JP2507897B2 - Imaging method of object by radio radiometer - Google Patents

Description

TECHNICAL FIELD The present invention relates to an imaging radiometer generally mounted on a mobile body such as an artificial satellite or an aircraft and used for remote sensing, and more specifically, it can be configured by a small antenna. An interferometer-type radiometer with a fixed antenna base line is moved relative to the target object to continuously receive radiated electromagnetic waves emitted from the target object, and aperture synthesis processing is performed on the coherence function obtained as the interferometer output. By applying
The present invention relates to an imaging method of an object by a radioradiometer capable of continuously detecting a wide range of targets with high spatial resolution in the moving direction.

[Conventional technology]

Conventionally, for passive remote sensing of the earth's surface from a mobile body such as an artificial satellite or an aircraft by radio waves, as shown in FIG.
A radio radiometer equipped with an actual aperture antenna that is determined by the size of is often used. FIG. 8 is a diagram for explaining the relationship between the dividing ability of the antenna A of the radiometer installed on the artificial satellite and the ground surface T, and B is the antenna beam.
P is the ground surface spatial resolution, D is the traveling direction of the satellite, S is the scanning direction, S _W of the antenna beam B shows the observed width of the ground surface T. Here, the positional relationship which is sufficiently far from the size of the device is represented by a dotted line. In this method, in the traveling direction D of the moving body, the antenna beam B moves accordingly,
Also, in the scanning direction S orthogonal to this, two-dimensional observation of the ground surface T is performed by electrical scanning in the case of antenna beam B scanning by antenna A mechanical rotation or antenna array. For this reason, the spatial resolution P on the ground surface T is determined by the antenna beam B, and the beam width is inversely proportional to the size of the aperture surface of the antenna A and proportional to the wavelength of the radio wave. Therefore, the antenna A must be increased or the wavelength of the radio wave must be shortened. In the passive remote sensing, since the optimum observation wavelength is almost determined by the physical information to be measured, there is a drawback that the antenna A must be made large in order to obtain a high ground surface spatial resolution P. The addition of a large antenna is limited due to reasons such as the difficulty of operation of the moving body and the obstruction of the mounting of other observation equipment. For example, the ground surface T of the currently launched satellite-based radiometer is limited. Spatial resolution P
The fineness of is about 10 km or more even in the frequency band of 10 GHz or more.

Further, conventionally, the aperture synthesis method using a radio interferometer based on the Fantittel-Zernike theorem as a method of observing incoherent signals is known as a method of realizing a spatial resolution equivalent to a large antenna without using a large antenna. is there. This method, as used in astronomical observations in radio astronomy, specifically combines two small antennas on the ground and measures the correlation between their output signals. Repeat for different combinations, or as shown in FIG. 9, while changing the relative positions of the _two small antennas A ₁ and A ₂ that make up the movable radio interferometer installed on the ground surface T, the same. Is performed so as to cover a wide area of the antenna A, and the two-dimensional correlation function (coherence function) thus obtained is subjected to aperture synthesis processing to obtain an antenna A having a high spatial resolution equivalent to the size of the antenna A. The radiation intensity distribution in the sky can be obtained by the virtual beam B _S created by. Note that C is a signal correlator.

With this method, even if the individual antennas A ₁ and A ₂ are small, the same high spatial resolution as that of an antenna of the same size as the maximum antenna spacing (baseline length) of the radio interferometer can be obtained. If so, it is necessary to measure the coherence function up to the longest possible baseline length. For this purpose, it is necessary to install many antennas on the ground or move the antennas over a wide area. Therefore,
As a method of reducing the number of antennas and moving as much as possible, a method known as radio astronomy is supersynthesis in which the base line of a radio interferometer with two antennas is rotated by utilizing the rotation of the earth.

In the case of the radio interferometer as shown in Fig. 9, this is because one of the radio interferometers that composes the radio interferometer for the electromagnetic waves coming from the fixed sky when the earth rotates while the antennas A ₁ and A ₂ are fixed on the ground. This is a method in which measurement is performed while another antenna is rotating around the antenna, and the annular coherence function can be measured in a wide area of the antenna A. In order to obtain the measurement data in a plane by this method, the antenna interval is changed and the above measurement is repeated.

[Problems to be Solved by the Invention]

As described above, the conventional method of aperture synthesis using a radio interferometer faithfully implements the Fannitel-Zernike theorem, but it takes time as represented by the radiation intensity distribution in the sky in radio astronomy. It was limited to objects that did not change enough to be observed. However, in applications such as remote sensing that observes the earth from low-orbit satellites, the satellites pass over the ground surface T for a short time, so they stop over and acquire data for aperture synthesis. It is impossible to do so, and it is difficult to realize the method of measuring the coherence function by moving a small number of small antennas A ₁ and A ₂ as used in the radio astronomy.

