GB1605315A - Improvements relating to targetlocating apparatus for aircraft - Google Patents

Description

(54) IMPROVEMENTS RELATING TO TARGET-LOCATING APPARATUS FOR AIRCRAFT (71) We, FERRANTI LIMITED, a company registered under the laws of Great Britain, of Hollinwood in the County of Lancaster do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to target-locating apparatus for aircraft and in particular to apparatus for enabling a pursuing aircraft to determine the range of, and perhaps other positional information about, another aircraft or flying missile which is to be intercepted or attacked. For convenience, the expression "fighter" or "fighter aircraft" will hereinafter be used to designate the pursuing aircraft and the term "target" the pursued aircraft or missile.

It is known to provide fighter aircraft with radar equipment arranged to scan the target with a directional pulsed transmission. From the orientation of the scanning aerial system and from the time of receipt of echoes of the transmitted pulses is derived information as to the direction in elevation and azimuth of the target and its range from the fighter. From this and further information. such as the airspeed of the fighter, is derived a display which enables the fighter pilot to fly a predetermined course relative to the target.

With such equipment a difficulty is experienced if the target emits a jamming radiation. The radar equipment of the fighter is still abie to derive the directional information but may be unable to measure the range so long as the jamming continues.

An object of the present invention is accordingly to provide, for fighter aircraft supplied with radar equipment of the kind described above. target-locating apparatus for deriving range information. as well as directional information, from a target flying level at a different height from that of the fighter, in spite of jamming radiation from the target.

Another object is to provide such apparatus which also derives one or more of the hollowing quantities: the bearing of the fighter from the target; the height difference between fighter and target; the closing speed of fighter and target; and the hypothetical time to go before collision between fighter and target.

In accordance with the present invention.

target-locating apparatus to be carried by a fighter aircraft for continuously deriving the unknown range R from the fighter to a target which is flying level at an airspeed U, as estimated by the pilot of the fighter, and at a different height from that of the fighter, includes radar equipment for continuously deriving information as to the direction in elevation and azimuth of the target relative to the fighter.

means for deriving from that equipment the elevational and azimuthal components we and Wa respectively of the sight-line spin, means for deriving the following additional information concerning the fighter, namely the true airspeed V, the elevational and azimuthal components e and a respectively of the angle between the sight line and the fore-and-aft line. the angle of incidence i between the fore-and-aft line and the flight line, and the climb angle c, the ang]es and c being measured in the elevational plane.

an electrical analogue computer for continuously deriving the said range R from the equations.

RWe-U(c±+i)coso+V(-+i) = 0.......(i) -a + Usin(# - a) + Vsin a = 0... ..(ii) where the second unknown quantity 6 represents the angle in plan between the courses of the fighter and the target. and means for supplying to the computer inputs which correspond to the quantities U, we, wa, and said additional information. - - The term "elevational" as used throughout this specification and claims should be understood as referring to a vertical plane containing the fore-and-aft line of the fighter.

and the term "azimuthal" as referring to a plane which is normal to the elevational plane and contains the sight line.

Similarly the term "angle of incidence" should be understood as meaning the component in the elevational plane of the angle between the fore-and-aft line and the flight line of the fighter.

When, as is usually the case, the radar equipment of the fighter is roll-stabilised, all three angles c, e. and i are measured in a true vertical plane, even when the fighter is banked.

By "sight line spin" is meant the angular velocity of the sight line with respect to the fighter.

In the accompanying drawings, Figures 1A and 1B are diagrams to illustrate the derivation of the formulae above stated, Figure 2 is a simplified schematic diagram of one embodiment of the invention, Figures 3 and 4 are diagrams of connections of parts of the apparatus shown schematically in Figure 1, and Figures 5 to 8 are diagrams showing supplementary equipment.

In all the diagrams the same reference numbers are used for corresponding components.

The invention relies on the above-stated simultaneous equations (i) and (ii) which, being dependent on instantaneous quantities, give continuous solutions whether the fighter and target are on straight-line or curvilinear courses.

The equations are valid so long as the angular quantities a, c, e, and i are small, as they usually are in an attack, and so long as the angular velocity quantities We and wa have appreciable values - that is, so long as the courses of the two craft do not lie in or very close to either the same azimuthal plane or the same elevational plane.

As an exception to this last requirement, it will be shown later that under certain conditions the range may be computed when Wa is approximately zero.

