it can be neglected by measuring the velocity of weak currents with the corresponding azimuthal resolution (r _x >> x _V ).

The formation of the luminance radar image in the SAR or IRS is determined by the exponential factor in expression (1), which can be represented as a function of the spatial coordinate (x), as well as the time coordinate (t = x / W _x ) and frequency coordinate (f = 2xW _x / λR _n ). In the latter case, the amplitude peak in expression (1) is as follows:

играет роль разрешающей способности по частоте. Таким образом, можно измерить сдвиг f_dy и без помощи интерферометра, что известно из работ [3, 4].where is the value

plays the role of frequency resolution. Thus, the shift f _dy can be measured without the help of an interferometer, as is known from [3, 4].

To measure both components of the velocity vector, it is proposed to use the azimuthal rotation of the radiation plane by a small angle (β ~ 15 °), i.e. artificial angle of demolition. The solution consists in the formation of two beams, each of which is projected both the measured speed of the reflector (V _x , V _y ), and the compensated speed of the apparatus (W _x ) and the speed of rotation of the Earth (W _E ) (Fig. 1). As follows from FIG. 1, the total radial Doppler shifts taking into account the speed of the vehicle W _x and the Earth's rotation speed W _E = W _E0 cosα (W _E0 = 462 m / s, α is the latitude of the place) - for the left and right beams of the antenna are

We introduce the compensation of the Earth's rotation speed W _E and the vehicle speed W _x by means of the corresponding frequency shifts of the signal in the left and right receiving channels:

и

.

and

.

Then, in accordance with expression (2), at the outputs of two independent IRSA channels, we obtain phase shifts

Adding and subtracting these shifts, you can get both speed components for each site with achievable accuracy at a given resolution:

, что в космических условиях составляет ~30 с, т.е. на два порядка превышает время синтеза. При малой скорости площадки, когда соблюдается условие V_x<r_x/Δt, данный метод, по-видимому, позволяет измерить вектор скорости течения. Расчеты показывают, что современный космический ИРСА сможет измерить вектор скорости течения с точностью ~3 см/с - при достижимом в этом случае размере симметричной площадки d=r_x~100 м [6]. Такие параметры являются намного лучшими по сравнению с параметрами, реализованными с использованием сдвига доплеровского спектра в РСА [3].There are certain restrictions. The time delay between measurements in two beams is

, which under cosmic conditions is ~ 30 s, i.e. two orders of magnitude longer than the synthesis time. At a low site velocity, when the condition V _x <r _x / Δt is met, this method, apparently, allows us to measure the current velocity vector. Calculations show that a modern space-based IRSA will be able to measure the current velocity vector with an accuracy of ~ 3 cm / s - with the achievable size of a symmetrical area d = r _x ~ 100 m [6]. Such parameters are much better than those implemented using the shift of the Doppler spectrum in SAR [3].

,

, а затем - составляющие скоростиNevertheless, the proposed two-beam method is advantageous for use not only in IRSA, but also in conventional PCA. In addition to the effect considered, i.e. Comparison of both components of the speed instead of one, it turns out to be unnecessary to compare the measured Doppler shift with the shift of a fixed site, because the second beam of the antenna plays this role. In this case, using expression (3) with the same compensation of the speeds of the vehicle (W _x ) and the rotation of the Earth (W _E ), we find for the left and right beams their total and difference Doppler shifts:

,

and then the components of speed

The functional diagram of the proposed method for measuring the current velocity vector in the SAR is presented in FIG. 2, where are indicated: 1 - two-beam antenna, 2 - mixers of the left and right channels; 3 - synthesizers of the left and right channels; 4 - reference signal generator; 5 - calculators of the output parameters of the left and right channels; 6 - calculator of the measured parameters with the formation of the azimuthal line of the brightness and speed images; 7 - multidimensional display; 8 - time synchronizer; 9 - sensor navigation parameters, 10 - input navigation parameters, 11 - input preset parameters of the radar image.

