GB2033691A - Improvements in or relating to the detection of a projectile - Google Patents

Abstract

Apparatus for detecting the passing of a projectile and for obtaining information as to at least one of, the velocity of the projectile Vo, the distance of closest approach of the projectile Lo, the time of closest approach of the projectile To, or other information relating to the movement of the projectile, comprises at least one radar device 2 arranged to provide a curtain of radiation across the intended direction of movement of the projectile, each device including means responsive to reflection of radar signals off the projectile to generate a detection signal representative of the doppler shift of the reflected radar signal as a function of time, said detection signal varying in frequency as the projectile passes through the radar curtain, there being a processing means for processing said doppler shifted signal to provide the desired information. There are at least two such apparatus spaced across the trajectory of the projectile, such that, by processing the desired information it is possible as by triangulating to pin point the position of passing of the projectile relative to a datum. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the detection of a projectile This invention relates to the art of detecting the passing of an object and relates particularly but not exclusively to apparatus for obtaining information concerning the passing of a projectile by radar means.

In the art of detecting the position of the trajectory of a projectile such as a bullet, there have been many proposals for determining the location of the trajectory of the projectile. The proposals have been put forward with the object of determining the position of the trajectory without having physically to inspect a target impinged by the projectile. One such proposal envisages the use of light beams which are projected to form a curtain through which the bullet passes and by determining the anges of incidence of light beams reflected off a projectile at two spaced apart light detectors, one can ascertain the position of the trajectory of the projectile. Another prior proposed system has used a series of acoustical detectors which detect the presence of the projectile shock wave generated as the projectile moves through the air towards the target.By appropriately processing the information received by the detectors it is possible to determine the position of the trajectory of the projectile.

The two aforementioned prior art systems, whilst being generally acceptable, have certain problems which are dependent in the first case on the particular nature of light and the capability of generating a suitable signal on receipt of the reflections and in the second case on the problems of changes in the projectile shock wave caused by unstable atmospheric conditions atthe instant of passing of the projectile.

The present invention has been devised to provide apparatus for ascertaining information as to the passing of an object such as a projectile by radar means and one embodiment envisages processing of the information so as to ascertain the expected position of the projectile striking a target The radar reflective characteristics of the projectile are not influenced by its light reflective properties or by atmospheric conditions.

According to one aspect of the present invention there is provided an apparatus for detecting the passing of a projectile and for obtaining information as to at least one of, the velocity of the projectile, V,, the distance of closest approach of the projectile, Lo, the time of closest approach of the projectile To, or other information relating to the movement of the projectile, comprising at least one radar device arranged to provide a curtain of radiation across the intended direction of movement of the projectile, and including means responsive to reflection of radiation off the projectile to generate a detection signal representative of the doppler shift of the reflected radiation as a function of time, said detection signal vary ing in frequency as the projectile passes through the curtain of radiation, and a processing means for pro cessing said detection signal to provide the desired information.

According to another aspect of this invention there is provided an apparatus for detecting the passing of a projectile and for obtaining information as to at least one of, the distance of closest approach, Lo, of the projectile to a datum, the time taken, To, on or after entering a curtain of radiation to reach the point of closest approach to the datum, the velocity, Vo, of the projectile, comprising at least one radar device arranged to provide a curtain of radiation across the intended direction of movement of the projectile, means responsive to reflection of radar signal off the projectile to generate a detection signal representative of the doppler shift of the reflected radiation as a function of time of the reflected radiation, said detection signal varying in frequency as the projectile passes through the radar curtain and wherein the line of sight velocity Vt of the projectile, which is the component of the projectile velocity in the direction of the apparatus detected by the apparatus, is determined over a period of time in order to obtain at least three line of sight velocity values, the said values being processed by processing means to determine the distance of closest approach, the time of closest approach and the velocity of the projectile, by solving the equation:

Preferably said radiation curtain is projected in a plane which is generally perpendicular to the intended direction of the trajectory of the projectile.

