Stagger compensated moving target detector

Abstract: A moving target detector (MTD) (23) in a pulse radar system (10) uses corrected weighting coefficients (W i ) to compensate for pulse stagger effect on transmitted pulses. Transmitted pulses are sampled (15, 17, 19, 20) when the radar is switched (32) to a test mode for determining a correction factor which is used to calculate (31) the corrected weighting coefficients (W i ) The radar return signals received through the antenna (18), duplexer (16), coupler (20), receiver (21) and quadrature detector (22) are processed in the MTD (23) by a finite impulse response (FIR) filter (24) using the stored (26) corrected weighting coefficients (W i ) calculated (31) for each sequence of transmitted pulses including block stagger and pulse stagger sequences. In the test mode, pulses from the transmitter (14) are routed from a coupler (15) by an operate/test mode controller (32) through a delay (17), attenuator (19) and the coupler (20) to the receiver (21), and from the quadrature detector (22) to the coefficient calculator (31) which also receives uncorrected coefficients D i . In the operate mode, the appropriate corrected coefficient (W i ) is selected from a memory (26) by a synchronizer (28) that triggers the transmitter (14).

Description: This invention relates to a moving target indication radar system comprising: means for transmitting pulses with staggered pulse repetition frequency (PRF); means for receiving return signals from said transmitted pulses; and a moving target detector having: filter means coupled to said receiving means for filtering quadrature samples of said received return signals of sequences of said transmitted pulses in accordance with weighting coefficients; means for storing weighting coefficients and supplying the weighting coefficients to the filter means; and means coupled to said storing means and said transmitting means for synchronizing the supplying of weighting coefficients for said filter means with said transmitted pulses in accordance with the pulse repetition frequency. The invention also relates to a method of operating a moving target indication radar system having a moving target detector. US-A-4137532 describes a variable interpulse period Doppler filter bank signal processor for a moving target indication radar system of the kind defined hereinbefore at the beginning in which the weighting coefficients are pre-computed and are stored in a read only memory that is addressed, during operation, by a variable interpulse period sequencer driven by a time interval counter. As is known in the art, when a staggered pulse repetition frequency (PRF) is used in a pulse-radar system, a voltage variation is induced in the transmitter power supply. Such a voltage variation modulates the amplitude and phase of the RF pulses. The filters of a moving target detector (MTD) in the radar system are designed to cancel clutter returns that do not contain this staggered modulation; hence, a limitation on clutter attenuation occurs as a result of such staggered induced modulation on the RF pulses. Radar systems in the prior art have attempted to deal with the staggered modulation problem by increasing the size of the filter in the transmitter power supply or by discarding power to maintain a constant power supply load which can amount to approximately 20% lost power. EP-A-0 153 130 describes a Doppler pulse radar system having a moving target indicator in which stationary clutter is removed from I and Q components of the received radar signals by two conventional MTI clutter filters whose outputs are corrected in respective corrector circuits so that two further MTI clutter filters can remove the remaining clutter from moving clutter sources. The corrector circuits produce respective signals from which the variation in amplitude and phase of the moving clutter components in the outputs from the stationary clutter cancelling MTI has been removed. The corrector circuits operate in response to correcting data generated on the basis of an assumed value of the average Doppler phase-shift of the moving clutter. WO 88/02493 describes a moving target indicator radar system in which small pulse-to-pulse imperfections, due for example to the use of a magnetron as the source of the radar pulses, are corrected by a filter coefficient calculation circuit that computes a separate filter coefficient for each pulse repetition interval by deriving a plurality of filter