GB2292493A - Monopulse radar - Google Patents

Abstract

In an air-to-surface radar ranging system employing a conventional four-element monopulse beam-split serial array (11) followed by a monopulse comparator (15) to extract sum, difference and nonsense channels, rolling of the aircraft and/or across-track sloping of the ground causes ranging uncertainties due to the left and right hand halves of the beam-split system "striking" the ground at different heights. It is found that when the left and right hand elevation difference signals are subtracted the result is a signal of magnitude proportional to the slope of the across-track surface and of polarity indicative of the slope direction. It is also found that this subtracted signal is given by the nonsense channel signal. <IMAGE>

Description

Air-to-Surface Radar Ranging system This invention relates to an air-to-surface raider ranging system employin a monopulse ttam-split arrangement.

A basic such arrangement is illustrated in elevation in Figure 1 and in plan view iD Figure 2.

Figure l(a) shows the split-beam characteristics of an aerial having two elevation elements. The beams split direction or crossover axis, i.e. the line of symmetry between the beams, is the only direction from which ground returns to the two elements can produce signals of equal magnitude and phase. The individual responses of the two elements are shown in Figures l(b) and l(c). It can be seen that as the transmitted pulse strikes the surface progressively, the magnitude of the returns increases and decreases as the point of reflection moves through the axis of the individual element, i.e. the most sensitive direction of the element.Since the axes of the two elements diverge slightly the peaks of the responses will occur from position (or at times) F and . ;.rhen the transmitted pulse is incident at point R, which lies on the boresight of the aerial, and iD particular on the above crossover axis, the two elements receive equal signals of the same phase. Coherent subtraction of the two signals therefore produces a signal of envelope waveform shown in Figure l(d), exhibiting a crossover or zero point N corresponding to the point R.A determination of the timing of the crossover point N in relation to the time of transmission of the radar pulse will therefore give a measure of the distance of the point R from the aircraft, the point R being clearly defined by the boresight of the antenna.

The above explanation assumes an infinitesimally narrow pulse and aD infinitesimally narrou beam in azimuth. In practice, the finite pulse length and beamidth imply that integration of the ground returns tzkes place and this integration is over round which, ignoring large discrete targets, in uncorrelated.

Consequently a noise-like characteristic is superimposed on the characteristic shown in Figure 1 which introduces an uncertainty in the crossover range. This effect is referred to as residual clutter and is one of the primary sources of range uncertainty in a ground ranging radar.

Figure 2(a) shows a plan view of the ground surface throughout its illumination by the incident pulse and it can be seen that the monopulse null arises from a line on the ground across the track of the aircraft.

When the aerial is rotated about its roll axis relative to the ground, or when the ground is sloping across the track of the aircraft, the effect is to twist the line L of null response, as shown in Figure 2(b).

The line is still straight but is no longer orthogonal to the beam pointing direction. This results iD the slope of the characteristic of Figure l(d) being reduced.

The system is, therefore, more sensitive to residual clutter, and the range error increases. In addition, the monopulse null on the ground is distributed in range, which provides a further degradation of ranging accuracy.

The depression snEle of the aerial boresight is typically quite small and even for small roll angles the effect pust described can increase the ranging error very significantly, e.g. from about 30 m to 20C m.

Effective rolling of the aerial about its roll axis (i.e. boresight) can also be caused by the use of the Elliott-Cassegrain Scanner, the construction of which is such that scanning in any direction out of both azimuth and elevation planes causes roll of the antenna about its roll axis. This could be avoided by a suitable gimbal arranEement but certain very significant advantages would be lost.

Correction for the rolling of the aerial caused by off-axis scanning can be achieved by electronic roll stabilisation. For this purpose, four aerial elements are employed, each having its own beam characteristic. The four are arranged in square formation, the beams diverging from the boresight so as to produce the beam arrannement of Figure l(a) in both elevation and azimuth. The four aerial element outputs are summed and differenced in known manner in a microwave comparator to produce "aircraft axes" azimuth and elevation difference signals and these are processed by a monopulse resolver to produce an elevation difference signal which is stabilised in space.

With such an arrangement, errors arising from non-alignment of the antenna azimuth axis and the across track surface will be due almost solely to the ground slope.

It is 6n object of the present invention to provide a radar ranch ng system in which an indication is derived of the disposition of the across tract surface relatIve to the elevation plane of the aerial and in which a control signal may be derived to correct the aerial stabilisation, mechanical or electronic, or to maintain the elevation monopulse split et all times normal to the across track surface.