The equations may be determined by trigonometry from the diagrams of Fig. 1A and Fig. IB, which show the configuration of the attack resolved on a vertical plane containing the flight line of the fighter and on a horizontal plane, respectively. In each diagram the instantaneous movement of the target B, as seen from the fighter A, is a rotation, about the fighter as centre, at a tangential velocity R cos a or Rwa cos (c + e + i), as the case may lie, these velcities being the resultants of the resolved U and V velocities of the two craft. The cosine terms are to cause the R terms to be projected onto the venical plane containing the flight line, or a horizontal plane, as the case may be.

Hence from Fig. la: Rie cos a - Usin (e + i) = 0 The sign convention is that the U and V terms are positive if they tend to rotate the line of sight in the clockwise direction. Thus the U term is here negative. The angle 6 is assumed to be such as to be zero, rather than 1800, when target and fighter are flying on parallel courses in the same direction.

It is assumed that the angular values are small enough for cos p to approximate to unity and the sine terms to approximate to the angular values. To replace the sine of an angle by the angle itself, in radians, introduces an error of only about 2% even when the angle is as much as 20 . Hence the above equation reduces to equation (i), as stated above.

Similarly from Fig. IB: Rwa cos (c + e + j) + Usin ( a) + Vedi c sin a = 0 Here both the U and the V terms are positive, as each tends to rotate the line of sight clockwise. As a is measured in the azimuthal plane, as above defined - namely, the plane normal to the elevational plane and containing the sight line -, the term p should more accurately be replaced by the expression tan -I (tan a + cos but for the small angles considered, the cosine term is practically unity. Thus 6 and a can be assumed to be co-planar.

Simplification of the equation thus derived gives equation (ii).

Equation (i) may conveniently be stated in the shorter form: Rwe - P = 0 .. ...(iii), where P = U(~ + ~ + i) cos 0 - V( + i) Similarly equation (ii) may be stated in the form: Rwa + Q = 0 ... ....(iv) where Q = Usin(# - a) + Vsin p.

By multiplying equation (iii) by Wa and (iv) by ilk we get: Rwe~3 - Pwa = 0 (v) Rwawe + Qwe = O ... .,.(vi) Subtracting to eliminate R: P-wa + Qllle = 0... ...(viz) In carrying out the invention in accordance with one form by way of example, an analogue computer for continuously deriving the range R of the target by solving equations (i) and (ii) in respect of the two unknown quantities 6 and R will now be described with reference to the schematic diagram of Fig. 2.

The left-hand side of equation (iii) is set up on a first group of stages which include a computer stage 11 in which the quantity -P is derived by elementary analogue components from signals representing the known input quantities U, V, e i and c together with a shaft rotation representing the unknown angle 6 derived from servo means 12, which may be in the form of a reversible servo motor. Stage 11, and the manner in which the known quantities are obtained for it, will be described in more detail later.

The output signal from stage 11 is applied as one of two input signals to a unity-gain amplifier 13 the output signal of which is applied to energise in push-pull a linear potentiometer 14 the wiper 141 of which is controlled in dependence on the quantity wa the azimuthal component of the sight-line spin. This last quantity is derived by a rate gyro, forming pan of a component 15, coupled to the directional aerial system of the normal radar equipment 16 carried by the craft. Wiper 141 is connected to supply one of the two input signals to a differential amplifier 17 to drive the 8 servo 12 by way of a phase-reversing switch 18, controlled by the sum of the quantities (c + c + i).

The output signal from amplifier 13 is also applied to energise in push-pull a dividing rheostat 19 the wiper 191 of which is controlled in dependence on the quantity A e, the elevational component of the sight-line spin.

Like the quantity wa, this is derived by a rate gyro. forming pan of a component 20, coupled to the radar equipment 16. Wiper 19' is connected to a Make fixed contact 21 of a changeover switch 22 the moving contact 23 of which is connected to the input of a high-gain amplifier 24. The output signal from the amplifier is applied as pan of a negative feedback loop to energise in push-pull a linear potentiometer 25 the wiper 25' of which is controlled in dependence on We by component 20. Wiper 251 is connected to the second input point of amplifier 13.

For setting up the left-hand side of equation (iv) there is provided a second group of stages which is somewhat similar to that just described and so need only be briefly indicated. The group includes a computer stage 26 in which the quantity +Q is set up from the known input quantities U, V and a. and from the computed angle 6 as supplied by servo 12. The output signal from stage 26 is applied as one of the two input signals to a unity-gain amplifier 27. The output signal from this is applied to energise in push-pull a linear potentiometer 31 the wiper 31 of which is controlled from component 20 in dependence on ze. The output signal from amplifier 27 is also applied to energise in pushpull a dividing rheostat 32 the wiper 321 of which is controlled from component 15 in dependence on sa Wiper 31' is connected to the second input point of amplifier 17. and wiper 32' to a Break fixed contact 33 of switch 22. The output signal from high-gain amplifier 24 is also applied to energise in push-pull a linear potentiometer 34 the wiper 341 of which is controlled from component 15 in dependence on -Wa and is connected to the second input point of amplifier 27.

