JPH08114672A - Active sonar - Google Patents

Abstract

PURPOSE: To prevent the reduction of a processing gain at the time of Doppler shifting by an extremely simple structure in an active sonar of an LFM (linear FM) correlation processing type. CONSTITUTION: Two transmission signals Q1, Q2 having reverse frequency deviation characteristics to each other are transmitted with a delay of a time t0. Reception signals P1, P2 of them are read by the matched times by memory circuits 1, 2, input to a correlation circuit 3 and correlation-processed. Thus, a gain reduction is eliminated, and the target recognition distance is not reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active sonar device, and more particularly to an LMF (linear FM) transmission type active sonar device for transmitting while changing the frequency of a transmission signal linearly with time.

[0002]

2. Description of the Related Art A conventional active sonar device of this type will be described with reference to FIG. An FM signal (LMF) in which the frequency of the transmission signal Q1 changes linearly with time
In the case of normal reception, the correlation circuit 3 calculates the correlation between the reception signal P1 that is a reflection signal from the target and the replica signal R that has the inverse characteristic of the transmission signal Q1. It detects high signals as strong echo signals.

The memory circuit 1 stores the received signal P1 and the memory circuit 2 stores the replica signal R.

When the received signal P1 is correlated with the LFM transmission signal Q1 within the range of f0 ± AHz with the transmission pulse width T seconds and the frequency f0 as the center, the replica signal R corresponding to the transmission signal characteristic is held in the memory circuit 2. In addition, the received signal P1 is held in the memory circuit 1, and the correlation circuit 3 detects the correlation between the outputs of these memory circuits 1 and 2.

When the LFM signal linearly modulated from a low frequency to a high frequency is used as the transmission signal Q1, the replica signal R has the opposite characteristic in the same band based on the transmission signal information. The signal is linearly modulated and stored in the memory circuit 2.

Based on the transmission signal Q1, the memory circuit 2 is thus
The replica signal R prepared in step 2 is subjected to correlation processing (integration processing) with the reception signal P1 in the correlation circuit 3 to obtain 2f0.
A peak with a high degree of c correlation is obtained.

Considering the gain of this correlation processing, "Principle of Underwater Acoustics" R.A. J. Corric, p. As described in 378, if the target is a stationary target and a reflected sound wave from this stationary target is received, the reception signal P
And the processing gain of the replica signal R is expressed as 10log 2BT (1). Here, B is the frequency shift width of 2 AHz, and T is the transmission pulse width (second).

In the LFM correlation processing method shown in FIG. 3, since the replica correlation method is used, f0 ± AHz
When LFM transmission is performed with, when the target moves and Doppler toft α occurs, the signal frequency band of the reception signal P1 becomes (f0 ± A) ± αHz, and the gain is deviated from the replica signal frequency f0 ± AHz. There is a problem that the distance at which the target is recognized becomes shorter and the distance at which the target can be recognized becomes shorter.

Incidentally, FIG. 4 shows the received signal P1 in which the Doppler shift α has occurred, and shows an example in which the received signal frequency band at this time becomes small and becomes BZ.

As a technique relating to the influence of the Doppler shift in such an active sonar device, Japanese Patent Laid-Open No. 59-59
There are those disclosed in JP-A-225375, JP-A-3-115879, JP-A-3-242586 and the like.

According to the technique disclosed in Japanese Unexamined Patent Publication No. 59-22537, by detecting the Doppler shift by the movement of the target from the time difference between the two correlation output signals, the movement of the target during LFM signal transmission can be detected. To know.

Further, according to the technique disclosed in Japanese Patent Laid-Open No. 3-115879, the detected signal level is compared with a predetermined level, converted into a 1-bit signal pulse of 1 or 0, and the order of the pulse train is verified. Therefore, Doppler discrimination is performed.

Further, according to the technique disclosed in Japanese Patent Laid-Open No. 3-242586, the LFM correlation is obtained by using two replica signals which are two signals obtained by shifting the transmission frequency to the high frequency and the low frequency by the same frequency. By doing so, Doppler detection is performed.

[0014]

In any of the above techniques, there is a drawback in that the circuit configuration becomes extremely complicated, which causes an increase in hardware and an increase in cost.

It is an object of the present invention to provide an active sonar device capable of obtaining the same processing gain as in the case where there is no Doppler shift with a very simple structure by eliminating the reduction of the correlation processing gain due to Doppler shift. .

