JPH0257876B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a synthetic aperture radar that is mounted on a flying object such as an aircraft and maps the ground.

First, the configuration and principle of a conventional synthetic aperture radar of this type will be explained. Figure 1 is a block diagram showing the configuration of a conventional synthetic aperture radar, Figure 2 is a diagram showing the geometric relationship between an aircraft or flying object equipped with the radar and a target, and Figure 3 shows the timing of received echoes. Figures 4 and 5 are diagrams representing range walk compensation. Each figure will be explained in order below.

Now, in Fig. 1, the IF (Intermediate
The CW signal in the Frequency) band is input to the pulse stretcher 2, where it receives a chirp signal (chirp bandwidth = B) whose frequency changes with time within a certain time T.
Or a coded pulse sequence can be obtained. At this time, it is assumed that the chirp signal or code pulse sequence is generated in synchronization with the trigger signal from the trigger signal generator 15. The output of the pulse stretcher 2 is the mixer 3.
RF (Radio) from local oscillator 4 stabilized by
After being mixed with a frequency (frequency) signal and increased in output by a high-output amplifier 5, it passes through a transmitter/receiver switch 6 and an antenna 7, and is radiated into space as a radio wave. The radio waves reflected by the target are inputted again to the mixer 3 via the antenna 7 and the transmitter/receiver switch 6, where they are input to the stabilized local oscillator 4.
It is mixed with the RF signal from the IF band and becomes an IF band signal.

The IF band signal is amplified by an IF amplifier 8, and then compressed in the range direction by a pulse compressor 9 to approximately 1/B in the case of a chirp signal and to a sub-pulse width τ _s in the case of a code pulse sequence.

The compressed signal is subjected to phase detection using the signal from the coherent signal generator 1 by the phase detector 10a. At the same time, the signal from the coherent signal generator 1 passes through a 90° phase shifter 11 and is input to the phase detector 10b.

The compressed signal inputted to the phase detector 10b is phase detected by the output from the 90° phase shifter. The outputs of the phase detectors 10a and 10b are input to gate circuits 12a and 12b, respectively, and after being gated, are input to the azimuth compression device 13.
The azimuth compression device uses the fact that the Doppler frequencies of radar echoes from targets existing in various azimuth directions are different to separate targets in the azimuth direction and improve azimuth resolution. The signals thus obtained are input to the display device 14 to obtain high resolution images in both the range direction and the azimuth direction.

In the inertial coordinate system xyz shown in Figure 2, a point target (P (for convenience of explanation, it is assumed that only one object exists in the xy plane, but the generality is not lost) in the xy plane is at a distance of r from the origin. , a point target P exists at an angle θ from the y-axis and is observed by a radar device R mounted on the aircraft.At this time, the aircraft flies parallel to the y-axis at a constant altitude h and a constant speed V, t=0
Assume that it exists at the coordinates (0, 0, h). As shown in FIG. 3, for the transmitted pulse (a) transmitted from the radar device R at t=0, the received echo (b・a) from the point target P at the output of the pulse compression device 9 is τ ₀ = 2/c√ ^{2 2} + ^{2 2} + ² (1) The signal is received by the radar device R after being delayed by the time given by 2/c√ 2 2 + 2 2 + 2 (1). Here C is the speed of light. Radar device R
If the repetition period of the transmission pulse is T, then the transmission pulse (I) transmitted from the radar device R at t=T is
For b), the received echo (lo b) from the point target P at the output of the pulse compression device 9 is τ ₁ = 2/c√ ^{2 2} + (-) ²
+ ² (2) is received by the radar device R with a delay of the time given by (2). Similarly, if NT is the synthetic aperture time, then τ _N =2/c√ ^{2 2} + (-
) ² + ² (3)
The echo from the point target P at the output of
is received. In this way, the echo from the point target P in the output of the pulse compression device 9 is transmitted to the gate circuit 1.
2a and 12b, it is necessary to move the gate position as the aircraft moves. For example, FIGS. 4 and 5 show this situation, and 100, 1, 100, 2, ..., 100,
N+1 is a memory such as a shift register, and T ₁ , T _{2 .} . . , T _L represent echo signals at the output of the pulse compressor 9 from L targets existing in the range direction. (In this case, L plural targets will be explained.) It is assumed that the above memory is inputted from the left and sequentially moved to the right.