Switch 22 is controlled by a relay 35 energised through a switching device 36 in dependence on the computed value of 6 so that when that angle is in the neighbourhood of 0 or 180 , and in consequence wa is too small for equation (ii) to be suitable for deriving R, the relay is energised. thereby switching contact 23 of switch 22 to engage contact 21. with the result that R is derived from equation (i), but so that when 8 has other than those values and in consequence cos 8 is too small for equation (i) to be suitable the relay is unenergised, contact 23 engages contact 33. and R is derived from equation (ii).

The function of phase-reversing switch 18 will be better understood after the more detailed circuitry has been described below. For the moment this stage may be considered as shoncircuited.

The output signal from amplifier 24. giving the computed value of R, is delivered over a channel 37. This may be considered as applied to whatever apparatus (not shown) is required to make use of the range information.

The operation of the equipment will now be described for the condition where the fighter is engaging a target from a direction such that 6 is, say, 45" and in consequence relay 35 is unenergised and contact 23 is engaging contact 33, as depicted in Fig. 2. It will be assumed to begin with that the quantities 6 and R, as derived by the computer, have values substantially in accordance with equations (i) and (ii). It will be appreciated that 6 and R will not normally have those values exactly, since a small error signal is needed in each case for the feedback loop.

In response to the input signals representing the known quantities and to the derived value of 6, supplied by servo 12 stage 11 derives the quantity -P by analogue methods and applies it as an input signal to amplifier 13. At the same time potentiometer 25, the wiper of which is controlled by the radar equipment 16 to represent the quantity we. Is energised from amplifier 24 by a signal representing R, with the result that a signal proportional to RMe is fed as the other input to amplifier 13. Theoutput signal from this amplifier therefore represents the left-hand side of equation (iii). This quantity is effectively multiplied at potentiometer 14 by the quantity to t represent the left-hand side of equation (v and applied as one of the input signals to amplifier 17. The left-hand side of basic equation land of the derived equations (iii) and (v) are thus set up in the computer.

Similarly with equations (ii). (iv). and (vi): the output signal from stage 26 representing Q is applied in addition to a signal representing Rwa (derived from amplifier 24 and potentiometer 34) as the inputs to amplifier 27. The output signal from this, representing the left-hand side of equation (iv), is multiplied by We at potentiometer 31 so that the secondinput signal to amplifier 17 represents the left-hand side of equation (vi).

The output signal from differential amplifier 17 accordingly represents the left-hand side of equation (vii) - an expression from which R has been eliminated by subtraction. This signal is applied to drive servo 12 in the sense to maintain the angular position of its output shaft in accordance with the derived value of 6.

From equation (vii) it will be seen that so long as this value of 6 is consistent with the basic equations (i) and (ii) the output signal from amplifier 17 is zero and servo 12 is unenergised.

For any other value of 6 an error signal is developed by that amplifier and applied to operate the servo in the direction for correcting 6. Hence the computer operates by continuous nulling of the error signal from amplifier 17 to maintain the value of 6 correct within the limits consistent with the necessity of there being a slight error signal, as already explained.

The value of R is computed from the fact that, in accordance with equation (iv), the output signal from amplifier 27 is zero (to within the limits required by the necessity for an error signal) when R has the correct value. Any other values of this signal result in a high-amplified signal from amplifier 24 fed back negatively through amplifier 27. The output of amplifier 24, as delivered over channel 37, is thus maintained proportional to R to a close degree of approximation.

Rheostat 32 is provided in addition to potentiometer 34, both adjusted in accordance with Wa. because Wa may change its sign. The negative feedback l5op formed by components 24, 34, 27 and 32 back to 24 must include component 34 for deriving the quantity Rwa. If rheostat 32 were not provided. this feedback loop would be stable when wa had one sign but would become unstable if we changed sign.

because of the reversal of the phase of the feedback signal at potentiometer 34. Rheostat 32 corrects this, because now when Wa changes sign, the input to amplifier 24 changes sign too, and this change counteracts that at potentiometer 34. Rheostat 32 also serves to compensate for a weak feedback signal due to a small value of wa; for in that case the resistance of rheostat 32 is small and its small attenuation of the signal compensates for the large attenuation at potentiometer 34. Similarly with the other feedback ]oop 24,25,13, 19 and 14.

