Sensitivity velocity control

Abstract: Surveillance radar for distinguishing wanted moving targets from unwanted moving targets by processing radar target return data has prestored evaluation criteria for accepting/rejecting the moving targets. The radar target return signals are passed by a receiver/exciter (40) to a digital signal processor (42) that outputs signals representative of ambiguous radial velocities of targets, signal-to-noise ratio, and range to a radar data processor (44). A true radial velocity estimator (22) and an apparent radar cross section estimator (24) of the radar data processor (44) determine the true radial velocity and apparent radar cross section. In the processor (44), a computer (26) evaluates these measurements based on a look-up table of preselected acceptable levels of minimum true radial velocity for each apparent radar cross section. The computer (26) declares a target acceptable when its radial velocity matches or exceeds the preselected minimum radial velocity for that radar cross section stored in memory. The acceptance criteria relates to the type of target (aircraft, missile, bird, etc.) being accepted or rejected. In general, the criteria accept large targets having low to high radial velocities. The smaller the apparent radar cross section of the target, the higher the true radial velocity must be for acceptance. The true radial velocity versus apparent radar cross section profile is intended to accept aircraft and missiles but reject birds.

Description: This invention relates to a method of distinguishing between wanted and unwanted moving targets, comprising generating an apparent radar cross section from radar target return data and using the apparent radar cross section in a test to determine whether the radar target return data represents an acceptable target, and to radar apparatus for carrying out the method. Surveillance radars have historically had difficulty distinguishing desired moving targets such as aircraft from undesired moving targets such as birds. The apparent radar cross section (RCS) of wanted targets is often less than the RCS of unwanted targets due to target fluctuations, multipath, radar antenna patterns and the size of aircraft and missiles. Techniques such as moving target indication (MTI) are insufficient to distinguish unwanted targets from wanted targets because the radial velocities (i.e., the component of velocity in the direction of the radar) of the unwanted targets often equal or exceed the ambiguous radial velocity of the desired targets. The primary means of rejecting unwanted targets in surveillance radars have been the use of sensitivity time control (STC), two-beam antenna techniques (which assume that unwanted targets are near the earth's surface) and clutter maps. These techniques are described in "Radar Handbook" by Merrill Skolnik, Editor in Chief, second edition, McGraw-Hill Publishing Company, 1990, pp. 15.65-15.70. Although each of these techniques provides some mitigation of the unwanted target problem, each has sufficient shortcomings to prevent achieving a desired performance level. An alternative method of eliminating unwanted targets in fire control radars is to eliminate all targets with absolute radial velocities below some value such as 120 knots. However, this solution is not available to surveillance radars with relatively low ambiguous velocities which are typically around 80 knots. JP-A-2025781 (see Patent Abstracts of Japan, Vol. 14, no. 169 (P-1032), 30th March 1990) describes a method and apparatus of the kind defined hereinbefore at the beginning in which the range of a target is not displayed if the radar cross section derived from a target return signal (after processing by a Doppler filter) is equal to or greater than a constant reference value. Accordingly, it is therefore an object of this invention to provide a method and apparatus for distinguishing wanted targets from unwanted targets in a surveillance radar. It is s further object of this invention to distinguish between wanted and unwanted targets based on a combination of true radial velocity and apparent target radar cross section. According to one aspect of the invention, a method of the kind defined hereinbefore at the beginning is characterised by the steps of: obtaining radar target return data representing a plurality of ambiguous radial velocities each measured with a different pulse repetition frequency; estimating a true radial velocity from a cluster of the said ambiguous radial velocities; testing said true radial velocity and said apparent radar cross section to determine whether a point defined by said true radial velocity and said apparent radar cross section in a plane having true radial velocity and apparent radar cross section as co-ordinates lies within or outside an accept/reject boundary defined in the said plane, the accept/reject boundary separating acceptable combinations of true radial velocity