JP2011237337A - Synthetic aperture radar device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a synthetic aperture radar device using an array antenna in which a high directional gain is held while holding high aperture efficiency and achieving an antenna pattern of a low side lobe.SOLUTION: At a time of transmission, all Nantenna elements 2 in an array antenna 1 are used. Npieces of antenna elements 2 are used in a manner so that the azimuth of the first side lobe of a transmission antenna pattern and the azimuth of a first null of a reception antenna pattern are matched with each other at a time of reception. Synthesized and received two beam signals are synthesized in a synthetic processing part 10, and a SAR image is generated in a SAR processing part 11.

Description

  The present invention relates to a synthetic aperture radar apparatus (SAR system) that uses a plurality of transmission / reception beams to perform observation while forming a low sidelobe antenna pattern while maintaining a high directivity gain.

  Conventionally, SAR is well known as a sensor capable of imaging the ground surface with high resolution regardless of weather such as clouds in the field of remote sensing using an artificial satellite or an aircraft.

  In the SAR system, observation is realized by transmitting a radio wave from an antenna toward the ground surface and receiving a radio wave reflected by the ground surface. In order to avoid imaging as ambiguity), a low-sidelobe antenna having excellent directivity is used as an antenna so that radio waves are irradiated only to a desired observation region.

  The antenna side lobe can be reduced by combining the apertures with appropriate weights. In this case, however, the aperture efficiency decreases and the gain decreases even with the same aperture area. It is known.

  In recent years, in order to realize high resolution of the SAR system, a multi-beam SAR system that forms a large number of received beams while widening the beam width by dividing and receiving the antenna aperture has attracted attention.

  Furthermore, in a SAR system using one transmission aperture and multiple reception apertures, the beam shape is optimized by combining the received signals from each aperture while performing appropriate weighting before A / D conversion. A pre-beam shaping technique has also been proposed (see, for example, Non-Patent Document 1).

N. Gebert, G.M. Krieger and A.M. Moreira, High Resolution Wide Swath SAR Imaging with Digital Beamforming-Performance Analysis, Optimization, System Design, EUSAR 2006, FIG. 1

  The conventional SAR system (synthetic aperture radar device) has a problem that even when the antenna beam is formed by the pre-beam shaping technique described in Non-Patent Document 1, it is impossible to avoid the above-described gain reduction due to the low side lobe. It was.

  The present invention has been made to solve the above-described problems, and optimizes and combines the size and numerical aperture of the transmission aperture and the size and numerical aperture of the reception aperture in the dual beam pre-beam shaping technique. Accordingly, an object of the present invention is to obtain a synthetic aperture radar apparatus that can realize a low sidelobe antenna beam and can acquire a SAR image with suppressed ambiguity without causing a decrease in antenna gain due to a decrease in aperture efficiency.

  A synthetic aperture radar apparatus according to the present invention is a signal processing apparatus for obtaining a SAR image by performing synthesis processing and SAR processing of two received signals, and is an array antenna in which a plurality of antenna elements shared for transmission and reception are arranged in parallel A distributor for transmitting radio waves using all of the plurality of antenna elements, a transmitter for radiating transmission radio waves from the plurality of antenna elements via the distributor, and receiving from each of the plurality of antenna elements Two combiners that combine two beams to generate two beam signals, two receivers that receive the two beam signals, and a combination process that combines the two received signals via the two receivers And a SAR processing unit that generates a SAR image using a combined signal from the combining processing unit, and the two combiners are beam patterns 1 at the time of transmission by a plurality of antenna elements. A first predetermined number of antenna elements positioned at both ends of the plurality of antenna elements are used at the time of reception so that the t-side lobe and the 1st null of the beam pattern at the time of reception by the plurality of antenna elements overlap, and the first Is set to about 70% of the number of the plurality of antenna elements.

  According to the present invention, the maximum gain observation using the antenna aperture with the maximum efficiency and the dual beam SAR observation using the two reception beams with the low side lobe can be performed.