Most preferably there are two or more spaced apart radar units which provide overlapping, but not necessarily coinciding, radar curtains, so that by comparing the information of one with the other such as by triangulating, the position of passing and/or trajectory of the projectory, or other information can be determined. Thus in a preferred embodiment of the invention there are two of said radar apparatus spaced apart along a datum line which extends across the intended direction of the trajectory of the projectile and there is further processing means for calculating from each from the information from each respective apparatus relating to distance of closest approach to that apparatus the position in two dimensions of passing of the projectile at the distance of closest approach to the datum.

Advantageously the apparatus may include a third such apparatus spaced in front of the said datum line and arranged with additional processing means for additionally enabling the determination of the trajectory of the projectile relative to a plane defined by the datums of all three apparatus and wherein there is even further processing means for calculating said trajectory from values of Lo orVo or To or combinations thereof from each of the apparatus.

Preferably said third apparatus is on a perpendicu lar line extending from the datum line between the first and second apparatus.

Advantageously the reflected radar signal is pro cessed by passing through a beat frequency determ ing unit to obtain the detection signal and the detection signal is in turn passed to memory device to store in the time values when the doppler shifted signal passes the zero axis thereby to store a representation of line of sight velocity Vt until required and wherein the memory output is connected with a maths processing means for calculating a signal representative of at least one of the velocity of the projectile Vo or a signal of the time of closest approach of the projectile To or a signal of the distance of closest approach Lo.

Conveniently the signals of the line of sight velocity of the projectile Vt are clocked off by a counter each time the doppler shift signal passes zero and the clock count is stored in said memory as representations of the line of sigh velocity Vt at the instant of each of said zero crossings.

The proposed apparatus makes use of a reasonable assumption in that for at least the closest ten or twenty metres from the radar unit, the projectile moves in a straight line with constant velocity through the radar curtain.

If the velocity of the projectile is 300 metres per second and a ten-GHZ signal is used for the radar unit, the maximum doppler frequency of the detection signal is 10Khz. With such a system the detection signal generated as consequence of the doppler shift is in the audible range and can be easily processed with known electronic circuit components. Suitable filters can remove any DC or low frequency components thereby disregarding radar reflections from stationary and slow moving objects so that only those reflections from the fast moving projectile can be studied.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated the invention will now be described by way of example with reference to the accompanying drawings in which: FIGURE 1 is a representation of a target in side perspective view with two radar units each forming a radiation pattern across the front of the target; FIGURE 2 is a front view of the target of Figure 1 showing the overlapping radiation patterns; FIGURE 3 is a side view of the arrangement shown in Figure 1; FIGURE 4 is a graph showing a typical signal generated by one of the detectors;; FIGURE 5 is a graph showing frequency verses time, of the frequency of the signals reflected by three projectiles passing through the radar radiation patterns at different line of sight distances, from the point of transmission of the radar signals, FIGURES 6 to 10 are diagrams used to explain mathematical calculations needed to extract the desired information from doppler shift signals generated at the radar units; FIGURE 11 is a block circuit diagram of electronic processing apparatus used with the embodiment of Figures 1 to 5; FIGURE 12 is a basic block circuit diagram of a tracking filter used in the radar units; FIGURE 13 is a signal diagram showing signals generated in the tracking filter of Figure 12; FIGURE 14 is a detailed circuit diagram of a filter which incorporated a L.P. filter, an A.G.C. circuit, a tracking filter and phase locked loop circuit;; FIGURE 15 is a block circuit of a zero crossing detector; FIGURE 16 is a detailed circuit of the tracking filter; FIGURE 17 is a block circuit of a central processing unit; FIGURE 18 is a block diagram ofsignals generated in central processing unit of Figure 17; and FIGURES 19 to 31 are diagrams used to explain mathematical calculations needed to extract the desired information concerning ascertainingthetra- jectory of the projectile.

Referring to Figure 1 two radar device 1,2 are used to determine the point at which a projectile passes (such as a bullet or shell fired at a target 3 in such a way that the bullet or shell passes over the radar devices 1, 2). A greater number of radar units may be used to increase the statistical accuracy of ascertaining the positions and or trajectory of the projectile.

