Low Elevation Angle Radar Tracking System

This invention is in the field of low elevationangle radar tracking, and specifically addressed to a system that enables such tracking to be accurately obtained.

One of the major requirements for radar systems including ship-based, land-based and airborne radar system, is to have a tracking system, which may be automatically controlled, for low angle targets such as aircraft.

Because of the earth's curvature, target aircraft are generally of the low-flying short to medium range aircraft, and medium to high flying aircraft at greater ranges.

To understand the problems encountered in the prior art, a brief description of angular elevation tracking heretofore utilized is necessary.

IN A PENCIL BEAM ELECTRONIC SCANNED RADAR SYSTEM, THE BASIC APPROACH USED FOR TRACKING TARGETS IN EITHER AZIMUTH OR ELEVATION ANGLES IS TO OBTAIN TWO MEASURES OF THE TARGET'S POSITION, "BEAM-SPLIT" THE RESULTANT MEASURES FOR ACCURACY, AND THEN FEED THE BEAMSPLIT INFORMATION TO A TRACK SMOOTHING COMPUTER ROUTINE WHICH TAKES OUT ANY SHORT TERM TRANSIENT MEASUREMENT ERRORS AND PRODUCES A SMOOTH PLOT OF THE TARGET'S ACTUAL POSITION. The measures are obtained by either a sequential lobing technique, that is firing two beams, one on each side of the expected predicted target position, or by a monopulse technique wherein two apparent receive beams equally spaced about the transmit beam position are formed. In the former case, the difference in the response of the two transmit beams is then used to provide a measure of how far from the predicted position the actual target is located. For example, the amplitude response of one beam will be greater than that of the other beam since the actual target may be closer to the position of the peak of said one beam, that is, close to the nose of such beam.

However, some of the problems encountered in any of the systems heretofore used were in prior attempts to permit low angle tracking, substantially unsuccessful. Such prior attempts ignored the problem of tracking long range targets and relied on an operator to track reacquisitions of short range targets. Though it may be possible to track short range targets in view of such targets being visible on a search PPI display, it is obviously both ineffective as well as inefficient when more than a few targets are simultaneously present.

Further, the following specific problems prevail in prior art tracking:

Low angle tracks beyond the clutter processing range, that is targets at long range, would get pulled off the target and follow clutter. This would often lead to track splitting situations and hence an observer would lose control of a tracking situation because track symbols would literally be running wild across the display face.

Tracks of low angle medium to long range targets would become lost. They would be pulled off the target track due to multipath effects which produced inaccurate amplitude responses from the beams. This inaccuracy is especially deleterious in a sequential lobing situation, such as is used in a typical radar system, since the two beams are fired at different times and hence have different multipath amplitude distortions.

Low angle short range tracks would be lost due to distortion of the transmit-receive beams. This is caused by part of the beam pattern to be directly in line with the ground or sea and create distortion due to the lower elevation portion of the beam interferring with the ground or sea surface. A distorted beam will not provide true amplitude response and hence the resultant beam splitting would indicate a wrong position for the target.

In order to allow the radar system to operate so as not to be troubled by the above problems, a "fix" was incorporated, which merely dropped all low angle (less than 0.3.degree. ) tracks. This, of course, was not a remedy for insofar as acquisition or tracking of low angle targets, but was rather an avoidance of the problems inherent to the system at that time.

Hence the system heretofore employed was one that did not permit successful tracking of low elevation targets.

Several papers given on this subject also attempted unsuccessfully to resolve the above-stated problem as to low angular targets, at the Proceedings of the International Conference at London, England on Oct. 23-25, 1973.

Paper A74-12376, entitled Accurate Tracking of Low Elevation Targets Over the Sea with Monopulse Radar was published in the publication of English Institution of Electrical Engineers, 1973, pages 160-165. This publication relates to a monopulse radar tracking of targets whose elevation is less than the radar beamwidth above a sea surface. Such tracking becomes unreliable since the radar system tries to null on a composite signal of the target and its image. It is shown that there is a continuous locus equilibrium positions which the radar may follow, as well as isolated regions of equilibrium about the image which can cause loss of tracking. Various schemes are discussed for maintaining track and obtaining elevation data on a low-flying target. The solution however requires three independent beams and hence a complex radar system.

Paper A74-12374, entitled The Low-Angle Tracking Problem, was published in the English publication of the Institution of Electrical Engineers in 1973, pages 146-153. This publication relates to the amplitude and angularspread characteristics of radar reflections from earth surface irregularities, examined on the basis of data obtained from communications experiments. The basic phenomena of surface relfection are shown to produce irregular and unpredictable components as well as the specular (image) reflections. The effects of such on radar tracking are evaluated by considering the combination of the target and reflection signals as a glinting target extended in azimuth and elevation but occupying the same range resolution cell as the desired target.

