[1]Un'l'ted States Patent Office 2@751@587 2,751,587 RANGE GATED AUTOMATIC GAIN CONTROL Homer G. Tasker, Van Nuys, and William T. O'Neil, La Creseenta, Calif., assignors to Gflfill.-n Bros., Inc., Los Angeles, Calif., a corporation of Califomia Application March 3, 1952, Serial No. 274,610 2 Claims. (Cf. 343-17.1) The present invention relates to an improved -ain control circuit for radar receivers, and, riore specifically, to improvements in automatic G. C. A. systens (autoihatic around , conttolled approach aircraft landing systems) whereiii it is desired to control the of ah intermediate frequency amplifier in a superi'ieterodyne receiver in accordailce with the amplitade of radar echoes from desired targets o-nly the arrangement herein fu-@ictioning generally to me@sure the amplitude of radar echoes from desired targets t6 control the gain of the interinediate frequency amplifier in an ;nverse relatiollship. The present invention contemplates ir.,iproved .@aii control circuitry which is reildered sensitive to echoes from. desired targets only, and, for that .-eileral purpose, the video developed in the output of the receiver is "range gated" by a control voltage or range gate developed in automatically tracking the desired target before being applied to a gain control stage, i. c., the intermediate frequency aniplifying stage of the radar receiver. Further, this circuitry has "riet-nory" and adjustable "learnin,-" and is thus particulaily advantageous in A. G. C. A. systems wherein ech6es from a corresponding plurality of aircraft in the aircraft approach zone to a landing field are received i@l periodic scaiining of the azimuth and elevation antenna beams. It is therefore an object of the prese-nt. invent;on to provide improved means and techniques for accompl;shing the results indicated above. A specific object of the present invention is to provide an improved automatic gain control circuit which serves to measure the amplitude of radir echoes and to control the gain of the radar receiver in an inverse relationship. Another specific object of the present inventioii is to provide an improved gain control circuit in which a control signal developed in accordance with the amplitude of the radar echoes for controlling the gain of the radar receiver is applied at the tin-ie radar echoes are being expected from desired targets. Another-specific object of the present invent-ion is to provide an improved gain control eircti,:t which serves to develop a control voltage for controllin,a the gain of a receiver, sucli control voltage being developed only as a result of echoes from desired targets so that echoes from undes@ired tatgets, sucli as clutter areas through which the antenna beam scans, have substantially no effect on the gaiii of the receiver. Another specific object of the present inventioii is to provide an automatic gain control circuit for a single radar rec&iver which serves to develop stibstaiitially sifriultaneously dataregarcting a pILirality of aircraf, iii aii approach zo-@ic to an aircra;t landing field as a related antenna beam scans through such approach zone, tile gain of such radar receiver being controlledcyclically in a degree dependent upon the partictilar echo Nvhich is received at that particular instant, so that relatively large echo signals, in general, have larger control effects than sinafler echo signals. Patented June 19, 1956 Another specific object of the present inve@ntion -is to provide a gain control system of the character described in the preceding paragraph in which adjustable memory means are ptovided and in which means are provided for adjusting the time during which such memory means may learn or acquire information of the rec&ived echoes. Another specific object of the present invenfion is . to provide an imi)roved gain control system in which a gating voltage, i. e., a range gate, is produced represen10 tative of the range of an aircraft from which echo signals are being derived in the form of videg, and in which such range gate is amplitude m6dulated in accordaiice with the intensity of @ucli echo signals, before being applied to a gain control element of the rad-ar receivar. 