GB2291541A - Radar beam-steering apparatus - Google Patents

Abstract

The apparatus processes the outputs of a strapdown radar phased array mounted in the nose of a homing missile and generates missile-to-target look-angle rotation-sensitive measurement residuals for a missile-to-target sightline spin-rate estimator, the outputs of which are used to guide the missile. The array steer direction is updated by transverse projection corrections ¦y - @¦,¦z - @¦, for the deviation between the target sightline (R Fig. 1) and the current array look direction (L Fig. 1), the corrections being formed using sum, and azimuth and elevation difference signals derived from the signals incident onto the array. A further embodiment (Fig 3) comprises a discrete phase shifter and electronic angle tracker and includes means for removing bias terms caused by the phase quantisation. Thus, smooth tracking between discrete beam positions can be achieved. <IMAGE>

Description

RADAR BEAM-STEERING APPARATUS This invention relates to homing missile guidance and in particular to missiles fitted with a strap-down radar phased array seeker which is to be used in conjunction with a missileto-target state estimator. State estimators are known in the art of missile guidance and are generally used to estimate missile-to-target sight-line spin-rate and other variables which characterise the dynamics of the missile. These estimates are used in a predictive homing guidance law so that the missile may be steered onto a collision course with its target. State estimators have been integrated successfully with optical and inçra-red seekers, but their co-operation with phased array seekers poses particular problems which this invention seeks to solve.Phased array seekers allow a radar beam to be steered electrically either side of the bore-sight(the direction normal to the seeker face) by producing a phase difference between the radiative elements of the seeker array. The profile of the radiated beam can be adjusted by controlling the amplitutes of signals at each element. In the conventional approach to estimator theory, it is generally assumed that the seeker makes measurements of one or more of the state variables being estimated eg. missile-to-target look-angle. Measurement residuals to drive the estimator are obtained by subtracting the estimated states from the measurements. The corrected estimated states are used for missile guidance. When a phased array is chosen as the seeker, measurement residuals cannot be obtained in this fashion because the array does not make direct measurements of a state variable.

It is an object of the invention to provide means for converting the outputs of a phased array seeker into look-angle rotation-sensitive measurement residuals suitable for applying to a spin-rate estimator. The outputs of the estimator may be used to steer the missile onto a collision course with its target and also to steer the main beam of the seeker to keep the target in the seeker's field of view.

Accordingly, the invention consists of beam-steering apparatus for a homing missile guidance system including means for converting the output from a phased array seeker into measurement residuals for a state estimator said means comprising: means for electrically steering the main beam of the seeker in response to existing demands from the state estimator corresponding to y and z where y and z are projections onto a set of orthogonal axes of a vector directed along the estimated missileto-target sightline; means for processing the output of the seeker in response to a detected target to generate signals indicative of the direction of the actual missile-to-target sightline with respect to the main beam;; and means for computing measurement residuals |Y~Y| and and Iz-zI where y and z are projections onto said orthogonal axes of a vector directed along the actual missile-to-target sightline by applying a factor to the said signals related to seeker array geometry and applying said residuals to the state estimator as correction terms for updating the values of y and z.

The invention has the advantage that such state estimators can be made compatible with seekers which are responsive to phase adjustments made either continuously or in discrete steps. When discrete phase adjustment is employed, the invention may provide means for removing any bias effects caused by the quantisation of phase. Additional means may be provided whereby an electronic angle tracker can be co-ordinated with the discrete phase adjustment to obtain a smooth, rather than step-like beam-steering motion and continuous, rather than quantised, correction terms to drive the estimator.

Two embodiments of the invention will now be described by way of example with reference to the drawings of which: Fig 1 is a projection of target and array beam direction on to a plane transverse to a missile's direction of motion, Fig 2 is a schematic block diagram of a first embodiment of the invention and Fig 3 is a schematic block diagram of a second embodiment of the invention.

