Angle Width Discriminator/Altitude Line Detector Radar

1. Field of the Invention

The present invention relates to a pulse radar with a scanning antenna to acquire targets; and, more particularly, to automatic acquisition of targets in an air combat situation prior to tracking the target or lock-on.

2. Description of the Prior Art

Automatic acquisition modes in non-coherent pulse-mode radars are plagued with interference from clutter returns arising from backscatter where the beam impinges upon the ground or from side lobes of the beam impinging the ground approximately below the antenna. Side lobe reflections from the ground below the antenna are especially troublesome because they are present for all pointing directions of the radar antenna in that some side lobe radiation is always reflected by the ground to the antenna. These radar return signals have been called "altitude lines" by radar operators because they cause a line to appear on a B-scope display at a range equal to the actual altitude during azimuth scans of the antenna. As the pilot tries to point his aircraft such that the target is within the antenna search scan pattern, he can get an unwanted radar tracking lock on the altitude line due to its presence before the target is illuminated by the antenna. If the altitude is less than the range or distance to the desired target, the altitude line will be acquired by the radar even if the antenna is illuminating the desired target. This is because the altitude line is interpreted to be greater threat by the automatic acquisition circuitry since it is nearer in range.

Previous attempts at solving the problem of an unwanted radar tracking lock on the altitude line included pulse width discrimination, sensitivity time control and gain tuning, and altitude line tracking and blanking. Of these approaches, only altitude line tracking has been successful. This has involved a special altitude line detection antenna scan and special tracking hardware. This approach adds significant time delays in the target acquisition process and adds to the radar hardware costs and complexity.

It is therefore desirable to provide a means and method for acquiring desired targets while at the same time rejecting the altitude line since the pilot must be confident that his radar has locked on the desired target for the automatic acquisition mode of the radar to be useful in combat.

It is further desirable that all returns from the side lobe, as well as the main beam, are processed by the same circuitry. By using the same circuitry, identical detection sensitivities and range measurement accuracies are maintained for both desired and undesired returns. This overcomes the problems encountered in those methods where separate circuitry is used to detect and track the undesired returns.

It is further desirable to reject target reports that extend over an antenna scan angle that is too large to be a valid target.

It is further desirable to select target reports which extend over a limited scan angle by the antenna indicative of a target having a valid target size.

It is further desirable to use target reports rejected during a prior scan as a basis for rejecting target reports of a current scan.

In accordance with the present invention, a method and means for operating a pulse radar is provided for acquiring a target comprising the steps of transmitting a sequence of signals from an antenna, the antenna radiating most of the electromagnetic energy in a narrow beam having a predetermined beam width, scanning the antenna to direct the beam along a predetermined path represented by a plurality of path increments connected in series, receiving reflected signals emanating from said antenna, integrating a plurality of the reflected signals arriving at the antenna at a predetermined time interval corresponding to a range cell and path increment for the directed beam to provide an output signal, thresholding the output signal to provide a target report at times when the amplitude of the output signal is greater than a predetermined value, repeating the steps of receiving and thresholding for a plurality of time intervals for each of a plurality of path increments for the directed beam, storing the target reports, comparing the occurrence of the target reports as a function of the plurality of path increments and range cell location with a plurality of predetermined target report patterns and selecting target reports having an occurrence matching one of said predetermined target report patterns indicative of a target having a valid target size.

The invention further provides performing the steps of transmitting, scanning, receiving, integrating, thresholding, repeating and storing during a second scan, storing rejected target reports of the first scan, rejecting target reports stored during a second scan when the target reports of the second scan occur within a predetermined distance of the range cell and path increment of the rejected target reports of the first scan, comparing the occurrence of the target reports from the second scan as a function of the plurality of increments and range cell location with a plurality of predetermined target report patterns, and selecting target reports having an occurrence matching one of said predetermined target report patterns indicative of a target having a valid target size.

FIG. 1 is a functional block diagram of one embodiment of the invention.

FIG. 2 is a diagram of a suitable antenna scan pattern for the embodiment of FIG. 1.

FIG. 3 is a diagram showing the special relationship of the main beam scan pattern of FIG. 2.