and apparent radar cross section from unacceptable combinations of true radial velocity and apparent radar cross section; and accepting the radar target return data for utilization if the said point represents an acceptable combination, or rejecting the radar target return data if the said point represents an unacceptable combination, the said boundary being such that for any true radial velocity there is a minimum value to a range of apparent radar cross sections with which combination is acceptable, and whereby a moving radar target that is small and slow is rejected, a moving radar target that is fast is accepted, and a moving radar target that is large is accepted. A preferred method also includes determining whether a no valid velocity estimate signal is generated by a radial velocity generator means which indicates a reject target, determining whether the apparent radar cross section is below a minimum value resulting in a reject target, and testing the radar target return signal in accordance with a velocity estimate quality signal to determine whether the target return signal is below a predetermined level of the velocity estimate quality signal resulting in a reject target. According to another aspect of the invention there is provided radar apparatus for distinguishing between wanted and unwanted moving targets, comprising: means for generating an apparent radar cross section from radar target return data; and means for using the apparent radar cross section in a test to determine whether the radar target return data represents a wanted target, characterised by: means for obtaining radar target return data representing a plurality of ambiguous radial velocities each measured with a different pulse repetition frequency; means for estimating a true radial velocity from a cluster of the said ambiguous radial velocities; and means for testing said true radial velocity and said apparent radar cross section to determine whether a point defined by said true radial velocity and said apparent radar cross section in a plane having true radial velocity and apparent radar cross section as co-ordinates lies within or outside an accept/reject boundary defined in the said plane, the accept/reject boundary separating acceptable combinations of true radial velocity and apparent radar cross section from unacceptable combinations of true radial velocity and apparent radar cross section, and for accepting the radar target return data for utilization if the said point represents an acceptable combination, or rejecting the radar target return data if the said point represents an unacceptable combination, the said boundary being such that for any true radial velocity there is a minimum value to a range of apparent radar cross sections with which combination is acceptable, and whereby a moving radar target that is small and slow is rejected, a moving radar target that is fast is accepted, and a moving radar target that is large is accepted. Preferably, radar apparatus embodying the invention includes means for determining whether a no valid velocity estimate signal is generated by the radial velocity estimating means indicating a reject target, means for determining whether the apparent radar cross section is below a minimum value resulting in a reject target, and means for testing the radar target return signal in accordance with a velocity estimate quality signal to determine if the radar target return signal is below a predetermined level of the velocity estimate quality signal. The reject boundary testing means preferably comprises memory means for storing the reject boundary. 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 flow chart of the present invention of a sensitivity velocity control in a surveillance radar; FIG. 2 is a graph of an apparent radar cross section versus an absolute radial velocity showing target accept and reject boundaries; FIG. 3 is a block diagram of a surveillance radar embodying the invention; and FIG. 4 is a block diagram of the radar data processor shown in FIG. 3 embodying the invention. Description of the Preferred Embodiment Referring to FIG. 1, a flow chart of the present invention of a method of Sensitivity Velocity Control (SVC) in a surveillance radar is shown for distinguishing wanted targets from unwanted targets based on processing a combination of a true radial velocity and an apparent target radar cross section (RCS). In general, the SVC program 10 of FIG. 1 evaluates these processed measurements against preselected accept/reject boundary data (FIG. 2) stored in a computer memory comprising acceptable levels of minimum true radial velocity for each apparent radar cross section. The invention is based on the following criteria: if a radar target is small and slow, it probably is an unwanted target; if the radar target is fast (no matter how small) or if the radar target is large (even if the radial velocity is low) it probably is a wanted target. Referring