15 Another specific object of the present invention is to provide iinproved circiiitry which serves to measure the amplittide of radar echoes and to contr6l the ga'm of the radar receiver in an inverse relationship, the circuitry incltiding adjustable "men-lory" and "learning" charac20 teristics whereby the gain control is parlially dependent tipon "i-einembdred" inputi and whereby the rate of respoii,,,.e to new iliput ampfitudes may be adjusied. Anothdr @pecific object of the present i-.ivention is to provide a range gated automatic gain control for a single 25 rzidar receiver 6perating in conjunction with azimuth and clevat-'@On antenna scanning, on a time sliaring basis. in.a horizolal and vertical plane so that the gain level establish-.d by an intqgrated value of radir hits on objects in the scanned area has a time copstant of variafion of the 30 order of 5 to 10 scan periods, such that ihe variitiori of receiver olitput is maiiitained r.-latively uniform at -least withiii 6 decibels when a, test signal is varied over a range of 40 decibi--Is, @ With the minii-num. signal being a typical.radar ebho.from a range of 5 miles. 35 Another spedific object of the present invention is to prrvide an improved gain control system in.which @the ga:in of a sin.- le radar receiver may be controlled in accordance with video signals developed froni a plurality of objects such as aircraft which are simultaneously 40 being tra@ked in range. Another specific obje@t of the present invention is to provide an impioved gain control systeni in which the gain of a ridar receiver may be controlled in accordance with substantidlly only ihe intensity of a desired video 45 si.gnal even though the radar receiver serves to develpp video sigiials of much larger intensity as a result of.- reflections from clutter areas. Another specific object of the preseit invention is to provide an improved gain control system in which a 50 video signal and a coriespondiiig range g@-te from an object tracked in range are combined in such anianner that the gain of a radar receiver is controlled for the duration of such range gate in an amount dependent upon the amplitude of such video signal. Another specific object of the present invention is to r)5 provide an improved gain. control system in which a desired video signal and a corresponding range gate from Ian object tracked in range are combined in such a mannerthat the range gate serves essentially two functi6ns, i. e. 60 (@) the range gate serves as a pedestal for the video signal so that such video signal, by a peak detection process, is distin@uished over other video signals even though they are of larger amplitude than such desired video signals ind (2) the range gate is amplitude modulated in accordance with the amplitude of the desired 5 video signal and then applied as a gain gate to controlthe gain of a radar receiver for -the duratio.n of such range gate. The features of the present invention which are believed to be novel. are set forth with particularity in the, 70 appended claims. This invention itself, both as to @its organization and manner of operation, together v@ith
[2]3 further objects and advantages thereof, may be best understood by reference to the forowing description taken in connection with the accompanying drawings in which: Figure I shows an improved range gated gain control circuit embodying features of the present invention. Figures 2 and 3 show, respectively, in schematic and block diagram form, the circuitry indicated in Figure 1 as range gated automatic gain control. Figure 4 is a schematic representation of a portion of the radar receiver indicated as "1. F. strip pulse gain control" in Figure 1. Figure 5 serves to illustrate the manner in which the receiving system shown in Figure 1 may be converted from conventional operation to an operation in accordance with the features of the present invention. Figure 6 illustrates the manner in which a single radar receiver, in an AGCA system, is controlled in gain in accordance with the range gates and video signals developed in a plurality of control channels. The radar system shown in Figure I serves to illustrate generally certain coniponents of an automatic ground controlled approach (A. G. C. A.) system. Figure 1 shows the conventional azimuth antenna 10 and elevation antenna 11, x@hich, on a time sharing basis, scan the approach zone to an aircraft landing field for purposes of transmitting pulsed energy and receiving resultant echoes from aircraft in such approach zone. For that purpose, pulsed energy from the transmitter 12 is transferred through the radar frequency Az-El switch 14 to either