FIG. 4 is a diagram of the altitude line clutter caused by side lobes from the antenna.

FIG. 5 is a typical B-scope display of the target in the presence of main beam clutter and altitude line clutter prior to rejecting the altitude line.

FIG. 6 shows video wave forms of reflected signals at the antenna.

FIG. 7 shows one embodiment for generating target reports from the video or reflected signals.

FIG. 8 shows a top level flow chart for the computer controlled automatic acquisition software.

FIG. 9 shows a detail flow chart for the computer controlled automatic acquisition software of box 94 in FIG. 8.

Referring now to the drawings, FIG. 1 shows one embodiment of the invention. Automatic acquisition radar 10 comprises antenna 12, radar system 14, radar processor 15 and signal detection unit 16. Radar processor 15 functions to control the scanning of antenna 12 by antenna position commands coupled over line 18 to an input of radar system 14. Radar system 14 functions to determine the pointing direction or provide resolver signals of antenna 12 and to couple the antenna resolver signals over line 19 to an input of radar processor 15. Radar processor 15 couples commands over line 20 to an input of radar system 14. Radar processor 15 couples a target range position over line 21 to an input of radar system 14. After automatic acquisition radar 10 has acquired a target, the target range position or the range of the target is supplied to a tracking circuit within radar system 14 which will cause the radar system 14 to track in range the desired target in response to a lock-on command from radar processor 15 over line 20. Radar processor 15 receives target reports from signal detection unit 16 over line 22. An interrupt command is generated by signal detection unit 16 and is coupled over line 23 to an input of radar processor 15, which functions to interrupt radar processor 15 to receive the detection reports on line 22. Radar processor 15 may receive start, initiate and halt commands for overall control of automatic acquisition radar 10 by a control operator or a control processor over line 24.

Radar processor 15 may have a plurality of registers, including an instruction accumulator, multiplier quotient, index, effective address, instruction counter, instruction, instruction buffer, memory operand, instruction register save, operation counter, block (MSB of address) and carry overflow register. Processor 15 may have an instruction set which includes at least an ADD, NAND, SUBTRACT, LOAD, STORE, INPUT, OUTPUT, CONDITIONAL BRANCH and SHIFT instructions for operating upon data transferred to the processor over lines 19 and 22 in accordance with the particular instruction in the instruction register of processor 15. Processor 15 has an input/output port responsive to control signals including HALT, INITIATE, CLEAR, and INTERRUPT coupled over lines 23 and 24. Processor 15 includes memory storage for both operand and instruction fetches. Processor 15 includes means for resetting all registers to zero in response to a clear control signal received on line 24. Processor 15 includes, for example, means for causing an instruction to be fetched and executed in response to an initiate control signal received on line 24. Processor 15 includes, for example, means for halting the fetching and execution of instructions in response to a halt control signal received on line 24. The input/output port of processor 15 includes logic for receiving, decoding and generating initiate, halt, clear and interrupt commands received on line 23.

Signal detection unit 16 functions to receive a video signal corresponding to the reflected signals received by antenna 12 over line 25. Signal detection unit 16 functions to generate target reports after processing received reflected signals which may include actual targets, the altitude line and main beam clutter. Specifically, signal detection unit 16 integrates a plurality of reflected signals arriving at the antenna 12 at a predetermined time interval corresponding to a range cell and path increment or antenna position where the main beam has been directed to provide an output signal. Signal detection unit 16 provides integration and thresholding for a plurality of time intervals for each of a plurality of path increments or antenna positions where the beam has been directed. The target reports are transferred to the radar processor 15 over line 22.

Antenna 12 functions to radiate most of the electromagnetic energy fed to the antenna into a narrow beam having a predetermined beam width, for example, six degrees.

Radar system 14 functions to transmit a sequence of signals which are fed to antenna 12 over lines 26 and 27 and to position or scan antenna 12 to direct the beam along a predetermined path represented by a plurality of path increments connected in series. Radar system 14 functions to receive reflected signals emanating and arriving back at antenna 12, to remove the carrier signal therefrom to provide a signal on line 25 indicative of the amplitude of the received reflected signals, such as a wide band video amplitude signal.

