GB2039187A - Monopulse radar apparatus - Google Patents

Abstract

A hybrid (amplitude-phase) monopulse direction finder uses overlapping divergent beams with spaced origins. Sum and difference signals ( SIGMA , DELTA ) are formed from the two received signals and the phase difference ( psi DELTA - psi SIGMA ) between the sum and difference signals, as well as the amplitudes (¦E DELTA ¦, ¦E SIGMA ¦) are determined (in 20). The manner in which these determined quantities vary with deviation of the signal direction from the boresight is previously known, Fig. 3, as a characteristic of the antenna and the actual direction is determined from said characteristic. A signal may be tracked around a direction off the boresight by adding phase offset ( psi 0) to the phase difference determination and bias voltage (V0) to the boresight error voltage (V1). <IMAGE>

Description

SPECIFICATION Monopulse radar apparatus This invention relates to radar apparatus and, in particular to monopulse radar apparatus.

In monopulse radar receiver direction finding systems, target angular coordinates can be derived by combining the output signals of four horns of a non-rotating monopulse antenna into signal combinations such as sum and difference signals and heterodyning these input signals to an intermediate frequency. Three or four input signals may be utilized with three or four separate channels and IF amplifiers to amplify and pass the input signals separately through the receiver. The IF input signals are detected and processed to obtain a measure of the relative amplitude of the signals received by the antenna and therefore, a measure of angular displacement of the antenna axis from the line of sight to the source of signals, that is, the target. Thus, there is generated the angle off-boresight of the antenna to the emitting target.

Known monopulse radar tracking systems assume either an ideal phase comparison or amplitude comparison monopulse. The conventional phase-comparison monopulse radar uses beams aimed parallel to each other and originating from antennas which are laterally displaced by several wavelengths. The conventional amplitude-comparison monopulse uses beams which are aimed in different directions and originating from antennas which have no appreciable laterial displacement, such as, feed horns for a parabolic reflector whose source of radiation appears to originate at the focal point of the reflector. In accordance with known mathematicai processing, the ideal phase-comparison monopulse radar error voltage falls exactly on the imaginary axis, whereas, the amplitude-comparison monopulse radar error voltage falls exactly on the real axis.

Use of a conventional phase or amplitude-comparison monopulse radar system in a missile has often required positioning the radar system in the nose cone area. However, the nose cone of a missile is a particularly desirable location for any number of devices, such as optical sensing devices, and alternate locations for the radar system would be desirable. But if the forward nose cone area is to be avoided, it is difficult to find a location suitable for a pure amplitudecomparison monopulse radar system wherein two antennas have diverging beam axes emanating from a single point. Indeed, even when two adjacent feed horns, each pointed in a slightly different direction, are used at the center of a parabolic reflector there is still some lateral displacement between the two beam originating points.A pure phase-comparison monopulse with parallel beams is difficult to locate because the missible body affects the radiated field and distorts the direction of the beams.

A monopulse radar system is known to process received signals by forming sum and difference signals and comparing only the magnitudes of the signals. In an amplitudecomparison radar, division of the difference signal by the sum signal yields a real ratio, since the difference signal and the sum signal are in phase or 180 out of phase. In a phase comparison radar, division of the difference signal by the sum signal yields an imaginary ratio, since the difference signal and the sum signal are either plus or minus 90 . The prior art neither teaches nor suggests compensating for variations from either a pure phase-comparison or a pure amplitude comparison monopulse radar system.Although processing sequences using relative amplitudes defined by real or imaginary numbers are known, there has been no teaching or suggestion that complex error voltages may be helpful in analyzing the signals from an antenna of a particular geometry. Attempts have been made to use complex error voltages to discriminate against multipath returns. For example, in "The Use of Complex Indicated Angles in Monopulse Radar to Locate Unresolved Targets", Proc, NEC 22, 1966, pp 243-48 S.M.

Sherman discusses the processing of comples error voltages generated by the presence of returns from two different directions. The radar used by Sherman for the multipath experiment was a conventional phase-comparison or amplitude-comparison monopulse. The Sherman article does not discuss using phase-comparison and amplitude-comparison monopulse together so that a single target is used to generate two signals which are analyzed in such a way as to establish and, if desired, track the position of the target with respect to the antenna.

