RU2447457C2 - Radar method for rapid diagnosis of ocean phenomena from space - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: quasi-mirror mode of scattering a microwave signal by the sea surface is applied using an extended "radio glare" with coaxial horizontal polarisation of radiation and reception; the centre of the viewing field is selected in the mirror point with equal angles of incidence and reflection γ₀ close to Brewster angle; the viewing zone near the mirror point is strobed (excluded from useful information), to form a small-scale field of heights of high waves and a mesoscale field of variations of the average ocean level, a cross-line interferometre is used, having an antenna base l₁ at angle of inclination of the base relative the horizontal of θ=γ₀, and a lengthwise-line interferometre with antenna base l₂ is used to form a small-scale field of orbital velocities of high waves and a mesoscale field of ocean currents. Minimum dimensions of the antenna bases, height of high waves, variations of the average level, orbital velocity of high waves and flow rate are calculated using given formulas. Two versions of realising the method for rapid diagnosis of ocean phenomena are disclosed. The first version is characterised by that, with the aim of global monitoring, two small apparatus are synchronously launched onto ordinary polar orbits, wherein the trajectories of said apparatus are longitudinally spaced apart, and from one of the apparatus, the ocean surface is irradiated with a weakly-directed antenna; the reflected signal is received by the other apparatus; upon transition from the ascending to a descending cycle, the transmitting apparatus becomes the receiving apparatus while the receiving apparatus becomes the transmitting apparatus; several pairs of apparatus on the same orbit are used to ensure the required frequency of radio images. The second version is characterised by that, with the aim of regional monitoring, the given part of the ocean is irradiated with a geostationary apparatus; the reflected signal is received at a low-orbit apparatus; viewing angles and orbit parameters of the apparatus are selected and matched, and to ensure the required frequency of radio images, several receiving apparatus on the same orbit are used.

EFFECT: rapid diagnosis of small- and mesoscale wave fields, flow rate and ocean level.

3 cl, 7 dwg

Description

The invention relates to radar surface of the ocean and can be used both for global monitoring of variable ocean phenomena (for example, geostrophic currents), and for the operational forecast of dangerous phenomena - tsunami waves, storm waves and extreme swell waves (the so-called “killer waves” "). The hydrometeorological service and organizations related to the economic development of marine areas (for example, navigation of ships, design and operation of offshore platforms), as well as organizations engaged in the study of the World Ocean are interested in using the invention.

The two-position principle of radar location of the sea surface is known [1], the work on single-position radar interferometry of the sea surface is also known [2, 3]. Single-position aircraft radars using synthesized aperture radio interferometers are successfully used to diagnose parameters of sea waves and currents [4, 5]. As for space-borne radar interferometers with oblique sounding of the surface, today the Tandem Terra SAR-X project is known [5], where two identical apparatuses in close orbits are used as an interferometer with a “rigid” antenna base, which is required for monitoring marine (non-stationary ) surface. However, oblique sounding from two devices “in one direction” leads to the same problems as in conventional (single-position) SARs: the low reflectivity of the surface and the inability to ensure a field of view of about 2000 km in the diagnosis of mesoscale ocean phenomena.

Patents are known that precede the creation of the Tandem TerraSAR-X system [6, 7, 8]. The first of three patents proposes a layout (in circular orbits) of a large number of satellites, alternately making up “bistatic pairs”, where one of the devices irradiates the Earth’s surface and the second device receives a reflected signal using a synthesized antenna aperture. The features of the proposed method of radar surface survey are that when synthesizing the aperture of the receiving antenna, it is necessary to enter with the necessary accuracy both the parameters of the irradiating signal and the parameters of the orbits.

The second patent (obviously, as the development of the first) proposes the composition and method of bistatic radar interferometry, where the distance between the transmitting and receiving spacecraft is used as an antenna base for a synthetic aperture interference radar (IRSA), and this distance is assumed to be large compared to judging by the given figures, the dimensions of the field of view are of the order of many thousands of kilometers. We consider this patent impossible if the author understands the word “holography” - namely the formation of radar images of the Earth's relief. It will be seen from what follows that the “rigid” antenna base of an interferometer operating over an extended surface consisting of many independent reflectors cannot be greater than a certain value. In our case (the wavelength of the signal is of the order of a centimeter, the height of the orbit is of the order of 1000 km, the width of the signal spectrum is of the order of 100 MHz) - the maximum size of the “rigid” transverse antenna base is of the order of units km.