Each radar device 1, 2 generates a radiation beam or pattern in the form of a narrow substantially upwardly directed curtain. The curtains lie in a common plane, each curtain being of triangular or sector shape, the apex of the sector being at the radar device, and the sectors being oriented so that the axes of the two sectors intersects. In Figure 1 it is assumed that the trajectory is perpendicular to the plane curtaining said radiation curtains.

The radiation beams are projected from radar units 1 and 2, such as manufactured by Varian Associates Inc., which operate in the 10-25 G. HZ range. The beams have up to 90" angle of divergence in a plane which is arranged parallel with a planar target 3. The radiation beams may be at any predetermined orientation to the target, provided that projectiles fired at the target will pass through the beam. When the beams are parallel with the target and the projectiles travel perpendicularly to the target the depth of each projected beam in the direction of the path of the projectile is determined by the angle of divergence of the projected beam i.e. the sum of the angles that the front and rear edges of the beam make with the vertical plane 11 on which the beam lies, and by the distance of the trajectory from the radar device. The angle of divergence of each beam is up to 30 . The wave shaping and guiding can be performed by known radar tech nology.

Figure 2 more clearly shows the overlapping nature of the radiation pattern generated by the radar devices 1,2. The patterns overlap one another so that for projectile 7 passing through the overlapping patterns there will be reflections of radiation back to each of the radar units 1 and 2.

The divergence of one of the projected beam in the direction of movement of the projectile is shown in Figure 3. It is to be noted from inspecting Figure 3 that the central plane 11 of the projected beam is substantially parallel with the plane of the face of the target 3 and approximately at right angles to the trajectory of the projectile 7. The central plane may however be at some angle to the target in the vertical plane. In some instances it may be desirable to projectthe beam at an inclined angle relative to the direction of movement of the projectile such that the point of closest travel from the trajectory to the detector is outside the beam. Such an embodiment is to be considered within the scope of the invention.

As a projectile such as a bulletorshell fired at the target 3 enters the front face of the beam the doppler frequency of the reflected signal is initially high since the projectile has a component of velocity directed towards the radar device, butthis doppler frequency decreases to zero at the point of closest travel of the projectile to the radar unit and as the projectile passes furtherthrough the beam the doppler frequency increases in the opposite direction, since the projectile now has a component of velocity directed away from the radar units. The point of closes travel will depend on the angle of incidence of the projectile with the target, and the position of the detector with respect to the target, and need not necessarily be a point on the central plane of the beam projected by the radar unit.The doppler shift is consequent on the fact that the projected beam radiates in an arc as diagrammatically shown in Figure 3 and the distance of the projectile from the radar unit when the projectile first enters the projected beam is further than when the projectile is at the point of closest travel to the detector. Figure 4 shows the general form of the expected signal which will be generated consequent on the doppler shift. The phase of the signal can be different to that shown.

Figure 5 shows a graph of frequency of doppler shift signal versus time of three different projectiles each travelling at the same velocity but passing at different distances, Lo, from the radar unit, each projectile travelling along a trajectory that is perpen dicular to the target.

Curve 1 represents the curve of the projectile where in the distance from the source is large.

Curve 2 is for a projectile which passes closer to the source than the projectile of Curve 1.

Similarly, Curve 3 is for a projectile which passes even closer to the source than the projectile of Curve 2.

It will be noted that the left-hand side and right hand side of the graph are mirror images about the point of closest travel for a projectile at a right angle to the central plane of a projected beam. Figure 4 likewise shows this by showing that the left-hand and right hand-sides are of identical shape. Thus, the trajectory of the projectile is perpendicular to the central axis of the projected radar beam. Thus, the ultimate speed of the bullet is determined by the frequency magnitude of the curve in Figure 5. For slower moving projectiles there will be a corres ponding lower frequency magnitude at which the curves asymtote.