[0016]

The active sonar apparatus according to the present invention is a transmission means for transmitting first and second LFM (linear FM) transmission signals having frequency sweep characteristics opposite to each other in this order in time series. And the first and second
Reception signal derivation means for generating first and second reception signals by reflection from the target by the LFM transmission signal so as to be temporally simultaneous, and first and second reception signal derivation means. And a correlating means for correlating the two received signals.

[0017]

The LFM transmission signals whose frequency shift characteristics are opposite to each other are transmitted in time series, and the correlation processing is performed by using the reception signals that are Doppler-shifted by the target speed respectively, so that the processing gain by the Doppler shift is obtained. The target distance is secured by preventing the decrease of

[0018]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention. In FIG. 1, at the transmission frequency f0 ± AHz and the transmission pulse width T seconds, the LFM transmission signal Q1 is first transmitted at the first time, and then the LFM transmission signal Q2 is sequentially transmitted in time series after t0 seconds. To do so.

In this case, the first and second LFMs
It is assumed that the frequency characteristics of the transmission signals Q1 and Q2 have opposite frequency deviation characteristics as shown in FIG.

Reflected reception signals P1 and P2 from the target by these LFM transmission signals Q1 and Q2 are input to the memory circuits 2 and 1, respectively, and are sent to the correlation circuit 3 in units of T seconds. At this time, the received signals P1 and P2 are sent to the correlation circuit 3 at the same time (same timing), and therefore the previously received received signal P1 is delayed by the memory circuit 2 for t0 seconds. Will be read.

Normally, when performing LFM correlation at a transmission frequency f0 ± AHz and a transmission pulse width T seconds, assuming a stationary target without Doppler shift, the frequency bandwidth B of the received signal is
0 and the processing gain L0 are expressed as B0 = (f0 + A)-(f0-A) = 2A ... (2) L0 = 10log (2.2AT) ... (3).

When performing LFM correlation with the configuration of FIG.
As shown in FIG. 2, the frequency bandwidth BX of P2 that is cross-correlated with the received signal P1 in which Doppler shift αHz occurs is BX = (f0 + A + α) − (f0−A + α) = 2A (4), and there is no Doppler shift. It will be the same value as when.

The processing gain LX is LX = 10log (2 · 2AT) = L0 (5), which is the same as the equation (3), and the Doppler shift has no effect.

The gain difference from the conventional system shown in FIG. 3 will be described below. In the conventional method, when the LFM correlation is performed with the transmission frequency f0 ± AHz and the transmission pulse width T seconds, the frequency bandwidth BZ of the reception signal bx that has caused the Doppler shift αHz is BZ = (f0 + A) −, as shown in FIG. (F0−A + α) = 2A−α (6), and the processing gain LZ when the Doppler shift occurs in the conventional method is LZ = 10log {2 (2A−α) T} (7) .

The processing gain difference when there is no Doppler shift is the difference between L0 of the equation (3) and LZ of the equation (7), and LZ-L0 = 10log {1-α / (2A)}. Since (8) and α ≧ 0 and A> 0, when LZ−L0 ≦ 0 and Doppler shift occurs, be sure to (8)
The gain will decrease according to the formula.

However, in the present invention, L in the equation (8) is
It can be seen that the same gain as 0 can be maintained.

[0028]

As described above, according to the present invention, only two transmission signals Q1 and Q2 whose frequency characteristics are opposite to each other are transmitted in time series, and the target recognition distance is reduced even in the reception signal P1 due to Doppler shift. Has the effect of not occurring.

Further, it is not necessary to generate and hold the replica signal R corresponding to the transmission signal Q1 in the receiving circuit, which contributes to the reduction of the load of the receiving circuit.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a correlation frequency bandwidth in the embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a conventional active sonar device.

4 is a diagram showing a correlation frequency bandwidth in the apparatus of FIG.

[Explanation of symbols]

 1, 2 Memory circuit 3 Correlation circuit Q1, Q2 Transmission signal P1, P2 Reception signal

Claims (3)

[Claims] 1. A transmission means for transmitting first and second LFM (linear FM) transmission signals having mutually opposite frequency sweep characteristics in a time series in this order, and the first and second LFM transmission signals. First by each reflection from the target by
And a received signal deriving means for generating the second received signals so as to be temporally simultaneous, and a correlating means for performing a correlation process of the first and second received signals generated and derived by the received signal deriving means. An active sonar device characterized in that 2. The received signal deriving means has a memory for storing the first and second received signals, respectively, and the memory is provided for reading the first and second received signals from the memory. The first received signal is delayed so that there is no time difference from the second received signal.
Active sonar device described. 3. The active sonar device according to claim 1, wherein the second LFM transmission signal is transmitted after a predetermined time delay from the transmission of the first LFM transmission signal.