In the third patent, filed with the participation of the German developer of the aforementioned X-ray system Tandem TerraSAR-X (Deutsches Zentrum fur Luft-und Raumfahrt, DLR), a scheme for visualizing the Earth’s surface, using two independent spacecraft with RSA, when they are brought to close located elliptical orbits with a small eccentricity and a center shifted relative to the center of the Earth. The distance between the devices ("base") does not change much, and the projections of this base on the surface change greatly. It seems to us that this patent is suitable for “soft” interferometry, where both devices independently create “phase portraits” of a stationary surface. The suitability of a patent for “hard” interferometry of a sea (non-stationary) surface should still be proved experimentally.

The prototype of the proposed method is a patent [9]. This patent proposes a method, which consists in using one spacecraft for independent formation of the fields of velocity and level of the ocean surface using an interferometer consisting of only two antennas spaced in the horizontal plane. At small viewing angles, left and right viewing zones are formed here, each of which has a width of about 0.2H at the orbit height H.

Thus, in four well-known patents, the fields of speed and level are proposed to be measured using interferometers located either on two independent spacecraft with large antenna bases, or on one device with small antenna bases (prototype). It seems that the prototype can use both the near-nadir (quasi-mirror) mode of scattering of the sea surface and the Bragg (at not very far from the vertical viewing angles) - however, the width of the field of view in any case remains narrow, which does not provide diagnostic requirements for extended ( mesoscale) ocean phenomena, as well as proper responsiveness.

The aim of the invention is the creation of a space radar tool for the operative diagnosis of small and mesoscale wave fields, current velocities and the level of the ocean surface, which requires a significant expansion of the side-view zone while maintaining the synthesis mode and a significant increase in the energy potential when isolating a phase difference signal.

The goal in this method, which consists in the radar operational diagnostics of ocean phenomena from space, using the illumination of the ocean surface from one device, the interference reception of the reflected signal by another device and the along-path synthesis of the aperture of the receiving antennas, is achieved by using the quasi-mirror mode of scattering of the microwave signal by the sea surface, using an extended “radio image” with coaxial horizontal polarization of the radiation and reception, the center of the field of view is selected in the mirror ke at equal angles γ ₀ of incidence and reflection are close to the Brewster angle, the viewing zone near specular point is excluded from the useful information, apply cross-Open Path interferometer antenna base l _1, with carbon base of inclination relative to the horizontal θ = γ _0, apply pipeline service interferometer antenna base l ₂ , the minimum dimensions of the antenna bases are selected from the relations

where λ is the radiation wavelength, H ₂ is the orbit height of the receiving apparatus, Δf _c is the width of the spectrum of the emitted signal, c is the speed of light, L _x is the size of the synthesized aperture, β ₁ (q) and β ₂ (q) are dimensionless coefficients, determined by the radar parameters and the available minimum background / noise ratio q, when processing difference-phase signals, the heights of large waves and average level variations are calculated from the ratio

- априорный фазовый набег на базе l₁, орбитальную скорость крупных волн и скорость течения вычисляют из соотношенияwhere N is the number of independent samples of a random phase difference φ _n , measured at a site of a given size, located at a viewing angle γ _n from the receiver,

- a priori phase incursion based on l ₁ , the orbital velocity of large waves and the flow velocity are calculated from the relation

- априорный фазовый набег на базе l₂.where W _x is the speed of the spacecraft,

- a priori phase incursion based on l ₂ .

The novelty of the proposed method lies in the use of a “quasi-mirror” mode of scattering of the sea surface (“glare path”) with appropriate optimization of the IRSA — an interference radar with a synthesized aperture — specifically for this mode. Such a system, as calculations show, is in principle capable of ensuring the formation of calibrated wide-format images of three most important fields: a) local heights of large waves and variations in the average level of the ocean surface; b) the local orbital velocity of surface waves and the average velocity of the surface flow; c) local and average slopes of surface waves. The listed fields should be formed in the form of independent radar images (calibrated radar images) with the necessary parameters of sensitivity, spatial resolution and the width of the field of view (up to 2000 km), and, very important, with a small number of small devices in the group.

The possibility of practical implementation.

