US3281841A

US3281841A - Signal system having improved oscilloscopic display means

Info

Publication number: US3281841A
Application number: US413830A
Authority: US
Inventors: Henri G P Forestier
Original assignee: Compagnie Francaise Thomson Houston SA
Current assignee: Compagnie Francaise Thomson Houston SA
Priority date: 1963-11-26
Filing date: 1964-11-25
Publication date: 1966-10-25
Anticipated expiration: 1983-10-25
Also published as: NL6413768A; GB1084034A; CH439419A; DE1272401B; NL144737B; FR1387128A; SE343147B

Description

Oct. 25, 1966 H. G. P. FORESTIER 3,231,841

SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANS Filed Nov. 25, 1964 5 Sheets-Sheet 2 nine pulses eLqhlf pulses nine pulses eight pulses A A A A E1 \f \f lllllll lllllll mum JL IL 1L JL 7 1 T0 T2 To 1- T0 T2 T9 LF LY Lr LY' Henri G. P Foresfier INVENTOR Oct. 25, 1966 H. G. P. FORESTIER SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANS Filed NOV- 25, 1964 5 Sheets-Sheet 5 Henri GP Foresfier I'NVENTOR.

1966 H. s. P. FORESTIER 3,281,841

SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANS Filed NOV. 25, 1964 5 Sheets-Sheet 4 28 V0 LTAGE GEARS I F44 PULSE GEN.

COMPARATOR VARIABLE 6P1 U RR 4 PHASE \r LOGIC C SHIFTER NTWK GP'Z PULSE \RADAR RCVR. PULSE GEN,

SPLITTER T CRT B 5 VOLTAGE CE CONTROL PULSE GEN 1 Lomc RT /1/" G52 urwx \RADAR PULSE GEN.) SPUTTER RcvR.

15 SAWTOOTH SINE-WAVE GEN. EA

KEY-PULSE GEN. CRT

Lro AND E2. EI|

KEY- PULSE GEN.

Henri GP Foresfier lNVENTOR r;

x {KM GENT Oct. 25, 1966 H. G. P. FORESTIER 3,281,341

SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANS Filed Nov. 25, 1964 5 Sheets-Sheet 5 AGENT.

3,281,841 SIGNAL SYSTEM HAVING IMPROVED OSCILLO- SCOPIC DISPLAY MEANS Henri G. P. Forestier, Paris, France, assignor to Compagnie Francaise Thomson-Houston, Paris, France, a French body corporate Filed Nov. 25, 1964, Ser. No. 413,830 Claims priority, application France, Nov. 26, 1963, 955,024 18 Claims. (Cl. 34313) This invention relates to systems utilizing oscilloscopic, or cathode-ray tube, devices for the display of repetitive signals, such as echo signals received from a target in a radar system.

In an oscilloscopic device an electron beam is caused to sweep the fluorescent screen of the device by applying suitable cyclically varying voltages to the deflection electrode means of the device, such as vertical and horizontal electrostatic deflection plates. Concurrently the intensity of the beam is controlled by applying to a beam control electrode, such as a Wehnelt grid, the electric pulses or signals to be displayed. These signals are then displayed on the screen as bright spots. In many applications, owing to the cyclic character of the scanning curve or raster swept out by the electron beam across the screen, coupled with the repetitive character of the signals displayed, the information conveyed by the positioning of the spots is non-univocal, and hence ambiguous. Thus, in radar Work where the signals to be displayed are echo signals reflected from a target in response to a continuous series of radar pulses transmitted towards the target, the display may present a continuous series of spots aligned in an array on the scanning raster, and indicating a series of target distances among which the true distance is not easily seen, the display becoming ambiguous. The need for such a repetitive and hence non-univocal display arises in present-day radar techniques owning to the very great range of target distances which a given radar system is required to monitor.

In many cases the non-univocal character of the display is of no practical consequence because the initial position of the target is known and the target may thereafter be tracked throughout its subsequent distance variations; that is, the information concerning true target distance is preserved through continuity. There are other cases, however, where the non-univocal character of the oscilloscopic display can lead to loss of information and consequent operational errors. One important such case occurs in connection with radar ranging systems of the socalled dual-transmission-rate type, In these systems, means are provided for transmitting the radar signals at several different repetition rates, switch-over from one rate to another being effected whenever the target distance approaches a value which is a whole multiple of the so-called critical distance associated with said one transmission rate. This critical distance is defined as the minimum distance at which echoes would be received at or about the same instant that a subsequent radar signal is being transmitted, i.e. critical distance= /2cT, where c is the velocity of electromagnetic waves and T is the reciprocal of the repetition rate or frequency of the transmitted radar pulses. When the target is situated at the critical distance or a multiple of it, the echoes tend to be confused or blanked out and operation is unsatisfactory, hence the utility of the dual-rate systems.

An example of such as dual-rate radar system, in which the switch-over between transmission rates is effected automatically in response to actual target distance and at an optimum instant avoiding loss of signal at the time of switch-over, is disclosed in my earlier co-pending application Ser. No. 337,352, filed on January 13, 1964.

In dual-rate radar systems of this general type the non- 3,281,841 Patented Get. 25, 1966 univocal character of the target-distance display has been found to produce difliculties. After the target has been successfully acquired and its true distance identified during the initial, so-called acquisition stage of the ranging procedure, and the system has been set to its automatic target-tracking mode, the information concerning true distance is preserved through continuity only so long as the transmission rate remains unaltered. However, at the first occurrence of automatic switch-over to the alternative transmission rate due to the targets approaching a multiple of the critical range, this information is liable to be lost and the acquisition procedure may then have to be performed all over again.

It is an object of this invention to provide a new type of oscilloscopic display for repetitive signal systems which will completely overcome the aforementioned difficulties inherent in the non-univocal or repetitive character of the display. Another object is to provide improved radio ranging systems, especially dual-rate radar ranging systems, which will be especially well-suited for the detection and ranging of remote, fast-moving targets, and will considerably simplify the procedures involved as well as reduce operational errors. A more specific object is to provide an improved automatic dual-rate radar-ranging system of the type disclosed and claimed in my co-pending patent application identified above, modified with a view to simplifying inter alia the performance of the target-acquisition stage of the procedure and to avoid loss of target during the subsequent, automatic tracking stage.

The above and further objects will appear as the disclosure proceeds.