antenna 10 or antenna 11, as the case may be. For purposes of initiating energy pulses in timed relationship, synchronizing pulses are transferred to the transmitter 12 from the synchronizer stage 15. The resulting echoes received on the azimuth antenna 10 or elevation antenna 11, as the case may be at any one partciular time, are transferred through the conventional tune-receive (TR) s,.vitch 18 to the crystal mixer stage 19 of a superheterodyne type of radar receiver, such crystal mixer serving to transform the received echo signals to a predetermined intermediate frequency, as is well understood in the art. Such signals of intermediate frequency are in conventional manner (with the movable contacts of switches 20, 21 engaging the upper contacts) amplified in the conventional intermediate frequency amplifier stage 22, and applied to the detector stage 24. The resulting video is amplified in the video amplifier 25 and applied to an intensity control electrode of the cathode ray tube or indicator tube 27. In accordance with the present invention (with the movable contacts of switches 20, 21 engaging the lower stationary contacts), a control signal developed from range gated video is applied to certain stages of the intermediate frequency amplifier 28 through which, of course, at this time signals are being transferred from the crystal mixer 19 to the detector stage 24. For this purpose, the video developed in the detector 24 is transferred to the range gated automatic gain control stage 31 wherein such video is combined with a range gate 32 de-.,eloped in the range tracking unit 34, i. e., the range gate 32 serves as a pedestal for the video to distinguish such video from other signals which possibly have larger amplitudes than such video. Jn general, the control signal in the form of a gain gate applied to the IF amplifier stage 28 is such that its gain is varied in an inverse relationship to the amplitude of the video developed at the output of the det ' 6ctor stage 24 so that the output signal is substantially c6nstant and all radar echoes are indicated with substantially constant intensity on the face of the cathode ray tube 27. .The range gate 32 is developed in the range tracking unit 34, details of which are described and claimed in the c6periding application of Alvin Guy Van Alstyne, Serial No. 244,111, filed August 29, 1951, and assigned to the some assignee, such copending application relating to 2,751,587 means and techniques whereby an aircraft in the approach zone may be "acquired" by a range tracking circuit incorporating a servo loop, such servo loop, after aircraft acquisition, serving to track the flight of an aircraft with respect to range and as a result of such tracking to develop the range gate 32. This range gate 32 is a gating voltage which is delayed with respect to the system trigger developed in the synchronizing unit 15, the delay being truly a measure of the distance or range of the air10 craft from the ground based radar equipment. For purposes of developing the range gate 32, there is applied to the range tracking unit 34 video from the detector stage 24 and a system trigger from the synchronizing unit 15. The manner in which the video applied to the stage 31 15 is combined with the range gate 32 is now described in -r,-ater detail with respect to Figures 2 and 3, Fi- re 3 ;eing a block diagram of the arrangement illustraotued in Figure 2. Video developed at the output of the detector stage 24 20 (Figure 1) is applied through the coupling condenser 1658 to the control grid of the pentode amplifier tube 1680 which has its cathode connected to ground thr6ugh the conventional resistance-condenser biasing arrangement 1656, 1655. The confral grid of tube 1680 is connected 25 to ground through the resistance 1657. The screen grid of tube 1680 is connected to the junction point of resistances 1652 and 1654 whicli provide a voltage dividing circuit, one terminal of resistance 1652 being connected to a +150 volt source and one terminal of resistance 1654 30 being grounded. The bypass condenser 1653 shunts resistance 1654. The anode of amplifying tube 1680 is connected to the +150 volt source through the serially connected resistances 1679 and 1651, the junction point of which is bypassed to ground through bypass con35 denser 1650. The amplified video voltage thus appearing on the anode of tube 1680 is transferred through coupling