We turn directly to the justification of the invention, i.e. two-position "marine" IRSA (interference radar with a synthesized aperture) using irradiation of the sea surface from one device, a "quasi-mirror" scattering mode and reception of the reflected signal by an interferometer installed on another device. The geometry of the two-position radar surface sighting is illustrated in the drawing - Fig. 1, where H ₁ , H ₂ - orbit heights, W _x - receiver velocity vector, l ₁ , θ - size and angle of inclination of the transverse antenna base, γ ₀ - mirror angle , R ₀₁ , R ₀₂ - the distance of the mirror point, R _n1 , R _n2 - the distance of the current point, the distance of the points for receiving antennas are indicated by a prime (R _n1 ^′ , etc.). The resolution area has dimensions r _x , r _y , the averaging area has dimensions d _x , d _y . The transverse antenna base l ₁ shown in the figure is used to measure either wave heights (at high resolution) or variations in the average surface level (when averaging at mesoscale sites). The longitudinal antenna base l ₂ necessary for measuring velocity fields is not shown in FIG. 1.

The practical implementation of the method with the location of the "rigid" antenna bases on one receiving device is presented in two versions. In the first embodiment, it is proposed to simultaneously launch two vehicles into equal-altitude polar orbits, the trajectories of which are separated in longitude - the drawing in Fig. 2. In the polar regions, the angles of inclination of the orbits will be very different, however, according to our calculations, the regime of quasi-mirror scattering by an excited surface at such orbits can be maintained up to latitudes of ± 75 °. When switching from an upward to a downward turn, the transmitting device becomes a receiving one, and the receiving one becomes a transmitting one, for which a weakly directed transmitting antenna and IRSA receiving antennas are installed on both starboard and starboard sides. To ensure a short repeatability time of the images, the entire group according to our calculations should consist of 5-6 pairs of similar devices.

An analysis of the ratios characterizing the parameters of the proposed method of radar survey of an excited sea surface leads to the following results. First of all, it was found out that feasible variants of a space two-position radar using the “quasi-mirror” scattering mode with oblique sensing do provide a high reflectivity of the surface: SEC from -5 to +5 dB in the field of view with a width of the order of 2H ₂ (doubled orbit height of the receiving device ), over a wide range of drive wind speeds. Even under calm conditions, the “glare path” persists, which follows not only from our calculations (based on empirical spatial spectra of wave slopes), but also from a number of publications on wave physics in the open ocean. Calculation diagrams of quasi-mirror scattering corresponding to this option of sighting and driving wind speeds of 4 m / s and 10 m / s are shown in the drawings - Fig. 3 and Fig. 4. Closed lines represent the ESR levels from +5 dB to -20 dB. The dimensionless coordinates m _x and m _{y are} determined by the distance from the working platform to the central (mirror) point, referred to the height of the orbit. Calculations show that when using the 3-cm range in the IRSA, the orbit height H = 800 km, the width of the viewing zone ± N (relative to the mirror point) is ensured by the level of SECR equal to 0 dB. If the working background / noise ratio in the IRSA channels is 30 dB, then the required average radiation power is ~ 40 W. In the calculations, the curvature of the Earth was taken into account.

The second implementation option is designed to provide regional diagnostics of ocean phenomena. It provides for irradiation of a given vast ocean area from a geostationary apparatus (GSKA) and the installation of IRSA on small receiving apparatuses (Figure 5). Reception of reflected signals, as in the first version, is carried out by a grouping of small low-orbit devices equipped with IRSA antennas, but only on one side. In this case, it is necessary to coordinate among themselves the parameters of the orbits and the angles of sight of the surface from both devices. Calculation diagrams of quasi-mirror scattering for this option (taking into account the curvature of the Earth) are also available, and with the same background / noise ratio in the IRSA channels, the required average radiation power from the HSCA is ~ 10 kW.

. Этот эффект позволяет использовать режим синтезирования апертуры приемных антенн при сохранении высокой частоты следования импульсов - тогда как в однопозиционных РСА для расширения зоны обзора приходится применять сложные многолучевые антенны на базе управляемых ФАР (фазированных антенных решеток). Выяснено также, что режим синтезирования апертуры, несмотря на иную физику рассеяния (по сравнению с брэгговской), не претерпевает существенных изменений.It was found that at the specular point y ₀ and in a certain region around it, the transverse resolution r _y sharply deteriorates - if we compare it with the resolution of a single-position radar at the same viewing angle. For our purposes, it turns out to be sufficient to exclude from the field of view only 20% of its width, but in return we get an extremely useful effect, namely stretching the horizontal range scale (yy ₀ ) over the entire viewing area with respect to the total slant range

. This effect makes it possible to use the synthesis aperture synthesis aperture mode while maintaining a high pulse repetition rate - whereas in single-position SARs, to expand the viewing area, it is necessary to use complex multipath antennas based on guided headlamps (phased antenna arrays). It was also found that the mode of synthesizing the aperture, despite a different scattering physics (compared to the Bragg one), does not undergo significant changes.