Paper A74-12377, entitled Multi-Frequency Complex Angle Tracking of Low Level Targets, was published in the English publication of The Institute of Electrical Engineers in 1973, pages 166-171. This publication states that radar tracking of a low-flying target becomes unsatisfactory when the target is at an elevation of less than about two thirds of the radar beamwidth above a fairly smooth surface. This is due to `glint` between the target and its image, which merge into an unresolved double target. The relative sum and difference signals can be processed to yield a complex indicated angle, but this complex angle cannot be used to determine the real angle uniquely without more information. The present paper describes a method for determination of the real angle by using complex angle measurements at more than one frequency. Digital computer simulation studies of the method indicate that it provides considerable improvement in low-level tracking over a smooth surface, but marginal improvement over rough surfaces. The limiting factor is frequently diffuse reflection from a rough surface, and it appears that this will also limit other methods of implementing complex-angle tracking.

It may be seen from the foregoing that the prior art radars and studies thereof have not provided accurate low elevation and angle target tracking.

In an attempt to overcome the problem of tracking multiple fast moving and low flying targets, a computer program was utilized in conjunction with the radar system. Though it was possible to track at lower elevation angles than heretofore with use of computer software, when used with the low angle mode for single beam tracking, a high false alarm rate of prevailing targets resulted.

Therefore, it is an objective of this invention to provide accurate tracking capability in a radar system for acquisition of low elevation and azimuth angle flying targets as well as acquisition of high angle targets.

Another objective is to provide such radar system with automatic tracking control to make possible handling of numerous targets at the same time.

A further objective is to enable such radar system to operate monopulse as well as to provide multiple sequential signal pulse outputs.

A still further objective is to enable the radar system to track short as well as long range targets.

Yet another objective is to prevent long range target clutter occurring by virtue of track splitting situations, and to prevent loss of low angle data within such clutter.

Still a further objective is to avoid multipath amplitude distortions particularly due to sequential lobing of two radar beams fired at different times and hence to avoid inaccuracies in tracking resulting therefrom.

Yet a further objective is to avoid loss of low angle short range tracks due to the aforesaid distortion of the transmit/receive beams by virtue of the beam pattern being directly in line with ground or sea surface, and thereby avoid beam splitting that results in wrong target position indication.

Briefly, according to this invention, since detection of received radar signals is performed in the radar system, two detection levels or modes of operation are utilized.

A first mode or detection level is utilized for normal tracking. A second mode or detection level is utilized for low elevation tracking of targets. In the second mode, where adjacent sequential elevation beams are used, detection of signals are required only on either of the two beams. In order to prevent false alarms in either of the tracking modes, the second mode of operation or detection level is set about 3db higher in amplitude than the first mode or detection level by means of suitable circuitry that may be built into the receiver detector or provided externally thereto.

FIG. 1 is a schematic diagram of a radar system in accordance with this invention.

FIG. 2 is a representation of sequentially switched antenna lobes in elevation by the radar system.

FIG. 3 is an algorithmic representation of several routines denoted as FIGS. 3a, 3b and 3c and converted into machine language to program a computer for rapid control and operation of the radar system. For convenience of discussion FIGS. 3a, 3b and 3c will be referred to hereinbelow as FIG. 3.

FIG. 4 is a detailed block schematic of the radar buffer shown in FIG. 1.

Referring to FIG. 1, a radar transmitter-receiver system is shown, inclusive of the inventive subject matter, used for elevation beam steering.

Stabilization data source 10 comprises the pitch and roll control unit for input to the radar system which feeds data into radar buffer 30 in conventional manner. Such stabilization data is provided as an input to digital computer 20 through buffer 30 in form acceptable by computer 20.

Computer 20 provides ten bit binary words to radar buffer 30 input corresponding to the required degrees of elevation of the radar antenna in accordance with table (1) hereinbelow generally illustrating several degrees of antenna elevation and their binary word equivalent, as exemplary.

Such stabilization data provided by source 10 is dumped into 10 bit shift register 302 for temporarily storing roll angle information and into 10 bit shift register 303 for temporarily storing pitch angle information. Input data to registers 302 and 303 is supplied to compensate angular deviation of pitch and roll with respect to reference plane coordinates, which coordinates are provided in computer program 100, and time multiplexer switch 304 will enable roll and pitch angle information to be supplied in sequence to computer 20. Registers 302 and 303 and multiplexer 302 are shown in FIG. 4.