The apparatus accordingly operates continuously in response to the input signals.

producing an output proportional to the range of the target, for display or utilisation as required.

It is necessary for the target to be flying at a different height from that of the fighter, for if the heights were the same the elevational component We of the sight-line spin would be zero and the equations (i) and (ii) insoluble.

If the conditions are such that 6 is close to 0 or to 1800 and in consequence equation (ii) is not reliable for the derivation of R, the consequent operation of relay 35 connects contact 23 to contact 21, so that R is derived from equation (i).

The operation is otherwise similar to that just described. Although only one of these equations is used for deriving R, the resulting value of R is supplied to both groups of stages which solve the equations in respect of 6.

Unity-gain amplifiers are used for components 13 and 27 so as to amplify power (to drive the servo motor 12) without modifying the voltage values, which must be solely dependent on the various quantities used in the equations.

Details of the stages which compute the quantities P and Q and of the various components from which the signals representing the known quantities are derived will now be described with reference to Fig. 3.

The estimated speed U of the target is obtained from a source 41 as an alternating voltage the amplitude of which is manually controlled by the fighter pilot in accordance with his estimation of that speed from his knowledge of the performance of the various types of target encountered. This voltage is applied in push-pull with respect to earth to energise a pair of leads 42. Allowance is made for the possibility that the direction of the target's flight relative to the fighter's cannot be determined; hence only the value of U is fed into the computer, its algebraic sign, indicating whether the target is flying towards or away from the fighter, being disregarded. The question of the signs of the various quantities and their representation by the phase of the corresponding electrical signals is discussed later.

The fighter's own airspeed V is obtained as another alternating voltage of varying amplitude from the usual airspeed indicating equipment indicated at 43, this voltage being applied in push-pull with respect to earth to energise a pair of leads 44.

The elevational and azimuthal components e and a, respectively, of the angle between the sight line and the fore-and-aft line of the fighter are derived from the movements of the radar scanning aerial or dish 45 in the form of rotations of shafts 46 and 47 respectively. As already mentioned, the corresponding components We and Wa of the sight-line spin are also derived from the radar equipment. by way of rate gyros. It should be appreciated that all these directional quantities may be derived whether the target is giving offjamming radiations or not - in other words, that the correct functioning of the apparatus of the invention is not dependent on the presence of such radiations.

The angle of incidence i between the fore and-aft line and the flight line of the fighter is in the form of a rotation of a shaft 51 controlled from known apparatus 52.

The climb angle c is best obtained from the equation Vc = dh/dt, where h is the height difference between the fighter and the target. As the target is assumed to be flying level the quantity dh/dt may be obtained in known manner from an aneroid capsule arranged to rotate a shaft through an angle proportional to the height: the required output proportional to the rate of change of height (dh/dt) is obtained from some sot of tachometer device which is driven by that shaft and generates an alternating voltage of amplitude proportional to the speed of rotation. Dividing this output by V, in any convenient manner. produces the quantity c. This part of the equipment is accordingly represented as a shaft 53 rotated in dependence on the value of c by some known apparatus 54, such as one containing an aneroid capsule as mentioned above, to which is applied an input from source 43 representing V.

Stage 11, for computing the expression P of equation (iii), includes a linear potentiometer 61, centre-tapped to earth, the wiper 611 of which is controlled in dependence on the effective elevation angle E, which in equation (i) is represented by the expression (e + e + i). To effect this control, shafts 53, 46 and 51, are coupled to wiper 611 by some convenient differential mechanical gearing (not shown) which applies to the wiper a movement which is the algebraic sum of the respective angular movements of those shafts. The potentiometer is energised in proportion to U from leads 42.

Wiper 611 is connected by way of a pushpull transformer 62 to energise a cosine potentiometer 63 the wiper 63' of which is controlled from servo 12 in dependence on 6.

A switch 64 is connected between wipers 611 and 63' to enable potentiometer 63 to be shortcircuited at will for a purpose to be explained later. The switch is normally open.

