Abstract:
A moving target indicator device comprising a first clutter removing circuit for responding to a radar having a variable transmit repetition time and for removing clutter having a Doppler speed of nearly zero from a radar received signal having stationary clutter and moving clutter. The device further includes a correction circuit to keep the amplitude components and the phase components of the first clutter removing circuit output substantially constant. A second clutter removing circuit removes clutter having a constant amplitude component and phase component from the output of the correction circuit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a pulse-radar system and more particularly to a moving target indicator which cancels the unwanted component signal (clutter) in a received signal and detects a moving target signal. 
     Conventionally, a pulse-radar system indicating a moving target such as an airplane is designed with a clutter-canceller or suppressor to remove stationary (ground) clutter from buildings or other surface irregularities and moving clutter, such as weather clutter from moving reflectors, such as rain or clouds and to detect and indicate the moving target. Among these clutter removing means, a moving target indicator system, hereinafter simply referred to as an MTI, is often used for removing stationary clutter. 
     Generally, an MTI system adapted as a radar system for detecting and indicating aircraft is used to remove or cancel a signal from a stationary clutter so as to indicate only a moving target. When the phases of transmitted R.F. pulses are compared with the phases of received R.F. pulses, the signal from a stationary clutter always has a constant phase, whereas the signal from a moving target always has a different phase for each pulse repetition period. Hence, phase-detection is carried out in response to signals from the same distance for the continued 2 pulse-repetition periods and the difference between the obtained video signals is taken so as to cancel out the video signal from a stationary clutter and to leave only the video signal corresponding with a moving target. This kind of MTI radar system is described in chapter 17, &#34;RADAR HANDBOOK&#34;, edited by Merill Skolnik, McGraw-Hill, U.S.A., 1970. 
     However, an ordinary MTI system on a moving carrier, such as a ship, cannot cancel out moving clutter from rain, fog, the sea surface, moving ground clutter from land, or the like. This is, because these signals have a different phase for each pulse repetition period, similar to said signals from a moving target. 
     An adaptive MTI system is suited for cancelling moving clutter, including relatively moving ground clutter as stated before, and for indicating only a moving target such as an airplane. Cancelling an echo from a moving object can be accomplished by varying and transferring Doppler frequency of moving clutter to a notch of an MTI filter. For this kind of conventional adaptive MTI radar system, a clutter-locking MTI system is utilized. The details thereof are described in the chapter 9, &#34;RADAR DESIGN PRINCIPLES&#34; McGraw-Hill, U.S.A., 1969. 
     A clutter-locking MTI system can remove moving clutter or stationary clutter by detecting an average Doppler frequency (or an average Doppler phase-shift) and locking the average Doppler frequency (or the average Doppler phase-shift) to a notch of the MTI filter. However, when stationary clutter and moving clutter are present, it is not possible to sufficiently remove both because the detected average Doppler frequency, that is, average Doppler phase-shift, is different from that of stationary clutter alone or of moving clutter alone. Further details will be explained in this respect. When both stationary clutter and moving clutter are present, the input signals of the clutter-locking MTI canceller are; ##EQU1## where V 1  and V 2  denote an input signal (radar signal) at the received time point and that after a radar-received repetition time T, respectively. The first and second terms of the equation (1) denote stationary clutter components including the Doppler frequency fd 1  (in this case, fd 1  =0) and moving clutter components including the Doppler frequency fd 2 , respectively. E 1  and E 2  denote amplitude components of the stationary clutter and the moving one and φ 0  denotes a phase defined on the basis of the existing location of clutter. Generally, E 1  ≠E 2  and fd 1  ≠fd 2 . 
     When one input signal is phase-shifted by the presumptive value φd=2fdT of the average Doppler phase-shift, the canceler output is: 
     
         ΔV=2E.sub.1 sin [π(fd.sub.1 -fd)T] cos [2πfd.sub.1 t+φ.sub.0 +π(fd.sub.1 -fd)T]+2E.sub.2 sin [π(fd.sub.2 -fd)T] cos [2πfd.sub.2 t+φ.sub.0 +π(fd.sub.2 -fd)T] 
    
     where there exists the condition of fd=fd 1  ≠fd 2  or fd=fd 2  ≠fd 1 . Thus, it is impossible to remove both stationary clutter and moving clutter. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a moving target indicator system which can distinguish a moving target in a region where both stationary clutter and moving clutter are present. 