coefficients each of which is equal to twice the kth sample of an average transmitter pulse sample less the kth sample of the transmitter pulse sample from an nth pulse repetition interval. These filter coefficients are used as the coefficients for a plurality of finite impulse response filters which act as a set of equalisers on stored traces having samples of the received radar signal. According to one aspect of the present invention, a radar system of the kind defined hereinbefore at the beginning is characterised by means operable in a test mode of the system for calculating in accordance with the formula W i =D i x (T 1 /T i ), where D i are uncorrected complex filter coefficients, T 1 is the first complex transmitter output pulse sample in a group of pulses to which the coefficients D i apply, and T i is the i th complex transmitter output pulse sample in the group, weighting coefficients (W i ) corrected for pulse stagger amplitude and phase errors on said transmitted pulses and supplying the corrected weighting coefficients (W i ) to the storing means for storage therein, and mode control means for determining whether the system is in the test mode or an operate mode, the mode control means including means for sampling said transmitted pulses and coupling said sampled transmitted pulses to the receiving means when the radar system is in the test mode; and means being provided for supplying a quadrature output from the receiving means to the calculating means, the calculating means being such as to calculate said corrected weighting coefficients based on quadrature samples of the sampled transmitted pulses and uncorrected weighting coefficient inputs (D i ) to the calculating means in the test mode. The transmitted pulses may comprise either or two types of staggered PRF, block stagger and pulse stagger. The block stagger transmitted pulses comprise a plurality of stagger blocks wherein the interpulse periods are equal within each of the blocks of the transmitted pulses and the interpulse period may be the same or vary for each of the blocks. The pulse stagger transmitted pulses comprise a varying interpulse period from pulse to pulse. According to another aspect of the present invention, there is provided a method of operating a moving target indication radar system having a moving target detector, comprising the steps of: storing in said moving target detector weighting coefficients; transmitting pulses with staggered pulse repetition frequency (PRF); receiving return signals from said transmitted pulses; filtering quadrature samples of said received return signals of sequences of said transmitted pulses in accordance with said stored weighting coefficients; and synchronizing the application of said weighting coefficients for filtering the quadrature samples in accordance with the pulse repetition frequency (PRF) of the corresponding transmitted pulses, characterised by the step of determining during a test mode of the radar system weighting coefficients (W i ) corrected for pulse stagger amplitude and phase errors on said transmitted pulses, the corrected weighting coefficients (W i ) being calculated in accordance with the formula W i =D i x (T 1 /T i ), where D i are uncorrected complex filter coefficients, T 1 is the first complex transmitter output pulse sample in a group of pulses to which the coefficients D i apply, and T i is the i th complex transmitter output pulse sample in the group, and in that the said storing of weighting coefficients comprises storing the corrected weighting coefficients (W i ), said step of determining the corrected weighting coefficients (W i ) comprising the steps of: switching to a test mode; sampling said transmitted pulses and coupling said sampled transmitted pulses to receiving means when said radar system is in said test mode; and calculating corrected weighting coefficients based on quadrature samples from said receiving means of said sampled transmitted pulses and uncorrected coefficient inputs (D 1 ) when said radar system is in said test mode. The method preferably comprises the step of transmitting pulses comprising a plurality of stagger blocks wherein the interpulse periods are equal within each of the blocks of the transmitted pulses, and the interpulse period may be the same or vary for each one of the blocks, or alternatively the step of transmitting pulses having a varying interpulse period from pulse to pulse. Brief Description of the Drawings Other and further