The antenna position information which is fed over line 19 to radar processor 15 indicates the position of the antenna relative to coordinates or position of the radar system. It is understood that the radar system 14 may be stationary or moving, such as in an airplane or ship. The coordinates of the radar system 14 are related to geographical coordinates of the earth through an inertial navigation unit 28 which provides suitable information over line 29 to radar processor 15. Radar processor 15 tags the detection reports with the antenna position or direction of the radar beam. The antenna coordinates or position of the radar beam is converted to space stabilization coordinates, such as geographical coordinates utilizing the information from inertia navigation unit 28 to facilitate correlating target reports as a function of a range cell and azimuth. With this arrangement, the motion of the air frame and the radar system 14 attached thereto is accounted for.

Radar processor 15 stores or places in memory target reports received on line 22. Radar processor 15 examines the occurrence of the target reports as a function of the plurality of path increments or antenna position and range cell location. Radar processor 15 rejects target reports indicative of a target which within at least one range cell extends over a plurality of path increments or antenna scan angle that is too large to be a valid target. Radar processor 15 selectes target reports indicative of a target having a valid target size, for example, from two to three target reports in an antenna scan path scanning at 120.degree./sec. with a 6.degree. wide beam without target reports for a predetermined scan path before or after the target reports.

FIG. 2 is a diagram of a suitably antenna scan pattern for the embodiment in FIG. 1. The elevation angles provided in FIG. 2 are referenced to the position of radar system 14. As shown in FIG. 2, the antenna 12 may scan as shown by line 30 from -3.degree. elevation to +45.degree. elevation, providing a total scan angle of 48.degree.. Line 31 shows a scan turn-around region which is above 45.degree. elevation where the antenna is moved from -2.degree. azimuth to +2.degree. azimuth. The antenna is then scanned from +45.degree. elevation to -3.degree. elevation, providing a scan angle of 48.degree., as shown by line 32. Curve 33 shows the antenna moving below -3.degree. elevation from +2.degree. to -2.degree. azimuth. The antenna is moved during scanning from line 30 to line 32 to provide better coverage of targets which may be at the edge of the beam during a particular scan. If the antenna 12 and radar system 14 are rotated about reference line 36 which is orthogonal to the elevation and horizontal azimuth directions, the scan pattern will traverse a volume of space to illuminate the target at shown by patterns in phantom lines. Rotation can be accomplished by an aircraft rolling.

In FIG. 2 the elevation direction is indicated by reference line 34, the azimuth direction by reference line 35. The axis of rotation for rotating the scan pattern is shown by reference line 36 and arrow 37. The scan pattern may be rotated counterclockwise or clockwise as shown by arrow 37 to illuminate a target. FIG. 4 shows an airplane and antenna 12 in relation to the axis of rotation 36. The angle of rotation from the vertical direction with reference to geographic coordinates is indicated by angle .phi. in FIG. 2. The scan pattern which has been rotated through an angle .phi. is shown in phantom by scan pattern 38.

FIG. 3 is a diagram showing the spatial relationship of the main beam scan pattern, such as line 30 or 32 of FIG. 2. The main beam, as radiated from antenna 12, scans through an elevation angle from -3.degree. to +45.degree.. As shown in FIG. 3, the beam width is approximately 6.degree. wide. In actual practice, the beam dwells on a target to get three target reports. Reflected signals arriving at 400 nanosecond time intervals are quantized by the signal detection unit 16 in FIG. 1. The 400 nanosecond time interval represents one range cell along a particular elevation angle .alpha.. In a conventional radar, the first 900 ft. from the antenna 12 may be ignored with the range cells running from 274 meters out to 15.36 kilometers, forming 256 contiguous range cells, each range cell being 60 meters long. Target reports from signal detection unit 16 indicate reflected signals were received at a time corresponding to a particular range cell and elevation angle.