     Another object of the present invention is to provide a moving target indicator which can sufficiently remove stationary clutter and moving clutter in a staggered-trigger radar system. 
     According to the present invention, there is provided a moving target indicator apparatus. The device comprises a first clutter removing means for responding to a radar having a variable transmit repetition time and for removing clutter having a Doppler speed of nearly zero from a radar received signal having stationary clutter and moving clutter. Further, the device includes a correction means to keep the amplitude components and the phase components of the first clutter removing means output substantially constant. A second clutter removing means removes clutter having a constant amplitude component and phase component from the output of the correction means. 
     Other objects and features of the present invention will be clarified from the following description with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a vector diagram of a radar receiving signal. 
     FIG. 1B is a vector diagram of signals obtained through frequency correction. 
     FIG. 2 shows an embodiment for generating amplitude and phase correction data by manually setting the presumptive value of the average Doppler phase shift of moving clutter. 
     FIG. 3 shows an embodiment for generating amplitude and phase correction data wherein the presumptive value of the average Doppler phase-shift of moving clutter is determined by stationary clutter cancelers. 
     FIG. 4 shows an embodiment for generating amplitude correction data based on the average amplitude of an input signal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention has two conventional MTI (stationary clutter removing means i.e., MTI filters) connected in series and a corrector which corrects the phase and amplitude components of the output of a first stage MTI filter in order to detect a moving target in the region where both stationary clutter and moving clutter exist. Specifically, the system according to the present invention includes an MTI canceller for removing the stationary clutter, a clutter-locking MTI canceler for removing the moving clutter not removed by the MTI canceller, and correcting means to correct the amplitude and phase variation caused due to a staggered-trigger system. It is possible to remove moving clutter with the MTI system described above when the radar transmit repetition time is always a constant time T. In practice, however, a staggered PRF MTI (Staggered Pulse Repetition Frequency MTI) system is often employed to improve blind speed by varying radar transmit repetition time. This has the disadvantage that moving clutter cannot be removed because the clutter-locking MTI canceller receives the input signal with a different amplitude-shift and phase-shift due to the repetition time difference. For example, when T 1  and T 2  are employed as a radar transmit times, stationary clutter and moving clutter signals supplied to the MTI canceller are; ##EQU2## The outputs of the MTI canceler are; ##EQU3## Stationary clutter (fd 1  ≃0) can be removed because sin [πfd 1  T 1  ]≃0, sin [πfd 1  T 2  ]≃0, however, moving clutter (fd 2  =0) cannot be removed. Therefore, the outputs of the MTI canceller are represented as follows; ##EQU4## 
     Similarly as stated before, one input signal is phase-shifted by the presumptive value of ##EQU5## of the average Doppler phase-shift of the moving clutter (this input signal is shown as V 2  &#39;) and is passed through the clutter-locking MTI canceller. The output ΔW of the clutter-locking MTI is shown: ##EQU6## Even when fd≃fd 2  ##EQU7## Hence, moving clutter cannot be removed. In the case that the radar transmit repetition time is constant as T 1  =T 2  (=T), it is obvious from the equation (6) that moving clutter can be removed because ##EQU8## The description stated above can also be applied to a vector processing MTI including an I (in-phase) channel and a Q (quadrature) channel. 
     As described above, in the staggered PRF MTI system, both the amplitude and the phase of the moving clutter in the output of the MTI canceller are varied with the Doppler frequency response characteristics. Thus, it has been hitherto impossible to remove moving clutter with a clutter-locking MTI. 
     The present invention is designed to remove moving clutter by correcting the moving clutter having amplitude-shift and phase-shift varied as stated above and supplying the corrected signal to the clutter-locking MTI canceler. 
     Following is a decription of amplitude-shift and phase shift correction in a triple staggered-trigger mode, where the trigger repetition times are T 1 , T 2 , T 3 . 
     In a radar signal passed through an MTI canceler (stationary clutter canceler), stationary clutter is removed leaving only moving clutter components. As stated before, the amplitudes and the phases of these moving clutter components vary in every staggered-trigger repetition by virtue of the Doppler frequency response characteristics of the stationary clutter canceler. 
     The following are the amplitude components A n+1 , A n+2 , A n+3 , A n+4  . . . (n; natural and positive integer) in every transmit trigger state (n+1), (n+2), (n+3) . . . as shown in formula (4). ##EQU9## The phase-shift components Δθ n+2 , Δθ n+3 , Δθ n+4 , Δθ n+5  . . . are changed as follows: ##EQU10## Thus, in the trigger state (n+1), ##EQU11## where θ n+1  =2πfd 2  t+φ 0  +πfd 2  T 1  +π/2 
     The amplitude- and the phase-shift components of the I and Q outputs obtained are corrected so as to have constant values without depending on the stagger-trigger time. 