The present invention has been made to solve the above problems, and an image of a target object can be obtained without rotating the antenna base line of the interferometer or moving the relative positions of a plurality of antennas constituting the interferometer. An object of the present invention is to provide a method of imaging an object with a radiometer.

[Means for solving the problem]

The method of imaging an object with a radioradiometer according to the present invention is basically a radio wave radiation consisting of an interferometer with a fixed antenna base line, which has a signal correlator and uses two antennas at a fixed distance. The radiometer is an interferometer that moves when the radiometer is moved by moving the meter on a moving object such as an artificial satellite and moving it with respect to the target object, receiving radio waves radiated from the ground surface, etc. The target is detected by measuring the change in output over a wide moving range and subjecting the obtained coherence function to aperture synthesis processing in the moving direction.

[Action]

In the present invention, high spatial resolution along the moving direction can be achieved without rotating the antenna base line of the interferometer or moving the relative positions of the plurality of antennas forming the interferometer, and without using a large number of antennas. Since this is realized and the observation portion can be continuously moved in the traveling direction, imaging of a wide range of targets can be obtained.

〔Example〕

 An embodiment of the present invention will be described in detail below.

Since the present invention was devised for remote earth sensing, description will be given assuming an artificial satellite as a moving body and a ground surface as a target.

An embodiment of the present invention is shown in FIG. Since this embodiment uses the relative movement of the artificial satellite or the like with respect to the ground surface, the high spatial resolution obtained by this is 1
Only dimension. The extension to the two-dimensional resolution will be described later. First, the operation will be described with respect to one dimension in the moving direction. Also, a form in which a plurality of base lines composed of a plurality of antennas are used at the same time is possible, but this can be easily expanded from the basic single base line of two antennas described below. Therefore, in FIG.
It shows the radio interferometer configuration of each antenna and the positional relationship with the ground surface. The object to be observed is a continuous surface,
For convenience of explanation of the operation, in FIG. 1, the explanation will be given by using the minute ground surface T as a target. Furthermore, since the locomotion of the low-orbit satellite with respect to the earth is an orbiting motion, its trajectory is strictly an arc shape, but it can be regarded as a linear motion locally, so the following explanation assumes a linear motion. And

The S _R circuit of the radio wave radiometer mounted on the artificial satellite is a radio wave interferometer composed of _two antennas A ₁ and A ₂ and a signal correlator C. The two antennas A ₁ and A ₂ are the radiometer S _R
Has a beam width capable of continuously receiving the radiated noise electromagnetic wave R from the ground surface T while moving from the position P ₁ to the position P ₂ along the moving direction of p. The antennas A ₁ and A ₂ are separated by a fixed base length (corresponding to the distance between the antennas) d in the vertical plane. The base line (segment connecting the antennas A ₁ and A ₂ ) L is installed at an angle α with respect to the vertical line, and the ground surface T is located on the extension of the base line L at the measurement start position P ₁ . Decide In addition, τ indicates a time delay.

The positions P ₁ and P ₂ are preferably symmetrical with respect to the ground surface T. The reason for this is as shown in FIG.
This is because the angle characteristic of the brightness temperature T _B representing the radiant intensity with respect to the incident angle θ _inc is flat in the portion close to the vertical incidence for both the vertical polarization V _P and the horizontal polarization H _P. Because of this, the position
Choosing P _1, P ₂ symmetrically minimizes variations in brightness temperature T _B of the ground surface T in measurement related to aperture synthesis is an important condition for maintaining the measurement accuracy. Also, the radiometer S _R
The oblique use of the baseline L of is a feature essentially necessary to obtain the maximum coherence function during the measurement from position P ₁ to position P ₂ . The output X ₀ of the signal correlator C is obtained as the signal correlation output of the _two antennas A ₁ and A ₂ which receive the noise radiation radio wave R radiated from the ground surface T. This output X ₀
Changes during the time that the radiometer S _R moves from the position P ₁ to the position P _2, so this is recorded as a time function, and the aperture synthesis processing is performed to obtain a reproduced image of the ground surface T. When the reception band of the radio wave radiometer S _R is wide, the time delay τ is added to the lower antenna A ₂ to adjust the time difference between the signals reaching the _two antennas A ₁ and A ₂ .
Since the artificial satellite is continuously moving with respect to the ground surface T, the above signal recording is continuously performed, and the same processing is performed on each part of the ground surface T to obtain a continuous reproduced image of the ground surface T. Obtainable. Further, the moving direction p of the moving body is not limited and can be reversed.