FIG. 4 is a diagram of the altitude line clutter caused by side lobes from the antenna. In FIG. 4, airplane 40 has an antenna 12 emanating a main beam 41 of electromagnetic radiation having a beam width of approximately 6.degree., directed in space at an elevation angle .alpha.. Antenna 12, shown out of scale in FIG. 4 for the purposes of illustration, radiates a significant number of side lobes. For example, if antenna 12 is a parabolic dish antenna it would have side lobes at an angle of 60.degree. to 90.degree. off the center reference line 42 of the main beam 41. FIG. 2 shows diagrammatically the area on the ground that contributes to the peak of the altitude line return. Analysis shows it to be a disc which subtends angles of 29.degree. and 17.degree. at the antenna 12 for radar altitudes of 1.83 kilometers and at 5.49 kilometers, respectively. Thus, the altitude line represents an average of returns or reflected signals from a large number of side lobes near 90.degree. from the main beam 41. As the antenna sweeps in elevation, the side lobes that are averaged change slightly because of the large subtended angle included. Thus, if an altitude line return exceeds the detection threshold at the beginning of an elevation scan, such as lines 30 or 32 in FIG. 2, it is likely to exceed the threshold for the entire elevation scan. In FIG. 4, the radius of the area on the ground that contributes to the peak of the altitude line return is described by equation 1: ##EQU1## where c is the speed of light, .tau. is the pulse width and h is the altitude. The angle .theta. subtended at the antenna by the side lobes may be estimated utilizing equation 2. ##EQU2##

FIG. 5 is a typical B-scope display of the target in the presence of main beam clutter and altitude line clutter. In FIG. 5 the ordinate represents range in kilometers and the abscissa represents antenna elevation angle .alpha.. As the antenna is scanned in elevation from -3.degree. to +45.degree., altitude line clutter appears on the B-scope and continues to appear as the main beam is scanned upwards. The altitude line 45 is shown in FIG. 5. A target 46 is illuminated by the radar main beam at an elevation angle of 29.degree. and a range of approximately 9 kilometers. An airplane at a range of 9 kilometers may be approximated as a point target. An area of main beam clutter 47 is shown when the main beam is at an elevation angle from 8.degree. through 21.degree. and a range of between 101/2 and 13 kilometers. The main beam clutter may return an exceptionally strong signal, for example, if the main beam was reflected off a mountainside. The main beam clutter may appear on the B-scope at the range it impinges upon the ground if the reflected clutter signals exceed the threshold amplitude of the detector. Since close-in targets would return a strong reflected signal, a sensitivity control means is normally provided to adjust the gain from 0 to 8 kilometers.

FIG. 6 shows a video wave form of reflected signals on line 25 of FIG. 1. The ordinate represents volts and the abscissa represents time. At time t0 a pulse of electromagnetic energy is radiated by antenna 12. Curve 50 at time t0 represents a small amount of signal leak-through from the transmitter to the video signal line 25. After the transmitted pulse, curve 50 goes back to zero volts. No signals are received until time t1 when the altitude line 51 occurs due to signals reflecting from the ground below the radar antenna 12. As can be seen in FIG. 6, the altitude line extends from t1 to t2. At time t3, a point target, such as an airplane, provides reflected signals as represented by curve 52. As shown in FIG. 6, the duration of curve 52 is relatively short. At time t4, main beam clutter is received which extends through time t5. As shown in FIG. 6, curve 53 has a relatively long duration as compared to a point target, curve 52.

FIG. 7 shows one embodiment for generating target reports. The video signal on line 25 is coupled to an input of pulse width discriminator 55 having an output coupled over line 56 to an input of comparator 57. A threshold voltage which may vary over time is coupled over line 58 to a second input of comparator 57. The output of comparator 57 is coupled over line 72 to an input of adder 59. The output of adder 59 is coupled over line 60 to a data input of memory 61 and an input of digital comparator 62. The output of memory 61 is coupled over line 63 to an input of adder 59. An address signal is coupled over line 64 to an address input of memory 61. A clear signal is coupled over line 65 to memory 61 and to counter 66, which functions to clear or dump the memory and counter to zero. A digital threshold signal, such as the number 5, is coupled over line 67 to an input of digital comparator 62. The output of digital comparator 62 is coupled to an input of AND gate 68. An output of counter 66 is coupled over line 69 to a second input of AND gate 68. The output of AND gate 68 is coupled over line 73 to an input of interface buffer 70 having an output coupled to line 22. An output of counter 66 is coupled to line 23 which functions to provide an interrupt signal. A clock pulse for each interpulse period is coupled over line 71 to an input of counter 66, which functions to advance the counter at the beginning of each interpulse period.