Fig 1 shows the axes Y and Z which form part of a rectangular system of co-ordinates X, Y, Z, the X axis (not shown) lying normal to the plane of the paper. The origin of this co-ordinate system defines the location of a phased array seeker which is defined to be in the YZ plane. Such a seeker would be fixed to the nose of a missile which in the co-ordinate system of Fig 1 would be travelling along a direction co-incident with the X axis. The main beam of the seeker (ie the direction of maximum sensitivity) is shown pointing along a line L. y and are the projections of the line L onto the Y and Z axes respectively. A line R defines the missile-to-target sightline.

This line is displaced from line L by angles  in azimuth and Ê in elevation,  lying in the XY plane and Ê in the XZ plane. y and z are the projections of R onto the Y and Z axes respectively.

It will be deduced from the diagram that  is a function of (y-y) and Ê is a function of (z-z). These quantities [(y-y) and (z-z) relate to the displacement of the target with respect to the main beam of the array and are used as the correction terms for a state estimator. These correction terms are applied to the existing values of y and z, as determined by the state estimator, to produce updated values of y and z and hence to steer the main beam of the seeker towards the deduced direction of the target.

These updated values can also be used to estimate sight-line spin-rate and other missile variables in a known manner.

In Fig 2 a state estimator 4 estimates the quantities y and which are converted to equivalent phase angle information by the converter 5. This information is fed into a continuously variable phase-shifter network 6. The phase-shifter network 6 communicates with a number of receiver elements comprising the phased array seeker 2 in order to steer the main beam in a direction corresponding to y and z. Nine elements are shown comprising the array but fewer or more elements could equally well be employed.The outputs from the elements, appropriately phase shifted with respect to one another by the network 6, are combined by a beam former 7 which provides four outputs V1, V2, V3 and V~, whose comparative amplitudes carry information regarding the residual phase relationships between the outputs and hence the position of a target within the beam pattern of the seeker 2.

Mathematical operations are performed on these four outputs by a sum-and-difference-combiner 8 which produces three outputs A, S and E. S is the sum of the four inputs, and the two difference terms A and E contain, respectively, information regarding the displacement of the target position from the main beam of the seeker in azimuth and elevation.

These three outputs are then mixed down to an intermediate frequency (IF) by means of three mixers 9a, 9b and 9c and a local oscillator 10 and then fed through an automatic gain controller (AGC) 11. The AGC 11 removes the effect of any increase in target signal amplitude as the missile-to-target distance decreases. A coherent demodulator 12 then removes the IF carrier by rectifying the signals A and E synchronously with Signal S.

The rectified DC outputs corresponding to the signals A and E are then passed through a low pass filter 13 which removes unwanted demodulation sum components and high frequency noise. The time constant of this filter is comparable with that of the AGC 11. The outputs of the filter 13, VA and VE, are related to the angles A and Ê which are, in turn, functions of the quantities (y-y) and (z-z). A scaler 14 applies the appropriate scale factor to VA and VE and outputs the correction terms (y-y) and (z-z) to the estimator so that the existing values of y and z may be updated.

In operation, as stated previously, the estimates y and are used to steer the main beam of the phased array seeker 2 (in addition to being used to estimate spin-rate etc for missile guidance).

The beam pattern of any phased array may be controlled by phase shifting the outputs of the individual elements with respect to one another. In the context of this invention, the main beam of the seeker 2 is steered by the phase shifter network 6 so that it lies along a direction defined by y and z.

To achieve this steering the converter 5 multiplies the values y and z by a factor dependent on the array geometry in order to provide the network 6 with the correct phase steering information.

A detectable target may lie anywhere within the steered beam pattern of the array and the amplitude of the sum of the elemental outputs of the array will give an indication of the target's position in the beam. This sum will be a maximum when the target lies on the main beam. However, merely summing the elemental outputs does not lead to a unique prediction of the target's position but by use of the beam former 7 and the combiner 8, measurements of the target's position in azimuth and elevation with respect to the steered main beam can be achieved.