In accordance with a basic aspect of the invention there is provided a signal system in which a first and a second signal train are produced at a first and a second repetition rate, respectively, said signal trains alternating for equal periods of time at a third rate which is the greatest common divisor of said first and second rates. Thus the signal trains coincide at regular intervals. Said signal trains are applied to a beam-control electrode, e.g. Wehnelt grid, of a cathode-ray tube. There is applied to the deflection means, e.g. plates, of the tube a cyclically varying voltage having a minor cycle period the reciprocal of said first repetition rate and having a major cycle period the reciprocal of said third repetition rate. The electron beam of the tube is thus made to sweep out a cyclic scanning curve (or raster) on the screen of the device which cyclic curve has a line repetition frequency equaling said first rate and has a scan repetition frequency equaling said third rate. Under these conditions, as will be shown hereinafter, the said signals will be displayed on the screen in alignment with two different loci or curves during said alternating periods of time. Where the signals to be displayed are radar echo signals from a common target or the like, the intersection of the two curves will provide an unambiguous indication of true target distance.

Exemplary embodiments of the invention will now be described with reference to the accompanying drawing. The description will make especial reference to cases where the invention is applied to a dual-rate radar system of the kind disclosed in the earlier copending application identified above, but it is to be understood that while this does constitute at present a preferred application of the invention, the usefulness of the invention is by no means limited thereto. In the drawing:

FIG. 1 is a functional diagram of part (the so-called keying section) of a dual-rate phase-shift follow-up radar system of the general type disclosed in my co-pending application, as modified in accordance with this invention;

FIG. 2 is a pulse timing diagram illustrating the timing of the two alternate trains of keying pulses and the uniform coincidence pulses, produced in the system of FIG. 1;

FIG. 3 is a diagram showing waveforms of beamdeflection volt-ages used in the system of FIG. 1 to produce spiral scanning in accordance with the invention;

FIG. 4 is a view of an oscilloscopic display provided in accordance with the invention when using a spiral raster;

FIG. 5 is a functional diagram of the same system as the one shown in FIG. 1, partly illustrating the so-called follow-up section and other details thereof;

FIG. 6 shows an osci-lloscopic display provided in accordance with the invention when using a sinusoidal raster;

FIG. 7 is a partial diagram showing how the system of FIG. 1 may be modified to provide the raster of FIG. 6;

FIG. 8 shows an oscilloscopic display provided in accordance with the invention when using a parallel-line raster.

As indicated, the invention is applicable to a dualpulse-rate radar tracking system provided with automatic switching means whereby the radar transmitter is switched from one keying frequency to another keying frequency whenever the distance of a target being tracked enters a range in which the echo from it would be received at about the same instant a subsequent radar signal is being transmit-ted, were the transmiter to continue sending at the former keying frequency.

A dual-rate radar system provided with automatic switching circuitry of this type was disclosed in my earlier co-pendin-g application Ser. No. 337,352, filed January 13, 1964. As there disclosed, means are provided for for generating two trains of keying pulses E1 and E2, at different frequencies so selected in relation to each other that the pulses in the respective trains intermittently coincide at periodic intervals. The pulses of only one of the two trains, Ell or E2, are applied as keying pulses to the radar transmitter at any given time in order to trigger the transmission of the radar pulses towards the target. The switch-over from one to the other keying-pulse train is performed automatically whenever the target enters a distance range in which continued transmission at the former pulse rate would prove unsatisfactory owing to near-coincidence between transmitted pulses and received echoes. Furthermore, means are provided whereby the effective switching action from the former keying-pulse train (say E1) to the other pulse train (say E2), will occur precisely at an instant at which the pulses of the two trains E1 and E2 coincide.

In other words, according to said co-pending application, whenever the tracking system of the radar senses that the target distance has entered a range of distances wherein continued transmission at the present pulse rate (say F1) would lead to unsatisfactory operations, said system issues a command signal, which states in efiect that prompt switchover to a different transmission pulse rate (F2) is in order. However, such switchover does not occur immediately. Logical circuitry is provided whereby the actual switchover to the alternative pulse trains (E2) awaits the occurrence of the next fol-lowing coincidence between the two pulse trains E1 and E2. At that instant, the keying-pulse train E1 is switched off from the keying input of the radar transmitter and the alternative pulse train E2 is switched on instead.

In accordance with the present invention, the procedure just described is modified, at least during the so-called target-acquisition stage of the radar tracking procedure, which is the preliminary stage of locating a designated target and presetting the tracking follow-up section of the radar system to enable automatic tracking of the target during the subsequent, tracking stage of said process.

During this preparatory acquisition stage, then, rather than switching the keying input to the transmitter between the two pulse trains E1 and E2 automatically in accordance with target distance as described above, the two pulse trains E1 and B2 are used in a. regular alternating sequence, the switching between the two trains being constantly effected every time the two pulse trains coincide.

As shown in FIG. 1, GEl and GE2 represent the two keying-pulse generators similarly designated in my copending application. Keying generator GE]. produces the keying-pulse train E1 at the first keying frequency or rate F1, and generator GEZ produces the keying-pulse train E2 at the second keying frequency or rate F2. As described in the copending application, the keying-pulse generators G131 and GEZ likewise produce, in the operation thereof, the two pulse trains Lrl and Lr2, which are at the same rates F1 and F2 as the respective keying-pulse trains E1 and E2, but are somewhat broader than they.

The pulse trains Lrl and Lr2 are applied to an AND- gate 9. The output of this AND-gate, therefore, is a pulse train Lrll, which is at the frequency F0, the greatest common divisor of F1 and F2. In other words, a pulse Lrtl appears at the output of AND-gate 9 every time the two pulse trains Lrl and Lr2 (or the two .pulse trains El and E2) coincide. The pulse train Lr0 from AND-gate 9 is applied (through a switch 32 closed at this time, and an OR-gate 34, which will both be later referred to in detail) to the common input of a bistable element 12, so as to switch it alternately between two states in each of which a respective one of its two outputs produces a voltage output. The outputs of the bistable element 12 are applied to respective AND-gates 13 and 13 whose other inputs receive the keying-pulse trains E1 and E2 respectively. The outputs from both AND-gates 13 and 13 are applied to an OR-gate 14 the output from which is applied to the keying input of radar transmitter RT. The components described above are contained in a logical network generally deseignated LN.

It will be evident that during those periods when bistable circuit 12 has its upper output energized the keying input to transmitter T receives E1 pulses (at repetition rate F1) from keying-pulse generator GEl, while during those alternate periods when bistable circuit 12 has its lower output energized the keying input to transmitter RT receives E2 pulses (at rate F2) from key-pulse generator GEZ. Since bistable element 12 is switched between its two states in step with the Lrt) pulses at the rate F0, which is the greatest common divisior of the pulse rates F1 and F2, it is seen that every time an LrO pulse occurs, the keying frequency applied to the radar transmitter RT is switched from value F1 to F2, or from F2 to F1, as the case may be.