It is a block block diagram which shows the whole system of the synthetic aperture radar apparatus which concerns on Embodiment 1 of this invention. It is explanatory drawing which shows the beam pattern at the time of full aperture transmission by the array antenna in FIG. It is explanatory drawing which shows the antenna pattern (transmission-and-reception beam pattern) at the time of full aperture transmission by the array antenna in FIG. It is explanatory drawing which shows typically the receiving opening in Embodiment 1 of this invention. It is explanatory drawing which shows the effect of low side lobe of the antenna pattern by Embodiment 1 of this invention. It is a block block diagram which shows the whole system of the synthetic aperture radar apparatus which concerns on Embodiment 2 of this invention. It is a block block diagram which shows the whole system of the synthetic aperture radar apparatus which concerns on Embodiment 3 of this invention.

Embodiment 1 FIG.
Hereinafter, the first embodiment of the present invention will be described in detail with reference to the drawings.
1 is a block diagram showing the entire system of a synthetic aperture radar apparatus according to Embodiment 1 of the present invention.

In FIG. 1, the synthetic aperture radar apparatus includes an array antenna 1, a transmitter 3, receivers 7 and 8, and a signal processing device 9 that generates a SAR image.
The array antenna 1 includes a plurality of antenna elements 2, a transmission distributor 4, and reception combiners 5 and 6.
The signal processing device 9 includes data files DF1 and DF2, a synthesis processing unit 10, and a SAR processing unit 11.

In the array antenna 1, N _TX (# 1 to #N _TX ) antenna elements 2 for transmission and reception are arranged in a linear array.
A transmission signal from the transmitter 3 is distributed by the distributor 4, supplied to each antenna element 2, and radiated as a transmission radio wave from each antenna element 2.

Reflected radio waves from the object are received by N _TX antenna elements 2, and among each received beam, N _RX antenna elements 2 (# 1 to #N) from the left end in FIG. _RX ) is input to one synthesizer 5.
Similarly, the reception beams of N _RX antenna elements 2 (#N _TX to (#N _RX-overwrap )) from the right end in the drawing are input to the other combiner 6.
Thus, the combiners 5 and 6 generate two beam signals B1 and B2 obtained by combining the N _RX received signals.

  The two beam signals B1 and B2 from the combiners 5 and 6 are input to the signal processing device 9 via the receivers 7 and 8, respectively, and the beam signals are input to the data files DF1 and DF2 in the signal processing device 9, respectively. Stored individually as received signals corresponding to B1 and B2.

The received signals of the two beam signals B1 and B2 stored in the data files DF1 and DF2 are input to the synthesis processing unit 10 including a reconstruction filter or a restoration filter and synthesized.
The combined signal generated from the combining processing unit 10 is input to the SAR processing unit 11 and is output from the signal processing device 9 as a SAR image.

  The array antenna 1 corresponds to an array antenna in general, and is not limited to a passive array antenna configured by only the antenna element 2, but may be a passive phased array antenna having a phase shifter in each antenna element 2. Further, each antenna element 2 may be an active phased array antenna provided with an amplifier.

FIG. 2 is an explanatory diagram showing a transmission beam pattern by the array antenna 1, and shows a beam pattern at the time of full aperture transmission by the N _TX array antennas 1.
FIG. 3 is an explanatory diagram showing an antenna pattern (transmission / reception beam pattern) during full aperture transmission, and corresponds to the transmission beam pattern of FIG.

FIG. 4 is an explanatory diagram schematically showing the reception aperture in the first embodiment of the present invention. By combining (Σ) the reception beams of the number of reception elements N _RX located at both ends in the array antenna 1. It shows that two beam signals B1 and B2 are obtained.
FIG. 5 is an explanatory diagram showing the effect of lowering the side lobe of the antenna pattern according to the first embodiment of the present invention. The amplitude level of the 1st side lobe P1 ′ after improvement is lower than the 1st side lobe P1 before improvement. You can see that.

As shown in FIG. 2, when all antenna elements 2 are used for uniform transmission, it is known that the transmitted / received antenna pattern has a SINC function shape as shown in FIG.
In this case, an antenna having a maximum aperture efficiency and a high gain can be realized. However, the level of the first side lobe P1 located beside the main lobe Mo is the highest, and the maximum gain is theoretically 13 [dB]. Is the limit.

  The beam width (width of the main lobe Mo) of the antenna pattern in FIG. 3 is proportional to the number of antenna elements 2 used, and the positions of the 1st side lobe P1 and 1st null Q1 are proportional to the number of antenna elements 2. Moving.