According to the present invention, an air-tosurface radar ranging system comprises an airborne monopulse radar including means providing range information from time of transmission of a radar pulse, in which the aerial arrangement provides an elevation beam-split characteristic at each of two spaced azimuth angular positions, each said characteristic having a crossover axis directed toward the surface, and includes means for producing a control signal representative of the time displacement between difference signals available from the respective elevation bea-split characteristics, the control signal providing an indication or control dependent upon the inclination of the said surface between the surface positions intercepted by said axes.

There may be included means for stabilising the elevation plane in space, the control signal being then dependent upon the across-track slope of the surface between said surface positions, with respect to the horizontal.

Alternatively, the StabiliEEtioD of the elevation plane X3y be modified by the control signal to naintain the elevation plane perpendicular to the surface between said surface positions.

The range information proviced by the system may be corrected by the control signal in an open-loop & rangement to suppress the effect of any across-track slope.

Freferably, the means for producing 8 control signal comprises a sum and difference comparator to which the signals received by a four-element aerial arrangement are applied and which provides a sum signal, an azimuth difference signal, an elevation difference signal, and a so-called nonsense channel signal, the latter signal being a function of the across-track slope of the surface between said surface positions.

An air-to-surface radar ranging system in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figures l(a) (b) (c) and (d) show an elevation beam split characteristic together with amplitude/range graphs for ground returns to the upper beam element and the lower beam element, and a difference signal/range graph; Figures 2(a) and (b) show plan views of the monopulse beams of Figure 1 intercepting horizontal ground and ground with an across-trac slope, respectively; Figure 3(a) shows a boresieht view of 8 rcur beem ::nonopulse alyroaching sloping ground; Figure 3(b) shows graphs of the ground return signals to the four aerial elements of Figure 3(a); Figure 4(a) shows the four beam cross sections at the instant that the boresight intercepts the slowing ground; Figure 4(b) shows graphs of the two elevation difference signals from the azimuth spaced elevation beam splits, together with a graph of the difference of these two signals; Figure 5 is a schematic ciabram of a mor.osulse comparator showing the standard input and output signals; Figure 6 is a block diagram of an air-tosurface radar ranging system; and Figure 6(a) is 8 diagram of the carrier aircraft orientation in relation to the ground surface and illustrating the various angles involved in Figure 6.

Figures 1 and 2 have already been described in relation to the prior art. Figure 3(a) shows an upright arrangement of elevation and azimuth beam split patterns such as would arise in a space stabilised system. In such a case, where the ground slopes across the track of the ranging aircraft, the four beams will strike the surface in seouence DCBA. (It. will be appreciated that the 'beams' do not actually strike the ground since they are not beams of radiation but characteristics of reception sensitivity against angle for each antenna element. The analogy nevertheless produces the correct analysis.) The output signals of the serial elements, if separately available, would be as shown in Figure 3(b), i.e. amplitude variations with time.

Each vertical pair, A, C and B, D, provides an individual elevation beam-srlit system as shown in Figure 1, the two systems being separated in azimuth.

Each pair of alternate signals in Figure 3(b) therefore would provide a respective difference signal such as that shown in Figure l(d).

The two elevation beam-split systems, being spaced in azimuth, will provide crossover points (N' in Figure l(d)) which arise at different times as a result of the across track slope. The time difference 6t between these points can be shown to be given approximately by: 6t = 4 r tan a . tan 111/2 where r is the range to the beam centre, i.e. where the aerial boresight intercepts the surface, a is the across-track ground slope, is the azimuth beam-split angle, c is the velocity of light, and e is the radar look-down angle.

In order to find the boresight range (r) therefore, the across track slope c would have to be known and the time displacement (6t) would have to be measured. Apart from the difficulty of measuring the time displacement, the foregoing pre-supposes that beam channels A, B, C and D can be processed separately, (or at least in rapid sequence b a comutated receiver).

However, it is well known that in ar.y beam differencing system, the system errors introduced are particularly sensitive to phase and amplitude errors which may be unintentionally introduced prior to the subtraction process, and that is why in all modern monopulse systems the beam subtraction process is performed as far forard in the system as possible, usually in a waveguide comparator immediately behind the 4-horn feeder.

A method of operation which meets this requirement, enabling a front end microwave beam comparator to be used is the 'toff-boresight" method which is well known for locating a target in relation to the boresight.