The beam former 7 performs further phase shifting and then summing operations on the elemental outputs and conveys the results to four output channels whose amplitudes are referred to in Fig 2 as V1, V2, V3, V4. The phase shifts are introduced so that each of these channels can be thought of as the outputs from four separate notional arrays whose main beams are distributed evenly around the direction of the steered main beam of the array 2. Because each of the four beams is comprised of contributions from all the array elements, any differences in element sensitivity characteristics will produce only second order errors in the measurements of target position. All four beams are collectively steerable by the network 6 bat their relative positions are fixed.

The four outputs from the beam former 7 are fed into the combiner which computes a sum S and two difference terms A and E; where S = V1 +V2 +V3 +V4 A = (V1 + V2) - (V3 + V4) and E = (V1 + V3) - (V2 + V4).

The number of outputs from the beam former 7 is not necessarily restricted to four; four being chosen here for convenience.

It can be seen from the above equations that S will be a maximum and A = E = 0 for the situation where a target lies on the mean beam of the seeker 2. A and E will be non-zero when a target lies off the main beam and hence can be thought of as error signals. Target off-main-beam direction is now defined by the amplitudes of the signals A and E.

The signals S, A and E are mixed down to a convenient intermediate frequency, passed through the AGC 11 and then into the coherent de-modulator 12. The rectified outputs from the de-modulator 12 are filtered to give outputs VA and VE which are scaled so that they can be fed back to the estimator as correction terms (y-y) and (z-z). The estimator uses these terms to make its next estimates of y and z and the process repeats itself, with the new values of y and z being used to phase-steer the beam pattern of the seeker 2.

A second embodiment of the invention will now be described with reference to Fig 3. This second embodiment shares many common features with the first, as will be evident from comparison of Figs 2 and 3. However, the second embodiment of Fig 3 includes a discrete phase shifter 6a instead of a continuously variable one (Ref 6 in Fig 2) and further comprises a beam switching controller 15, an electronic angle tracker (EAT) 16 and a computing element 17. All other components shown in Fig 3 operate in the same fashion as in the case of the first embodiment.

Because the phase shifter used in this embodiment is capable of adjusting phase in discrete steps only ie the phase adjustment is quantised, the exact values of phase required to steer the main beam in accordance with the values of y and z supplied by the estimator cannot be achieved solely by means of the converter 5.

Hence, the beam switching controller is employed to choose the nearest available quantised steered main beam position to the demands y and z and the residual steering adjustment is made by the EAT 16. In this way the desired correction terms (y-y) and (z-z) appear at the output terminals of the scaler 14.

In operation, the beam switching controller 15 selects permitted projection values ya and za (which are as close as possible to y and z) and feeds the values to the converter 5.

The restricted set of phase values made available by the phase shifter 6a will, in general, prevent the selection of precisely the correct phase shift at all elements for achieving the projection values ya and za. Hence the output of the array will be biased due to these phasing errors. However, the beam switching controller can calculate these errors (if the phase shifter 6a is well calibrated and stable) and selects ya and Za so that these errors are minimised. Bias correction terms BA and BE are used by the EAT 16 to compensate for these phasing errors.

The computing element 17 computes the residual terms (y-ya) and (Z~za) which are also fed to the EAT 16.

The beam former 7, combiner 8, local oscillator 10, mixers 9a, 9b, 9c and the AGC 11 all operate in the manner previously described. The EAT 16 applies the final adjustment to the terms A, S and E to account for the phasing errors and the residual terms (Y-Ya) (z-za) Those skilled in the art of missile guidance will be familiar with an EAT as a means for steering a beam by using post-beam-forming network cross-coupling equivalent to a phasemodulation of the array outputs. In this embodiment of the invention however, the output values from the AGC 11 are corrected by the EAT 16 by adding a proportion of the sum term S to the respective terms A and E. The outputs A' and E' from the EAT 16 are thus related to the values A and E by the following equations: A' = A + gA S E' = E + gE S where gA = K(y - ya) + BA = = K(z - za) + BE and K is a scale factor related to the geometry of the array.

The signals A' and E' are applied to the demodulator 12 then filtered, scaled and fed back to the estimator 4 as before.