As a result of this action the sequence of keying pulses applied to transmitter RT proceeds according to the time pattern shown in FIG. 2. In this figure it has been assumed that the alternate repetition frequencies F1 and F2 used are in the ratio Fl/F2=9/ 8. In the other words, the following relationships exist between the two keyingpulse rates F1 and F2 and the pulse rate F0 of the pulses Lr0 from AND-gate 9:

F0, as already indicated, being the greatest common divisor of F1 and F2.

The above assumption is in agreement with the conditions in one actual embodiment of the invention, wherein the following values were approximately used: F1=576; F2:512; hence F0=64.

The lower line in FIG. 2 indicates the Lr0 pulses produced from AND-gate 9, which are separated in time by equal intervals Ti) (with T 0:1/F0). The upper line in the figure indicates the keying pulses applied to radar transmitter RT. As shown, during a first T0 period the pulses applied by logic network LN to the transmitter are the E1 pulses at the rate F1. The LrO pulse appearing at the end of this first T0 period acts to switch the output from the pulse train E1 to the pulse E2 at the repetition rate F2. Thus during the second T0 period E2 pulses at repetition rate F2 are applied to the keying input of trans mitter RT. At the termination of this second T0 period the Lr0 pulse occurring at the output from AND-gate 9 acts to switch the logic-network output back from pulse train E2 to pulse train E1; and so on repeatedly. It will then be evident with the above numerical assumptions, that during one set of alternate T0 periods (herein the odd T0 periods) the radar transmitter will receive at its keying input a series of nine E1 pulses at the higher repetition frequency F1 and during the other set of alternate T0 periods (herein the even-numbered set) the transmitter will receive a series of eight E2 pulses at the lower repetition frequency F2. The period T2 separating a pair of the E2 pulses is, of course 9/8 the period T1 separating a pair of the E1 pulses.

In the numerical example referred to above, where Fl=576, F2:5l2 and F0=64, the time periods T0, T1 and T2 have the following values in milliseconds:

TO=15.6255 ms.; Tl=1.736 ms.; T2=l.953 ms.

Returning to FIG. 1, a cathode-ray tube CRT is schematically shown for the display of the radar signals. The tube CRT includes a control electrode, e.g. a Wehnelt grid, schematically indicated at CE, a pair of vertical deflection electrodes VD and a pair of horizontal deflection electrodes HD. Video signals from the radar receiver RR are applied in a conventional manner to the control electrode CE by way of connection through an amplifier EA. In order to display these video (echo) signals upon the screen of the cathode-ray tube, the voltages applied to the vertical and horizontal deflection electrodes VD and CD are controlled in accordance with this invention in the manner now to be described.

There is provided a vertical deflection modulator VM the output from which is applied by way of an amplifier VA to the vertical-deflection plates VD; and there is provided a horizontal-deflection modulator HM the output from which is applied by way of an amplifier HA to the horizontal-deflection plates HD. The deflection modulators VM and HM each have a

carrier input

16 and 17 respectively, which are both connected to receive the E1 pulses from keying generator GE1. It is noted however that the carrier input 16 to vertical-deflection modulator VM has a delay circuit 18 interposed therein which imparts a 90 delay to the E1 pulses applied to that modulator with respect to the E1 pulses applied to horizontal modulator HM.

Each of the deflection modulators VM, HM further has a modulating input connected to the output of a sawtooth generator SG. Generator SG has a control input 20 connected to receive Lr0 pulses from the output of AND- gate 9, so as to emit a linear sawtooth wave at each occurrence of an Lr0 pulse.

With the arrangement described, the voltages applied by amplifiers VA and HA to the vertical and horizontal deflection plates VD and HD of the cathode-ray tube will have the waveforms shown at UV and UH respectively in FIG. 3.

It will be seen that the horizontal deflection voltage UH (upper curve) is a waveform composed of a series of sawtooth cycles each having a duration T 0=1/F0 since F0 is the modulating-envelope frequency applied by sawtooth generator SG to the modulating input of modulator HM. Each sawtooth cycle is composed of a pseudo-sine-curve of linearly increasing amplitude with the period of each pseudosinusoid being given by Tl=1/Fl since F1 is the carrier frequency applied to input 17 of modulator HM. The vertical deflection voltage UV (lower curve) is a waveform of identical shape but displaced in time one fourth the minor period T1 with respect to the horizontal deflection voltage UH.

Under these conditions the trace of the electron beam in the cathode-ray tube CRT when subjected to the joint action of the two deflection voltages UV and UH will sweep out on the screen of the tube an Archimedean spiral S as indicated in FIG. 4. Each scanning cycle, i.e. the time period required for the spot to sweep out the complete multi-turn spiral arc and return to its initial point 0 is the major period T0=1/F0 (about 15.6 milliseconds in the above-mentioned example). The pitch (or line repetition period) of the spiral, i.e. the time period required for the spot to travel from any point of the spiral to the corresponding point on the immediately adjacent turn of the spiral situated on the same radial line such as OR, is the minor period Tl=1/Fl (about 1.7 ms. in the practical example).

It will be understood that the spiral scanning pattern just described, wherein the scanning cycle frequency is F0 and the line repetition or pitch frequency is F1, is used according to the invention both when the radar system is transmitting at the repetition rate F1 (keying pulses E1) and when transmitting at the repetition rate F2 (keying pulses E2). Consider first those periods, the odd-numbered T0 periods in FIG. 2, when the system transmitter is using the keying pulses E1 (repetition rate F1). Then successive echo signals received from the target (assumed to be stationary) will appear as luminous spots a, a, a" etc. on the spiral raster S displayed on the radar screen, all aligned along a common radius such as 0R. This is evident when one considers that as each spot on the screen is produced by a video signal applied to control electrode CE over an amplifier EA connected to the receiver output 15, when the target is stationary these video signals occur with the same repetition frequency as the transmission repetition frequency F1.

However, during those periods, i.e. the even-numbered T0 periods in FIG. 2, when the system transmitter is using the keying pulses E2 at the repetition rate F2, then the video signals are applied through amplifier EA with this same repetition rate F2, different from the scanning frequency which is still F1, so that the spots b, b, b etc. indicative of successive echoes from the target will now line up along a spiral are 2, which differs from the scanning spiral S.