The present invention provides a monopulse radar apparatus for determining the angular position of a target including: an antenna array having a pair of radiator elements ror receiving signals from the target, said radiator elements being spaced from one another so as to have beam patterns originating at spaced points and having beam patterns with diverging main axes; a hybrid coupler means coupled to said antenna array for receiving a signal from each of said radiator elements and for generating a sum signal and a difference signal from the two signals received from said antenna array; a phase and amplitude detecting means coupled to said hybrid coupler means for determining the phase difference between the sum and difference signals and for determining the amplitude of each of the sum and difference signals, and a processing means coupled to said phase and amplitude detecting means for comparing the phase difference between the sum and difference signals and the amplitude of the sum and difference signals with the characteristics of the antenna array to determine the relative angular position of the target with respect to said antenna array.

The present invention further provides a method of acquiring a target comprising the steps of: receiving a signal from a target at each of a pair of spaced antennas with beam patterns having angularly divering main axes with respect to a boresight; generating a sum and a difference signals from the two signals received by the pair of spaced antennas; and determining the relative phase between the sum and difference signals and the amplitude of the sum and difference signals; and comparing the relative phase and amplitude of the sum and difference signals to known antenna characteristics so as to determine the position of the target relative to the pair of spaced antennas.

Embodiments of the invention make use of the fact that in general case of a monopulse radar system the generated difference and sum signals are phasors, expressed by complex numbers, and that they may have arbitary phase relative to each other. Thus, in addition to considering the relative magnitudes of the signals being processed, the relative phase relationship of the signals is considered.

The hybrid monopulse radar system described in accordance with the preferred embodiment uses radiated beams aimed in different directions which originate from antennas which also are displaced laterally and employs a self-generated complex error voltage for tracking. The error voltage generated by the beams of the hybrid system falls in the complex plane and contains both real and imaginary parts. The error voltage crosses the real axis when the off-boresight target position coincides with a maximum or minimum amplitude point of the sum pattern. The monopulse radar system uses this complex error voltage to track a single target.

A radar system in accordance with an embodiment of this invention can be conformally mounted on the sides of a missile. The beams formed by the conformal array do not look through a radome; thus the loss of pointing accuracy associated with the radome is eliminated.

The forward ene of the cylindrical missile body is not required for the conformal antenna and is available for other uses, such as an electro-optical sensor. Suitable beams also can be formed by a planar or conformal phased army.

In order that the present invention may be more readily understood an embodiment therefore will now be descriibed, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a monopulse radar in accordance with an embodiment of this invention; Figure 2 is a plan view of the radiation pattern in a plane including the boresight of a monopulse radar in accordance with an embodiment of this invention; Figure 3 is a polar plot of the complex error voltage versus the angle off the antenna boresight axis to determine target angle in accordance with an embodiment of this invention; Figure 4 is a graph of error voltlage magnitude versus target angle for three discrete phase offsets 20', 40 and 60";; Figures 5a to 5d give antenna beam patterns, similar to Fig. 2 which vary in divergence with frequency; Figures 5e to 5h are related to Figs. 5a to 5d, respectively and are a graphical representation of error voltage versus target angle relative to antenna boresight axis depecting the angular region over which a useful error signal occurs by a heavy horizontal line; Figures 5j, 5k, 5m and 5n are related to Figs. 5a to 5d, respectively and are a graphical representation of the sum and difference channel complex voltage relationship; Figure 6 is a graphical representation of experimental data plotted on the cordinates of phase angle offset versus target azimuth versus error voltage in accordance with an embodiment of this invention;; Figure 7 is a graphical representation of error voltage versus target azimuth angle for a plurality of phase angle offsets in accordance with an embodiment of this invention; and Figure 8 is a graphical representation of scan angle from an antenna system boresight versus frequency of the signal applied to the antenna system and the characteristic of an endfire antenna beam width.

Referring to Fig. 1, a monopulse radar system 10 includes a pair of spaced antennas 11 and 1 2 connected to two input ports of a sum and difference hybrid 14. The two output ports of sum and difference hybrid 14 are connected to a pair of mixers 1 6 and 1 7 which receive a signal from a local oscillator 1 5 to produce an intermediate frequency. A transmitter 1 3 is connected to an injector 23 coupled to sum and difference hybrid 14 to provide a signal to be transmitted. A phase sensitive detector 20 has two input ports connected to mixers 16 and 1 7.

A phase offset adjuster 1 8 is coupled to the signal flow between mixer 1 6 and phase sensitive detector 20. Phase offset adjuster 1 8 can be used to regulate the phase difference between the sum and difference signals from sum and difference hybrid 14. Phase offset adjuster 1 8 is typically used so that monopulse radar system 10 can track off the boresight of the system which can for example, be mounted on a missile and be aligned with the axis of the missile.