The radar video signal on line 25 is processed through pulse width discriminator 55 which may, for example, include a bipolarizing delay line to permit pulse width discrimination against wide returns, such as clutter. The video signal from discrete targets, even those riding on clutter, will pass through the bipolarizing pulse width discrimination circuitry 55. The bipolar video is fed to a comparator that has an output of a "logic one" if the video exceeds a predetermined threshold which is based on allowable false alarm rates. These decisions or the output of comparator 57 are integrated on a range cell by range cell basis by memory 61 and adder 59 for sixteen interpulse periods. The results of the integration are fed to an input of digital comparator 62 for further processing and memory 61 is cleared (dumped) at the conclusion of the sixteen interpulse periods (IPP's). This integrate and dump detection process is repeated every sixteen milliseconds, for example.

The detection criteria utilized is based on achieving a probability of false alarm greater than 10.sup.-10 and less than 10.sup.-5. In addition, for a detection probability which is greater than 0.5 and less than 0.9 then M.sub.optimum is approximately equal to 1.5.times. square root of k for a fluctuating target, where k equals the number of detections possible and M.sub.optimum equals the number of detections necessary to have the probability of detection and the probability of false alarm to be within their desired ranges. M.sub.optimum is equal to 6 detections. Thus, digital comparator 62 functions to find which range cells integrated up to six or more hits out of the sixteen interpulse periods. The integrate and dump target detection scheme will detect both target and altitude line returns. Target returns will tend to be in one or two adjacent range cells since the range cells are matched to the transmitted pulse width. The altitude line returns will be in one or more contiguous range cells, depending on the orientation of the air frame to the terrain below it and on the actual altitude of the aircraft since the sensitivity time control of threshold signal 58 is applied to returns out to about 6.4 kilometers. By looking for leading edges of groupings of threshold detections in range, targets and altitude line returns can be identified with a single range cell number. The single range cell number may be utilized to minimize overloading the interface line 22 in FIG. 1.

The integrate and dump detector works for sixteen interpulse periods which at the completion after sixteen milliseconds, interface buffer 70 sends the first or nearest four detected leading edge range numbers over line 22 to an input of radar processor 15 for correlation with detections from other beam pointing positions in the elevation scan.

In operation, radar processor 15 commands antenna 12 to scan through the pattern shown in FIG. 2. Assuming that the antenna scans at 120.degree. per second and that the effective detection beam width is 6.degree., a point target in space, such as an airplane, should be detected in at most three reports from the single unit 16. A valid target then will generate one of the following patterns of reports where X equals detection and 0 equals no detection: (X, 0, 0), (0, X, 0), (0, 0, X), (X, X, 0), (X, 0, X), (0, X, X), or (X, X, X). Radar processor 15 looks for one of the preceding patterns except the patterns (X, 0, 0), (0, X, 0) and (0, 0, X) which are sacrificed to improve rejection of the main beam clutter. Flight test data has shown that these patterns often occur for main beam clutter returns, whereas the other patterns containing at least two detection reports occur almost exclusively for valid targets only. Radar processor 15 performs the following correlation task:

(1) Look for the first detection followed by three reports with no detections at the same range .+-.60 meters.

(2) Look for one or more detections in the previous two reports prior to the first detection occurring at the same range .+-.60 meters.

(3) If step (2) is successful, look for no detections at the same range .+-.60 meters for five consecutive target reports starting with three reports back from the starting point in step (1).

(4) If step (3) is successful, stop the antenna, position it to the last detection position and command the radar to commence track lock sequence. If steps (2) and (3) are not successful, continue the antenna scan pattern. In other words, looking back at time of the target reports, there should be three intervals with no detections, followed by a pattern, such as provided above, followed by five consecutive reports with no detections as shown in Table I.