Computer 20, by receiving the stabilization data, is enabled to compute the exact elevation of the horizon.

Radar buffer 30 in the return path and resulting from an output of the detector, to be hereinbelow described in greater detail, provides detection data in binary form as an input to computer 20 for processing thereby.

Radar buffer 30 converts the ten bit binary elevation words into ten bit frequency words provided as input to exciter 40. Exciter 40 is designed to provide 1024 discrete frequencies, each frequency representing a specific elevation position of the radar antenna corresponding with the elevation position range shown in table (1). Accordingly, there are 1024 beam positions covering an 80.degree. elevation range in 0.078 degree steps.

The computer adds a half beamwidth or a value of 6 of the 0.078 degree steps to the computed horizon value, and this is the lowest elevation angle sent to buffer 30.

At the appropriate time, the elevation word in terms of frequency is sent to exciter 40 for selection of the required discrete frequency. The selected frequency is fed at low RF power to a suitable amplifier within transmitter 50, and output of transmitter 50 at high power level is fed to radar antenna 60.

Antenna 60 has a frequency scan characteristic as to elevation, so that the selected frequency by exciter 40 determines the elevation angle, as shown in exemplary fashion in table (1), at which the antenna beam, as at 70, is transmitted and/or received. Antenna beam 70 is shown in terms of the earth's horizon at 0.degree. , the angle above the horizon indicated as +.phi..degree., and the angle below the horizon during pitch of the ship bearing the radar system as being -.phi..degree..

It should be noted that when the radar system is installed on a ship the horizon will be constantly changing with respect to the ship's pitch and consequently the zero reference portion of angle .phi. as at 70 will be adjusted with change in pitch.

Referring to both FIGS. 1 and 2, and with specific reference to FIG. 2, a pair of antenna lobes A and B as conventionally transmitted by a monopulse radar system at a predetermined sequential period are shown in elevation, and are representative of antenna beam 70.

Though several methods are used for transmitting and receiving radar energy, in the final analysis it is necessary to sequentially switch radar antenna 60 to provide lobe pair A and B in a predetermined sequence.

Lobes A and B will therefore not be present at the same instant of time, but generally lobes A and B will be usable to detect higher flying targets at which time switch 94 will be positioned by means of flip-flop 305, shown in FIG. 4, providing output signal at 97 which activates and switches switch 94, at contact 95 in order to obtain normal offset signal and hence a lower threshold value as seen from table (4)hereinbelow. Either Beam A or B may be used to detect low flying targets in which case switch 94 will be positioned at 96 in order to obtain an offset signal higher by 3db than the offset signal when switch 94 is set at 95 position, and consequently obtain a threshold level higher by 3db than the threshold level when switch 94 is set at position 95, also as seen from table (4) hereinbelow.

It is therefore obvious that selection of high or low threshold levels will be controlled by buffer 30 and may be predetermined by appropriate time sequence controlled by computer 20.

One method used in conventional monopulse radar systems is to receive the sequentially switched antenna lobes A and B in elevation. Another method used is to transmit the sum of the lobes as a single beam and receive either the resultant of the sum and difference of the received beam pair. However, in the latter instance two antenna lobes result anyway, so that insofar as developments of the novel approach herein, the circuitry within the detector to be hereinafter described would generally be applicable and representative of either method.

A signal reflected from a target as sensed by antenna 60 is fed by such antenna to receiver 80.

The output of receiver 80 is generally fed to several components of radar detector 90, as inputs to mean level detector 91 and comparator 92. The output of mean level detector 91 provides a voltage level input to summing circuit 93.

Data from comparator 92 is processed by computer 20, however such data is inputted to flip-flop 306 shown in FIG. 4 to pass or reject the data in accordance with the logic as stated in the truth table at (5) relative flip-flops 305 and 306. Data from flip-flop 306 is passed to computer 20 for processing same, and data from flip-flop 305 provided by computer 20 is passed at 97 to switch 94 to change its position as required.

Threshold level switch 94 is illustrated as a two position mechanical switch (but may be an electronic type switch), which is positioned either at position 95, which is the normal operational threshold level position, or at position 96 which is the position for low altitude operation of the radar system.

An offset signal level K.sub.1, which may be a series of binary pulses such as those comprising a five bit binary word, is supplied from within detector 90 to terminal 95. Likewise, an offset signal level K.sub.2, also a binary word, is supplied to terminal 96 of switch 94. Switch 94 is controlled as to its position at either 95 or 96, by means of switch command at 97 in order to select threshold T.sub.1 or T.sub.2.