  Since the received signal observed by the SAR system is received by combining antenna patterns at the time of transmission / reception, the number of antenna elements 2 on the receiving side is optimized, and the first side of the transmitting antenna pattern (FIG. 2). By superimposing the direction of the 1st null Q1 of the reception antenna pattern on the position of the lobe P1, it is possible to reduce the 1st side lobe P1 in the total transmission and reception (FIG. 3).

By the above method, it is possible to improve the maximum 3 dB with respect to the reduction of the first side lobe P1 in the total transmission and reception (FIG. 3).
At this time, how to obtain the optimum number N _RX of antenna elements 2 at the time of reception relative to the number N _TX of antenna elements 2 at the time of transmission is as follows.
First, a variable x _ATX is obtained from the following equation (1) using the number of transmitting antenna elements N _TX .

Since equation (1) cannot obtain an analytical solution, it is possible to obtain the variable x _ATX using a numerical solution method such as NEWTON method, but it is not exact, and the following equation (2) is used. The variable x _ATX may be set.

  In the equation (2), the coefficient α is an arbitrary numerical value of about 0.6 to 0.8, and is a value determined in consideration of the gain amount and the like along with the reduction of the side lobe level.

Next, the number N _RX of the antenna elements 2 at the time of reception is obtained from the variable x _ATX obtained by the expression (1) or (2) and the following expression (3).

Here, when an approximate solution using the coefficient α (= 0.6 to 0.8) in the equation (2) is used as the variable x _ATX , the antenna element 2 at the time of reception obtained from the equation (3) is used. The number N _RX is generally represented by the following formula (4).

From Expression (4), the ratio (N _TX : N _RX ) of the number of antenna elements 2 at the time of transmission / reception is generally expressed as follows.

N _TX : N _RX ≒ 1: α

Therefore, as shown in FIG. 1, with respect to a plurality of reception beams combined by the combiners 5 and 6, the number of antenna elements 2 that overlap at the time of reception is _set as _Noverwrap and the signals are distributed and input to the combiners 5 and 6, respectively. By providing the antenna element 2 to be used, the number N _TX of antenna elements 2 used at the time of transmission and reception remains the same, and the relationship of Expression (4) can be satisfied.
Note that the number _Noverwrap of the antenna elements 2 that overlap the two combiners 5 and 6 at the time of reception is obtained by the following equation (5).

Here, when an approximate solution using the coefficient α is used as the variable x _ATX , the overlap number N _overwrap is generally expressed by the following equation (6).

From the equation (6), the overlap number _Noverwrap is approximately 0.2 to 0.6N _TX .

  As described above, the synthetic aperture radar apparatus according to Embodiment 1 (FIG. 1) of the present invention uses a plurality of antennas shared for transmission and reception in order to obtain a SAR image by synthesizing and SAR processing two received signals. Array antenna 1 in which elements 2 are arranged in parallel, distributor 4 for transmitting radio waves using all of the plurality of antenna elements 2, and transmitting radio waves from a plurality of antenna elements 2 through distributor 4 Transmitter 2, two combiners 5 and 6 for generating two beam signals B 1 and B 2 by combining the received beams from the plurality of antenna elements 2 into two sets, and two beam signals B 1 and B 2 SAR image is generated by using the two receivers 7 and 8 that receive the signal, the synthesis processing unit 10 that synthesizes the two reception signals via the two receivers 7 and 8, and the synthesized signal from the synthesis processing unit 10 SAR processing unit And a 1, a.

The two combiners 5 and 6 have a plurality of beam patterns so that the first side lobe P1 of the beam pattern at the time of transmission by the plurality of antenna elements 2 and the 1st null Q1 of the beam pattern at the time of reception by the plurality of antenna elements 2 overlap each other. N _RX (first predetermined number) antenna elements 2 positioned at both ends of the antenna element 2 are used during reception.
The number N _RX (first predetermined number) used at the time of reception is set to about 70% of the number of antenna elements 2 (N _TX pieces).
The _Noverwrap (second predetermined number) antenna elements 2 located at the center of the plurality of antenna elements 2 overlap when receiving the two beam signals B1 and B2.