Thus, the necessary wide viewing area is provided with an acceptable transverse resolution dictated by the spectral width of the emitted signal, longitudinal resolution is provided by synthesizing the aperture of the receiving antennas, and the formation of level and velocity fields of the ocean surface is provided by an interferometer with a transverse and longitudinal bases mounted on the receiving device. At the same time, it is precisely the high ESR of a “quasi-mirror” surface, which is 3 orders of magnitude higher than the ESR of the sea surface during Bragg scattering, which makes it possible to significantly reduce the size of the antenna bases placed on the receiving device.

As a result of the analysis and calculations performed with respect to interferometers operating in the “quasi-mirror” exposure mode from the second apparatus, the relations were obtained for the maximum achievable level and velocity fluctuation sensitivity, from which the minimum dimensions are selected (for given orbital and apparatus parameters of the system) antenna bases.

The dependence of the normalized level fluctuation sensitivity on the available energy potential q (background / noise ratio) and radar parameters (dimensionless coefficient β ₁ ) is shown in Fig.6, it is written as

Where

and the normalizing value (level sensitivity as q → ∞) is

In expressions (1-3) are indicated: q - energy potential (minimum background / noise ratio); l ₁ - the size of the transverse antenna base; c is the speed of light; θ is the angle of inclination of the antenna base to the horizon; γ _n is the angle of sight of the site from the receiving apparatus; λ is the radiation wavelength; Δf _c is the signal spectrum width; H ₂ - the height of the orbit of the receiving apparatus; m = (yy ₀ ) / H ₂ - parameter that determines the position of the sighted area on the horizontal axis; d is the size of the sighted (symmetrical) site;

(m=0,2-2) - удельное (на 1 м²) число независимых отсчетов сигнала в пределах визируемой площадки. Из графиков фиг.6 видно, что с ростом энергопотенциала q растет и область рабочих величин β₁, при которых потеря чувствительности относительно предельной (3) еще допустима. Принимая за допустимую величину потерь 1 дБ, при q=10 (20 дБ) получим (β₁)_мин ~0,6, а при q=100 (40 дБ) получим (β₁)_мин ~0,1.

(m = 0.2-2) is the specific (per 1 m ² ) number of independent samples of the signal within the sighted area. From the graphs of Fig.6 it is seen that with an increase in the energy potential q, the range of working quantities β ₁ also increases, at which the loss of sensitivity relative to the limit (3) is still permissible. Taking 1 dB as the permissible loss value, for q = 10 (20 dB) we obtain (β ₁ ) _min ~ 0.6, and for q = 100 (40 dB) we obtain (β ₁ ) _min ~ 0.1.

The dependence of the normalized velocity fluctuation sensitivity on the available energy potential and radar parameters (dimensionless coefficient β ₂ ) is presented in Fig. 7, and is written as

Where

and the normalizing value (speed sensitivity as q → ∞) is

In expressions (4-6) are indicated: Ф (β ₂ ) is the probability integral, L _x is the size of the synthesized aperture, l ₂ is the size of the horizontal antenna base, W _x is the speed of the device, the remaining notations do not differ from the above. From the graphs of Fig. 7 it can be seen that the choice of the parameter β _{2 is} more favorable compared to β ₁ : for q = 10 (20 dB), you can choose (β ₂ ) _min ~ 0.4, and for q = 100 (40 dB) we have (β ₂ ) _min ~ 0.05.

Thus, using expressions (2) and (5), one can find the minimum dimensions of the antenna bases installed on the receiver, where the viewing angle of the site can be replaced by the viewing angle of the mirror point γ ₀ :

The optimal algorithm for processing quadrature signals generated at the output of the correlator is known and consists in the independent accumulation of difference-phase samples of the sine and cosine components, followed by their division into each other and the introduction of an a priori (varying in viewing angle) coefficient. At the same time, variations in signal intensity are strongly suppressed, however, for accurate reproduction of the measured quantity (level or speed), a priori phase shift compensation at the antenna base is also necessary. The heights of large waves (when averaging at small sites) and the average level variations (when averaging at mesoscale sites) are found from the relation

- априорный фазовый набег на базе l₁. Орбитальная скорость крупных волн и скорость течения (на соответствующих площадках) находятся из соотношенияwhere N = d ² N ₀ is the number of independent samples of a random phase difference φ _n , measured on a symmetrical site of a given size d, located at a viewing angle γ _n from the receiver,

- a priori phase incursion based on l ₁ . The orbital velocity of large waves and the current velocity (at appropriate sites) are found from the relation

- априорный фазовый набег на базе l₂.Where

- a priori phase incursion based on l ₂ .