By measuring the beat frequency as a function of time one can derive at least three parameters at each transmitter/detector as shown in Figure 6 as follows: 1. Velocity of Projectile (Vo) 2. Time of closes approach (To) which is the time, after entering the radar curtain, at which the projectile reaches the point of closest approach. This may be measured relative to the instant when the projectile first enters the radar beam or it may be measured relative to some other point such as zero crossing of the doppler shifted signal across the time axis (see Figure 4 which shows several zero crossing points) 3.Distance of closest approach (Lo) Referring now to Figures 6to 10 it isto be understood that the following symbols signify the following factors: P = Projectile t = transmitter/detector (i.e. radar unit 1 or 2) D = distance from point of closest approach Lt = distance to transmitter/detector Vt ='line of sight velocity, i.e. the component of the velocity of the bullet at any instant in the direction of a line of sight drawn between the bullet and the radar unit. Doppler beat frequency relates to the line of sight velocity Vt of the projectile by V '"C ' where C is the velocity of light and F0 is the frequency of the radiation generated by the radar units.

It is also noted that each cycle of beat represents a change in the distance to the detector of one half a wave length.

Figure 6 shows the relationship of the various functions.

The following relationships apply: D=Vo(To -t) Vt=VOCasO

And one can determine v (t) from the beat frequency f beat (t) as f (t)= beat (t) . C fo So we need to solve

for measurements of the mean velocity of successive intervals of 1 ms or so.

In order to solve this equation to find a solution of eitherVo, To and Lo, we need three measurements of Vt from each detector unit, as there are three variables in the equation. Thus by simply determining the line of sight velocities Vt at different times from the same detector represented for example by the zero crossings of the doppler shifted signals - see Fig. 4 - we can find the values of Vo, To and Lo.

With a large computer one would use regressional analysis etc; using simple computing the following anomies

Since (To - T) is a linearly varying function with time, so the right hand side of the equation represents a straight line with time.

This leads to the following processing sequence.

(1) From samples f beat (t) calculate v (t) (2) Choose a value for v0 and calculate values of 1 A(t) = [(Vo/V(t) - 1] (3) Fit a straight line Bt + C of slope B and offset C to the values of A(t) and determine the rms residuals of the fit (4) Choose another value for v0 and fit a line.

(5) The correct value for vO is then that value which produces the best fit to a straight line (i.e. smallest rms residual). The accuracy of the value of v0 is found from the sensitivity of the fit to the choice of vO.

(6) Now A(t) = Bt + C = vL (T0-T) Lo (7) So from the parameters B and C of the best fitting line

From these calculations we can determine Vo, Lo and To from each of the doppler signals at the respective transmitter/detectors.

Given 2 transmitter/detectors to determine projectile position.

CASE 1 When the value for To is essentially the same for both transmitter/detectors (see Figs. 7 and 8) Then projectile travelled at right angles to plane of the projected beams of the detectors.

ThenX2=D-X1 X12 + Y2 = L12 and X22 + Y2 = L22 So X12 - X22 = L12 - L22 X12(DX12) = L12 - L22 -D2 + 2DX1 = L12 - L22 D X, = D + L12 - L22 SoX1 2D andthenY=[L21 - X,2]( CASEII The values of To are different so the line of travel was not 90 . (See Fig. 9) We still want the co-ordinates of the point in the plane containing the detectors.

We still assume that the flight was parallel to the ground.

Then in plan view we calculate D= [D2 - v02 (t1 - t2)2]from triangle (See Fig. 10) i.e. Look in direction of travel and determine

So in the plane of 1 & 2 we have D.X' X] = Y- [L12 ~ X32] Assuming the projectile comes in at any angle (but still in a straight line) then at least 3 detectors are needed. This will be discussed later.

Figure 11 shows in very broad schematic form, the electronic circuitory for ascertaining information as to the projectile from one radar detector.

The detector DETI is shown connected with pre amplifier 100 and filter 101 which in turn apples the reflected signal to a zero crossing detector 103. The zero crossing detector 103 is in turn connected with a memory 104 which includes a central processinig unit C.P.U. The memory 104 is in turn used to store the doppler frequency information as a count each.

time the doppler shifted signal crosses the zero axis.