  Thus, the antenna gain reduction due to the reduction in aperture efficiency is achieved by optimizing and combining the size and numerical aperture of the transmission aperture with the size and numerical aperture of the reception aperture in the dual beam (B1, B2) pre-beam shaping technique. Therefore, a low sidelobe antenna beam can be realized and an SAR image with suppressed ambiguity can be acquired.

Embodiment 2. FIG.
In the first embodiment (FIG. 1), the antenna element 2 located at the center of the plurality of antenna elements 2 is used as an overlap element at the time of reception. However, as shown in FIG. Alternatively, a second predetermined number of antenna elements 2A dedicated for reception may be additionally provided outside the both ends of the two to eliminate the overlap elements.

FIG. 6 is a block diagram showing the entire system of the synthetic aperture radar apparatus according to Embodiment 2 of the present invention. The same components as those described above (see FIG. 1) are denoted by the same reference numerals as those described above, or A detailed description will be omitted by adding “A” after the reference numeral.
The reception-only antenna element 2A may be a transmission / reception shared antenna element, or may be configured to be OFF-controlled only during reception.

6 differs from the above-described configuration (FIG. 1) only in that an antenna element 2A dedicated for reception is added.
The configuration shown in FIG. 6 provides the same operational effects as described above, and the overlap element in FIG. 1 is not necessary.

Embodiment 3 FIG.
In the first and second embodiments (FIGS. 1 and 6), the reception beams from the plurality of antenna elements 2 are combined into two sets of beam signals B1 and B2, and then received by the two receivers 7 and 8. However, as shown in FIG. 7, each received beam is received via the same number of receivers 7B as the number N _TX of antenna elements 2, and two beam forming arithmetic processing units 19 and 20 in the signal processing device 9B Two received signals may be generated.

FIG. 7 is a block diagram showing the entire system of the synthetic aperture radar apparatus according to the third embodiment of the present invention. Components similar to those described above (see FIG. 1) are denoted by the same reference numerals as those described above, or A “B” is appended to the reference numeral and the detailed description is omitted.
In FIG. 7, instead of the pre-beam shaping technique described above (FIG. 1), a DBF method for receiving received beams from all antenna elements 2 is applied.

In FIG. 7, the array antenna 1B includes a plurality (N _TX ) of antenna elements 2 and a distributor 4, and each antenna element 2 is individually connected to a plurality of (N _TX units) receivers 7B. Has been.

The signal processing device 9B includes a plurality of (N _TX ) data files DF individually connected to each receiver 7B, two beam forming arithmetic processing units 19 and 20, a synthesis processing unit 10 and a SAR similar to those described above. And a processing unit 11.
In this case, each reception beam from the plurality of antenna elements 2 is received via the same number (N _TX units) of receivers 7B as the number of antenna elements 2, and is input to the signal processing device 9B as a plurality of beam signals. .

In the signal processing device 9B, beam signals (data) from the respective receivers 7B are stored in the respective data files DF, beam forming processing is performed by the beam forming arithmetic processing units 19 and 20, and two beam signals are supported. Generate a received signal.
Hereinafter, the two beam signals (received signals) are combined by the combining processing unit 10 and the SAR processing unit 11 generates a SAR image.

  As described above, the signal processing apparatus according to Embodiment 3 (FIG. 7) of the present invention uses a plurality of antenna elements shared for transmission and reception in order to obtain a SAR image by synthesizing and SAR processing two received signals. In order to radiate transmission radio waves from the plurality of antenna elements 2 via the distributor 4 and the distributor 4 for transmitting radio waves using all of the plurality of antenna elements 2. Transmitter 3, a plurality of receivers 7 B that individually receive reception beams from a plurality of antenna elements 2, and reception signals corresponding to two beam signals from a plurality of beam signals via the plurality of receivers 7 B. Two beam forming arithmetic processing units 19 and 20 to be generated, a synthesizing processing unit 10 for synthesizing two received signals from the two beam forming arithmetic processing units 19 and 20, and a synthesized signal from the synthesizing processing unit 10 The SAR processing unit 11 for generating a SAR image using, and a.