Next, it is shown that this method achieves an operation almost equivalent to that of a basic two-antenna interferometer with a variable base line that faithfully realizes the Fancittel-Zernike theorem, and that it has the same spatial resolution. It is shown below.

FIG. 3 shows the angular relationship between the antennas A ₁ and A ₂ and the ground surface T which moves relative to them in the system fixed to the radiometer S _R. The angle formed by the base line L with respect to the vertical direction is α, and the direction of the ground surface T measured from the base line L is θ. The angle range of θ for observing the ground surface T is from 0 to the maximum angle θ shown by the straight lines B ₁ and B _2.
Set to _max . Although the value of θ _max is theoretically arbitrary, it is desirable that θ _max = 2α as described above. This condition means that the range of directions in which the antennas A ₁ and A _{2 view the} ground surface T is set symmetrically with respect to vertical incidence. Also,
The effective maximum value of the inclination angle α of the base line L is 45 °, and at this time θ _max = 90 °, the reception range is up to the direction orthogonal to the base line L. In addition, N has shown the vertical direction.

In order to roughly evaluate the spatial resolution, FIG. 4 shows an operation explanatory diagram in comparison with a system of a variable base length interferometer having two basic antennas A ₁ and A ₂ . This considers a system in which the line (line of sight) connecting the radiometer S _R and the ground surface T is fixed. The relative movement between the radiometer S _R and the ground surface T has the same effect as the rotation of the base line L with respect to the ground surface T (position P _e1 to position P _e2 ), and the coherence function that changes with it has a signal correlator. Obtained from the output X _{0 of} C. The antenna distance d, which changes with the rotation of the base line L, is divided into a line-of-sight direction component and a component orthogonal to this direction, and normalized by the wavelength (λ) as follows:
Expressed as u.

In FIG. 4, S _L is the line-of-sight direction, P _e1 and P _e2 are relative position changes of the antennas A ₁ and A ₂ , ξ is an angle measured from the line-of-sight direction S _L , and K ₁ and K ₂ are the line-of-sight. shows the slope of the ground surface T with respect to the direction S _L.

The coherence function obtained as the interferometer output is a function of the antenna spacing u and w. Antenna spacing w in line-of-sight direction S _L
Since the change of does not contribute to the spatial resolution with respect to the other surface T having the line-of-sight direction S _L , if the time delay τ corresponding to the equation (2) is inserted into the output of the antenna A ₂ to compensate for the time, Except that the inclination of the ground surface T on the extension of the line-of-sight direction S _L changes with the movement of the total S _R , a one-dimensional basic variable baseline length interferometer as shown in FIG. It is almost equivalent to observing the target object in front. If the signal bandwidth is narrow, phase compensation can be used instead of time compensation. In this case, phase correction can be performed at the same time as aperture synthesis processing is performed on the coherence function. In the following, a rough evaluation of the resolution will be described by approximating the basic variable baseline length interferometer of FIG.

Fourth coherence function C (u) obtained by a function of the brightness temperature distribution angle measured from immediately below direction xi] near the ground surface T as an output of the radio interference meter for a T _B (xi]) as shown in Figure follows as become.

Here, m = sin ξ (4). Since T _B (ξ) and C (u) are in a Fourier transform relationship, the function T _B (ξ) which is a reproduced image of the brightness temperature distribution is obtained by performing an inverse Fourier transform on the measured coherence function as follows. You can ask.

Since C (u) is a complex conjugate function, the normalized antenna interval u can be processed by the equation (5) by measuring only the positive region. In such aperture synthesis processing, the spatial resolution can be evaluated by the maximum antenna spacing (u _max ) of the interferometer. According to the equation (1), u is the maximum value u when θ = 2α.
take _max , so Is. Assuming that the window function of the aperture synthesis processing of the formula (5) is rectangular, the obtained angular resolution is 0.88 / u _max.
Is. Therefore, if the altitude of the artificial satellite equipped with this radiometer S _R is h, the ground surface resolution δ _a is expressed by the following equation.

Specifically, let us calculate the ground surface resolution assuming a low-orbit satellite mounting system. As an example, the antenna spacing d
= 1000 wavelengths, altitude h = 700 km. The antenna spacing is 10 m at 30 GHz and about 210 m at 1.4 GHz. The ground surface resolution is δ _a = 616 m when the inclination of the baseline is α = 45 ° and δ _a = 1.8 Km when α = 10 °.

Equation (7) is the approximate resolution on the ground surface T evaluated by approximating a basic variable baseline interferometer, and as shown in Equation (6), an antenna with a length of dsin (2α) Is equivalent to using. Since it is difficult to formulate a strict spatial resolution, the result of a specific numerical calculation example is shown in FIG.