The memory 104 in turn is connected with computer 105 through an interface to determine information such as Vo Lo and To from the zero crossing times.

The computer 105 has a maths determining section 107 for performing this function. The information from Section 107 is fed to a further section 109 to determine the co-ordinates of the position of passing of a projectile when information from DET2 and DET3 when similarly processed to obtain values of theirVo Lo & o.

The particular signal processing involved can best be explained by considering the following problem.

Given that we have the doppler beat frequency (shown in Fig. 4) we wish to find the time intervals between each of the zero crossings.

Now referring to Fig. 11, we can see that doppler frequency at Point A is passed into the zero crossing detector 103 after Filter 101. When the first pulse from the zero crossing detector 103 occurs, the memory 104 is started. The next zero crossing pulse is then suitably counted into the memory 104.

Hence, we can say that the first address in memory contains an absolute time value T1. The next address in memory contains an incremental time value T2, not an absolute value. Hence, to obtain the time value of the second interval, we subtract (digitally) T1 from T2to obtainthe absolute time duration of the next interval.

The information stored in memory is periodically accessed on the data bus buy a C.A.T. computer.

In more detail, the Filter 101 is a tracking filter known generally in the radar arts and is arrnngedto.

track over a 2û0hz range at 10 khz, the maximum expected doppler frequency where the projectile entersthe radar curtain. The purpose of the Filter 101 is to enable the gain of the system to be reduced until such time as a projectile enters the radar curtain and then the gain can be increased over a narrow band 200hz which tracks along with the doppler frequency. Pre amp 100 includes athreshold detector, also known in the radar arts, which does not allow signals to pass until they have reached a predeter mined level.

A detailed description of the tracking filter 101 fol lows with reference to Figures 12, 13 and 14. Initially the signal from a Schottky diode detector in the receiver of the radar unit pre amp 100 is low pass filtered to eliminate false target signals.

The signal is then held at a constant signal level by an A.G.C. circuit (See Fig. 15). The tracking filter then maintains 'lock' on the signal A in the following manner. We have assumed for known velocity pro jectilesthatthe maximum doppler shift be 10khz and we have chosen that the bandwidth of the tracking filter 100 to be 200hz. The tracking filter 100 tracks on the principle that zero phase shift occurs through the low pass filter when resonance is reached.

A waveform diagram showing the operation of the tracking filter is shown in Figure 13 at the various signal points shown in Figure 12. Figure 14 outlines the operation of the tracking filter 100. We see that when the signals B and C are 90" out of phase, then the pulses at D on the exclusive OR 203 output occur equally about the 5 volt level and so the integrator 205 output is effectively 5V.

For signals nearly 1800 out of phase integrator 205 output is effectively 10V. For signals with nearly 0 phase shift the integrator 205 output is effectively 0 volt and so on. The integrator 205 output is applied to the gate of a FET shown in Figure 15 which has the effect of shifting the filter until the signals at B & C are 90" out of phase.

Finally in order to avoid noise spikes and hence eventual false zero crossings a phase locked loop is used to follow the signal at B. The phase locked loop 4046 is not responsive to noise variation due to the low bandwidth used and so by taking the zero crossing output from a voltage control oscillator the signal is effectively clear of spikes. (see Fig. 15) A detailed circuit diagram of the zero crossing detector 103 is shown in Figure 16.

The zero crossing filter operation is shown broadly in Fig. 15 and in detail in Fig. 16. The circuit is arranged such that at each zero crossing represented by the 0" 5 volt or 10 volt signals B - F shown in Figure 13 there will be an output pulse which is fed to a central processing unit (C.P.U.) in the memory 104.

The central processing unit (C.P.U.) is responsible for all of the data and address control and also for timing ofthe pulses needed.