This can be established mathematically as follows.

Let us first determine the polar equation of the scanning spiral S swept by the electron beam during each scan cycle T0. The equations for the horizontal and vertical deflecting voltages (FIG. 3) can be respectively written as follows:

t y UV: (rO-I-H cos t T-1*O+H- -TO+H-FO-t Eliminating the parameter t:

H T1 770+Z'fi6 Equation 4 is the equation of an Archimedean spiral S in which the spiral pitch, i.e. the radial distance between corresponding points of adjacent turns such as the points a, (1, etc. marked out on radius OR in FIG. 3, is (T 1/TO)H. During those periods when the system is operating at keying frequency F1 so that consecutive echo signals from a stationary target are spaced apart by the constant period T1, if a first of these echoes occurs at a 7 time 1'1 it will be displayed as a spot (a) having the polar angle 1=21rt1/ T 1, and the next following echo will occur at the time t1=t'1+T1 and will be displayed as a spot (a') having the polar angle The two polar angles are seen to differ by the quantity 010'l=21r, and hence the two consecutive echoes are positioned on a common radius O-R, as earlier stated.

Next consider the case where the transmitter is using the key pulses E2 at the repetition frequency F2, i.e. during the even-numbered T0 periods in FIG. 2. The consecutive echoes from the stationary target are now arriving at intervals of T2 (wherein T2 T1). A first of these echoes appearing at a time t'2 produces a spot on the scanning spiral S say at b, having the polar angle The next echo occurs at time (t'Z-l-T 2) and produces a spot b having the polar angle t 2 T2 T1 The difference between the two polar angles as The corresponding difference between the vector radii of the two points b, b, as deduced from the second Equation 3 is Ar=H(T2/TO).

The above values of A0 and Ar remain constant when applied to any two consecutive points such as b and b; b and b; and so forth. Hence these points are all laid out along a spiral 2 whose equation can readily be obtained by noting that the dilference Aqb between the polar angles of any two consecutive points as measured along the spiral 2 is equal to the difference A0 between the polar angles of the same two points as measured along the spiral S, minus 360". That is Spiral 2 is therefore such that whenever the polar angle is increased by the vector radius of the spiral 2 increases by A =H T 2/ T0) The equation for such a spiral, as can be instantly verified, must be of the form l 2M0 T2- T1) This can be rewritten P=Z fi (o) or, putting Fl /F0=a and F2/F0=b,

H 1 a m (6) In the numerical example considered above, where a=9 and b=8, Equation 6 becomes H P' (s o) (7) Such is the equation of this spiral 2 shown in FIG. 4.

In the above Equations 5, 6 and 7, the quantity 4m depends on the target distance.

The principal results of the above mathematical analysis can be summarized as follows:

When in accordance with this invention two different keying repetition frequencies (F1, F2) are used alternately for radar transmission and a spiral scanning raster is used for displaying the echo signals on the radar screen,

8 said spiral raster having a line repetition frequency constantly equal to one (P1) of said alternate keying frequencies and a scan recurrence frequency constantly equal to the greatest common divisor (P0) of both keying frequencies used, then:

(1) during those periods when said .one keying frequency (F1) is being used for transmission, the echo signals from a stationary target all line up on a common radial line; and

(2) During those alternate periods when the other keying frequency (F2) is being used for transmission, the echo signals from the stationary target all line up on a spiral diiferent from (and radially expanded with respect to) the scanning spiral at the intersections of said second spiral with the successive turns of the first, or scanning, spiral.

At each instant of switchover from one to the other of the two keying frequencies F1 and F2, i.e. at the termination of each of the successive T0 periods (see FIG. 2), the position of the spot formed by the echo from the common target must necessarily be the same whether given by the one or the other keying frequency. Hence at each instant of switchover the echo spot is positioned at the intersection of radius OR (the locus of spots given by the keying frequency F1) and spiral Z (the locus of spots as given by keying frequency F2). This partic ular spot, therefore, appears as a double spot, i.e. a spot of enlarged diameter, as indicated at A, and is thus readily distinguishable from the neighboring spots.

In this manner the ambiguity which would otherwise be present as to the particular spot representing actual target position is avoided.

It will be understood that the type of ambiguity which the system of the invention serves to avoid is essentially due to the inherently repetitive character of the scanning curve and of the signals. Thus, if we consider a radar system transmitting continuously at a keying repetition frequency of F1, and a display using a spiral scanning raster at the same line repetition frequency of F1 so as to provide the spiral S as in FIG. 4, then a given target at a distance of x kilometers will be displayed as an array of spots corresponding to distances differing from one another by quantities of /2 c/Fl kilometers, that is, in the above example, quantities of 260.4 kilometers. From an observation of the radar screen it will therefore be impossible to say whether the target distance is x, or xi260.4, or xi520.8, or xi78l.2 km., etc. When such a scanning scheme is used with a single-frequency radar system the above uncertainty is, usually, of no consequence, because once the automatic tracking follow-up section of the radar system has been set manually to track a designated target (e.g. by the phase-shift follow-up method described in the copending application referred to above), it will continue automatically to keep track, there being no reason for the system to lose track of the target at any time. On the other hand, in the case of a system using several different transmission keying frequencies, the automatic tracking section of the system would tend to lose track of the target at the instant of switchover from one particular keying-pulse train i.e. the one used at the time of the manual setting of said tracking section, to the other keying-pulse train should the target enter a distance range requiring such switchover as explained in the copending application. The display system described herein was primarily designed to overcome this difliculty.

It will be understood from the foregoing that during the initial, so-called acquisition stage of operation of the improved radar system, in which stage the transmission keying rate is continually alternating between the two values F1 and F2 at intervals of T0 as described above and shown in FIG. 2, the true distance of the target is made known unambiguously to the radar operator as the particular distance indicated by the enlarged (doublepoint) spot A, and no other of the radial array of spots 9 a, a, a", appearing on the screen. The operator can then proceed to set the tracking section of the system manually so that it will track the unambiguously determined target, this manual setting operation being accomplished through the following means forming part of this invention.