The two beam forming arithmetic processing units 19 and 20 are configured such that the first side lobe P1 of the beam pattern at the time of transmission by the plurality of antenna elements 2 and the first null Q1 of the beam pattern at the time of reception by the plurality of antenna elements 2 overlap. The beam signals from the receiver 7B corresponding to the N _RX (first predetermined number) antenna elements 2 located on both ends of the plurality of antenna elements 2 are used.
Further, the number N _RX (first predetermined number) of the receivers 7B used by the beam forming arithmetic processing units 19 and 20 is set to about 70% of the number of antenna elements (N _TX ).

Thereby, all the N _TX antenna elements 2 in the array antenna 1 are used at the time of transmission, and the direction of the 1st side lobe P1 of the transmission antenna pattern is matched with the direction of the 1st null Q1 of the reception antenna pattern at the time of reception. As described above, combining and receiving are performed using N _RX antenna elements 2, and two beam signals (received signals) can be combined by the combining processing unit 10.
Therefore, as described above, it is possible to obtain a synthetic aperture radar apparatus using an array antenna that maintains a high aperture efficiency, maintains a high directivity gain, and is compatible with a low sidelobe antenna pattern.

  In the first to third embodiments, the case where the SAR system is used has been described. However, it goes without saying that the present invention can also be applied as a low sidelobe antenna in a radar system using other dual beam antennas.

1, 1B array antenna, 2, 2A antenna element, 3 transmitter, 4 distributor, 5, 6 combiner, 7, 7B, 8 receiver, 9, 9B signal processor, 10 combine processor, 11 SAR processor 19, 20 Beam forming arithmetic processing unit, B1, B2 beam signal, DF, DF1, DF2 data file, Mo main lobe, number of _Noverwrap overlap, number of N _RX receiving elements, number of N _TX transmitting elements, P1 1st side lobe Q1 1st null, 1st side lobe after improvement of P1 ′.

Claims (4)

A signal processing apparatus for obtaining a SAR image by combining and receiving two received signals.
An array antenna having a plurality of antenna elements shared for transmission and reception;
A distributor for transmitting radio waves using all of the plurality of antenna elements;
A transmitter for radiating transmission radio waves from the plurality of antenna elements via the distributor;
Two combiners that combine two received beams from the plurality of antenna elements into two sets to generate two beam signals;
Two receivers for receiving the two beam signals;
A synthesis processing unit that synthesizes two received signals via the two receivers;
A SAR processing unit that generates a SAR image using a combined signal from the combining processing unit;
With
The two combiners are:
The 1st side lobe of the beam pattern at the time of transmission by the plurality of antenna elements and the 1st null of the beam pattern at the time of reception by the plurality of antenna elements overlap.
Using a first predetermined number of antenna elements located at both ends of the plurality of antenna elements during reception;
The synthetic aperture radar apparatus, wherein the first predetermined number is set to about 70% of the number of the plurality of antenna elements.   2. The synthetic aperture radar according to claim 1, wherein a second predetermined number of antenna elements located in a central portion of the plurality of antenna elements overlap when receiving the two beam signals. apparatus.   The synthetic aperture radar apparatus according to claim 1, wherein a second predetermined number of antenna elements dedicated to reception are arranged in parallel at both ends of the plurality of antenna elements. A signal processing apparatus for obtaining a SAR image by combining and receiving two received signals.
An array antenna having a plurality of antenna elements shared for transmission and reception;
A distributor for transmitting radio waves using all of the plurality of antenna elements;
A transmitter for radiating transmission radio waves from the plurality of antenna elements via the distributor;
A plurality of receivers that individually receive the received beams from the plurality of antenna elements;
Two beam forming arithmetic processing units for generating reception signals corresponding to two beam signals from a plurality of beam signals via the plurality of receivers;
A synthesis processing unit that synthesizes two received signals from the two beam forming arithmetic processing units;
A SAR processing unit that generates a SAR image using a combined signal from the combining processing unit;
With
The two beam forming arithmetic processing units are:
The 1st side lobe of the beam pattern at the time of transmission by the plurality of antenna elements and the 1st null of the beam pattern at the time of reception by the plurality of antenna elements overlap.
While using a beam signal from a receiver corresponding to the first predetermined number of antenna elements located on both ends of the plurality of antenna elements,
The synthetic aperture radar apparatus, wherein the first predetermined number is set to about 70% of the number of the plurality of antenna elements.

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