If a detection at a given range is rejected by the angle width discriminator, all detections at that range .+-.120 meters are rejected for the remainder of the antenna scan. This improves performance against fading altitude line returns, clutter and noise. The angle width discriminator or sliding window algorithm is a target finding scheme based on beam width discrimination, while at the same time, rejecting altitude line returns without interrupting the elevation acquisition scan of the antenna 12.

The altitude line detector algorithm or previous scan rejection algorithm performed by radar processor 15 utilizes data attained from the previous scan of antenna 12 to reject undesirable detections occurring in the present scan bar. It is based on the beam width discrimination criteria as follows. Undesirable detections are defined as those at any range number which have occurred five or more times during any scan bar. A scan bar is shown in FIG. 2 by lines 30 or 32. Radar processor 15 causes antenna 12 to scan at the rate of 120.degree./sec. The sample reporting rate from signal detection unit 16 for each integration period is 1 per 25 milliseconds, which precludes a valid target from being reported for more than three angular samples. Therefore (allowing a margin of safety) any range numbers reported five or more times in a given scan represent undesirable targets. The lowest range number which is detected five or more times per scan bar is considered to be an altitude line return. All other range numbers which are detected five or more times per scan bar are considered to be undesired clutter returns. Every range number satisfying the "five or more" criteria is rejected during the next scan bar. The testing process occurs during the beam turnaround at the end of each scan bar. The results are available in time for use during the upcoming scan bar. New data is available for every scan bar after the first. The range rejection width is 300 meters. The lowest previous scan rejection range number (considered to be the altitude line) will be used to determine an optimum direction to sweep the radar system 14 range gate during the lock-on process for a valid target. The radar system 14 range gate will be swept from near range to far range except when the lowest previous scan rejection range value is less than the range value of the valid target. In this case, the radar system 14 range gate will be swept from far range to near range. Thus, the radar system 14 range gate will always sweep through the target first; before it can encounter the lowest previous scan rejection range, which is most likely the altitude line.

This invention is amenable to both hardware and computer software implementations. A computer program or software has been written for controlling radar processor 15 entitled, "Computer Controlled Automatic Acquisition". The software is written in the milli-computer assembly language, a description of which has been published by Westinghouse Electric Corporation. Targets are found by software by processing or comparing the range gate history buffer data with predetermined patterns. It is the object of this software to find real targets as opposed to altitude dependent returns, main beam clutter or noise. This is done by a twostep process: (1) sliding window search or angle width discrimination and (2) range blanking or altitude line detection.

Sliding window search or angle width discrimination is a method of comparing the eleven most recent samples, each sample being an integration and dump by signal detection unit 16, and determining whether the detection from three samples ago (if any) is a valid target. This method is capable of finding a target within 100 milliseconds of the time it was detected. Because each sample represents a different angle (25 milliseconds is about 3.0.degree. at the 120.degree. per sec. scan rate), this method is equivalent to angular discrimination. Angular discrimination is a definition of a target by the fact it can only span a small angle. The sliding window describes a target as a small group of hits (less than 7.degree.) bracketed by misses before and after. The sliding window search is a five step procedure:

Step 1. Check the lowest range position at t3 (three samples back). If there is no valid range at that position, then continue the scan and wait for the next sample; otherwise, go to Step 2.

Step 2. Compare the range at t3 to all of the ranges from t0 (current), t1 and t2. If there are any values within 60 meters of t3, then return to Step 1 until each of the four possible range detections is checked; otherwise, go to Step 3.

Step 3. Compre the range at t3 to all of the ranges from t7, t8, t9 and t10. If there are any values within 60 meters of t3, then save the range value at t3 in the sliding window reject blanking range (to be described later) and return to Step 1 until each of the four possible range detections is checked; otherwise, go to Step 4.

Step 4. Compare the range at t3 to all the ranges at t6. If there are any values within 60 meters of t3, then set flag (used in Step 5) and go to Step 5.

Step 5. Compare the range at t3 to all of the ranges at t4 and t5. If there are any values within 60 meters of t3, then this range has passed all the sliding window criteria and may be a target. If there are no values near t3, then go back to Step 1 until each of the four possible range detections is checked.