The control electrode CE of the cathode ray tube CRT (see FIG. 5) has applied to it over a connection 22a socalled marker pulse which is a pulse recurring at the repetition frequency F0, the same as the repetition frequency of the Lri) pulses mentioned above. These marker pulses however are derived not from the AND gate 9 producing said Lrtl pulses, but from a similar AND gate (not shown) forming part of the selector logic network L in the follow-up section of the system, here schematically shown. As fully disclosed in the afore-mentioned copending application, the said logic network L produces a train of pulses called L'c in said application, which repeat at the repetition rate F0, i.e. the greatest common divisor of the keying rates F1 and F2, but which are variably delayed with respect to the similar pulses (herein called Lrtl) produced in the keying section of the radar system. The delay is produced through the action of a variable phase shifter schematically shown herein and fully described in the copending earlier application. In accordance with the present invention, the said delayed pulses L'c at the repetition rate F are used to provide marker pulses for the display, and are for this purpose passed through a marker-pulse amplifier MA and through an elongator-and-splitter circuit PS, of any suitable conventional design, such that the output of the circuit PS, when applied by line 22 through amplifier MA to the control electrode CE of the cathode-ray tube, will appear on the screen as a split, elongated streak or marker index shown at MI in FIG. 4. Since the marker pulse recurs at repetition rate Ft), the resulting marker index MI is displayed at a particular position along the spiral S and this particular position is determined by the variable delay imparted to the L'c pulses by the variable phase shifter g5. Thus, the radar operator, through selective adjustment of the phase shifter 4 is able to shift the marker index MI along the sweep spiral S until it frames or straddles the spot A indicative of true target distance, as indicated in dotted lines at MI in FIG. 4. To enable this adjustment, the tracking follow-up servo-motor M (FIG.

), which as described in the copending application normally serves to operate the phase shifter under control of the error voltage supplied from time-comparator device C, is here shown as beiing manually operable. For this purpose a reverser switch 26 is interposed in the connection 24- from comparator C to motor M. The switch 26 when moved from its full-line position (the normal position during automatic tracking mode of operation of the system) to its dotted-line position during the initial acquisition mode of system operation, connects the motor input to an auxiliary voltage source AS through a potentiometer 28. In this position of the switch 26, manual operation of the potentiometer control knob 39 will actuate motor M to modify, through gearbox B, the mechanical setting of phase shifter 1; and thereby shift the marker index MI along the spiral scan curve S on the radar screen. After the operator has rotated knob 36 so that the marker index MI has framed the true-distance spot A as shown at MI, he moves the switch 26 to its automatic-tracking position shown in full lines. At this time the phase shift imparted by the phase shifter to the L'c pulses, and simultaneously imparted thereby to the follow-up pulse trains P1 and P2 generated by the follow-up pulse generators GP1 and GPZ, exactly equals the time required by the radar signals to travel from transmitter RT to the target and for the echo (or response) signals to travel back from the target to receiver RR. Hence, as explained in the copending application, the follow-up pulses P or P, as the case may be, applied by follow-up logic L to comparator C coincide in time with the echo signals applied to the comparator from receiver RR, and so long as this condition obtains the follow-up servomotor M remains stationary. In case of a discrepancy between the comparator inputs due to target movement or system disturbance, the comparator applies an output voltage to motor M causing the motor to readjust the setting of phase shifter until a new equilibrium position has been reached. Automatic tracking of the target thus proceeds generally as described in the copending application.

It will be understood that during this automatic tracking stage of the procedure, the fixedly alternating switching action between the keying-pulse trains E1 and E2 at the repetition frequencies F1 and F2, as explained above with reference to FIG. 2 and as used in the initial acquisition stage, is no longer effected. Instead, the switching between the two keying frequencies is now automatically commenced under control of target distance in the manner disclosed in the copending application. That is, keying-pulse trains at one rate, say F1, would continue to be applied by network LN to the keying input of transmitter RT (and follow-up pulse trains P1 at the same rate would continue to be applied by follow-up network L to comparator C) for as long as the target does not approach a distance which is an integral multiple of the critical distance c/2F1, at which time the readings would become unreliable. When the target does enter such a critical distance range, the keying frequency is switched to the other of the two available values, F2, both in the keying section and in the follow-up section of the system, all as explained in the copending application.

In this connection it is important to note that the logic network here designated LN and described with reference to FIG. 1 as performing a basic function of the present invention, namely the continual, uniformly-timed switching between the alternate pulse trains E1 and E2 at equal T0 intervals, has considerable circuitry in common with the selector logic network called L in the copending application and shown in detail in FIG. 4 of that application. For convenience, the elements common to the two networks are designated by the same numerals in both figures.

In actual practice, in cases where the present invention is applied to a radar system of the type disclosed in the prior application and as described above, there would be used a common logical network capable of being switched between the two circuit conditions respectively shown in FIG. 1 of the present application and FIG. 4 of the prior one.

Accordingly, the binary circuit 12 is shown in FIG. 1 as having its input alternatively controllable, by way of OR-gate 34 earlier referred to, through the pair of automatic-

control connections

36, 38 which may be regarded as connected to the outputs of the respective AND gates called 11 and H in FIG. 4 of the copending application. When the switches 40 interposed in the connections 36 and 33 are open, and the switch 32 interposed in the connection from AND gate 9 to OR gate 34 is closed, then the logic network LN operates, in the manner described above in detail, to cause regular alternation between the two states of binary circuit 12, and hence between the two pulse trains E1 and E2 applied to the keying input of transmitter RT, at the rate of the L0 pulses. When on the other hand switch 32 is open and switches 40 are closed, the binary circuit 12 is switched between its two states in dependency on target distance, and the switching between the two keying-pulse trains E1 and E2 is similarly made dependent on target distance, in accordance with the operating mode described in the co-pending application.

The switches 32 and 49 here shown for clarity as separate, ganged switches may in actual practice be replaced by a single switch performing an equivalent function. The operation of the switching means 32-40 is preferably ganged with the operation of the manual switch 26 (FIG.

), as indicated by the mechanical connection 44 shown in both FIGS. 1 and 5, so that the radar operator, on moving switch 26 from its full-line position to its dottedline position on completion of the target-acquisition stage of the procedure described above, simultaneously switches the network LN from the circuit condition shown in FIG. 1 hereof to a condition similar to that shown in FIG. 4 of the earlier copending application, in order to provide for automatic interchange between the keyingpulse trains under control of target distance during the automatic tracking stage of the procedure.

In FIG. 5, the deflection-voltage control means shown in detail in FIG. 1 and described in connection with that figure are schematically shown as the block DC.

As stated earlier, in a display system according to this invention the echo spot indicative of the true distance of a target appears in the form of an enlarged-diameter spot A by virtue of being the geometric intersection of the two echo loci OR and 2. While such an enlarged-diameter double spot will generally be found sufficient to permit the radar operator to pick out the true echo without any hesitation from among the neighboring spots of lesser diameter and/ or lesser brightness, the invention contemplates the provision of means for rendering such recognition even easier and more positive.