condenser 1616 to the control grid of the coincidence amplifying tlibe 1614 which has its control grid returned to 40 ground through resistance 1617. The cathodes of tubcs 1613, 16114 are both grounded and the anodes of fubes 1613, 1614 are each connected to a +150 volt source through resistance 1615. The video signal thLIs devcloped on the anode of tube 1614, of positive valite, is 45 mixed with a voltage developed by the positive range gate 32 applied through condenser 1600A to the control grid of the blocking oscillallor stage 1600, 1610. The tube 1600, in its quiescent state, is normally nonconducting, since its control grid is maintained at a relatively 60 large negative pofential, being connected to the junction point of resistances 1611, 1612, which form a voltage dividing network connected between a -150 volt sourcc and ground. The cathode of ttibe 1600 is grounded. The anodes of tubes 1600, 161.0 are both connected through @,s5 the transformer winding 1603A and resistance 1601 to a +300 volt source, fhe junction point of winding 1603A and resistance 1601 being bypassed to ground through condenser 1602. The cathode of tube 1610 is returned to ground through resistance 1609; and the control grid 60 of tube 1610 is connected to a -150 volt source through serially connected transformer winding 1603B and resisfance 1607. In order to provide some control over the width of the pulse developed in the blocking oscillator stage, an adjustable condenser 1606 has oiie of its ter6,,; minals grounded and the other one of its terminalscoiinected to the junction point of winding 1603B and resistance 1607. The condenser 1606 is shunfed by resistance 1608. The blocking oscillator stage 1600, 1610 serves to de70 velop v(yltages in the secondary w@@nding 1603C and 1603D' which are applied, on the one hand, as a reference pulse, for purposes described later, to the control grid of the mixer tube 1661, and on the other hand, to the conttol grid of tube 1613 fo achieve a coincident coiidition. - Foi75 that purpose, transformer winding 1603C ',ias ol 6.6f i I ts
[3]-5 termi @ inals grounded and theiother.,oxLeof,its ferminals conAppted Arough resistance.1676A to the anode and - -cojitrol ,grid of tube'1076 and to the confrol grid of tube .1661, such contr6i grtd :being return-ed to ground through the 'Prystal rpetifier 1662. Tra;n4@foriner l6O-3D @has one @of 5 I s te inals groppoe@d ;and the other one of- it@ - terminals c,o.nnected throuah registance 1604 To the control gfid of tilbe 1613 and also to the anode of the clipper, limiter or claniping tube 1605 which has its cathode - grounded. B-y these means, a bipolar valtage represented at 1604A 10 -is developed at the junction point of wind@'-Iig 1603D and resisrance 16,04; but, the positive portion of such voltage 1604A is substantially removed by the'liniiter tube 1605, as '-indicated at i6O4B. The ne@gaiive pulse thus transferred @to the control @rid of tube .1613 causes a positive 1.5 puls,e to a,ppear at the anode ol' tube '1613 which is mixed at such anode -with the video pulse to achieve a wave form a-t the anodes 1,613, .1014 o@ the character illustrated at 1604C. Th.-is composite wave form 16,04C is transferred -through coupling , condenser 1618 to the peak - detector 20 stage comprising -tubes 1621 1622 connecti@d in parallel with their confrol grids connected directly to their anodes. The parallel connected Tesistance 1619 and crystal - rectifier 1620 are connected between ground and the anodes of @tubes 1621, 1622. 25 @it is noted that the blocking oscillator &tage 1600, 16-10 -ser,ves to generate a -gafe corresponding in time to one of the respective range gates and that -the gates so - generated ha,v.e high Tates of rise and decline. Some adjustment of pulse duration approxi-mating plus @or @minus 40,Yo is ef- 3,0 fe.cted by variable capacitive loading of blocking oscillators provided by the adjtisfable condenser 1606. The .coincident.cir.cuit@consisting of the two triodes .1613, 1614, h4ving a common plate load in the forrn of r-esistance 1,6-15, serves as a @video "@-mphasizer" during the duration 35 of the range gate. Subsequent peak detection effected ' by d@e tubes 1621, 1622 @affords -a method of determining cqipbined gate and video maximum amplitude, it being -o,b@scryed that the -ate component is constant and therefc!r,e compensable through a compensation process de- 40 sc ibed later. TJie output