Stage ]1 also includes a linear potentiometer 65 which is centre-tapped to earth and energised in push-pull from leads 44. Its wiper 651 is controlled by suitable differential gearing (not shown) in dependence on the algebraic sum of the angular movements of shafts 46 and 51.

representing the quantities e and As the computer is energised from alternating sources the algebraic sign of a term which is represented by an electrical quantity is represented by the in-phase or counterphase condition of that quantity with respect to some reference phase. For convenience, the phase which represents the positive or the negative algebraic sign will be referred to as the posnive phase or as the negative phase. as the case may be. It is to enable the output signal from the wiper to reverse in phase as the controlling quantity passes through zero that the various potentiometers of the computer are centre-tapped to earth and energised in push-pull.

The output from wiper 63' is thus proportional to U (e + e + i) cos 6, and that from wiper 65' to V (e+ i). The positions of the wipers with respect to the earthed points of the respective potentiometers are such that these terms are respectively negative and positive.

Hence the combined output as applied to stage 13 represents the quantity -P.

Q is derived in positive phase from the expanded form of equation (ii): RWa + U sin 6 cos a - U cos 6 sin a + V sin a = 0 (viii) Stage 26 accordingly includes a cosine potentiometer 71 and two sine potentiometers 72 and 73 having respective wipers 711, 721, and 731 controlled by shaft 47 in dependence on a.

Potentiometers 71 and 72 are energised in pushpull from leads 42 and potentiometer 73 from leads 44. As a never in practice exceeds a value of the order of + 50 . each of these three potentiometers may conveniently be restricted to that range.

The stage also includes a sine and a cosine potentiometer 74 and 75 which are energised from wipers 711 and 721 by way of push-pull transformers 76 and 77 respectively; the wipers 741 and 751 are controlled from servo 12 in dependence on 6.

The output from wiper 741 is thus proportional to U sin 6 cos a that from wiper 751 to U cos 6 sin a. and that from wiper 731 to V sin a. The wipers are located so that the output from wiper 751 is of negative phase and the outputs from wipers 741 and 731 of positive phase, in accordance with the slgns of the corresponding quantities in equation (viii). Hence the combined output as applied to stage 27 represents the quantity + Q.

Switching device 36 by means of which relay 35 is controlled in dependence on the value of 6 includes a moving contact 81 which is rotated by servo 12 so as to engage one or other of two arcuate contacts 82 or 83 when 6 is in the region of 0 or 180" respectively. Contact 81 is earthed and contacts 82 and 83 are connected in common through the coil of relay 35 to a source of potential. Consequently relay 35 is energised only when contact 82 or contact 83 is engaged by contact 8], that is, only when 6 is in the region of ()' or I 80C Phase-reversing switch 18 is rendered necessary because the sign of U is unknown, as already indicated: hence equations (i) and (ii) admit of an incorrect value of R. It can be shown that the correct value of R is denved when the sign (phase) of the error signal from amplifier 17 has a predetermined relationship to the sign of the effective elevation angle E, represented by the expression (e + e + i). If the relationship is not correct, the phase of the error signal is such as to operate servo 12 in the direction for driving 6 away from its true value, thereby increasing the error signal instead of nulling it. It is to prevent such unstable operation that switch 18 is provided. This component is in the form of a double-pole changeover switch connected between amplifier 15 and servo 12 and so operated by the combined shafts 53, 46 and 51 that the error signal has the sense required for stability. Whenever E changes sign, therefore, switch 18 changes over, thereby reversing the phase of the error signal to maintain its predetermined relationship to the sign of E.

A spurious solution may be computed when making a tail attack within a part of an angular range in the region of 35 in azimuth (as above defined), the exact value of the range depending on the speed advantage V/U of fighter over target. Such misoperation may be avoided by working to a simplified equation derived from equation (i) in reliance on the fact that at such angles cos 6 may be regarded as unity. Equation (i) thus becomes: RWe-U(c+e+1)+Vĕ+I)=0.. (ix) Switching device 36 may then be arranged so that relay 35 is energised when 6 is in that region, with the result that R is derived from equation (i). The simplified form of that equation is brought into use merely by closing switch 64 to shon-circuit potentiometer 63 and so represent cos 6 by unity.

The equation (i) equipment for the push-pull energisation of the We and Wa potentiometers is shown in Fig. 4.

The output from amplifier 13 is applied in push-pull form with respect to earth by way of leads 85 to potentiometer 14 and rheostat 19.

The wiper 191 of the latter is earthed through a resistor 86 the value of which is small compared with the resistance of that pan of the rheostat which is normally in circuit. The distance of wiper 191 from the zero position is in direction dependence on the value of We' as controlled from component 20 in the manner already described. Accordingly the current through resistor 86, and hence the potential applied to amplifier 24, is directly proportional to the voltage output from amplifier 13 and inversely proportional to the quantity We The output signal from amplifier 24 is similarly delivered in push-pull over leads 87.

from which connections are made to energise potentiometers 25 and 34.