features and advantages of the invention will become apparent in connection with the accompanying drawings wherein: FIG. 1 is a block diagram of a radar system having staggered compensation in the moving target detector in accordance with the present invention; FIG. 2 is a functional block diagram of an FIR filter showing the W i inputs for the filter weighting coefficients; FIG. 3 shows an example of the relative timing of transmitter and receiver radar signals for block stagger for a four pulse FIR in pulse-radar systems; and FIG. 4 shows an example of the relative timing of transmitter and receiver radar signals for pulse stagger and a four pulse FIR in pulse-radar systems. Description of the Preferred Embodiment Referring to FIG. 1 there is shown a block diagram of a pulse-radar system 10 having staggered compensation in a moving target detector 23 in accordance with the present invention. A transmitter 14 generates a staggered pulse sequence in accordance with trigger pulses from a synchronizer 28. Transmitter power supply 12 provides power to the transmitter 14. The transmitter output is coupled via a directional coupler 15 to a duplexer 16 which provides the transmitted pulses to an antenna 18. Return signals are picked up by the antenna 18 and coupled to a receiver 21 via duplexer 16 coupled to a directional coupler 20. The received return signals are amplified and passed on to a quadrature detector 22 comprising phase detectors and analog-to-digital converters known to one skilled in the art for collecting quadrature data samples of the received return signals. The quadrature data samples are coupled to the moving target detector (MTD) 23 comprising finite impulse response (FIR) filters 24 coupled to a post processor 30 which is required for further radar signal processing. The error due to power supply 12 modulation on the received signals being processed by the FIR filters 24 is compensated by corrected weighting coefficients stored in a digital coefficient selector memory 26, the output of which is coupled to the FIR filters 24. The selection of the corrected weighting coefficients in the digital memory 26 is determined by a pulse interval address generated by synchronizer 28 which generates the transmitter trigger signals. The MTD 23 also comprises a coefficient calculator which is coupled to the coefficient selector memory 26. When the radar system 10 is switched from an operate mode to a test mode by an operate/test mode controller 32, the coefficient calculator determines corrected weighting coefficients (W i ) based on samples of transmitted signals and uncorrected filter coefficients (D i ), and the calculated corrected weighting coefficients are stored in the coefficient selector memory 26. Referring to FIG. 2, a functional representation of each of the FIR filters 24 is shown. All of the complex signal samples (S i ) which are the radar returns from a number of transmitted pulses, n, are stored in a digital memory for processing after all of the return signals have been received; this memory is represented functionally by a tapped delay line 36. Selected return signal samples (those from the same target range) are retrieved from memory (delay line 36) and multiplied by a corresponding corrected weighting coefficient (W i ) in multipliers 38 l to 38 n . These weighting coefficients have been stored in the digital coefficient selector memory 26, and a particular set of coefficients is selected by a pulse interval address provided by synchronizer 28 for the interpulse periods that were used when collecting the signal samples. The complex outputs (P l to P n ) from the multipliers 38 l to 38 n are added together by a plurality of summers 40 l to 40 n-l to produce the complex filter output, S o . The complex multiplier is represented mathematically as follows: If the received signals, S = Sx + jSy and the filter weight, W = Wx + jWy then, S * W = (Sx + jSy) * (Wx + jWy) S * W = (SxWx - SyWy) + j(SxWy + SyWx) Hence, a single complex multiply requires four numbers as inputs to each of the multipliers 38 l to 38 n and two numbers are generated at the outputs P l to P n . The complex filter output, S o , may be expressed as follows: Referring to FIG. 1, the uncorrected filter coefficients (D i ) used for a particular group of pulses are corrected by the coefficient calculator 31 to compensate for the small amplitude and phase errors on the transmitted signal resulting from the staggered PRF using the following