As shown in FIG. 1, the lower output of binary element 12, i.e., that output thereof which is energized during those alternate Ti} periods when E2 keying pulses are being utilized, is applied to the input of a ringing oscillator Rt). When energized, oscillator R0 produces a small-amplitude output wave at a frequency which preferably is approximately the reciprocal of the pulse width of the radar pulses used in the system. The output from oscillator R0 is connected to the output of vertical-deflection amplifier VA through a 90-ph ase shifter or delay device, and is applied to the output of horizontal-deflection amplifier I-IA directly.

As a result of this arrangement, during those alternate T0 periods when the keying pulses E2 at repetition rate F2 are being used to key the transmitter RT, a smallamplitude, high-frequency wobble voltage, produced by ringing oscillator R0, is superimposed over the normal spiral deflecting voltages applied to the vertical and horizontal deflector plates of the cathode-ray tube, so that the echo spots displayed during those T0 periods, which as earlier described are arrayed along the spinal 2, appear as open circles rather than points (see FIG. 4). The particular echo which lies on the intersection with radius OR and hence represents true target distance, appears as a dotted circle and thus provides a clear and unmistaketable indication of true target distance.

The improved radar scanning and display system of the invention is susceptible of a number of embodiments and modifications other than those disclosed. One especially important class of modifications involves the type of scanning raster used, which may be other than the spiral ras-ters heretofore considered. As one example of such a modification, FIG. 6 illustrates the type of display obtained with the system of the invention when using a sinusoidal rather than a spiral raster. Here the cyclic sweeping or scanning curve 5, instead of being an Archimedean spiral as the similarly designated curve in FIG. 4, is a sine-curve. As before, the repetition frequency of this curve, i.e. the frequency at which the entire curve S is repeatedly swept out by the electron beam across the cathode-ray screen, is F0, the greatest common divisor of the two keying pulse rates F1 and F2, while the dine repetition frequency, i.e., the frequency at which corresponding points such as a, a, a" of consecutive sine cycles are scanned by the beam, in one of the two keying frequencies used, herein F1.

A sinusoidal scanning raster of this kind can be obtained by the means shown in the partial diagram of FIG. 7 in which components corresponding to components presentin FIG. 1 are similarly designated. As shown,

t2 the horizontal-deflection amplifier HA is fed with a sinewave voltage at the frequency Fl, supplied by sine-wave generator SW whose input receives the E1 pulses from keying-pulse generator GEll. The vertical-deflection amplifier VA is fed with a linear sawtooth wave having the cycle repetition frequency F0, the greatest common divisor of the keying frequencies F1 and F2, as derived from sawtooth generator SG having its input connected to the output of AND-gate 9.

With such an arrangement, the equations for the horizontal and vertical deflection voltages can be written as follows:

L 1/ l/o (8) Equations 8 are the parametric Cartesian equations for the scanning curve S. The quantities x and y are constants of the sawtooth generator SG and sinewave generator SW and define the boundaries of the scanning raster as shown in FIG. 6.

In those alternate T0 periods when the keying pulses at frequency F1 are being used, it is obvious that the echo spots a, a, a, a' are all aligned on a common line RR parallel to the y axis as shown. In those other alternate Ttl periods when the keying pulses at frequency F2 are being used, it can be shown that the echo spots b, b, b", b', are all arrayed on a sine curve 2 the Cartesian equation of which can be written 2 l 5 x sin 1r(a b)y (9) where a and b have the meanings .precedingly given and no is determined by the actual target distance. When target distance varies the sine curve 2 is bodily displaced parallel to the y coordinate axis, just as in FIG. 4 the spiral E was in the same circumstances bodily rotated. It will be noted that the diflerence (a-b) between the ratios of the respective keying frequencies to their common divisor frequency, F0, is according to the invention preferably selected equal to unity, as in the numerical example previously considered where a=9 and 12:8, in order that the differential sweep curve herein called 21 shall present a minimum number of intersections with any line such as O-R (FIG. 4) or R'-R (FIG. 6) over the extent of the major period T0.

The above results can be easily generalized to the broad case of a periodic curve of any character whatever being used as the cyclic scanning raster, provided the line repetition frequency of the curve is made equal to one (Fl) of the alternate keying frequencies used for radar transmission according to the invention, and the scan recurrence frequency of the curve is made equal to the other (F2) of said alternate keying frequencies. It can then be stated, for this broad case, that:

(1) During those periods when one keying frequency (F1) is being used for transmission, the echo signals from a stationary target all line up on a common line parallel to one coordinate (the repetitive coordinate) axis of the scanning raster; while (2) During those alternate periods when the other keying frequency (F2) is being used, the echo signals are all arrayed upon a differential curve of similar nature to that of the scanning curve, but expanded with respect to the scanning curve in a direction parallel to the aforementioned repetitive coordinate axis, at the intersections of said second curve with the firs-t, i.e. scanning curve; and consequently (3) The unique, or quasi-unique, intersection of the expanded differential curve with the said common line parallel to the repetitive coordinate taxis will provide an unambiguous indication of the ture target distance.

As an additional example of a type of oscillographic display with which the invention is usable, FIG. 8 shows a parallel-line scanning curve or raster. In this embodiment the scanning curve swept out by the electron beam at the scan recurrence rate F6 is a family of parallel lines S, inclined to the x-axis of the oscilloscope screen. The line repetition frequency is the keying rate F1. During those T periods where the keying-pulse rate F1 is being used, the echo spots are aligned as at a, a, a along a line R'R parallel to the y axis. During the alternate Tl) periods when the pulse rate F2 is used, the echo spots are aligned as at b, b, b" along a line 2. The intersection A of 2 with R-R provides the unique indication of true target distance. The means for developing deflection voltages capable of providing a display of the type shown in FIG. 8 will be readily conceived by those familiar with the art from the explanations previously given herein.

Clearly, in each of the modified embodiments of the invention shown in FIGS. 6, 7 and in FIG. 8, wobble.- voltage means similar to those described in connection with the first embodiment may be provided in order to display circles around the spots of one of the arrays, if this is desired.

It should be understood that the novel method of oscilloscopic display provided in accordance with the invention, while being of considerable value when applied to dual-rate radar systems such as the automatic dual-rate, phase-shift follow-up system specifically described herein as an exemplary embodiment of the invention, is susceptible of many other applications, such as in monitoring a chain of separate radar stations transmitting at different frequencies, as well as in other cases where repetitive signals are to be displayed. While in the embodiments described both pulse rates F1 and F2 were inherently available in the signal system with which the oscilloscopic display systems is associated, it should be understood that in other applications of the invention only one such pulse rate (say F1) may be present in the signal system, in which case the companion pulse rate (F2), as well as the coincidence pulse rate (F0) would be developed especially for the purposes of the invention.