Claims (6)

1. Beam steering apparatus for a homing missile guidance system including means for converting an output from a phased array seeker into measurement residuals for a state estimator said means comprising: means for electrically steering the main beam of the seeker in response to existing demands from the state estimator corresponding to y and z where y and z are projections onto a set of orthogonal axes of a vector directed along the estimated missile-to-target sightline; means for processing the output of the seeker in response to a detected target to generate signals indicative of the direction of the actual missile-to-target sightline with respect to the main beam;; and means for computing measurement residuals |Y~Y| and and where y and z are projections onto said orthogonal axes of a vector directed along the actual missile-to-target sightline by applying a factor to the said signals related to seeker array geometry and applying said residuals to the state estimator as correction terms for updating the values of y and z.

2. Beam steering apparatus as claimed in claim 1 in which the means for processing the seeker output comprise: means for introducing relative phase shifts between the outputs of each of the elements comprising the seeker array and for combining the phase-shifted outputs to form a plurality of output channels; means for forming sum and difference terms from said output channels; and a coherent demodulator for rectifying the difference terms in order to generate signals indicative of the direction of the actual missile-to-target sightline with respect to the main beam of the seeker.

3. Beam steering apparatus as claimed in claim 2 in which the means for electrically steering the main beam of the seeker comprise a continuously variable phase shifter network for generating relative phase differences between each of the seeker elements to correspond to the values of y and z.

4. Beam steering apparatus as claimed in claim 2 in which the means for electrially steering the main beam of the seeker comprise a phase shifter network permitting only quantised phase differences between the seeker elements and further comprising means for generating permitted values ya and Za close to y and z and for applying correction terms corresponding to j-y,l and IZ-Za! to the said difference terms.

5. Beam steering apparatus as claimed in claim 4 further comprising means for applying to the said difference terms bias correction terms to compensate for the quantised phase differences between the seeker elements preventing exact correspondence with the values ya and za.

6. Beam steering apparatus substantially as hereinbefore described with reference either to Figure 2 or to Figure 3 of the drawings.

6. Beam steering apparatus substantially as herein described with reference either to Fig 2 or to Fig 3 of the drawings.

Amendments to the claims have been filed as follows 1. Beam steering apparatus for a homing missile guidance system including: a phased array seeker the relative phasing of signals applied to whose elements defines the direction of the seeker beam relative to the missile body and the outputs from whose elements indicate the direction of a target relative to the beam and hence to the missile body; a phase shifter network for applying relative phase information to the seeker elements; means for electrically steering the seeker beam comprising a state estimator for generating signals to be applied to the phase shifter network via a converter for producing a desired phase differential between the elements of the seeker; means for generating a plurality of signals providing information bout the position of the target within the beam of the seeker and for applying said signals to a combiner having three outputs two of which relate to the displacement qf the target from the beam centre in orthogonal directions; and means for processing said three outputs and for applying the processed outputs to the state estimator as correction terms whereby the state estimator can determine an updated beam direction.

2. Beam steering apparatus according to Claim 1 in which one output of the combiner is derived from the sum of the signals applied to the combiner and the two outputs of the combiner which relate to the displacement of the target from the beam centre are derived from difference terms derived from said signals.

3. Beam steering apparatus according to Claim 2 in which the phase shifter network applies to the seeker elements phase information corresponding to signals generated by the state estimator.

4. Beam steering apparatus according to Claim 2 in which the phase shifter network permits only quantised phase differences between the seeker elements, the apparatus further including: a beam switching controller arranged to convert signals generated by the state estimator to signals corresponding to the closest permitted relative phases of the seeker elements; and an electronic angle tracker for applying correction terms to the outputs of the combiner in order to compensate for the effect of any bias introduced by the restriction in seeker element phase differences, said correction terms being determined by error signals generated by said beam switching controller.

5. Beam steering apparatus according to Claim 4 i which the correction terms applied by the electronic angle tracker to the outputs of the controller are constituted by a proportion of the sum output of the controller being added to each of the difference outputs, said proportion being determined by the geometry of the array.