In FIG. 4, (a) shows that the echo signals from L targets when transmitted at t=0 are stored in the memory 100,
(b) shows that the echo signals from L targets when transmitted at t=T are stored in the memory 10.
0 and 2 are stored. Similarly, the echo signal transmitted at t=2T is stored in memories 100 and 3, and at t=NT, the echo signal is stored in memory 100 and 3.
00, N+1. As is clear from FIG. 4, the echo signal from the target from t=0 to t=NT captures the target in front of the aircraft equipped with radar device R, as shown in FIG.
Assuming that the synthetic aperture time is short, as the distances between these targets and the radar device R become shorter, they gradually move to the left. For this reason, it is necessary to transfer echo signals from the same target within the memory so that they come to the same range bin as shown in FIG. As a result, in the azimuth compression device 13, information for the same target is transferred from the memory 100,1 to the memory 10.
0 and N+1 can be made to exist in the same range bin, so by applying a predetermined phase compensation to the information of each of the (N+1) identical range bins, it is possible to compress the information in the azimuth direction.

The present invention provides a switch within the pulse compression device in order to overcome these drawbacks, and will be described in detail below with reference to the drawings.

FIG. 6 shows an embodiment of the present invention, in which 21 is a tapped delay line or a tapped shift register, 16 is a phase shifter, and 17 is a device for calculating the sum of outputs from each phase shifter, such as an adder. Conceivable. 18 is the output of the IF amplifier 8 in FIG. 1, 19 is the output of the pulse compression device 9 in FIG. An example of something that can change the point is a switch. 21
Each tap in the chirp signal has a chirp bandwidth B
It is assumed that the encoded pulse sequence is delayed by a time corresponding to the sub-pulse width τ _s .

Now, in Fig. 2, as the radar device R mounted on the aircraft moves, the received echo from the point target P becomes τ _o = 2/c√ ^{2 2} + ( −
) ² + ² (4) It is received by the radar device R with a delay of the time given by (4). If (N+1) pulses are transmitted during the synthetic aperture time, then based on the delay time τ N/2+ _{1 when n=N/2+1,} N _G =τ _o −τ _{N/2+ 1} /τ ₀ (5) Find the deviation N _G in the number of gates given by: Here, [ ] is a Gaussian symbol, τ ₀ is 1/B for the chirp signal, and τ _s for the code pulse sequence.

First, the output 18 of the IF amplifier is input to a tapped delay line or tapped shift register 21. At this time, when N _G in equation (5) is maximum, that is, when n = 0, all the contacts of each switch 20 are tilted to the left as shown in FIG. 6, and as the value of N _G decreases by 1, each switch Move the contact point 20 to the right side of the current location. If the outputs of these switches 20 are passed through a phase shifter 16 whose phase is set in advance by the sign of the chirp signal or code sub-pulse and the sum is calculated by an adder 17, a constant time period relative to the transmitted pulse can be obtained regardless of the movement of the aircraft. A compressed pulse can be created at a delayed position. This output 19 is used as an input to a conventional phase detector 10a, 10b. All switching of the switch 20 is controlled in accordance with a control signal generated by the control device 22 based on the results obtained by calculating equations (4) and (5).

As described above, in the synthetic aperture radar according to the present invention, a switch is provided for each tap output in the tapped delay line or tapped delay line in the pulse compression device, and the switch is used to adjust each tap output as the radar mounted on the aircraft moves. By switching, the radar echoes from the target can be located in the same range bin, which has the advantage of facilitating signal processing for azimuth compression.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a conventional synthetic aperture radar, and FIG. 2 is a diagram showing the geometric relationship between an aircraft or flying object equipped with the radar and a target.
FIG. 3 is a diagram showing the timing of received echoes, FIGS. 4 and 5 are diagrams showing range walk compensation, and FIG. 6 is a diagram showing a pulse compression device according to the present invention. 1 is a coherent signal generator, 2 is a pulse stretcher, 3 is a mixer, 4 is a stabilizing local oscillator, 5 is a high output amplifier, 6 is a transmission/reception switch, 7 is an antenna,
8 is an IF amplifier, 9 is a pulse compression device, 10 is a phase detector, 11 is a 90° phase shifter, 12 is a gate circuit,
13 is an azimuth compression device, 14 is a display device, 15
is a trigger signal generator, 16 is a phase shifter, 17 is an adder, 18 is the output of the IF amplifier, 19 is the adder output,
20 is a switch, 21 is a delay line with taps or a shift register with taps, 22 is a control device, R is a radar device, P is a point target, a, a, b, b, a, c,
A, d are transmission pulses, b, a, b, b, b, c,
b, d is received echo, 100, 1 to 100, N+
1 is memory. It should be noted that the same or corresponding parts in the figures are indicated by the same reference numerals.

Claims (1)

[Claims] 1. In a synthetic aperture radar that uses a pulse compression device to increase the resolution in the range direction, the pulse compression device includes a delay line with taps or a shift register with taps, a switch attached to the output end of each of the above taps, and each of the above switches. a phase shifter attached to the output end of the phase shifter, and an adder installed at the output end of the phase shifter to adjust the shift according to the control signal from the control device as the radar device mounted on the aircraft or flying object moves. A synthetic aperture radar characterized in that the connection between the phase converter and the tap is switched by the switch.