of the @dio e @conne-ete-d tubes ,r I d 1621, 1622 is @a _pplied through a variable stop series re sistaq(;i@ 1623, 1624, 1,625, 1626 and 1627 to the mov@bie' contact of the single-pole double-throw Az-El relay s.witch 1649. where azimufli and elevation functions are 45 set@cted and' 4pplied, on @L time @sharing basis, to the azimuth rhannpl,apd i,@levation channel, i. e., to the contr,oj grid of fube 1629 and tjie control grid of tube 1645. The c@pac;itor 1630A, in the a7imuth function, and the c4pacitor 1646A, i th e e aton functi(yn, rc@spectively 50 n e l,v I ar chax-ged jirough the resistance st4@ps deternuned by ttm @etti4g of fhe multioosition switch 16$2. These capacitors 1630A and 1646A provide integrated input voltages with variable "leaming" characteristics to thecathode fqHpweT tubes 1629, 1630 on the one hand and 1645, r,5 1646 on thp other hand. As in,.dicated previously, the "Ipamin time" for these capacitors is varied thro .9 - ugh suitable range@ by adjustment of the "learning time" switch 1682 The condenser 1630A is connected in sh t with re- 60 uxi sistance 1630B, whereas similarly resistance 1646B is in parall I with condenser 1646A. For fhe purpose at hand, resistances lo@23, 1624, 1625, 1626 and i6f7 are respecti,vely 2200 ohms, 1000 ohins, 470 ohms, 220 ohms and 100 ohms. Condensers 1630A and 1646A are each 3300 65 microf ads and resistances 1630B and 1646B are each . , ?Lr - 4.7 megohms. The output of the azimuth cathode follower 1629, 1630 is applied to the condenser 1638, while similarly the output of the elevation cathode follower 1645, 1646 is ap70 plied to the condenser 1647. The decay time of the charge in condensers 1638, 1647 is determined by the parallel resistance steps determined by the position of the multiswitch 1637. The azimuth and elevation functions @re then combined,by the Az-El relay switch 1649 76 and @pplied to -tne.control,,grid of the cathode lollower tube 1673. For the above nientioned purposes, the -azimuth decqy -time Aetermining resistances 1631 1632, 1633, 1634, 1635 and.163-6 @are respectivi@ly I 000 ohms 4.7 megohms, 3.3 niegoh=, 3,.3 megohms, 1.5 me@gohms and 1.5 --megohms; whereas, the -elevation decay time determining @resistances 1639, 1640, 1.641, 1642 and 1643 and 1644 are respectivoiy 1.5 me@gohi-ns, 1.5 meoohms, 3.3 megohins, 3,3 megohms, 4.7 megbhms, and 1000 ohms, andconclemers 1638 and 1647 are tach 2 microfarad condensers. The cathode follower 1673 has its anode connected to a +300 volt source and its cathode connected to a - 150 volt source through serially coilnected ressistances 1674, 1675. The resistance 1674 constitutes a so-called level adjustingresistor,and for that,,purpose has its.adjustable tap connected to the cathode of the diode 1676 to accomplish a mixing function in stage 1661. The control grid and anode of the diode connected tube 1676 are both connected through the afor-e-menfioned resistance 1676A to the ungrounded @terminal -of winding 1603C; while the cathode of tube 1676 is connected to grouild through condenser 1677. Also, the control grid and anode of tube 1676 are both zonnected tothe control grid of the cathode follower tube 1661, such control grid being returned to ground thro-ugh the crystal rectifier 1662; and the cathDde of tube 1,661 is returned to ground through the load resistance 1663. The signal at the anode of tube 1676, it is -noted, is the negative-goin portion of the origin.-d 9 bipolar pulse 1604A occurring at essentialy the same time as the combined positive surge and.video pulse ilIListrated at 1604C. Combination of the two signals thus affords a measurement of the video level, with the pulses generated in the bloclcing oscirator stage IL600, 1610 serving as a reference and being modiilated in inteusity in accordance with the intensity of the video signal. The result is that a si,gnal of positive polarity is developed - on the cathode of tube IL661, having a duration equal to the range gate but modulated in arnplitude. The signal .of positive polarity thus developed across the load reista 1663 is applied through coupling condenser 1664 s nce to the control grid of the pentode amplifying tube U67 Which has its control grid ai2d cathode returned to ground ftough resistances 1665, 1666 respectively. The anode of tube 1667 is connected through serially connected resistances IL168, 1669 and 1670 to a +300 volt source with the junction point of resistances 1668, 1669 bein.