     It is assumed that reference amplitude and phase values are given as A 0  and θ 0 , respectively, and B n+1  and θ n+1  are given below: ##EQU12## Thus, the I and Q channel components have constant of, 2E 2  A 0  cos θ 0  respectively and 2E 2  A 0  sin θ 0 . In other words, the amplitude and phase components are independent of the difference of the Doppler shift fd 2  of the moving clutter and the trigger period. 
     In the next trigger state (n+2), ##EQU13## are given, and the following corrections similar to those of the previous trigger state, are carried out for the I and Q components. ##EQU14## In consideration of equations (12), (13), the following components independent of fd 2  and the trigger time are obtained. 
     I components: 2E 2  A 0  cos θ 0   
     Q components: 2E 2  A 0  sin θ 0   
     Generally, for amplitude and phase correction in the trigger stage (n+m), if ##EQU15## are assumed, the following corrections are required: ##EQU16## As a result, both components will be expressed as; I components: 2E 2  A 0  cos θ 0   
     Q components: 2E 2  A 0  sin θ 0  These components can be regarded as stationary clutter. Since the Doppler frequency fd 2  of the moving clutter is unknown, the presumptive value fd thereof is utilized as the corrected signal. The corrected component signals are made to pass through the clutter-locking MTI canceller to remove moving clutter. 
     FIG. 1 is a vector diagram showing the above-stated relationship. In FIG. 1A, A n+1 , A n+2 , A n+3 , A n+4 , A n+5  denote the signal amplitudes in the trigger states (n+1), (n+2), (n+3), (n+4), (n+5), respectively, and Δθ n+1 , Δθ n+2 , Δθ n+3 , Δθ n+4 , Δθ n+5  the phase-shifts from the signals in one previous trigger states for the trigger states (n+1) (n+2), (n+3), (n+4), (n+5), respectively. As described above, shifting the phases of the states (n+1), (n+2), (n+3) and (n+4) by Δθ n+1 ,Δθ n+1  +Δθ n+2 , Δθ n+1  +Δθ n+2  +Δθ n+3 , and Δθ n+1  +Δθ  n+2  +Δθ n+3  +Δθ n+4  makes signals with the stationary phase θ 0 . The varying amplitudes in every trigger state can also be made constant by correcting with B n+m  in equation (14). In other words, correction on the basis of equation (15) is accomplished in the present invention. FIG. 1B shows a vector diagram of the signals obtained through the phrase and amplitude correction. In each state, the amplitudes and the phases of the signals are made to be constant values 2E 2  A 0  sin θ 0 . 
     Now, one embodiment of the present invention will be described in consideration of the principle of this invention. 
     Referring to FIG. 2, stationary clutter cancelers 11 and 12, composed of the well-known delay circuit and subtractor circuit, remove stationary clutter from the signals I in  (I component given as E cos φ) and Q in  (Q component given as E sin φ). The I component signal and the Q component signal from which stationary clutter has been removed (i.e., stationary clutter-free signals) are expressed as A n+m  · cos θ n+m  and A n+m  · sin θ n+m , respectively. 
     Amplitude and phase correctors 13 and 14 correct the input signal on the basis of equation (15) to obtain a signal with constant amplitude and phase and supplies the corrected signal to the moving clutter cancelers 15 and 16. The moving clutter cancelers 15 and 16, comprising the same circuit as the stationary clutter cancelers 11 and 12, remove the corrected moving clutter. Then, the I and Q component signals from which the stationary clutter and the moving clutter have been removed are combined in a I/Q combiner 17 and the combined signal is output to a display (not shown) as an indication video signal. 
     A phase-correcting data generator 19 generates a phase-variation signal Δθ n+m  in each trigger repetition period, due to the differences in duration of the trigger repetition periods. The correction signal Δθ n+m  corresponds to the average Doppler frequency fd which is related to moving clutter velocity. An amplitude-correcting data generator 20 similarly generates a correction signal B n+m  to normalize the amplitude varied in every trigger repetition at a predetermined value. An integrator 21 integrates the phase-variation signal Δθ n+m  in every trigger repetition which is output from the phase-correcting data generator 19 and outputs the signal ##EQU17## in equation (15). A SIN&amp;COS converter 22 generates SIN (ΣΔθ) and COS (ΣΔθ) in response to the integrating data ##EQU18## A correcting data generator 23 transmits correcting data B n+m  · sin (ΣΔθ) and B n+m  · cos (ΣΔθ) to the amplitude and phase correctors 13 and 14 in response to the outputs from the amplitude-correcting data generator 20 and the SIN&amp;COS converter 22. 