Phase offset adjusted 18 is electrically adjustable and operates at the I.F. frequency. Adjuster 1 8 is shown in the sum channel, but could also be positioned in the difference channel. An adjustment to the output of phase sensitive detector 20 is generated by an error bias voltage generator 22 which is coupled to a subtractor 21. Subtractor 21 is also coupled to the output of phase sensitive detector 20 to produce an output error voltage.

When system 10 is used in connection with a missile, a missile yaw rate gyro 1 9 can be coupled to transmitter 1 3 to aid the capture of targets at + 15" or less from boresight. As further discussed later, missile yaw rate gyro 1 9 varies the frequency sweep rate of variable frequency transmitter 1 3 so that the angular divergence of the antenna beams decreases at the angular rate equal to the missile yaw rate (i.e., the antenna array boresight yaw rate), thereby keeping the beam pattern centered on the target because of the error signal produced by the divergent sum and difference beams.

The system shown in Fig. 1 is particularly advantageous for capturing and tracking off the boresight of the antenna system. A simpler system can be used if only the position of the target with respect to antenna 11 and 12 is desired because the output of sum and difference hybrid 14 contains sufficient information to determine position. The angular position of the target can be determined from the relative phase and magnitude between the sum and difference signals in conjunction with known characteristics of the antenna.

Referring to Fig. 2, there are shown the antenna beam patterns of antennas 11 and 1 2 which are endfire traveling wave antennas conformally mounted on opposite sides of the surface of cylindrical missile body to achieve the required amplitude and phase separation of the beam patterns. The beam patterns such as shown in Fig. 2 are used as models to calculate the sum and difference complex voltages corresponding to signals received at the antenna. Table I shows the results of computations of complex sum and difference voltages made at 2" target angle intervals.

TABLE I AZIMUTH ANGLE E, EA IPA 0 1.038 0" 0 2 1.020 2" .314 65.7" 4 .957 8" .619 69.3" 6 .840 18.8" .888 73.5" 8 .705 36.8" 1.111 78.7" 10 .634 65.5" 1.255 84.4" 12 .699 96.7" 1.303 91.9" 14 .851 119.3" 1.243 101.8" 16 .966 135.1" 1.114 114.4" 18 1.008 148.6" .961 128.9" 20 .925 160.8" .754 145.4" 22 .747 172.3" .517 163.2" 24 .531 181.7" .303 185.3" The following equations are used to determine the complex sum and difference voltages from the signals at an antenna A and an antenna B:: Ez = EAei2 + EBe E= E,d- E,e-j% where zero Sinus A= A the phase angle D = antenna separation in wavelengths A ss = Target angle EA = field strength at angle > of antenna A EB = field strength at angle f of antenna B In operation, the complex voltages shown in Table I can be processed by a system such as that shown in Fig. 1 to derive useful error signals for both capture of the target and off boresight tracking of the target if a continuous adjustment is made of the error bias voltage and of the phase shift between the difference and sum channel voltage.The locus of the calculated difference to sum channel voltage ratio is shown in Fig. 3 as a function of target angle for +0, the phase angle offset, equals zero degrees. The phase of the difference channel voltage relative to the sum channel voltage varies between - 20" and + 70" for positive target angles and between + 160 and + 250" for negative target angles. The importance of this feature is that the difference channel voltages occupy a separate phase region for positive target angles than negative target angles. If +0 is varied through 360" of phase, the curve will rotate counter clockwise through 360" relative to tthe coordinate axes and will result in a family of error voltage curves.

TARGET CAPTURE The adjustment of phase offset adjuster 1 8 determines the ease with which targets are detected at various azimuth angles. A family of error voltage curves for a particular embodiment of this invention is shown in Fig. 4 for phase shifter settings of {0 = + 20 , + 40" and + 60 .

A choice of IPo = 40", for example, produces an error voltage versus target angle curve which is positive from boresight to the system sensitivity limit at the right edge of the sum beam and, negative for negative target angles from boresight to the left edge of the sun beam. These sensitivity limits establish the capture angle for fixed antenna beams. Scanning the beams will increase the capture angle range as will be shown later in the discussion.