- bypassed to ground through condenser 1671. The screen grid of tube 1667 is connected to a +150 volt source. The amplified signal, of inverted negative polarity developed on the anode of tube 1667, is applied through coupling condenser 1678 to the resistaiice 1672, the gate appearing across resistance 1672 appearing at 1672A. This gate consisting of negative-going pulses, of variabl6 amplitude, bears an absolute relationship to the amplitude of the video target pulses which are measured. These negative-going pulses, which constitute a gain gate, are applied to the intermediate frequency amplifier stage 28, as ind;cated in Figure I and shown in detail in Figure 4 the gate being appeed to terininal 1708A in Figure 4 to effect modulation of the gain of the interinediate frequency amplifier. It is noted, from the above description, that the condensers 1638 and 1647 impart "memory" to the azimuth and elevation functions respectively, since these condensers "remember" the amplitude information derived from the previous scans. The learning and memory features of this circuitry are considered novel. It is observed, following the peak detector stages 1621, 1622, that the charging capacitors 1630A, 1646A have adjustable "leaning" time as a result of the adjustable switch 1682. However, the time constant may not, as a practical matter@ be made long enough without unreasonably large values of resistance for adequate scan-to-scan mem-ory; and yet provide sufficient rapid learning time
[4]7 without requiring unreasonably large charging currents. The first charging circuit involving condensers 1630A and 1646A is therefore maintained to regulate learning time and does not have long-time holding characteristics. The recuired long-time holding characteristics are achieved by circuitry involving the condensers 1638, 1647 which comprise the cathode impedence of different cathode followers. The latter circuit is referred to as the "memory follower," and is able, by virute of the characteristics of the cathode follower, to charge the relatively large memory condensers 1638, 1647 in close duplication of the peak detector's own char.-ing circuit. These cathode follower circuits, on the one hand 1629, 1630 and on the other hand 1640, 1646, are independent of the decline of the peak detector capacitor voltage 1630 and 1646A respectively. The decay time constant resistance is therefore a factor in determining the scan-to-scan memory. Referring to Figure 4, the gain gate, as mentioned previously, is applied to terminal 1708A to modulate the gain of the intermediate frequency amplifier. The gain of the intermediate frequency amplifier stage may be manually adjusted in the Az-El functions by positioning the two taps on the voltage dividing resistance 1712A which has its opposite terminals connected to opposite terminals of source 1712B which has its positive terminal grounded. The voltage on either one of these taps is transferred through the movable contact of the single-pole double-throw relay switch 1712C to the lead 1712E. 'I'he negative potential on this lead 1712E, i-,l general, establishes the "static level" of amplification in the intermediate frequency amplifier since such negative voltage is applied to the control grid of variable mu tubes. The parallel connected resistance 1712 and condenser 1711 are connected between ground and the lead 1712E. The received signals from the crystal mixer stage of the superheterodyne receiver are applied to the input of ,,i stagger-tuned band pass amplifier stage 1697A, the outpiit circuit of wbich is coupled to the control grid of tube 1691. The control grid of tube 1691 is coniiected through the tunable coil 1692 and coil 1710 to the negative lead 1712E, the junction point of coils 1692 and 1710 being bypassed to ground throiigh condenser 1709. The cathode of tube 1691 is returned to ground throtigh the conventional biasing resistance-condenser combination 1693, 1694. The screen grid of tube 1691 is connected through suitable choke coils 1713, 1722 and 1724 to -,i +120 volt source. The anode of tube 1691 is coniiecte(.1 through resistance 1697 to the same so-urce through tl-ic aforc-mentioned coils 1714, 1722 and 1724. The amplitied voltage thug appearing on the anode of tube 1691 is applied through condenser 1695 to the resonant coil 1702 which has one of its terminals grounded. The resonant coil 1702 is link-coupled