JPS6350777A - Synthetic aperture radar apparatus - Google Patents

Abstract

PURPOSE:To obtain various distance resolving powers by predetermined BPF at a receiving time, by changing the band width of a transmission signal corresponding to desired distance resolving power. CONSTITUTION:The frequency of a reference clock signal is divided into transmission pulse frequency in a control part 2 and an impulse gate signal G of a predetermined pulse width is sent out to a charp pulse generating circuit 1. Further, counting is performed in the timing of the signal G to also send out a gate signal G1 rising and falling at a predetermined delay time to the generating part 1. Furthermore, control signals S1, S2 for cutting off the frequency component corresponding to the signal band with of the signal G11 are sent out to a receiving part 6. The generating part 1 transmits a transmission charp pulse P changing the signal band width corresponding to desired distance resolving power through a transmission part 3 and an antenna part 5. The receiving part 6 receives the receiving signal from an isolator 4 and selects BPF corresponding to the signal band of the pulse P on the basis of the signals S1, S2 to supply the output signal thereof to a signal processing system. At this time, by changing the pulse width of the signal G11 of the control part 2 by the variable order from the outside, the frequency band width of the pulse P is changed to obtain various resolving powers.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD The present invention relates to a synthetic aperture radar device, and more particularly to a synthetic aperture radar device mounted on a flying object of an airborne Isoya artificial satellite.

Conventional technology A side-looking radar mounted on a flying object such as an artificial satellite moves while emitting radio waves to the ground surface on the side, and synthesizes a series of received data, making it effective with a relatively small diameter antenna. Synthesizer-to-loreg devices capable of combining large-diameter antennas are well known.

Although it is desirable to make the transmission pulses used in such a synthetic aperture radar as short as possible from the viewpoint of improving distance resolution, the opposite conditions are required from the viewpoint of signal detection.

Due to this background, the transmitted pulse is usually linear FM (L
(inear FM), utilizes a frequency modulated wave by linear FM called so-called chirb, and in the case of reception, energy is concentrated at one point through a circuit whose frequency vs. delay time characteristics are opposite to that of the transmitted pulse, such as a distributed delay line. It is extracted as a sharp pulse.

The resolutions in the azimuth and distance directions of such a synthetic frontage radar are shown as follows.

That is, the azimuth resolution δ is approximately expressed as D/2, where D is the dimension of the antenna in the azimuth direction, and the resolution δ in the traveling direction and perpendicular distance direction of the flying object is expressed by the following equation (1).

δ, = (C/2)(2,8/π)(1/△f)(
1/cos θ) (1) In equation (1), C is the radio wave propagation speed, Δ is the transmission signal frequency bandwidth, and θ is the elevation angle.

FIG. 7 is a geometry diagram of the synthetic frontage radar, and the elevation angle θ shown in the diagram is given by equation (2).

θ= cos”(((Re +H)/Re) ・si
nα)...(2) In equation (2), Re is the earth's radius, H is the flying object altitude, and α is the off-nadir angle. Using equation (2), the following (
3) Equation is obtained.

δ, = (C/2)(2,8/π)(1/Δf)−
(Re/(Re +H)) (1/sin α) −・
-(3) When the altitude and off-nadir angle of the flying object are constant, the following equation (4) can be derived from equation (3).

δ・Δf=)(・・・・・・(4) Here, K−(C/2)(2,8/π)(1/Δf)・(Re
/(Re +)-1)) (1/sin α)-(4
) (4) means that the synergistic value of distance resolution δ and frequency band Δf is constant 1i! as long as the flying object altitude and off-nadir angle are constant. ! This means that it becomes . In other words, distance resolution is inversely proportional to frequency bandwidth.

In the conventional synthetic frontage radar system, a large number of SAW distributed delay lines are required to obtain the desired distance resolution, and the distance resolution cannot be changed.

The conventional synthetic frontage radar described above has the following problems. That is, when the signal bandwidth of the transmission signal used is fixed, there is a drawback that only one type of distance resolution can be obtained.

Further, in the case where the signal bandwidth is changed in accordance with the desired distance resolution, there is a drawback that a large number of SAW distributed delay lines must be arranged.

An object of the present invention is to provide a synthetic frontage radar device that can obtain various distance resolutions with a simple configuration without using a large number of SAW distributed delay lines.

A synthetic frontage radar device according to the present invention comprises chirped pulse generating means whose signal frequency bandwidth can be freely changed, and control means which generates a gate signal for setting the signal frequency bandwidth of the chirp pulse. , receiving means for selectively extracting and deriving frequency components of the received signal obtained by transmitting the chirp pulse according to the signal frequency bandwidth.