The C.P.U. is shown in detail Fig. 17 and there are two modes given i.e. Read and Record. In the read mode the C.P.U. allows the C.A.T. computerto gain access to the address and data buses by enabling input/output buffers. In the record mode the zero crossing pulses are uninhibited and proceed to clock the address counter 7493 shown in Figure 17. The 7493 counter has been used with the criteria of using various combinations of divide by N outputs to control writing to RAM clock resetting and buffer control. The waveforms from the 7493 address counter are shown on the switching diagram of Figure 18.

Also we see that after a number of clock pulses the 7493 counter is reset by a flip-flop i.e. DM107. This is necessary to prepare the pulse synchronizing for the next time interval.

The timing involved is basically that when a zero crossing occurs the recorded count representing the time between zero crossings is latched and the counting with Ram is inhibited. The clock pulse of 7493 in the C.P.U. are used to clear the counters. At the start of the next zero crossing the address counter is incremented and the process repeats.

In orderto determine the trajectory of the projectile it is necessary to have at least three detectors A, B and C as described hereinafter. With three detectors A, B, and C we wish to find the x,y co-ordinates (xOIyO) and the angles a and p which define the objects straight line path. (See Figs. 19-22) The object moves towards increasing z (refer Fig.

20) Projecting on the z plane (Refer Fig. 21) Positive a implies it moves to lower values of Y Projecting onto the x z plane (Refer Fig. 22) Positive p implies object moves to lower values of x.

To solve, one must take pairs of detectors AB and BC to find the distance of closest approach to the x and y axes respectively and the angles 8 and + respectively which the path makes to the plane perpendicular to the x and y axes respectively.

PAIR A, B One cannot be sensitive to angles of rotation about the x axis when one measures only the lines of closest approach of the trajectory to A & B. Consider then the case when line of closest approach to the x axis is vertical in Fig. 23.

Base Line DAB Closest to A is LA attA Closest to B isLB attB Refer now to Fig. 17 DAB = vo2 (tB - tA)2 + r (tA2 ~ YAB2) < + (LB2 - vAB2) 2 DAB is a 4th order equation so solve iteratively i.e.

select values of YAB such that: (1) vo2(tB - tA)2 + [(LA2 - AB2)5' + (LB2 - AB2)2 DAB2 +, o For relationship of DAB,0, etc. refer to Fig. 18 Hence find YAB (2) sin = vo(tB - tA) DAB so have value and sign ofO

and so finally have found XAB.

PAIR B, C is similar with (See Figs. 26 and 27) DBC, LB, LC, tB, tC, and Vo known.

Find XBC for which: (1 ) vo2 (tB ~ tC)2 + [(LB2 ~ XBC2) + (LC2 - xBC2)2 - DBC2=0 Hence find XBC

We see that for A, B it is impossible to determine rotations about the x axis. Similarly for B, C about the z axis. So introduce angles a and p and find values for a and p which are consistent with all the measurements.

So take Fig. 20 and rotate a into pages. (See Fig.

28) In the plane of the trajectory, perpendicular to YAB one has: (See Fig. 29) So So we have Yo = Cos a (1) xO = XAB + YAB tan a tan o (2) For BC one has: (See Fig. 30) Edge view looking to +x axis i.e. arrow comes out of page at angle p.

In Plane of trajectory perpendicular to XBC (See Fig. 30) XBC x0 = Cosss (3) y0 = YBC + XBC tan ss tan (4) Solving this analytically, (1) to (4) are four equations in the four unknowns (XO, yO, p) To solve equations (1) to (4):

20 = XAB + tan 0 (Yo2 - YAB2)12 i.e. (20 - xAB)2 = tan2 H (V02 - YAtt2) (5) Similarly eliminate tan p to get: (Vo - BC)2 = tan2 a (x02 - xBC2) (6) (Y0 - YBC)a xo2 = XBC2 + tan # tan2 a and take +ve values of xO.

Substitute in tI

So square both sides and get a fourth order equation for Y0.

Equations (1) to (4) can be solved by an iterative procedure, which can be processed by the computer previously referred to with the following equations after suitable computer programming.