Consider first the outputs from a simple two beam comparator system as a point source traverses the main beam. The output-versus-angle functions are roughly a single cosine function for the sum channel output and a single sine function for the difference channel. 3ecause of the linearity of the sine function in the beam crossover region, the position of a target which is not quite on boresight may be accurately estimated from the sige of the difference channel signal and its ratio to the sum channel signal, provided the usual constants of proportionality involving beam widths etc., are known. This is a well established technique known as "off boresight measurement".This principle is allied to across-track ground slope measurement in accordence with a preferred embodiment of the invention, as follows.

Referring to FiLure L, as tbe short tri. St.: t ted pulse proceeds towards te slopiDr ground, the instant illustrated in the drawing will arise, i.e. returns will be received from the point where the aerial boresight intercepts the ground, point 0. At this instant the range, of the point 0, will be indicated by te normal elevation difference channel signal, i.e. (A+B) - (C+D), exhibiting a crossover.

If the elevation beam-splits A, C and B, D were treated separately, they would show sources, or targets, at that instant, which were above and below the horizontal through the point 0 and which produced positive and negative signals respectively, proportional to the height, above and below this horizontal, of these sources, i.e.

of the ground level.

This is shown diagrammatically by the signal vectors V1 and V2 in Figure 4(a). As the radar pulse traverses the projection of the beam cross-section along the ground, the separate signals V1 and V2 execute separate "single cycle sine waves" (representing their difference channel characteristics) but displaced in time from one another according to the ten a law stated above.

One way of avoiding the complication of having to accurately measure the time difference between the separately generated (and somewhat noisy) sine wave like cross-over (difference) signals is to subtract them electrically and make use of the fact that the difference between two similar sine waves serrated in time is a cosine wave.

i.e. Sin eXt - Sin ^(t - bt) = 2 Cos ft(t + #t). 'in8bt 7 This quantity is a cosine wave of atnplitude 2 Sin #St.

7 Thus the vector difference of the signals V1 and V2 gives a quantity which is proportional to the sine of the above time displacement 6t and thus to the sine of half the across-track ground slope (tan a).

This quantity also has a sign which depends on the direction of the slope, left to right or vice versa.

This relationship with the ground slope applies only if the monopulse axes are stabilised in space to maintain the azimuth axis horizontal. Otherwise the quantity V1 - V2 is related to the angle between the unstabilised elevation plane and the across-track disposition of the ground (a-B in Figure 6(a) ).

Thus, in a basically stabilised system a measure of actual across-track ground slope can be obtained directly, and in an unstabilised system a direct measure of the across-track slope referred to the monopulse axes can be obtained directly.

The difference signal V1 - V2, for most practical purposes directly proportional to across-track ground slope, could be derived, as described above, from the individual beams. As also stated above however, it is highly desirable to employ a mocrowave comparator early in the system to minimise the effects of phase and amplitude errors. It can be shown that this compara tor can be made to provide the rround slope siLnsl as follows Consider a 4 horn feeder with a straightthrough type monopulse comparator and the usual associated + 900 phase shifters. Let the four horns 1, 2, 3, 4 (respectively producing the four microwave beams ABC and D) give rise to signal amplitudes a, b, c and d.Such a conventional 4-beam monopulse comparator is shown diagrammatically in Figure 5 and may consist of four waveguides in square formation with 3 dB couplers between adjacent guides and the 900 phase shifters in diagonally opposite elides.

Such a comparator is commonly used simply to produce a sum signal, an azimuth difference signal, and an elevation difference signal, for off-boresight target tracking. The fourth output channel of the comparator is commonly known as the 'nonsense channel' since it produces a signal of the form (c+b) - (a+d), i.e. the difference between diagonal pairs. This channel is usually terminated in a matched load and ignored.

However, by rearranging the nonsense channel signal to read (c-a) - (d-b) it is seen to contain the signal V1 - V2 As mentioned above, this signal contains information on the magnitude and sign of the acrosstrack ground slope in a 'mean earth stabilised' system.

In such a system it is therefore employed to further correct the electronically stabilised monopulse axes to 'local remote earth stabilised' and so remove one of the principal remaining errors in radar air-to-round ranging.

A system of this kind is illustrated in Figure 6. A four-element monopulse aerial 11 is driven, for scanning purposes, by a scanner drive unit 13. The four elements are coupled to a monopulse comparator 15, shown in greater detail in Figure 5. The sum channel output of the comparator is applied to a mixer 17 by way of a duplexer 19, the difference channels and nonsense channel being applied directly to similar mixers. The sum channel is fed by way of the duplexes 19 from a transmitter 21 in conventional manner and the mixers 17 are supplied with a signal from a local oscillator 23.

The four intermediate frequency output channels are then provided with I.F. amDlifiers 25.