It should also be observed that whereas in the examples here disclosed the higher of the two pulse rates, termed F1, was used as the line repetition rate of the scanning curves S, this is not essential, since the lower of the two pulse rates may Well be used for that purpose. The only difference would be that the differential curve 2 would be reversed in position with respect to the positions shown.

What I claim is:

1. A signal system comprising means for producing a first signal train at a first repetition rate;

means for producing a second signal train at a second repetition rate;

means for alternately enabling said signal trains for equal periods of time at a third repetition rate which is the greatest common divisor of said first and second rates;

an oscilloscopic display device including electron-beamproducing means, deflection electrode means, beamcontrol electrode means and a screen; means for applying said first and second signal trains to the control electrode means of the display device;

means for developing a cyclically varying voltage having a minor cycle period the reciprocal of said first repetition rate and a major cycle period the reciprocal of said third repetition rate; and

means for applying said voltage to the deflection electrode means whereby said beam will sweep out on the screen a cyclic scanning curve having a line repetition frequency equaling said first rate and having a scan repetition frequency equaling said third rate, and

whereby said first signals will be displayed on a linear first locus composed of the corresponding points of successive cycles of said cycle scanning curve while said second signals will be displayed on a second locus comprising another curve intersecting said linear locus.

2. The system claimed in claim 1, including means operative during those alternate periods of time when said second signals are produced for applying to said deflection electrode means a relatively high-frequency, small-amplitude additional voltage whereby to dilferentiate the display of said second signals from the display of said first signals.

3. A signal system comprising means for producing a first signal train at a first repetition rate;

means for producing a second signal train at a second repetition rate;

means for alternately enabling said signal trains for equal periods of time at a third repetition rate the greatest common divisor of said first and second rates;

means for developing cyclically varying spiral-deflection voltages having a minor cycle period the reciprocal of said first repetition rate and a major cycle period the reciprocal of said third repetition rate; and means for applying said voltages to the deflection electrode means whereby said beam will sweep out on the screen a cyclic spiral scanning curve having a line repetition frequency equalling said first rate and having a scan repetion frequency equalling said third rate, and 7 whereby said first signals will be displayed as the intersections of successive turns of said spiral scanning curve with a common radial line of the screen while said second signals will be displayed as the intersections of successive turns of said spiral scanning curve with another spiral curve expanded with respect to the scanning spiral.

4. The system claimed in claim 1, wherein said cyclically varying voltage includes a vertical and a horizontal deflection-voltage component, said voltage components being cyclically varying in accordance with said minor and major cycles respectively, where-by said scanning curve is a curve cyclically repetitive along one Cartesian coordinate, said first locus is a line parallel to said one repetitive coordinate and said second locus comprises another curve generally similar to said scanning curve but expanded with respect thereto in a direction parallel to said one coordinate.

5'. The system claimed in claim 1, including means for applying to said deflection electrode means a marker signal train at a repetition rate equaling said third rate, thereby to display said marker signal train as a marker index on the screen of the device, and means for adjusting the time relationship of said marker signal train with respect to said first and second signal trains whereby said marker index may be brought into coincidence with the intersection of said loci.

6. A signal system comprising means for generating a first train of control signals at a first repetition rate;

means for generating a second .train of control signals at a second repetition rate; means for deriving from said first and second signal trains a third train of signals at a third repetition rate the greatest common division of said first two rates;

means controlled by said third signal train for automatically switching between said first two signal trains whereby said signal trains are enabled during respective periods of time alternating at said third repetition rate;

means for deriving from said first and second controlsignal trains a first and a second train of resulting signals having the same repetition rates as said first and second control-signal trains respectively but time displaced with respect thereto;

an oscilloscopic display device including electron-beamproducing means, deflection electrode means, control electrode means and a screen;

means for applying said resulting-signal trains to said control electrode means of said device for display on said screen; means for developing at least one cyclically varying voltage having a minor cycle period the reciprocal of said first repetition rate and a major cycle period the reciprocal of said third repetition rate; and

means for applying said voltage to the electrode means whereby said beam will sweep out on the screen a cyclic scanning curve having a line repetition frequency equaling said first rate and a scan repetition frequency equaling said third rate, and

whereby said resulting signals will be displayed on the screen at the intersections of said scanning curve with a line indicative of said time displacement during periods when said first signal trains is enabled and at the intersections of said scanning curve with another curve during periods when said second signal train is enabled and the intersection between said line and said other curve will provide an unambiguous identification of said time displacement.

7. The system defined in claim 6 including means operative when one of said first and second signal trains is enabled for applying to the deflection electrode means a high-frequency small-amplitude additional voltage whereby to differentiate the display of said second signals from the display of said first signals.

8. The system defined in claim 6, including means for varying said voltage to control the beam for sweeping out a spiral curve having a line repetition frequency equaling said first rate and a scan repetition frequency equaling said third rate, whereby said line is a radial line and said other curve is a spiral expanded with respect to said first spiral.

9. The system defined in claim 6, including means for varying said voltage to control the beam for sweeping out a scanning curve which is periodically repetitive along one Cartesian coordinate of the screen whereby said line is parallel to the repetitive coordinate and said other curve is a curve similar to but expanded with respect to said scanning curve.

19. A radio ranging system comprising a transmitter having a keying input for controlling the repetition rate of signals transmitted thereby toward a target;

a receiver for receiving response signals from the tarmeans for generating a first train of keying pulses at a first rate;

means for generating a second trains of keying pulses at a second rate;

coincidence means deriving from said first and second pulse trains a third pulse train at a third rate the greatest common divisor of said first and second rates;

selector logic means having inputs connected to said keying-pulse-generating means and output connected to said keying input of the transmitter;

said logic means further having a controlling input connected to said coincidence means whereby to apply said first and second keying-pulse trains to said keying input for respective periods regularly alternating in accordance with said third rate;

an oscillographic dispsay device including ele:tron-beamproducing means, deflection electrode means, beamcontrol electrode means and a screen;

means connecting said receiver to said control electrode means to apply said response signals thereto for display on said screen;

voltage-developing means having inputs connected to said first keying-pulse generating means and said coincidence means for developing at least one cyclically varying voltage having a minor cycle period the reciprocal of said first repetition rate and a major cycle period the reciprocal of said third rate;

said voltage-developing means having a voltage output connected to said deflection electrode means of the display device whereby said electron beam will sweep out on the screen a cyclic scanning curve having a line repetition frequency equaling said first rate and a scan repetition frequency equaling said third rate, and

whereby said response signals will be displayed on the screen at the intersections of said scanning curve with a line indicative of target distance during periods when said first keying pulses are applied and at the intersections of said scanning curve with a different curve during periods when said second keying pulse train is applied, and the intersection between said line and said different curve will provide an unambiguous identification of target distance.