The formation of the azimuthal line of the brightness and vector-speed images is as follows. Signals from the two-beam antenna (1) entering the mixer (2) where they are given to compensating the frequency shifts F _E and F _x. Then each of these signals goes to the synthesizer (3), where the reference signal from the generator (4) also arrives. Then, in each channel, a calculator (5) comes into play, generating video signals with amplitudes proportional to the amplitude and Doppler shift of the response of the moving platform. As a result, two pairs of signals are formed for the left and right rays, which determine, in addition to the intensity of the reflected signal, the modulus and direction of the velocity vector. The calculator (6) separates the velocity components (V _x , V _y ), calibrates them, calibrates the signal intensity, and generates a line of brightness and speed images taking into account the delay when probing the same area with left and right beams. The signals thus formed are fed to a multidimensional display (7), which in one way or another displays simultaneously the intensity (brightness) of each image element, the magnitude and direction of the element's speed. To ensure consistency and accuracy of measurements, a time synchronizer (8) and a navigation parameter sensor (9) are used to control the azimuthal scan of the display, the delay Δt varying in range, the frequency of the reference signal and generating frequency shifts F _E and F _x . When measuring the velocity vector in the IRSA, where the antenna has two sections, the functional diagram of FIG. 2 does not change - of course, with more complex circuit diagrams of the antenna itself (1), synthesizers (3) and the synchronization unit (8), which is known from the literature [6].

Information sources

1. Objects of radar. Detection and Recognition, ed. A.V. Sokolova / M., Radio engineering, 2007, chapter 4: Radar image of the target during aperture synthesis with ultra-high resolution synthetic-aperture radar, p. 117-128.

2. Pettersson M.I. Detection of Moving Targets in Wideband SAR // IEEE Trans, on Aerospace and Electronic Systems, 2004, v. 40, No. 3, pp. 780-786.

3. Dostovalov M.Yu., Neronsky LB, Pereslegin S.V. The study of the velocity field of ocean currents according to the phase-measuring data obtained by the SAR of the ERS spacecraft // Oceanology, 2003, v. 43, No. 3, p. 473-480.

4. Neronsky L. B., Dostovalov M. J., Pereslegin S. V. The extended algorithms for Doppler centroid estimation // Proc. EUSAR-2004, Ulm, Germany, May 2004, v. 2, pp. 709-712.

5. Romeiser R., Suchand S., Hartmut R., Steinbrecher U., Grimier S. First Analysis of TerraSAR-X Along-Track InSAR-Derived Current Fields // IEEE Trans, on Geoscience and Remote Sensing, v. 48, No. 2, pp. 820-829.

6. Pereslegin S.V., Halikov Z.A. Signal processing in synthesized aperture radars during restoration of velocity fields of the Earth’s surface // Izv. Universities. Radiophysics. 2014, No 1, p. 1-13.

7. Martin Suss, Werner Wiesbeck. Side-looking synthetic aperture radar system / ER Patent, Number 1.241.487. B1, Data of filing 03/15/2001.

8. Takashi Fujimura. Along-track interferometric synthetic aperture radar / US Patent, Number 5.945.937, Data of patent Aug. 31, 1999.

9. Pereslegin SV, Zakharov A.I., Halikov Z.A., Ivonin D.V., Dostovalov M.Yu., Shapron A. Method for measuring the radial speed of a reflector in a side-scan radar with a synthesized aperture // Patent for invention No. 2537788, priority 09/10/2013.

10. A.I. Baskakov, T.S. Zhutyaeva, Yu.I. Lukashenko. Locational methods for the study of objects and environments / M., IC "Academy", 2011, chapter 5.

Claims (1)

A method of measuring the velocity vector field of ocean and river currents with a synthetic aperture space radar using interference or Doppler methods for measuring radial velocity, characterized in that two independent beams are formed in the antenna, symmetrically deflected in azimuth by an angle of ± β, in each of the independent channels, programmable along the trajectory shift of the carrier frequency by a certain amount depending on the angle β, the angle of sight of the site, the speed of the device and the speed of rotation of the Earth and at this latitude, after the synthesis of the azimuthal radiation pattern and measuring the speed of the site in each channel, a pair of signals are obtained whose amplitudes reflect the scattering intensity and the velocity vector of the site located at a given range, by comparing the amplitudes of the two pairs of speed signals, the radial and tangential components of the speed are calculated each site, form the azimuthal line of the brightness and vector-speed image of the area on a multidimensional display, taking into account the delay, determine adjustable apparatus speed, angle β and angle of sight of the site.

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