Similar arrangements are made for the pan of the computer which solves equation (ii).

Other valuable information may be obtained from the computer as follows.

The bearing (6 - a) in plan of the fighter from the target, by which is meant the angle between the projections on a horizontal plane of the line of fight of the target and of the sight line from the target to the fighter, see Fig 1 B, is readily obtainable since both 6 and a are represented in the computer. Where for example these quantities are represented by shaft rotations, as already described, the required difference may be obtained as shown in Fig. 5 in a simple analogue stage in which a differential gear 91 derives a shaft rotation proportional to the difference between the rotation of shaft 47, representing a and of the shaft from servo 12, representing û; the resultant movement is applied to any convenient form of display apparatus 92 on which the required bearing is indicated, This is itself a valuable item of information during an interception.

The height difference h. the time derivative of which is used to derive c as already explained, may itself be derived by multiplying the range R by the effective elevation angle E or (c + e + i) This may be achieved as shown in Fig. 6 in a simple analogue stage in which a linear potentiometer 93, centre-tapped to earth, is energised in push-pull over leads 87 from the R amplifier 24, the wiper 931 being controlled from thee, e, and i shafts 53, 46, and 51. The voltage of wiper 931 is then proportional to h. and may be applied to any convenient form of display 94 by which this quantity is indicated, The closing speed dR/dt is obtainable from the equation: dR/dt = U cos (6 - a) - V cos a = U cos 6 cos a + U sin 6 sin a - V cos a .. (x) As the right-hand side of this equation is similar in form to the U and V portion of equation (viii) the solution may conveniently be derived from the potentiometers 71 to 75 on which the latter expression is set up. All that is required is an additional wiper on each of potentiometers 74, 75, and 73 in the positive cosine, positive sine. and negative cosine locations respectively - see Fig. 7, in which the additional wipers are indicated at 7411,7511, and 7311. The combined outputs from the first two of these wipers is clearly proportional to U cos 6 cos a + U sin 6 sin a subtraction from this of the V cos a voltage on wiper 7311 gives the required output proportional to the closing speed, which may be displayed on some convenient instrument 95.

The derivation of the closing speed as just described enables a further item of important information to be derived - namely, the hypothetical time T to collision, that is, the time still to go before the fighter's position, projected on a horizontal plane, coincides with the target's position projected on the same p]ane. T at any given moment is equai to the range at that moment divided by the closing speed - in rotational form, T = R . dR/dt.

This may be evaluated as shown in Fig. 8.

Here a voltage proportional to R is applied to a dividing rheostat 96 the wiper 96' of which is controlled by a shaft 97. Shaft 97 is rotated in dependence on the voltage dR/dt. which may be derived as described with reference to Fig. 7 and converted to a shaft rotation by a servo 98.

Wiper 961 is earthed by a resistor 99 of low value compared with the normal value of the energised part of the rheostat and is connected by way of an amplifier 100 to a suitable display instrument 101, The dividing action of rheostat 96 is similar to that of rheostat 19 described with reference to Fig. 2, the wiper potential being maintained at a value proportional to R + dR/dt; this value of T is displayed on instrument 101.

T may alternatively be evalued in reliance on the fact that this information is usually required when the fighter is in the tail cone of the target and in consequence the angle 6 can be assumed to be zero. By flying level for the few moments required to compute T, and at a height difference sufficient to give We an appreciable value, the pilot can make c zeib also. Equation (thus becomes simplified to: Rwe - U(~ + i) + V(~ + i) = O ...(xi) Hence RVve = (U - V) (e + i) But under these conditions (U - V) = dR/dt; on substitution, therefore, we get: R + dR/dt, = T, = (e + )) + We The quantity (e + i) may readily be available as a combined voltage, as is indicated below: this voltage may therefore be applied to energise a dividing rheostat the wiper of which is controlled in dependence on We and so develops a potential proportional to T, the apparatus operating in a similar manner to that of Fig. 8.

Once T has been derived it need not be computed further. for its subsequent diminishing value may be indicated by a conventional clock.

thus allowing the pilot to pursue the attack without having to continue to fly level to zeroise the quantity c.