The selection of tPo = + 40" phase adjustment appears to be the best compromise between achieving maximum error voltage slope at boresight and widest capture angle for an assumed sensitivity of 0.2 volts. A + 40" phase adjustment will yield 0.136 volts/ degree at boresight and a capture angle + 22". A + 60" phase adjustment will yield the best tracking capability of 0.148 volts/degree at boresight, but the capture angle will reduce to + 14". The best capture angle of + 23"; occurs with a phase adjustment of + 20"; however the tracking error slope at boresight is reduced to 0.1 07 volts/degrees.

SEARCH AND ACQUISITION WITHIN + 50" Another application of a system in accordance with one embodiment of this invention is to search for and capture a target within + 50' of the missile axis, then steer the missile axis to the target by using the antenna error voltage. The diagrams in Fig. 5 help to understand the use of traveling wave antennas having beam steering ability by sweeping frequency. The diagrams in Figs. 5a and 5d show two antenna beams, A and B, at various scan angles from the missile axis. The widest scan angle corresponds to the lowest frequency.Antennas A and B are connected to a hybrid in the same fashion as antennas 11 and 1 2 shown in Fig. 1 and various frequencies are applied from a source such as transmitter 1 3. Figs. 5j, 5k, 5m and 5n show the sum and difference channel complex voltage relationships corresponding to the beam scan angle positions shown at left. Note that for the beam position of Fig. 5a, where the beams are approaching coincidence, the sum and difference voltages are approaching the quadrature relationship of those in a pure phase comparison monopulse system. The arrowheads on these curves correspond to increasing target angle.Figs. 5e through 5h show the curves for error voltage versus target azimuth angle with approximately iko = 45 phase adjustment in the sum channel. The heavy solid line along the azimuth axis of each diagram depicts the angular region over which a useful error signal occurs. A target within any of these regions will produce an error signal enabling the missile axis to be steered toward the target. The angular extent of these regions corresponds approximately to the sum pattern 10 dB beam width which varies from 40' at zero azimuth to 7" at 45" azimuth for typical antennas. The following sequence is typical to search for, acquire and track a target.

To search, first, roll missile through 1 80" while repeatedly sweeping frequency from f, to f4; second, detect target on the peak of a sum beam; and third, store roll angle and frequency.

To acquire, first, halt the search, command the missile to roll to the position of the target and set the frequency applied to the antenna to direct the peak of the antenna beam pattern at the position of target; second, switch to track mode; third, actuate the missile to respond to an error signal by starting a yaw maneuver which will move the missile axis toward the target; fourth, measure the yaw rate and command a final frequency sweep to scan the beam angle at the yaw rate toward the missile axis; and fifth, stop frequency scan at f4.

To track, first, tracking will be done at f4; second, the phase offset of the sum channel voltage can be adjusted to improve tracking accuracy; and third, off-boresight tracking can be done at this time to develop lead angles relative to the target of up to + 20".

OFF-BORESIGHT TRACKING The method for off-boresight tracking can be understood by referring to Fig. 6 and to the block diagram in Fig. 1. Fig. 6 was generated from Fig. 3 and shows the phase adjustment iPo versus the target angle for fixed values of voltage, V,, the output of phase sensitive detector 20.

For example, if IPo = 40", V, is zero at zero degree target azimuth, + 0.4 volt at + 2.8 degrees target azimuth; + 0.8 volt at + 5 degrees target azimuth, etc. If V, is fixed at + 0.4 volt the target azimuth is + 3.3" at +0 = 20", + 4.6" at +0 = 0", + 6.6" at +0 = - 20" and + 8.6" at Ao -40".

For off-boresight tracking at target azimuth angles between 0" and 2.8", the phase offset can remain fixed at Iso = 40" and V, can be biased by setting V0 at a level between 0 and + 0.4 volt which will yield a zero error voltage (V, - V,) at the desired target azimuth. For tracking between 2.8" and 10" target azimuth, V0 can remain fixed at + 0.4 volt while the phase offset is adjusted between IPo = 40" and - 60" to yield a zero error voltage (V, - V,) at the desired target azimuth. Off-boresight tracking is possible with combinations of V0 and +0 other than those chosen in the foregoing example and extending the example further will demonstrate a tracking capability of + 20".