After the target has pssed through the sliding window search successfully, it is compared to a table of seven blanking ranges. It if is not within 240 meters of any of these ranges, then it is a valid target and a target acquisition mode is initiated. These seven blanking ranges are:

1. SWR--Sliding Window Reject--from the sliding window target finding technique described above, any range that passes Steps (1) and (2), but fails Step (3). This range is blanked out for eight turnarounds (half scans). A maximum of three can exist at the same time.

2. PSR--Previous Scan Reject--any range (.+-.120 meters) that appears five or more times during one half scan. This range is blanked out for twenty turnarounds (half scans).

3. PSR2--Secondary PSR--same as PSR but greater range. Blank out this range for five turnarounds (half scans).

4. PRRA--Automatic Previous Rejection Reject--after an attempted acquisition, if after holding full action on for its full duration (one second) and range lock is not on. Use the attempted acquisition range for blanking for the next three turnarounds (half scans).

5. PRRM--Manual Previous Rejection Reject--after a range skipover is initiated by pressing the nose wheel steering button while the radar is locked onto an acquired target. Use the range of the original target acquisition for blanking the next two turnarounds (half scans).

When a target is found, the software enters the target acquisition mode:

1. The predicted range voltage which corresponds to the detected target less 300 meters is sent to the radar via a D/A channel.

2. The position of the target in space is continually computed and sent to the antenna servo. This is done by using the antenna resolver input and inertial navigation unit 28 attitude inputs from when the target was detected and the current values of the inertial navigation unit 28 inputs to compute the current position of the target and antenna coordinates.

3. One hundred milliseconds after the target is detected, full action is set high and the predicted range analog output is set to 300 meters beyond the target. This results in a range sweep through the target.

4. The full action discrete output is held high for a total of 900 milliseconds. If the radar does not lock onto the target during this time, this range is inserted into the range blanking table as described above.

If the radar is locked on a target and the latched automatic acquisition input discrete is on and the nose wheel steering button is pressed, then range skipover is intiated:

1. While range lock is still high, the range input value is saved.

2. The radar range lock is broken by outputting the target reject output discrete for 100 milliseconds.

3. The antenna is held at its current position.

4. The current range values from the target detector are compared to the saved range value. If there is a detection which is at least 300 meters greater than this target and less than 5 miles, then the target acquisition sequence is initiated on target.

5. If there are no valid targets greater than this range, then resume the scan but do not start saving any detector data for about 115 milliseconds; this allows time for the antenna to get up to speed.

FIG. 8 shows a top level flow chart for the computer controlled automatic acquisition software. At the top of FIG. 8 is entry point 77, from which point the radar processor 15 goes from the operational executive to the computer controlled automatic acquisition program. Radar processor 15 passes through entry point 77 periodically, for example, from 10 to 100 cycles per second. From entry point 77, radar processor 15 is programmed to compute the current antenna angles in degrees from the resolver signals which are expressed in sine and cosine voltage. The antenna angles are referenced to the airframe coordinate system. From box 78 radar processor 15 is directed to one of four paths, namely, Nos. 0 through 3, depending upon the number stored indicating the mode to be processed. If no number is stored, then No. 0 will be selected. Mode No. 0 on line 80 begins the start-up function by sending the antenna to the starting point of a scan. When it is near a starting point for the scan, the mode stored in mode processing box 79 is switched to No. 1. Radar processor 15 follows along line 81 to box 82, where it returns to the operational executive.

When radar processor 15 re-enters entry point 77, it computes the current antenna angles in degrees in box 78 and goes to box 79, where Mode No. 1 is stored and directs the programming path to follow along line 83, which is the search mode. Radar processor cycles through Mode No. 1 every 25 msec. At box 84, it asks the question, "Is the elevation of antenna 12 in the turnaround region?". If the answer is "yes", then path 85 is followed to box 86, wherein radar processor 15 searches the last scan for the altitude line and puts this value in the range blanking table. Radar processor 15 then follows path 87 to box 82 and returns to the operational executive.