The apparatus of the above-described embodiments may, be modified in various waves within the scope of the invention. For example.

it IS sometimes more convenient to derive the quantities e and a as voltages from potentiometers actuated by the radar dlsh. rather than as shaft rotations. Similarly' the quantities c and i may more conveniently be derived as voltages. Two or more of such voltages may be combined in a unity-gain amplifier.

Alternatively, any such voltages may be convened singly or in combination to shafl rotations, in the usual servo manner. Thus the expression (e + e + j) occurring in equation (i may be obtained by adding the corresponding voltages in a unity-gain amplifier and converting the output to a shaft rotation by means of a simple servo system to control the wiper 61 1 of the U potentiometer 61 and phase reversing switch 18. Various other details of the analogue components of the computer may be modified in a similar manner.

Where more convenient. the computer may be energised from direct-current sources. the positive and negative algebraic signs being represented by the polarities of the electrical quantities rather than by their phases.

WHAT WE CLAIM IS: 1. Target-locating apparatus to be carried by a fighter aircraft for continuously deriving the unknown range R from the fighter to a target which is flying level at an airspeed U, as estimated by the pilot of the fighter. and at a different height from that of the fighter, including radar equipment for continuously deriving information as to the direction in elevation and azimuth of the target relative to the fighter. means for deriving from that equipment the elevational and azimuthal components We and w, respectively of the sight-line spin. means for driving the following additional information concerning the fighter, namely the true airspeed V, the elevational and azimuthal components e and a respectively of the angle between the sight line and the fore-and-aft line, the angle of incidence l between the fore-and-aft line and the flight line, and the climb angle c. the angles i and c being measured in the elevational plane. an electrical analogue computer for continuously deriving the said range R from the equations.

RWe-U)c+e+I)cos6+Vĕ+1')=0.. ...(i) RWa - Sin(6 - a) + Vsin a = 0 . ...(ii) where the second unknown quantity 6 represents the angle in plan between the courses of the fighter and the target, and means for supplying to the computer inputs which correspond to the quantities U. We u a. and said additional information. - - 2. Apparatus as claimed in claim ] wherein said computer includes a first group of stages for deriving an output signal proportional to the lefthand side of equation (X) multiplied by wa.

a second group of stages for deriving an output signal proportional to the left-hand side of equation (ii) multiplied by we a difference amplifier for deriving an error signal dependent on the difference between said output signals, servo means for controlling the value of 6 as supplied to both said groups of stages. and connections for applying the error signal to drive said ser 0 means in the sense to null the error signal said stages including components for deriving the value of R when the error signal is nulled.

3. Apparatus as claimed in claim 7 where the sense of U' is not known. wherein said connections for applying error signal include a reversing switch operable in response to changes in the sense of the quantity (e + e + i) to reverse the sense of the error signal. thereby ensuring that the error signal has the sense required for stability.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   derived as described with reference to Fig. 7 and converted to a shaft rotation by a servo 98.

   Wiper 961 is earthed by a resistor 99 of low value compared with the normal value of the energised part of the rheostat and is connected by way of an amplifier 100 to a suitable display instrument 101, The dividing action of rheostat 96 is similar to that of rheostat 19 described with reference to Fig. 2, the wiper potential being maintained at a value proportional to R + dR/dt; this value of T is displayed on instrument 101.

   T may alternatively be evalued in reliance on the fact that this information is usually required when the fighter is in the tail cone of the target and in consequence the angle 6 can be assumed to be zero. By flying level for the few moments required to compute T, and at a height difference sufficient to give We an appreciable value, the pilot can make c zeib also. Equation (thus becomes simplified to: Rwe - U(~ + i) + V(~ + i) = O ...(xi) Hence RVve = (U - V) (e + i) But under these conditions (U - V) = dR/dt; on substitution, therefore, we get: R + dR/dt, = T, = (e + )) + We The quantity (e + i) may readily be available as a combined voltage, as is indicated below: this voltage may therefore be applied to energise a dividing rheostat the wiper of which is controlled in dependence on We and so develops a potential proportional to T, the apparatus operating in a similar manner to that of Fig. 8.

   Once T has been derived it need not be computed further. for its subsequent diminishing value may be indicated by a conventional clock.

   thus allowing the pilot to pursue the attack without having to continue to fly level to zeroise the quantity c.

   The apparatus of the above-described embodiments may, be modified in various waves within the scope of the invention. For example.

   it IS sometimes more convenient to derive the quantities e and a as voltages from potentiometers actuated by the radar dlsh. rather than as shaft rotations. Similarly' the quantities c and i may more conveniently be derived as voltages. Two or more of such voltages may be combined in a unity-gain amplifier.