The following method can be used to pinpoint an off-boresight target known to be within + 20" of boresight. Illustrated in Fig. 7 are curves of V, versus target angle for various values of +0. During initial target acquisition +0 should be set at + 40". A target between zero and 20" azimuth will produce a positive error voltage as shown in Fig. 7. The next step toward pinpointing the target position is to set +0 = - 80". If the error voltage remains positive the target is between + 10.5" and + 20".If the error voltage is zero the target is at + 10.5", and if the error voltage is negative the target is between zero and + 10.5". The target angle can be exactly determined after iko has been set to - 80" by using the error signal to select preprogrammed values of iko and V0 until a null occurs as in the previous example. A positive error would direct a search through the program in one direction while a negative error would direct a search in the opposite direction. The foregoing discussion about targets at positive angles applies equally to targets at negative angles. Capture and tracking in the orthogonal plane has been left out of the discussion but would be accomplished in the same manner using another set of antennas.

Experimental Results A test was conducted to show that the required complex error voltage curve could be generated by antennas 11 and 1 2 when illuminated by a single target. The antenna beam centers were aimed 30" apart and the antennas were spaced 2.8 wavelengths apart for the test.

The test model consisted of two endfire antennas conected to a sum/difference hybrid and mounted on opposite sides of a 2.8 wavelength diameter cylinder. Measurements were made of amplitude and phase versus target angle at the sum and difference terminals of the hybrid connecting the two antennas. A single transmitting horn was used as a beacon target.

Measurements of relative amplitude and phase versus target angles were made at the sum and difference ports of the hybrid using a Scientific-Atlanta Model 1 750 Phase-Amplitude receiver.

The measured data was analyzed to determine the difference-to-sum port voltage ratio and the corresponding phase difference at each target angle. A plot of these results generally appears the same as and confirms the analytical results presented in Fig. 3.

A second experiment was performed which demonstrated the ability to scan the beam position by changing the applied frequency. The experiment was conducted with the test equipment connected to one antenna only. The frequency was varied between 7.8 GHz and 9.4 GHz in 0.1 GHz steps. The VSWR relative to 50 ohms, the gain relative to an isotropic radiator and the radiation pattern were measured at each frequency. The measured patterns were analyzed at each frequency for beam width, scan angle and sidelobe level. The results of the measurements and analysis are given in Fig. 8.

The results shows that the scan angle could be varied smoothly between 1 5" and 50" by varying the frequency between 8.4 GHz and 9.3 GHz. Over this same frequency range, the VSWR was less than 2.6 (1.67 average). The gain was greater than 1 3 dBi (15 dBi average) and the sidelobe level was less than - 6dB (- 11.3 dB average). These results were obtained with an existing antenna whose design was not based upon a beam scanning requirement. An optimization of this design for beam scanniny will improve all performance parameters.

Various modifications and variations will no doubt occur to those skilled in the various art to which this invention pertains. For example, a particular antenna configuration for achieving the desired antenna beam pattern may be varied from that disclosed herein. Similarly, the particular configuration of the tracking apparatus may be varied from that disclosed herein. Also, the particular method of off-boresight tracking by means of the complex error voltage may be varied from that disclosed herein. These and all other variations which basically rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention.

Claims (11)

1. A monopulse radar apparatus for determining the angular position of a target including: an antenna array having a pair of radiator elements for receiving signals from the target, said radiator elements being spaced from one another so as to have beam patterns originating at spaced points and having beam patterns with diverging main axes; a hybrid coupler means coupled to said antenna array for receiving a signal from each of said radiator elements and for generating a sum signal and a difference signal from the two signals received from said antenna array;; a phase and amplitude detecting means coupled to said hybrid coupler means for determining the phase difference between the sum and difference signals and for determining the amplitude of each of the sum and difference signals, and a processing means coupled to said phase and amplitude detecting means for comparing the phase difference between the sum and difference signals and the amplitude of the sum and difference signals with the characteristics of the antenna array to determine the relative angular position of the target with respect to said antenna array.

2. A monopulse radar apparatus as recited in Claim 1 wherein said phase and amplitude detecting means determines the sum signal amplitude in accordance with the following equation; = = EAeiS/2 + EBe-;+/2 and the difference signal amplitude in accordance with the equation: E"= EA'2 - Ee -j2 wherein + is (27rD/A) sin f; D/A is the separation distance between said radiator elements in wavelengths; + is the target angle; EA is the field strength at angle f of one of said radiator elements; EB is the field strength at angle + of another of said radiator elements.