These iteratively solved for a and p by choosing a and calculating sec p and tan p and hence two values of p. Alter a until the two equations give the same value ofp. Given a and p, (1) and (3) simply gives (xo, Yo). With measurement errors, one will have to accept the values of a or p which best satisfies the pair of equations.

In order to reduce the time taken for processing of the signals when the trajectory is being determined, it may be preferable to use four or even more detectors thereby simplifying the mathematical computation and also increasing the statistical accuracy of the final values of the co-ordinates.

Claims (10)

1. An apparatus for detecting the passing of a projectile and for obtaining information as to at least one of, the velocity of the projectile, Vo, the distance of closest approach of the projectile, Lo, the time of closest approach of the projectile To, or other information relating to the movement of the projectile, comprising, at least one radar device arranged to provide a curtain of radiation across the intended direction of movement of the projectile, and including means responsive to reflection of radiation off the projectile to generate a detection signal responsive of the doppler shift of the reflected radiation as a function of time, said detection signal varying in frequency as the projectile passes through the curtain of radiation, and a processing means for processing said detection signal to provide the desired information.

2. An apparatus for detecting the passing of a projectile and for obtaining information as to at least one of, the distance of closest approach, Lo, of the projectile to a datum, the time taken, To, on or after entering a curtain of radiation to reach the point of closest approach to the datum, the velocity, Vo, of the projectile, comprising at least one radar device arranged to provide a curtain of radiation across the intended direction of movement of the projectile, means responsive to reflection of radar signals off the projectile to generate a detection signal representative of the doppler shift of the reflected radiation as a function of time, said detection signal varying in frequency as the projectile passes through the radar curtain and wherein the line of sight velocity Vt of the projectile, which is the component of the projectile velocity in the direction of the apparatus is determined over a period of time in order to obtain at least three line of sight velocity values, the said values being processed by processing means to determine the distance of closest approach, the time of closest approach and the velocity of the projectile, by solving the equation: Vo2 = (ToT) Vt = (V02 (To - T)2 = Lg2)f

3. An apparatus as claimed in claim 2 wherein said radiation curtain is projected in a plane which is generally perpendicular to the intended direction of the trajectory of the projectile.

4. An apparatus as claimed in any one of the preceding claims wherein there are two of said radar apparatus spaced apart along a datum line which extends across the intended direction of.the trajectory of the projectile and there is further processing means forcalculating from each from the information from each respective apparatus relating to distance of closest approach to that apparatus the position in two dimensions of passing of the projectile at the distance of closest approach to the datum.

5. An apparatus as claimed in claim 4 including a third such apparatus spaced in front of the said datum line and arranged with additional processing means for additionally enabling the determination of the trajectory of the projectile relativeto a plane defined by the datums of all three apparatus and wherein there is even further processing means for calculating said trajectory from values of Lo or Vo or To or combinations there from each of the apparatus.

6. An apparatus as claimed in claim 5 wherein said third apparatus is on a perpendicular line extending from the datum line between the first and second apparatus.

7. An apparatus as claimed in any one of the preceding claims wherein the reflected radar signal is processed by passing through a beat frequency determining unit to obtain the detection signal and the detection signal is in turn passed to memory device to store the time values when the doppler shifted signal passes zero axis thereby to store a representation of line of sight velocity Vt until required and wherein the memory output is connected with a maths processing means for calculating a signal representative of at least one of the velocity of the projectile Vo or a signal of the time of closest approach of the projectile To or a signal of the distance of closest approach Lo.

8. An apparatus as claimed in claim 7 wherein the signals of the line of sight velocity of the projectile Vt are clocked off by a counter each time the doppler shift signal passes zero and the clock count is stored in said memory as representations of the line of sight velocity Vt at the instant of each of said zero crossings.

9. An apparatus for detecting the passing of a projectile and for obtaining information as to at least one of, the velocity of the projectile Vo, the distance of closest approach of the projectile, Lo, the time of closest approach of the projectile To, or other information relating to the movement of the projectile, substantially as hereinbefore described with reference to the accompanying drawings.

10. Any novel feature or combination of features disclosed herein.