   Alternatively, any such voltages may be convened singly or in combination to shafl rotations, in the usual servo manner. Thus the expression (e + e + j) occurring in equation (i may be obtained by adding the corresponding voltages in a unity-gain amplifier and converting the output to a shaft rotation by means of a simple servo system to control the wiper 61 1 of the U potentiometer 61 and phase reversing switch 18. Various other details of the analogue components of the computer may be modified in a similar manner.

   Where more convenient. the computer may be energised from direct-current sources. the positive and negative algebraic signs being represented by the polarities of the electrical quantities rather than by their phases.

   WHAT WE CLAIM IS: 1. Target-locating apparatus to be carried by a fighter aircraft for continuously deriving the unknown range R from the fighter to a target which is flying level at an airspeed U, as estimated by the pilot of the fighter. and at a different height from that of the fighter, including radar equipment for continuously deriving information as to the direction in elevation and azimuth of the target relative to the fighter. means for deriving from that equipment the elevational and azimuthal components We and w, respectively of the sight-line spin. means for driving the following additional information concerning the fighter, namely the true airspeed V, the elevational and azimuthal components e and a respectively of the angle between the sight line and the fore-and-aft line, the angle of incidence l between the fore-and-aft line and the flight line, and the climb angle c. the angles i and c being measured in the elevational plane. an electrical analogue computer for continuously deriving the said range R from the equations.

   RWe-U)c+e+I)cos6+Vĕ+1')=0.. ...(i) RWa - Sin(6 - a) + Vsin a = 0 . ...(ii) where the second unknown quantity 6 represents the angle in plan between the courses of the fighter and the target, and means for supplying to the computer inputs which correspond to the quantities U. We u a. and said additional information. - -
2. 2. Apparatus as claimed in claim ] wherein said computer includes a first group of stages for deriving an output signal proportional to the lefthand side of equation (X) multiplied by wa.

   a second group of stages for deriving an output signal proportional to the left-hand side of equation (ii) multiplied by we a difference amplifier for deriving an error signal dependent on the difference between said output signals, servo means for controlling the value of 6 as supplied to both said groups of stages. and connections for applying the error signal to drive said ser 0 means in the sense to null the error signal said stages including components for deriving the value of R when the error signal is nulled.
3. 3. Apparatus as claimed in claim 7 where the sense of U' is not known. wherein said connections for applying error signal include a reversing switch operable in response to changes in the sense of the quantity (e + e + i) to reverse the sense of the error signal. thereby ensuring that the error signal has the sense required for stability.
4. 4. Apparatus as claimed in either claim 2 or

   claim 3 wherein each of said components for deriving R includes a high-gain amplifier, a stage for deriving a quotient output signal proportional to the left-hand side of equation (i) or equation (ii), as set up in the computer when the error signal is nulled, divided by We or as the case may be, connections for applying this quotient signal as input to said amplifier, and a feedback connection for applying the output signal from said amplifier negatively to the input thereof, thereby nulling said quotient signal.
5. 5. Apparatus as claimed in claim 4 wherein a changeover switching device is provided to enable R to be derived from either of said equations, said device being operable under the control of said servo means so that when 6 is in the region of zero degrees or 180 degrees R is derived from equation (i) and when 6 has other values R is derived from equation (ii).
6. 6. Apparatus as claimed in either claim 4 or claim 5 wherein each of said feedback connections includes a stage for deriving the product RWe or Rwa, as the case may be, for the derivation of the sald output signal from the appropriate one of said groups of stages.
7. 7. Apparatus as claimed in any of the preceding claims wherein an analogue stage is provided for deriving an indication of the bearing in plan of the fighter from the target from the difference between said quantities 6 and ss.
8. 8. Apparatus as claimed in any of the preceding claims wherein an analogue stage is provided for deriving an indication of the height difference between fighter and target from the product of the quantity R and the algebraic sum of the quantities, c, e. and i.
9. 9. Apparatus as claimed in any of the preceding claims wherein an analogue stage is provided for deriving an indication of the closing speed dR/dt of the fighter to the target from the equation dRldt = U cos (e - 3) - V cosa.
10. 10. Apparatus as claimed in claim 9 wherein an analogue stage is provided for deriving the time T to go to collision between fighter and target from the equation T = R + dR/dt.
11. I I. Apparatus as claimed in any of the preceding claims wherein arrangements are provided for representing the term cos 6 in equation (i) by unity at will.
12. 12. Target-locating apparatus substantially as hereinbefore described with reference to the accompanying drawings.