The next time radar processor 15 enters entry point 77, it computes the antenna angles in box 78, since the mode is still in No. 1, it follows path 83 to box 84. If the answer to box 84 is "no", the program follows path 88 to box 89. At box 89, radar processor 15 puts the new target detector and angle data into history buffers. The target data comes from signal detection unit 16. Path 90 is followed to box 91, which asks the question, "Is the antenna elevation in the valid target region?". If no, then path 92 is followed, which returns radar processor 15 to the operational executive box 82. If the answer to box 91 is "yes", then radar processor 15 uses the sliding window angular determination and range blanking to find a valid target at box 94. Radar processor 15 then follows path 95 to box 96. If a target has been found from the processing in box 94, then switch to target acquisition mode No. 2 at box 79. Radar processor 15 follows path 97 to operational executive box 98, which returns processor 15 to the operational executive.

Radar processor 15 returns to entry point 77, computes the current antenna angles in box 78 and goes to mode No. 2 at 79. Mode No. 2 is the target acquisition mode and follows path 99 to box No. 100. At box No. 100, radar processor 15 positions the antenna over the target by sending proper angle information to radar system 14. Radar processor 15 follows path 101 to box 102, where radar processor 15 sends range information to radar system 14, which attempts to lock-on target by sweeping range and applying full action. Radar processor 15 follows path 103 to box 104, which returns radar processor 15 to the operational executive.

In the event that the target locked onto is undesired, such as a friendly aircraft, but with an enemy aircraft some distance beyond the friendly aircraft, mode No. 3 is used, which is set by a discrete signal sent by the radar observer. The next time radar processor 15 passes through boxes 77, 78 and 79 mode 3 is selected which is termed the skipover mode. Radar processor 15 follows path 105 to box 106 to break the radar lock on the current target. Radar processor 15 at box 107 asks the question, "Is there a targer further out?". If "no", path 108 is followed to set the mode to No. 1 at box 79, then follows path 110 to operational executive box 98 and returns to the operational executive. If box No. 107 is "yes", then path 111 is followed to box 112, which sets the mode in box 79 to No. 2 for target acquisition. Radar processor 15 then follows path 113 to operational executive 98. The operational executive noted by boxes 82, 104 and 98 are returning radar processor 15 to the operational executive so that radar processor 15 might perform other tasks under software control unrelated to computer controlled automatic acquisition.

A detailed description of the equations and algorithms to be solved by the computer controlled automatic acquisition (CCAA) software to implement the flow chart in FIG. 8 is as follows:

FIG. 9 shows a detailed flow chart for the computer controlled automatic acquisition software for implementing box 94 of FIG. 8. In FIG. 9 the box designated with the letters "D3SETSCN" directs radar processor 15 back to the operational executive. The box designated with the letters "D3SWRST" directs radar processor 15 to perform range blanking.

The invention provides a method for operating a pulse radar for acquiring a target comprising the steps of transmitting a sequence of signals from an antenna, said antenna radiating most of the electromagnetic energy in a narrow beam having a predetermined beam width; scanning the antenna to direct the beam along a predetermined path represented by a plurality of path increments connected in series; receiving reflected signals emanating from the antenna; integrating a plurality of the reflected signals arriving at the antenna at a predetermined time interval corresponding to a range cell and path increment for the directed beam to provide an output signal; thresholding the output signal to provide a target report at times when the amplitude of the output signal is greater than a predetermined value; repeating the steps of receiving and thresholding for a plurality of time intervals for each of a plurality of path increments for the directed beam; storing the target reports; comparing the occurrence of the target reports as a function of the plurality of path increments and range cell locations; and selecting target reports indicative of a target having a valid target size.

Reference is made to a computer program listing attached hereto as Appendix 1, pages 1 through 31, to provide a detailed understanding of the software that may be utilized to practice one embodiment of the present invention.

Although not considered essential, attention is directed to a description of the listing language of Appendix 1 which is entitled "Computer Programming Manual For Operational Flight Program" and published in April, 1978 by Westinghouse Electric Corporation in Volumes I and II, and identified as Contract Data Requirement list A00M, on Air Force Contract No. F42600-76-C-0236 and available from the Defense Documentation Center, Cameron Station, Alexandria, Va. 22314. ##SPC1## ##SPC2## ##SPC3##