In a roll stabilised system the azimuth aDd elevation difference signals are applied to a sine/cosine resolver 27 which can be rotated under the control of an angle drive input 29 to re-orientate the elevation plane, the shift of the plane corresponding directly to the angular input control. A roll-angle reference source 31, an inertial platform, provides an output proportional to the angle n (see Figure 6(a) ) between the aerial elevation plane and the vertical. This error signal would normally be supplied directly to the sine/cosine resolver to maintain the elevation plane effectively vertical and thus independent of aircraft roll.

In the present embodiment the roll angle error signal is applied by way of a mode switch 33.

The roll stabilised elevation channel output from the sine/cosine resolver is applied to an elevation phase secsitive-detector 35 together with the sum signal, to provide a difference signal such as that of Figure l(d).

This signal has a crossover, or zero value, which mould hio an uncertainty dependent upon the across track ground slope. A narrow pulse marking this crossover is then provided by a crossover pulse generator 37 which detects the zero crossing. This pulse is applied to range determining circuitry 39 for measurement of the elapsed time since the pulse transmission and the range is indicated accordingly.

os so far described, this system is conventional, and suffers from across track ground slope as described earlier.

The narrow crossover pulse is also applied to a gate 41, enabling it for the duration of the pulse. The other input of gate 41 is provided as follows. The sum I.F. signal and the nonsense channel signal are applied to a phase sensitive detector 43 which produces a scaled version of the 'cosine' sum signal shown in Figure 4(b).

The central peak of this signal. will have a magnitude determined by the time separation of the two difference signals of which it is composed, and thus by the ground slope. The polarity of the signal will be determined by the sequence of the left and right hand difference signals of Figure 4(b) and thus by the slope of the ground, leftto-right or vice versa.

The 'cosine' signal input to gate 41 is effectively sampled by the narrow pulse from the crossover pulse generator to produce an output pulse of magnitude and polarity representative of the across-track slope.

The system has been described so far with the sine/cosine resolver 27 in operation and controlled by the roll angle reference source 31. Thus the elevation plane of the electronically corrected beam-split patterns has been maintained vertical. The output of gate 41 is proportional to the angle between the uncorrected aerial elevation plane and the across-track surface, that is, to ( -ss) (see Figure 6(a) ). If the mode selector switch 33 is switched to the right therefore, the elevation plane will effectively be rotated in accordance with the angle (a-), reducing this angle to zero, and thus making the effective elevation plane normal to the across-track surface in the area of the target.

The crossover range will then be corrected to the same value for all positions across the track, the oblique characteristic of Figure 2(b) being corrected to that of Figure 2(a).

In determining a particular target range, the operator locks the radar on to the target, operating initially in the roll-stabilised mode using the reference source 31. Once the target is clearly acquired the operator switches to the local ground stabilised mode providing a much more certain range determination.

In addition to clarifying the range determination it may be desirable to provide at the same time an indication of the across-track slope. This is achieved by a subtractor circuit 45. The roll angle reference source 31 provides a measure of the elevation plane angle ss while the gate 41 provides a measure of the angle ( -b). The output of the subtractor 45 is therefore the angle a, the across-track ground slope. This may be displayed on a ground slope indicator 47.

Claims (6)

,CLAM

1. An air-to-surface radar ranging system comprising an airborne monopulse radar including means providing range information from the time of transmission of a radar pulse, in which the aerial arrangement provides an elevation beam-split characteristic at each of two spaced azimuth angular positions, each said characteristic having a crossover axis directed toward the surface, and including means for producing a control signal representative of the time displacement between difference signals available from the respective elevation beam-split characteristics, the control signal providing an indication or control dependent upon the inclination of the said surface between the surface positions intercepted by said axes.

2. A ranging system according to Claim 1, including means for stabilising said elevation plane in space, said control signal being then dependent upon the across-track slope of the surface between said surface positions, with respect to the horizontal.

3. A ranging system according to Claim 2, wherein the stabilisation of said elevation plane is modified by said control signal to maintain the elevation plane perpendicular to the surface between said surface positions.

4. A radar system according to Claim 2, wherein the range information provided by the system is corrected by said control signal in an open-loop arrangement to suppress the effect of any.across-track slope.

5. A ranging system according to any preceding claim, wherein said means for producing a control signal comprises a sum and difference comparator to which the signals received by a four-element aerial arrangement are applied and which provides a sum signal, an azimuth difference signal, an elevation difference signal, and a so-called nonsense channel signal, the latter signal being a function of the across-track slope of the surface between said surface positions.

6. A radar system for air-to-surface ranging, substantially as hereinbefore described with reference to the accompanying drawings.