formula: W i = D i *(T l /T i ) where D i are the uncorrected complex filter coefficients, T l is is the first complex transmitter output pulse sample in the group, T i is the i th complex transmitter pulse sample in the group and W i are the compensated or corrected filter weights. These transmitter output pulses are measured periodically (depending on the system application and the method of staggered PRF) by the operate/test mode controller 32 switching the radar system 10 to the test mode of operation and they are used to compute the new corrected weighting coefficients, (W i ), which are stored in memory 26 for use in the normal operate mode of the radar. An example showing a particular set of numerical values for the uncorrected weights, transmitter samples, and corrected weights is shown in Table 1. The values are chosen arbitrarily to illustrate the process. When any uncorrected weighting coefficient (W i ) is multiplied by its corresponding transmitter sample (S i ) the result is equal to S l , thus proving that the weighting coefficients remove the signal distortion injected by the tranmitter. Uncorrected filter coefficient weights: Dx1 = 1.00 Dy1 = 0.00 Dx2 = 1.00 Dy2 = 0.00 Dx3 = 1.00 Dy3 = 0.00 Dx4 = 1.00 Dy4 = 0.00 Transmitter samples: Tx1 = 1.01 Ty1 = 0.10 Tx2 = 1.00 Ty2 = 0.00 Tx3 = 0.98 Ty3 = 0.10 Tx4 = 0.90 Ty4 = 0.20 Corrected filter coefficient weights: Wx1 = 1.00 Wy1 = 0.00 Wx2 = 1.01 Wy2 = 0.10 Wx3 = 1.03 Wy3 = -0.003 Wx4 = 1.09 Wy4 = -0.13 Referring again to FIG. 1, the radar system 10 is placed in a test mode by the operate/test mode controller 32, to measure the transmitter output samples (T 1 to T i ) for each transmitted pulse, and these measurements are used to compute the corrected filter weights (W i ) in the coefficient calculator 31 which are (as described above) used by the FIR filters 24 in the normal operate mode. In the test mode a sample of the transmit pulse is coupled from the output of transmitter 14 via the operate/test mode controlled to a time delay 17 and attenuator 19 and then coupled to the receiver 21 input via directional coupler 20. The transmitter samples are amplified and quadrature detected in receiver 21 and quadrature detector 22 which is the same apparatus used in the radar system operate mode. When in the test mode, the quadrature detector 22 output is switched to the coefficient calculator 31 by the operate/test mode controller 32 for calculating the new corrected filter coefficient weights (W i ). Referring now to FIG. 3 and FIG. 4, there are two methods illustrated for accomplishing the stagger compensated MTD. PRF stagger may be applied either as block stagger as shown in FIG. 3 or pulse stagger as shown in FIG. 4. In the block stagger method a block of pulses are transmitted with equal interpulse intervals. The signals received from this block of transmissions are stored and processed as a single batch. The interpulse spacing varies from block to block but the processing remains the same with the exception of the corrections placed on the filter weights. Each processing batch uses its own set of corrected weights for processing the signals in that batch. These corrected weights will cancel the transmitter modulation induced on the transmitter output by the power supply's response to the particular interpulse periods used. The second method referred to as pulse stagger employs pulse-to-pulse stagger. Here the interval between pulses is not held fixed as it was in block stagger but rather the interpulse interval varies from pulse to pulse. Four contiguous pulses comprise a processing batch as shown in the example of FIG. 4. After the next pulse is transmitted and its signal returns are received and stored, another processing batch is initiated. This new batch combines the returns from the last four transmitted pulses, and since the pulses have different relative positions in time, that processing batch requires a different set of corrected weights from the preceding batch. Therefore, in the pulse stagger method a new set of corrected filter weights is obtained from the digital coefficient selector memory 26 at the start of each processing batch. This concludes the description of the preferred embodiment. However, many modifications and alterations will be obvious to one of ordinary skill in the art without departing from the spirit and scope of the inventive concept. Therefore, it is intended that the scope of this invention be limited only by the appended claims.