Accordingly, either threshold level T.sub.1 or T.sub.2 will be provided at 98 as input to radar buffer 30. Hence, either T.sub.1 or T.sub.2 threshold will be provided to buffer 30 by summing the signal obtained at 99 with the detected means level voltage provided to summing circuit 93, the output of summing circuit 93 being fed to comparator 92 to provide either T.sub.1 or T.sub.2 threshold. Threshold levels T.sub.1 and T.sub.2 are defined by the following equational relationships:

For a better understanding of the threshold level requirements as inputs to buffer 30, the following table shows signals required in terms of beams A and B for the two modes of operation of the radar system.

In the normal operational mode, detection of both beams A and B at threshold level T.sub.1 is required to provide date output from buffer 30.

In the low altitude operational mode, detection of either beams A or B at threshold level T.sub.2 is sufficient to provide output from buffer 30.

No detailed circuits are required for any of the components discussed, such as components 10, 20, 30, 40, 50, 60 and 80 as these are known in the art. As to radar detector 90, the components thereof such as 91, 92, 93 and 94 are each individuallyknown in the art as circuits, however the method of switching between different threshold levels to provide one pair of beams to be of greater mangitude than the other is not known in the art.

Referring to FIGS. 1, 2 and 3, and particularly to FIG. 3, algorithms from which a computer program designated at 100, are shown; algorithms 100 are directed to routines to check if single beam tracking is valid, for track smoothing for elevation, and for beam firing routine.

The actual computer 20 used was UNIVAC MODEL 1230.

The computer language used is SIMON SYCOL which is a UNIVAC COMPILER language. The actual computer program implementing algorithms 100 is submitted with this specification.

The basic concept of the single beam low elevation tracking technique is to allow a track's range, azimuth and amplitude data to be updated from only one of the two sequentially-lobed beam pairs A or B. In addition, the technique limits the lowest point at which the beam may be fired in elevation. The decision to use a single beam's return, or both beam returns, is based on logic criteria which is described hereinbelow.

The logic criteria are functions of target range, maximum clutter processing range of the system, target elevation, and return signal amplitude. As can be seen from the logic criteria and the algorithmic flow diagrams of the FIG. 3, other less critical steps are also taken. For example as the target goes lower in elevation, the two sequential beams are brought closer together. Although this does make the beam splitting curve less sensitive, it has the advantage of keeping the lower elevation beam from being distorted and hence allows some measure of elevation angle determination through beam splitting.

In the following rules which delineate the single beam low elevation tracking technique, the particular numerical values shown are typically applicable for radar system.

If the target elevation angle, .phi. < 0.6.degree. which corresponds to about 0.6 beam widths, this angle permits single beam tracking.

In order to prevent the lower beam from ever being <0.44.degree. , the beam pair center is never <0.56.degree. .

If the target is at short range (less than 30 nmi) perform single beam tracking only if the target amplitude is sufficiently high to be representative of a real target. This is accomplished by determining if at least one of the last two updates had an amplitude > 20 db. This criterion prevents false alarm problems, that is tracks running away on noise or on noise spikes.

If the target range is greater than the effective clutter processng limit, for example long range tracking, and .phi. < 0.6.degree. and if it is certain that clutter processing is required to detect the target, then the target track is dropped. Such targets are prevalent at long range and are flying low over land and are therefore very low priority tracks for the system. If clutter processing is not required for these long range targets then single beam tracking is permitted.

Fix the amount of beam movement from update to update to small change, for example 0.078.degree. ,when the target elevation angle is < 0.7.degree. ,such as when multipath can have an effect. Eventually, by following the direction, although not the magnitude, of the beam split indications, tracking of the target will be successful. The update rate must be adequate such that target movement does not escape the beam movement avoiding the production of misses.

When .phi. <0.7.degree. decrease the spacing between the two elevation beams, using 1/2 the previous spacing.

When single beam tracking is permitted, a check of the returns of each beam is made. If either one has a return which correlates with the track, that single return is used to update the track. The track's elevation angle is kept constant. If both beams have correlating returns, then beam splitting and track update proceeds as normal, except, as discussed above, in that the amount of elevation movement permitted is restricted. The essence of this point is that single beam tracking allows, but does not require, a detection from a single beam to be used to keep a target in track.

FIG. 4, shows the three inputs and four outputs therefrom corresponding to FIG. 1. As can be seen, the components are standard storage or shift registers, flip-flops and a time multiplexer, all known in the art, to provide the means for interfacing with components of FIG. 1 designated as 10, 20, 40, 92, and 94. It should be noted that the computer program includes the handling of the various inputs and outputs shown in FIG. 4.