11. The system claimed in claim 10, including control means connected to said receiver responsive to target distance, and said logic means having a second controlling input connectable to said control means for selectively applying said first and second keying-pulse trains to the keying input of the transmitter in dependency on target distance, and operator-controlled switching means displaceable between a first position in which said first controlling input is connected to the logic means and a second position in which said second controlling input is connected to said logic means.

12. The system claimed in claim 10, including means for applying to said deflection electrode means a marker pulse train at a repetition rate equaling said third rate, thereby to display said marker pulse train as a marker index on the screen of the device, and means for adjusting the time relationship of said marker pulse train with respect to said first and second pulse trains whereby said marker index may be brought into coincidence with said intersection.

13. The system claimed in claim 12, further including an automatic-tracking follow-up section connected to said receiver, said marker pulse train being generated in said follow-up section, a follow-up motor in said follow-up section movable to track a target and simultaneously alter the time relationship of said marker pulse train, and an energizing input for said motor, said operator-controlled switching means when displaced to said first position simultaneously connecting said motor input to adjustable voltage means for adjustably altering the time relationship of said marker pulse train until said marker index has been brought into coincidence with said intersection between said line and said different curve, said switching means when displaced to said second position connecting said motor input in a servo-loop with said receiver for automatically tracking the target.

14. The system claimed in claim 19, including means connected for operation by said logic means for applying during the alternate periods of application of said second keying-pulse train a relatively high-frequency, smallamplitude additional voltage to said deflecting electrode means whereby to display small circles around the intersections of said scanning curve with said different curve.

15. The system claimed in claim 10, comprising means for varying said voltage to control the beam for sweeping out a first spiral whereby said line will be a radial line and said difierent curve will be a second spiral expanded with respect to said first spiral.

16. The system claimed in claim 10, comprising means for varying said voltage to control the beam for sweeping out a curve which is periodic along the one Cartesian and second repetition rates are so selected that the dif- 10 ference between the ratio of one of said first and second rates to said third rate and the ratio of the other of said first and second rates to said third rate is one.

No references cited.

CHESTER L. JUSTUS, Primary Examiner.

LEWIS H. MYERS, Examiner.

R. D. BENNETT, Assistant Examiner.

Claims

10. A RADIO RANGING SYSTEM COMPRISING A TRANSMITTER HAVING A KEYING INPUT FOR CONTROLLING THE REPETITION RATE OF SIGNALS TRANSMITTED THEREBY TOWARD A TARGET; A RECEIVER FOR RECEIVING RESPONSE SIGNALS FROM THE TARGET; MEANS FOR GENERATING A FIRST TRAIN OF KEYING PULSES AT A FIRST RATE; MEANS FOR GENERATING A SECOND TRAINS OF KEYING PULSES AT A SECOND RATE; COINCIDENCE MEANS DERIVING FROM SAID FIRST AND SECOND PULSE TRAINS A THIRD PULSE TRAIN AT A THIRD RATE THE GREATEST COMMON DIVISOR OF SAID FIRST AND SECOND RATES; SELECTOR LOGIC MEANS HAVING INPUTS CONNECTED TO SAID KEYING-PULSE-GENERATING MEANS AND OUTPUT CONNECTED TO SAID KEYING INPUT OF THE TRANSMITTER; SAID LOGIC MEANS FURTHER HAVING A CONTROLLING INPUT CONNECTED TO SAID COINCIDENCE MEANS WHEREBY TO APPLY SAID FIRST AND SECOND KEYING-PULSE TRAINS TO SAID KEYING INPUT FOR RESPECTIVE PERIODS REGULARLY ALTERNATING IN ACCORDANCE WITH SAID THIRD RATE; AN OSCILLOGRAPHIC DISPLAY DEVICE INCLUDING ELECTRONBEAM-PRODUCING MEASN, DEFLECTION ELECTRODE MEANS, BEAM-CONTROL ELECTRODE MEANS AND A SCREEN; MEANS CONNECTING SAID RECEIVER TO SAID CONTROL ELECTRODE MEANS TO APPLY SAID RESPONSE SIGNALS THERETO FOR DISPLAY ONSAID SCREEN; VOLTAGE-DEVELOPING MEANS HAVING INPUTS CONNECTED TO SAID FIRST KEYING-PULSE GENERATING MEANS AND SAID COINCIDENCE MEANS FOR DEVELOPING AT LEAST ONE CYCLICALLY VARYING VOLTAGE HAVING A MINOR CYCLE PERIOD THE RECIPROCAL OF SAID FIRST REPETITION RATE AND A MAJOR CYCLE PERIOD THE RECIPROCAL OF SAID THIRD RATE; SAID VOLTAGE-DEVELOPING MEANS HAVING A VOLTAGE OUTPUT CONNECTED TO SAID DEFLECTION ELECTRODE MEANS OF THE DISPLAY DEVICE WHEREBY SAID ELECTRODE BEAM WILL SWEEP OUT ON THE SCREEN A CYCLIC SCANNING CURVE HAVING A LINE REPETITION FREWUENCY EQUALING SAID FIRST RATE AND A SCAN REPETITION FREQUENCY EQUALING SAID THIRD RATE, AND WHEREBY SAID RESPONSE SIGNALS WILL BE DISPLAYED ON THE SCREEN AT THE INTERSECTIONS OF SAID SCANNING CURVE WITH A LINE INDICATIVE OF TARGET DISTANCE DURING PERIODS WHEN SAID FIRST KEYING PULSES ARE APPLIED AND AT THE INTERSECTIONS OF SAID SCANNING CURVE WITH A DIFFERENT CURVE DURING PERIODS WHEN SAID SECOND KEYING PULSE TRAIN IS APPLIED, AND THE INTERSECTION BETWEEN SAID LINE AND SAID DIFFERENT CURVE WILL PROVIDE AN UNAMBIGUOUS IDENTIFICATION OF TARGET DISTANCE.