Claims: A moving target indication radar system comprising: means (12,14,28) for transmitting pulses with staggered pulse repetition frequency (PRF); means (21) for receiving return signals from said transmitted pulses; and a moving target detector (23) having: filter means (24) coupled to said receiving means (21) for filtering quadrature samples of said received return signals of sequences of said transmitted pulses in accordance with weighting coefficients; means (26) for storing (23) weighting coefficients and supplying the weighting coefficients to the filter means (24); and means (28) coupled to said storing means (26) and said transmitting means (12,14) for synchronizing the supplying of weighting coefficients for said filter means (24) with said transmitted pulses in accordance with the pulse repetition frequency, characterised by means (31) operable in a test mode of the system for calculating in accordance with the formula W i =D i x (T 1 /T i ), where D i are uncorrected complex filter coefficients, T 1 is the first complex transmitter output pulse sample in a group of pulses to which the coefficients D i apply, and T i is the i th complex transmitter output pulse sample in the group, weighting coefficients (W i ) corrected for pulse stagger amplitude and phase errors on said transmitted pulses and supplying the corrected weighting coefficients (W i ) to the storing means (26) for storage therein, and mode control means (32) for determining whether the system is in the test mode or an operate mode, the mode control means (32) including means (15,17,19,20) for sampling said transmitted pulses and coupling said sampled transmitted pulses to the receiving means (21) when the radar system is in the test mode; and means (22) being provided for supplying a quadrature output from the receiving means (21) to the calculating means (31), the calculating means (31) being such as to calculate said corrected weighting coefficients based on quadrature samples of the sampled transmitted pulses and uncorrected weighting coefficient inputs (D i ) to the calculating means (31) in the test mode. A radar system according to claim 1, characterised in that the mode control means (32) comprises: means (15) arranged to couple said pulses from said transmitting means (12,14) to delay and attenuator means (17,19) when set to said test mode, the delay and attenuator means (17,19) then supplying to the receiving means (21) signals having characteristics of said return signals. A radar system according to claim 1 or 2, characterised in that said transmitting means (12,14,28) is such as to transmit a plurality of stagger blocks wherein said interpulse periods are equal within each of said blocks of said transmitted pulses and said interpulse period may be the same or vary for each of said blocks. A radar system according to claim 1 or 2, characterised in that said transmitting means (12,14,28) is such as to transmit pulses with staggered PRF having a varying interpulse period from pulse to pulse. A method of operating a moving target indication radar system having a moving target detector, comprising the steps of: storing (26) in said moving target detector (23) weighting coefficients; transmitting (12,14,28) pulses with staggered pulse repetition frequency (PRF); receiving (21) return signals from said transmitted pulses; filtering (24) quadrature samples of said received return signals of sequences of said transmitted pulses in accordance with said stored weighting coefficients; and synchronizing (28) the application of said weighting coefficients for filtering (24) the quadrature samples in accordance with the pulse repetition frequency (PRF) of the corresponding transmitted pulses, characterised by the step of determining (32,31) during a test mode of the radar system weighting coefficients (W i ) corrected for pulse stagger amplitude and phase errors on said transmitted pulses, the corrected weighting coefficients (W i ) being calculated in accordance with the formula W i =D i x (T 1 /T i ), where D i are uncorrected complex filter coefficients, T 1 is the first complex transmitter output pulse sample in a group of pulses to which the coefficients D i apply, and T i is the i th complex transmitter output pulse sample in the group, and in that the said storing of weighting coefficients comprises storing (26) the corrected weighting coefficients (w i ), said step of determining the corrected weighting coefficients (W i ) comprising the steps of: switching (32) to a test mode; sampling said transmitted pulses and coupling (15,17,19,20) said sampled transmitted pulses to receiving means (21,22) when said radar system is in said test mode; and calculating (31) corrected weighting coefficients based on quadrature samples from said receiving means (21,22) of said sampled transmitted pulses and uncorrected coefficient inputs (Di) when said radar system is in said test mode. A method according to claim 5, characterised in that said method further comprises the step of transmitting pulses comprising a plurality of stagger blocks wherein said interpulse periods are equal within each of said blocks of said transmitted pulses, and said interpulse period may be the same or vary for each one of said blocks. A method according to claim 6, characterised in that said step of calculating corrected weighting coefficients for return signal processing is performed for each of said blocks. A method according to claim 5, characterised in that said method further comprises the step of transmitting stagger pulses with said staggered PRF having a varying interpulse period from pulse to pulse. A method according to claim 8, characterised in that said step of calculating corrected weighting coefficients for return signal processing is performed for each predetermined batch of said staggered pulse return signals.

Inventors: WARD HAROLD R, WARD, HAROLD R.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1990
Main classification: G01S13
IPC codes: G01S7, G01S13