Example Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of a synthetic aperture radar device according to the present invention. The configuration of the embodiment shown in FIG. 1 includes a chirp pulse generating section 1.t11 and a control section 2. Transmission unit 3. Isolator 4. Antenna part 5. The device includes a receiving section 6, of which the chirp pulse generating section 1, the control section 2, and the receiving section 6 are directly related to the present invention.

Transmitter 3 and isolator 4. In addition, the antenna section 5 constitutes a known radar system mounted on a flying object as a coherent radar. The chirp pulse generating section 1 generates a transmission chirp pulse whose signal bandwidth is changed to correspond to a desired distance resolution, and supplies this to the transmitting section 3.

The transmitting section 3 amplifies the transmitted chirp pulse to a predetermined level and supplies it to the antenna section 5 via the isolator 4. In this embodiment, the chirp pulse generating section 1 and the transmitting section 3 are configured independently, but there is no problem in having an integrated configuration as well.

FIG. 2 is a block diagram showing the chirp pulse generating section 1 in detail, and FIG. 3 is a diagram showing an example of its operating waveform. The chirp pulse generating section 1 shown in FIG. 2 includes a reference signal generating circuit 11. Mixer 12, SAW distributed delay I!
J13. It is configured to include a mixer 14 and the like. The reference signal generating circuit 11 has a frequency f shown in FIG. 3(A). A CW (continuous wave) reference signal is generated and supplied to the mixer 2.

This mixer 2 receives a gate signal G1 as a first gate pulse out of two gate pulses G and G2 supplied from a control section 2, which will be described later.

As shown in FIG. 3(A), this gate signal @G1 has a pulse width τ and a constant repetition period (transmission period).

The CW wave, which is the reference signal, is pulse-modulated by the mixer 2 to become a pulse having the pulse width τ of the gate signal G1, and then supplied to the 5AW (surface acoustic wave) distributed delay line 13. The frequency spectrum of this pulse modulated wave, which is the output of the mixer 2, is as shown in FIG. 3 (8), and has a bandwidth of 1/τ centered on the reference signal frequency f0.

The SAW distributed delay line 13 receives the pulse modulation signal thus supplied and outputs its impulse response waveform for each pulse modulation signal input. The output waveform of this SAW delay line 13 is shown in FIG. 3 (A>) with the time axis enlarged, and the waveform corresponds to the characteristics of this delay line 13 (see FIG. 5).This impulse The response waveform is then supplied to the mixer 4 and shaped by the gate signal G2 as a second gate pulse.This shaping changes the impulse response waveform output from the SAW distributed delay line 13 into the shape shown in FIG. 3 (△).
The pulse width of the gate signal n G 2 shown in is extracted. The output of mixer 4 extracted in this way is: is supplied to the transmitter 3 as a transmission chirp signal having a center frequency of 1 and a signal bandwidth determined by the rise time t1 and fall time t2 of the pulse of the gate signal 2.

As is clear from the above, the frequency bandwidth of the transmitted chirp signal can be determined by changing the pulse width (t2-tl) of the gate signal G.

FIG. 4 is a block diagram showing in detail a portion of the control section 2 shown in the embodiment of FIG. 1. The control section 2 shown in FIG. 4 includes a clock generation circuit 21. Clock frequency divider circuit 22. Impulse generation circuit 23. Gate generation circuit 24. It is composed of a flip-flop circuit 25 and a control signal generation circuit 26.

Clock generation circuit 21 generates a reference clock signal and supplies it to clock frequency division circuit 22 . The clock frequency dividing circuit 22 divides this into the transmission pulse repetition frequency and supplies it to the impulse generating circuit 23.
3 thereby generates an impulse with a predetermined pulse width and sends this to the chirp pulse generator 1 as a gate signal G1.

The gate generation circuit 24 includes a counter circuit, a pulse generation circuit,
It is equipped with a gate circuit, etc., receives a clock frequency division signal of the number of repetitions set in consideration of the determination accuracy of the gate signal G2, starts counting at the timing of the gate signal G1, and also starts counting at the timing of the gate signal G1. .

In response to this, the count is stopped and a logic value "1" is output at each time.

In other words, at delay times t and t2, an output with a logical value of "1" is generated, and this is sent to the flip-flop circuit 25.
supply to.