In FIG. 6, in order to compare the example of the radiometer system of the present invention (d = 1000 wavelength, α = 45 °, altitude = 700 Km) with a radioradiometer having an antenna of the same base line length d. The point image response on these ground surfaces T is shown. The point image response is an example of image formation on a single minute symmetrical object, and the smaller the spread of the image formation result and the closer to the point, the higher the spatial resolution. The vertical axis of FIG. 6 represents the imaging intensity and the horizontal axis represents the spatial spread. In the case of the radioradiometer S _R of the present invention,
As shown by the curve S _a, it has a small peak in the side lobe region, but it has a spatial resolution equivalent to that of the huge antenna with a real aperture shown by the curve f _a . By using the present invention, it is possible to realize a fine spatial resolution in the moving direction p of the moving body by using one set of small antennas instead of the huge antenna.

Although the above description relates to one dimension in the moving direction p of the moving body, two-dimensional spatial resolution is necessary for the radioradiometer S _R for remote sensing mounted on an artificial satellite or the like.

As shown in FIG. 7, a plurality of small antennas a are arranged in a direction orthogonal to the moving direction p to form two-dimensionally expanded antenna elements A _a 1 and A _a 2, and two antenna elements in front and rear are provided. By constructing a radio interferometer for each combination of antennas, it is possible to easily expand to a system that can measure a two-dimensional coherence function, and realize two-dimensional spatial resolution by aperture synthesis. In addition to this method, it is easy to expand the two-dimensional spatial resolution by using an antenna having directivity in a direction orthogonal to the moving direction such as a multi-beam antenna. Furthermore, even if the two-dimensional expansion is performed, the spatial resolution in the moving direction p is realized by the small antennas of the interferometers that are separated by a constant interval.
As shown in FIG. 7, it is practical that it can be mounted on the satellite body with little influence on it. Moreover, since a large antenna is not required in plan view, it can be easily mounted on an aircraft.

〔The invention's effect〕

As described in detail above, the present invention receives an electromagnetic wave radiated from a target object while moving an interferometer type radioradiometer having a fixed antenna base line composed of a plurality of fixed antennas and a signal correlator with respect to the target object. , A coherence function obtained by measuring the change in output with movement is subjected to aperture synthesis processing in the movement direction to detect targets distributed over a wide range with high spatial resolution in the movement direction. A small antenna that can be mounted on a small moving body and that has little effect on other devices can be used to achieve high spatial resolution that was not possible with conventional technology. It is extremely useful as a method.

[Brief description of drawings]

FIG. 1 is a schematic elevation view showing an embodiment of the present invention, and FIG. 2 is a schematic diagram showing an incident angle characteristic of microwave luminance temperature on the ground surface which is mainly targeted by a radiometer used in the present invention.
FIGS. 3 and 4 are diagrams for explaining the corresponding operation, FIG. 3 is a diagram in which the coordinate system is fixed to the radioradiometer, and FIG. 4 is the coordinate system in which the radioradiometer is the ground surface. FIG. 5 is a diagram fixed to the line of sight connecting the two, and FIG. 5 is a one-dimensional interferometer with a basic variable base length composed of two antennas which is referred to in order to approximately explain the performance of the present invention in FIG. Schematic of No. 6,
FIG. 7 is a diagram showing an example of numerical calculation of point image response for comparing the spatial resolutions of the radioradiometer of the present invention and the radioradiometer using a large real aperture antenna according to the conventional method. FIG. Sketch drawing showing an example of expanding to a dimension,
Fig. 8 is a diagram showing the relationship between the radiometer antenna mounted on an artificial satellite used for conventional ground surface remote sensing and the ground surface resolution, and Fig. 9 has been conventionally used for observing astronomical objects in radio astronomy. It is an explanatory view of a radio interferometer installed on the ground. In the figure, S _R is a fixed base line interferometer type radiometer, A ₁ and A ₂ are antennas, d is a base line length, C is a signal correlator, L is a base line, X ₀ is an output of the signal correlator, and τ is Time delay, α is the inclination angle of the baseline, P ₁ and P ₂ are the measurement start and end positions, p is the moving direction, T is the ground surface, and R is the radiated noise electromagnetic wave.

Claims (1)

(57) [Claims] 1. An electromagnetic wave radiometer of an interferometer type having a fixed antenna base line composed of a plurality of fixed antennas and a signal correlator, receives electromagnetic waves emitted from the target object while moving with respect to the target object, and is accompanied by the movement. The coherence function obtained by measuring the change in the output is subjected to aperture synthesis processing in the moving direction, and the target of the radioradiometer characterized by detecting the target distributed in a wide range with a high spatial resolution in the moving direction. Imaging method.