3. A monopulse radar apparatus as recited in Claim 1 or 2 further comprising an off boresight tracking means including: a phase offset means coupled to said hybrid coupler means for adjusting the relative phase between the sum and difference signals; and an error bias voltage means coupled to said hybrid coupler means for applying a bias voltage for adjusting the magnitude of an error signal which is a function of the magnitude and phase of the sum and difference signals and the phase adjustment of said phase offset means, said error bias voltage means being adjusted so that the location of the target is determined with respect to an offset axis which diverges from the boresight of said antenna array thus permitting off boresight tracking of the target, the magnitude of the divergence of the offset axis being determined by the adjustment provided by said phase offset means and said error bias voltage means in accordance with the characteristics of the antenna array.

4. A monopulse radar apparatus as recited in Claim 3 wherein said off boresight tracking means includes: a phase sensitive detector means having an input coupled to said phase offset means and said hybrid coupler means for receiving the phase adjusted sum and difference signals and having an output coupled to said error bias voltage means for supplying a voltage to be adjusted by said error bias voltage means.

5. A monopulse radar apparatus as recited in Claim 4 wherein: said phase offset means includes means for adjusting the phase diference between the sum and difference signals to be 90" so that there is a maximum sensitivity to changes in magnitude of the received signal.

6. A monopulse radar apparatus as recited in Claim 5 further comprising control means coupled to said phase offset means and said error bias means for varying the adjustment of said phase offset means and said error bias voltage means so that the offset axis is pointed at the target and the output of said phase sensitive detector as adjusted by said error bias voltage means is at a minimum.

7. A monopulse radar apparatus as recited in any one of claims 3 to 6 further comprising: frequency variation means coupled to said antenna array for altering the divergence of the beam patterns of said radiator elements thus varying the acquisition angle of said antenna array.

8. A monopulse radar apparatus as recited in Claim 7 further comprising: a missile yaw rate means coupled to said frequency variation means to vary the frequency sweep rate so that the angular divergence of the antenna beams decreases at the angular rate equal to the antenna array boresight yaw rate, thereby keeping the beam pattern centered on the target as the antenna array guides toward the target because of the error signal produced by the divergent sum and difference beams.

9. A method of acquiring a target comprising the steps of: receiving a signal from a target at each of a pair of spaced antennas with beam patterns having angularly diverging main axes with respect to a boresight; generating a sum and difference signal from the two signals received by the pair of spaced antennas; and determining the relative phase between the sum and difference signals and the amplitude of the sum and difference signals; and comparing the relative phase and amplitude of the sum and difference signals to known antenna characteristics so as to determine the position of the target relative to the pair of spaced antennas.

10. A method as recited in Claim 9 further comprising: off-boresight monopulse radar tracking by establishing an offset diverging axis with respect to which the position of the target can be determined the offset diverging axis being established by adjusting the relative phase between the sum and difference signals and biasing the magnitude of the sum and difference signals so that a null is obtained with the target positioned on the offset diverging axis.

11. A method as recited in Claim 10 wherein adjusting the relative phase between the sum and difference signals to obtain phase compensated sum and difference voltages includes determining the point of maximum sensitivity of the ratio of the difference signal to the sum signal thereby pointing the offset axis at the target.

1 2. A method as recited in Claim 11 further comprising combining the phase compensated sum and difference voltages to obtain an error voltage and combining the error voltage with a biasing voltage, used to bias the magnitude of the sum and difference signals to obtain a signal indicative of the positioning of the target with respect to the offset axis when considered in conjunction with the antenna beam characteristics.

1 3. A method as recited in Claim 1 2 further comprising a step of acquiring the target by frequency sweeping to vary the frequency of the signal applied to the spaced antennas and change the angular divergence between the main axes of radiation between the spaced antennas thus varying the sensitivity of the antenna with respect to the boresight of the antenna.

1 4. A method as recited in Claim 1 3 wherein the step of acquiring the target by frequency sweeping includes varying the rate of frequency sweeping as a function of the antenna boresight yaw rate so that the angular divergence of the antenna beams decreases at the angular rate equal to the antenna boresight yaw rate, thereby keeping the beam centered on the target as a missile carrying the antenna guides toward the target because of the error signal produced by the divergent sum and difference beams.

1 5. A method as recited in any one of Claims 11 to 14 wherein adjusting the relative phase between the sum and difference signals includes achieving an orthogonality between the voltage phasor of the sum signal and the tangent to the error curve, in the complex plane, of the comparison of the relative phase and amplitude of the sum and difference signals to known antenna characteristics.

1 6. A monopulse radar apparatus for determining the angular position of a target, substantially as hereinbefore described with reference to the accompanying drawings.

1 7. A method of acquiring a target substantially as hereinbefore described with reference to the accompanying drawings.