to FIG. 3, a functional block diagram of a Radar System 30 is shown which employs the Sensitivity Velocity Control Program 10 in a Radar Data Processor 44. The Radar System 30 comprises a Transmitter 38 and a Receiver/Exciter 40 which are coupled via a Duplexer 34 to an Antenna 32 for transmitting signals to targets and for receiving returned signals. The Antenna Control Unit 36 is coupled between the Antenna 32 and a Digital Signal Processor (DSP) 42. The Antenna Control Unit 36 is used to control and report the positioning of the Antenna 32; the DSP 42 forms the base data from analog signals received from the Receiver/Exciter 40 such as the Ambiguous Radar Velocities, Signal To Noise (S/N) Ratio and Range. The Radar Data processor 44 coupled to such base data from the DSP 42 processes such base data to determine wanted targets from unwanted targets. The base data received from the DSP 42 is processed by the Sensitivity Velocity Control Program 10 in the RDP 44. The wanted targets are coupled to a User Computer 45 for utilization such as target tracking and to a User Display 46 which displays the wanted or accepted targets. Referring now to FIG. 4, a block diagram of the Radar Data Processor 44 is shown comprising a True Radial Velocity Estimator 22 and an Apparent Radar Cross-section Estimator 24, both of which have outputs coupled to a Computer 26 comprising the Sensitivity Velocity Control Program 10. The True Radial Velocity Estimator 22 receives up to five Ambiguous Radial Velocities, each measured with a different pulse repetition frequency, from the DSP 42, and it determines the True Radial Velocity. It also generates a Velocity Estimate Quality signal or a No Valid Velocity Estimate signal which is used by the SVC program 10 along with the True Radial Velocity to perform a first test (Test A 14) on the radar target return data. The True Radial Velocity Estimator 22 examines how closely the ambiguous radial velocity estimates cluster at each possible true radial velocity. The true radial velocity is determined where a cluster is found. The Velocity Estimate Quality signal is based on how well the ambiguous radial velocity estimates cluster at the estimated true radial velocity. The Velocity Estimate Quality signals range from 1 to 4 with 4 being the best quality. If the ambiguous radial velocity estimates do not cluster at any true radial velocity, then a No Valid Velocity Estimate signal is generated. The Apparent Radar Cross Section Estimator 24 receives the S/N Ratio (SNR) signal and the Range signal from the DSP 42 and generates the Apparent Radar Cross Section signal which is connected to Computer 26 and used by the SVC program 10 to perform a second test (Test B 16) on the radar target return data. To determine the apparent radar cross section (RCS), the Radar System 30 is characterized by a Reference SNR for a Reference RCS at a Reference Range. Then the measured Range and SNR are used to calculate the apparent RCS in accordance with the following formula: Apparent RCS = ( Range/Reference Range ) 4 ( Reference SNR/SNR ) ( Reference RCS ) Referring again to FIG. 1 and also FIG. 2, the Sensitivity Velocity Control Program 10, which is located in a memory of the Computer 26 (FIG. 4), tests the radar target return data to distinguish between wanted targets such as aircraft and unwanted targets such as birds. When START SVC 12 occurs the first step performs a Test A 14 on the Apparent Radar Cross Section, True Radial Velocity and the No Valid Velocity Estimate signals. The radar target return data is determined to be a Reject Target 21 if Test A 14 determines that the No Valid Velocity Estimate signal is "active" and if the Apparent Radar Cross Section is less than -3 dBSM. If the radar target data passes Test A 14, the second step performs the Test B 16 on the Apparent Radar Cross Section and Velocity Estimate Quality signals. Test B 16 determines that if the Velocity Estimate Quality signal is less than 2 and the Apparent Radar Cross Section is less then -10 dBSM, then the radar return target data is a Reject Target 21. If the radar target data passes Test B 16, then the third step performs Test C 18 on the True Radial Velocity and Apparent Radar Cross Section signals. Test C 18 determines if the radar target return data is within a predetermined boundary, and if it is, then the target data is a Reject Target 21; otherwise an Accept Target 20 has been determined. FIG. 2 is a graph of the Apparent Radar Cross Section versus the Absolute Radial Velocity showing a typical predetermined boundary between an Accept Target 20 and a Reject Target 21. As pointed out hereinbefore, the Sensitivity velocity Control Program 10 is based on the criteria that if the radar target is small and slow, it probably is an unwanted target, and if the radar target is fast (no matter how small) or if the radar target is large (even if the radial velocity is low), it probably is a wanted target.