Information sources

1. Rubashkin S.G. et al. Reflection of radio waves by the ocean surface during bistatic radar using two satellites // Radio Engineering and Electronics, 1993, issue 3, p.447-453.

2. Pereslegin S.V., Sinitsyn Yu.P. The restoration of the mesoscale field of the ocean level in the space side-scan radar interferometer // Electromagnetic waves and electronic systems, 1998, No. 5, p. 44-50.

3. Sinitsyn Yu.P., Pereslegin S.V. Potential accuracy and optimal algorithm for reconstructing the mesoscale topography of the sea surface with a side-scan space radar // Earth Research from Space, 2000, No. 1, pp. 51-57.

4. Shulz-Stellenfleth I. and others. Sea Surface Imaging with an Across-Track Interferometric Radar: The SINEWAVE Experiment // IEEE Trans. on Geosc. Rem. Sens., 2001, v. 39, No. 9, pp. 2017-2028.

5. Siegmund R., Vinguan Bao, Lehner S., Mayerle R. First Demonstration on Surface Currents Imaging by Hybrid Along- and Cross-Track Interferometric SAR // IEEE Trans, on Geosc. Rem. Sens., 2004, v. 42, No. 3, pp. 511-519.

6. Krieger G., Moreira A., Fiedler H., HajnsekL, Werner M., Younis M., ZinkM. "TanDEM-X: A Satellite Formation for High-Resolution SAR Interferometry," IEEE Transactions on Geoscience and Remote Sewnsing, Vol. 45, No. 11, Nov. 2007, pp. 3317-3341.

7. US Patent G01S 13/90, No 4.602.257, July 22, 1986. Method of satellite operation using synthetic aperture radar addition holography for imaging, inventor Grisham W.H.

8. US Patent G01S 13/90, No 6.452.532, September 17, 2002. Apparatus and method for microwave interferometry radiating incrementally accumulating holography, inventor Grisham W.H.

9. US Patent G01S 13/90, No. 6.677.884, January 13, 2004. Satellite configuration for interferometric and / or tomographic remote sensing by means of synthetic aperture radar (SAR), inventors Moreira A., Krieger G., Mittermaier J .

10. US Patent G01S 13/90, No 11.235.42, May 18, 2005. Method for producing map images of surface sea current velocity vectors and altimetric radar system using the method, inventor Buck C.H.

Claims (3)

1. A radar method for the operational diagnostics of ocean phenomena from space, using the illumination of the ocean surface from a transmitting spacecraft, interference receiving a reflected signal by a receiving spacecraft and synthesizing the aperture of the receiving antennas, characterized in that a quasi-mirror microwave signal scattering mode is used when extracting the difference-phase signal the sea surface, using an extended "radio image" with coaxial horizontal polarization of radiation and reception, center ony review selected specular point at equal angles γ ₀ of incidence and reflection are close to the Brewster angle, the viewing zone near specular point is excluded from the useful information, apply cross-Open Path interferometer antenna base l _1, with carbon base of inclination relative to the horizontal θ = γ ₀ use a long-track interferometer with an antenna base l ₂ , the minimum dimensions of the antenna bases are selected from the relations

where λ is the radiation wavelength, H ₂ is the orbit height of the receiving apparatus, Δf _c is the width of the spectrum of the emitted signal, c is the speed of light, L _x is the size of the synthesized aperture, β ₁ (q) and β ₂ (q) are dimensionless coefficients, determined by the radar parameters and the available minimum background / noise ratio q, the optimal algorithm is used when processing difference-phase signals, eliminating the influence of signal intensity on the measured parameter, large wave heights and average level variations are calculated from the ratio

where N is the number of independent samples of a random phase difference φ _n , measured at a site of a given size, located at a viewing angle γ _n from the receiver,

- a priori phase incursion based on l ₁ , the orbital velocity of large waves and the flow velocity are calculated from the relation

where W _x is the speed of the receiving spacecraft,

- a priori phase incursion based on l ₂ . 2. The method according to claim 1, characterized in that two spacecraft are synchronously launched into polar orbits, the trajectories of which are distant in longitude, the ocean surface is irradiated with a weakly directed antenna from the transmitting spacecraft, the reflected signal is received by the receiving spacecraft. 3. The method according to claim 1, characterized in that the specified ocean area is irradiated from a geostationary transmitting spacecraft, the reflected signal is received on a low-orbit receiving spacecraft.

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