The threshold control signal is provided as shown in FIG. 1 by switching between positions 95 and 96 of switch 94, which threshold levels T.sub.1 or T.sub.2 are fed at 97 to switch 94 through flip-flop 305 by input from computer of a particular logic level at 20 which logic level is temporarily stored in flip-flop 305 (FIG. 4) until command by computer is issued to switch to another of the binary logic states to select the other threshold as needed. It is obvious that flip-flop 305 is provided to enable temporary storage of a logic state until required to change position of switch 94.

Further, table (4) shows operational modes involve threshold levels, and that the threshold control necessitates the change of position of switch 94 and the threshold control level with each change in antenna elevation.

Input data coming from pitch and roll stabilization unit 10 is shown in FIG. 1 as being inputted into buffer 30. The specification already states that the stabilization data is in form acceptable by computer 20. The binary form of data, since we are using a conventional digital computer is well known in the art. It is therefore inherent in the needs of this system that some storage of the binary data, coming from unit 10, as provided by shift registers 302 and 303 will have to be included, and that a time switching device such as time multiplexer switch 304 to switch the roll angle data words and the pitch angle data words, would have to be provided in the buffer so as to feed such words into the computer for storage and use. It should be noted that computer 20 is able to correct for pitch or roll by correcting the antenna angle by taking the data temporarily stored in registers 302 and 303 in sequence, accomplished by multiplexer 304. As to the 10 bit frequency word provided as input from the computer to exciter 40, the only requirement would be elevation word information in binary form sent by computer 20 through exciter 40 and ultimately to antenna 60.

With respect to flip-flop 306 of FIG. 4, this flip-flop only provides a dealy or storage of binary pulse, in terms of the binary state temporarily stored therein for providing detection data in binary form to computer 20. It is inherent by the showing in FIG. 1 of an input of detector threshold levels T.sub.1 or T.sub.2, discussed above, and an output of detected data in binary form through the buffer, implying temporary storage of the binary state before sending it on to computer; any binary storage device would do for this purpose but a single one such as flip-flop 306 is shown as satisfying this situation and implied storage function. Components herein are shown in the Radar Handbook by Skolnik, 1970, McGraw Hill, N.Y. Transmitter 50 is shown at pages 7-44 (Section 7.5) to 7-48. Antenna 60 is shown at page 13-3 (FIG. 3). FIG. 3 also shows buffer 30 in block form combined with computer 20, and transmitter 50 and a frequency generator corresponding to exciter 40. In general, pages 13-1 through 13-27 show standard ways of building the frequency scanned radar system.

With respect to buffer 30, it is a standard circuit used for isolating several components of the system from each other as shown in FIG. 4. It happens to be a convenient location for utilizing therein a means for switching switch 94 to one of two positions. In the illustrated example an electro-mechanical switch 94 is used having contacts 95,96 and 99 which would be relay contacts, a relay coil thereof would be triggered, symbolically shown at 97; alternatively a solid state switch could be used. The important consideration is that the programmed computer activates the switch at the appropriate time to achieve the desired condition, and hence the computer, based on input information received as to angle .phi. level, will command switch 94 to operate as required. Both the flow diagrams FIGS. 3a-3c and the actual computer program used, show the part the computer plays in this regard. As to component 10, this is shown in Radar System Engineering by Ridenour, Radiation Laboratory Series, Vol. 1, at page 308, McGraw Hill, N.Y. 1947, entitled Shipboard Antenna Stabilization. Unit 10 calculates the true horizon at two orthogonal points (box and midship) by use of a gyroscope described on page 308. This data is sent through unit 30 to unit 20 so that the next radar beam is fired not in relative but in true direction space. It should be noted that the computer tracks in true range, elevation and azimuth coordinates. The pitch and roll units measure the deviation in degrees from horizontal. The computer orders a beam fired at a selected elevation and azimuth position. The buffer corrects these angles based on the pitch and roll angles detected, as aforesaid. The buffer 30 is simply a temporary storage unit that interfaces with the real time radar and the programmable computer unit 20. Exciter 40 is simply a group of oscillators providing the 1024 different frequencies.

It should be noted that the Skolnik textbook at page 22-13 (Section 22.3)discusses the problem of reflections at low elevation angles. It states that very poor performance occurs when the beam is pointed one beamwidth above the surface. However demonstration by actual flight testing, good performance at greater than 0.44 beamwidths above the surface, was exhibited with the instant system.