Claims: A method of distinguishing between wanted and unwanted moving targets, comprising generating (42,24) an apparent radar cross section from radar target return data and using (26,10) the apparent radar cross section in a test (10) to determine whether the radar target return data represents a wanted target (29), characterised by the steps of: obtaining (42) radar target return data representing a plurality of ambiguous radial velocities each measured with a different pulse repetition frequency; estimating (22) a true radial velocity from a cluster of the said ambiguous radial velocities; testing (26,10) said true radial velocity and said apparent radar cross section to determine whether a point defined by said true radial velocity and said apparent radar cross section in a plane having true radial velocity and apparent radar cross section as co-ordinates lies within or outside an accept/reject boundary defined in the said plane, the accept/reject boundary separating acceptable combinations of true radial velocity and apparent radar cross section from unacceptable combinations of true radial velocity and apparent radar cross section; and accepting the radar target return data for utilization if the said point represents an acceptable combination, or rejecting the radar target return data if the said point represents an unacceptable combination, the said boundary being such that for any true radial velocity there is a minimum value to a range of apparent radar cross sections with which combination is acceptable, and whereby a moving radar target that is small and slow is rejected, a moving radar target that is fast is accepted, and a moving radar target that is large is accepted. A method according to claim 1, characterised in that the step of estimating (22) a true radial velocity comprises forming estimates from the said plurality of ambiguous radial velocities, determining the closeness with which the said estimates cluster, and determining the true radial velocity where a cluster is formed, or generating a signal indicating rejection of the radar target return data if there is no cluster. A method according to claim 2, characterised by the step of testing (16) together the apparent radar cross section and a further signal representing the said closeness to determine acceptability of the radar target return data before testing (18) the true radial velocity and the apparent radar cross section. A method according to claim 2 or 3, characterised by determining (26,10) whether said apparent radar cross section is below a test minimum value and, if so, classifying the radar target return data as indicating a reject target. Radar apparatus for distinguishing between wanted and unwanted moving targets, comprising: means (42,24) for generating an apparent radar cross section from radar target return data; and means (26,10) for using the apparent radar cross section in a test to determine whether the radar target return data represents a wanted target (29), characterised by : means (42) for obtaining radar target return data representing a plurality of ambiguous radial velocities each measured with a different pulse repetition frequency; means (22) for estimating a true radial velocity from a cluster of the said ambiguous radial velocities; and means (26,10) for testing said true radial velocity and said apparent radar cross section to determine whether a point defined by said true radial velocity and said apparent radar cross section in a plane having true radial velocity and apparent radar cross section as co-ordinates lies within or outside an accept/reject boundary defined in the said plane, the accept/reject boundary separating acceptable combinations of true radial velocity and apparent radar cross section from unacceptable combinations of true radial velocity and apparent radar cross section, and for accepting the radar target return data for utilization if the said point represents an acceptable combination, or rejecting the radar target return data if the said point represents an unacceptable combination, the said boundary being such that for any true radial velocity there is a minimum value to a range of apparent radar cross sections with which combination is acceptable, and whereby a moving radar target that is small and slow is rejected, a moving radar target that is fast is accepted, and a moving radar target that is large is accepted. Radar apparatus according to claim 5, characterised in that the means (22) for estimating a true radial velocity comprises means for forming estimates from the said plurality of ambiguous radial velocities, means for determining the closeness with which the said estimates cluster, and means for determining the true radial velocity where a cluster is formed and for generating a signal indicating rejection of the radar target return data if there is no cluster. Radar apparatus according to claim 6, characterised in that the means (26,10) for using the apparent radar cross section in a test includes means (10) for testing (14) together the apparent radar cross section and a further signal generated by the estimating means (22) to determine acceptability of the radar target return data, the said further signal representing the said closeness. Radar apparatus according to claim 6 or 7, characterised by means (26,10) for determining whether said radar cross section is below a test minimum value and, if so, classifying the radar target return data as indicating a reject target. Radar apparatus according to any one of claims 5 to 8, characterised in that said testing means (26,10) comprises memory means for storing said accept/reject boundary.

Inventors: SHRADER WILLIAM W, SHRADER, WILLIAM W.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1992
Main classification: G01S7
IPC codes: G01S13, G01S7