The flip-flop circuit 25 is driven by a clock signal received from the clock frequency divider circuit 22, and uses the two logical "1" levels output from the gate generation circuit 24 as a set signal and a reset signal, respectively, to calculate the difference between delay times t1 and t2. is the pulse width, rises with a delay time t, and has a delay time t
A gate signal G2 that falls at 2 is generated and sent to the chirp pulse generator 1.

The gate generation circuit 24 supplies a control signal corresponding to the gate signal G2 to the control signal generation circuit 26.

The &I 111 signal generation circuit 26 supplies the control section @S1 and the Luli control signal $2 to the reception section 6. The control signal S1 and the control signal S are control signals for cutting out a frequency component corresponding to the signal bandwidth of the gate signal G2.

In FIG. 5, the line segments indicate delay time vs. frequency characteristics, and correspond to a preset range of the delay time vs. frequency characteristics of the SΔW distributed delay line 13 shown in FIG. This preset range is set corresponding to the desired maximum distance separation, and in the case of FIG.
, ~ fIWln ff1aX , and the corresponding delay time is t ~ tax 1o. Also, in FIG. 5, f. 1.1 corresponds to the center point of the line segment in the reference signal frequency data, so f is f
-f, 1/2. Also, to is tmao m
It becomes 1/2 of ax man -1. In addition, these frequencies
Both mln and time data are processed in digital quantities.

Frequency f + Δf/2 = f2 and f. −Δf/2−f
When data of is desired, the delay times corresponding to these f2 and f are t2 and tl, respectively, where t 1i o = i o t 2. These delay times 1. 1 is the signal bandwidth Δf and the center frequency f when the delay time vs. frequency characteristic shown by the line segment is determined. can be easily determined by data on Now, the signal bandwidth corresponding to the predetermined distance resolution is Δf
Considering the case where the delay time changes to -, in this case, the delay time t1 changes to tl- and t2 changes to t2-.

FIG. 6 is a block diagram showing in detail the receiving section 6 shown in the embodiment of FIG. 1. The receiving section 6 in the figure includes a low Wl sound amplifier 61. Divider 62. It is composed of a combiner 63 and band wavers 601 to 60n.

The low noise amplifier 61 amplifies the received signal, which is the output signal of the isolator 4, to a predetermined level and sends it to the divider 62.
supply to. The divider 62 supplies the received signal from the low-noise amplifier 61 to the band transducer 601 to the band transducer 60n. Further, the divider 62 uses the control signal 1 from it, II control unit 2, etc. to divide the band Iff? corresponding to the signal bandwidth of the transmission chirp pulse. A received signal is supplied to the door transducer 60i. The output signals of the bandpass filters 601 to 60n are supplied to a combiner 63. The combiner 63 selects the output signal of the bandpass filter j1360i corresponding to the signal bandwidth of the transmission chirp pulse based on the control signal S2 from the control unit 2, and supplies the selected output signal to a signal processing system (not shown).

By making the pulse width of the gate signal G2 generated by the gate generation circuit 24 of the 111@ section 2 shown in FIG. 4 variable by an external variable command, the pulse width shown in FIG.
Since the gate width t1 to t2 shown in the bottom row of changes,
Frequency bandwidth Δf of transmission chirp pulse = f − f2
becomes changeable.

Therefore, as is clear from equation (4), it is possible to change the distance resolution δ as necessary.

As described in detail, according to the present invention, one SAW distributed delay line is used to change the signal bandwidth of a transmitted signal in accordance with a desired distance resolution, and a predetermined bandpass filter is used at the time of reception. By using this, it is possible to realize a synthetic aperture radar device that can reproduce images with various distance resolutions with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a concrete block diagram of the chirp pulse generator shown in FIG. 1, and FIG.
A) and (B) are operational waveform diagrams of the chirp pulse generator shown in Figure 2, Figure 4 is a specific block diagram of the control unit shown in Figure 1, and Figure 5 is a diagram showing the characteristics of the SAW distributed delay line. , FIG. 6 is a concrete block diagram of the receiving section of FIG. 1, and FIG. 7 is a geometry diagram of the synthetic aperture radar device. Explanation of symbols of main parts 1...Chirp pulse generation section 2...Luri control section 3...Transmission section 6...Reception section

Claims (1)

[Claims] a chirped pulse generating means whose signal frequency bandwidth can be freely changed; a control means which generates a gate signal for setting the signal frequency bandwidth of the chirp pulse; and a control means for generating a received signal obtained by transmitting the chirp pulse. A synthetic aperture radar device comprising: receiving means for selectively extracting and deriving frequency components according to the signal frequency bandwidth.