     There will now be described an example of manually setting a presumptive value fd of the average Doppler frequency (fd 2 ) of the moving clutter. For example, a radar operator watches a video indicator displaying the output of the I/Q combiner on the radar screen and arranges the output from a presumptive Doppler frequency (fd) generator 18 to reduce clutter on the screen as much possible. fd from the fd generator 18 is fixed at a suitable frequency minimizing clutter on the screen. 
     The phase-correcting data generator 19 sets a reference phase value θ 0  equal to θ n+1 . The phase variation (presumptive value) Δθ n+1  generated in the trigger state (n+1) is therefore Δθ n+1  =θ n+1  -θ 0  =0. Using the well-known data fd, T 1 , T 2 , T 3 , the generator 19 generates the following variations (presumptive values): ##EQU19## 
     The amplitude-correcting data generator 20 generates the following data to correct the amplitude variation in each state: 
     The amplitude-correcting value (presumptive value) B n+1  in the trigger state (n+1) is: ##EQU20## 
     As is clear from the equations (15) to (17), the amplitude and phase correctors 13 and 14 give the moving clutter signal with constant amplitude and phase through processing for the amplitude component: ##EQU21## and through the processing for the phase component: ##EQU22## 
     The embodiment shown in FIG. 2 discloses an example of generating amplitude and phase correction data through a manual operation, however, it is possible to automatically design the construction of generating the correction data. 
     FIG. 3 shows the embodiment where the correction data is generated on the basis of the presumptive value θ n+m  of the average Doppler phase-shift of the moving clutter determined by the outputs of stationary clutter cancelers 11 and 12. In FIG. 3, the same numerals as in FIG. 2 denote the same constituent elements therewith. Only the difference between the embodiments shown in FIGS. 2 and 3 will be described. A phase-correcting data generator 19A determines the phase-shift Δθ n+m  by utilizing the I and Q component signals of the outputs of the cancelers 11, 12 in the trigger states (n+m) and (n+m-1) according to the following equation. ##EQU23## 
     Next, the presumptive value Δθ n+m  (≃Δθ n+m ) of the average Doppler phase-shift of the moving clutter in the distance R 0  is given as: ##EQU24## Here, the average of N samples of the moving clutter phase-shift between the distance (R 0  -N/2) and (R 0  +N/2) in the transmit trigger state (n+m), except the distance R 0 , is regarded as the presumptive value of the average Doppler phase-shift of the moving clutter. The same processing with the embodiment shown in FIG. 2 is carried out to obtain an integrated value, SIN component and COS component from the obtained presumptive value Δθ n+m . 
     An amplitude-correcting data generator 20A determines ##EQU25## according to Δθ n+m  =πfd(T j-1  +Tj) in response to Δθ n+m  obtained in the phase-correcting data generator 19A, and then supplies it to a correcting data generator 23. 
     FIG. 4 is the composition block of another embodiment for generating correction data. In this embodiment, the phase-correcting data generator 19A is identical with that of the embodiment shown in FIG. 3, however, the amplitude-correcting data generating method is different from that in FIG. 3. 
     An amplitude-correcting data generator 20B is composed of an amplitude calculator 201B, an averaging circuit 202B and a reciprocal number calculating circuit 203B. The amplitude calculator 201B develops ##EQU26## in response to receipt of the I components (A n+m  · cos θ n+m ) and Q components (A n+m  · sin θ n+m ) and then provides this output as amplitude components. The averaging circuit 202B develops the average amplitude value A n+m  (≃A n+m ) in response to receipt of those amplitude components according to the equation; ##EQU27## Next, the reciprocal number calculator 203B provides the output of the amplitude-correcting data generator 20B as the reciprocal number of A n+m  or ##EQU28## and supplies it to the correcting data generator 23. 
     When the outputs of the cancelers 11 and 12 are corrected according to the correction data obtained as stated above, the phase components and the amplitude components are kept constant so that moving clutter is removed by the moving clutter cancelers 15 and 16.