through suitable lin'l- co@,ipling 1702A to the input of the push-puu amplifying stage 1703, 1704 having the input coils 1700, 1701. The stige 1703, 1704 constitutes a pulse mcdulator, tli. push-pull oittput of which is modulated by the ,ipplicat'on of the negative-going pulses t6 the aforementioned teri-ainp.1 1708A developed in the circuitry iflustratcd in F'igure 2, and applied as a biasing voltage to the control grids e-f both tubes 1703 and 1704. Follo,,ving amplificiition by the balanced stage 1703, 1704, the !F output is link-coupled by link coupling ineans 1714A to another stagger-tuned band pass amplifier stage which includes tubes 1717 and 1731 A. The amplif-@ed video modulated by the range gated video ap-, peariiig on the anode of tube 1731A is applied through coupliti- coiidenser 1725 to the crystal detector 1726 and the detected video appcars on the output terminal 1727A. The detected video appearing on such terminal 1727A is applied as shown in Vigure 1, to the stage 31 a,nd range trackiiig unit 34. It is, tiius seen that a closed loop is pr6vidi@d for controlling the gain of the IF strip 28. 2,751,587 It is noted, as indicated above, that the time constant of the circuit including one or more of the resistances 1623, 1624, l@25, 1626, and 1627, is commensurate with the duration of the range gate, whereas, the time constant of the circuit associated on the one hand with condenser 1638, and on the other hand, the condenser 1647, is commensurate with the scanning time of the azimuth and elevation antenna beams, it being noted that the azimuth antenna scanning period is 1/4 of a 10 second and likewise the elevation antenna beam scanning time is '/,'t of one second. In the operation of the circuitry described above, it is observed that the range gate serves two essential purposes, i. e., it serves: (1) to select the particular piece 15 of video with which the apparatus is concerned, and (2) it is used in a modulation process to control the gain of the ainplifier for a period beginning immediately before the desired video and ending after the desired video. The first of these functions, i. e., the selection function, 20 is accomplished by combining the video and range gate as illustrated at 1604C so that the important video is placed on a pedestal and then peak detected. By thus placing the important video on a pedestal, the desired video is distinguished from other undesired video signals 25 which, in fact, may have larger amplitudes than the desired video. Especially, when the radar beam scans through a clutter area. In such case, without the range gate serving as a pedcstal, the amplitude of the undesired large video signals 30 would control the value of voltages developed on thecondensers 1630A and 1646A. To assiire against such undesired result, the desired video signal is placed on a pedestal, i. e., on the range gate wliich is of sufficiently large amplitude to cause the 35 combined amplitudes of the range gate and the desired video to be much greater than the amplitude of undesired video. To aid in the selection process and general exclusion of the effects of random noise effects, the I-,arning "time" of the condensers 1630A and 1646A, 40 determined by the position of the switch 1682, is commensurate with the duration of the range gate. The second function of the range gate, as indicated above, is for purposes of developing a gain gate indicated as such in Figure 2. The gain gate may be considered 4,3 to be an amplitude modulated range gate, such range gate being modulated in amplitude depending upon the value of voltage developed on the condensers 1638 and 1647 as the case may be. It is noted that these condensers 1638 and 1647 are 50 associated with circuitry of relatively large time constant, so that the voltages developed on these condensers vary inappreciably during three or four antenna beam scanning periods. Figure 5 illustrates a relay switching arrangement 35 whereby either the conventional IF amplffler '1732, or the AGCA IF strip 1733, identical with the strip 28 in Fi-,Ure 1, may be substituted by the application of 28 volts to the parallel connected relay coils 1735, 1734. Whi . le the system as described above pertains to the 60 co,trol of a radar receiver by a video signal and a range gate developed due to reflections from a single aircraft in an approach zone to an aircraft landing field, Figure 6 illustrates the manner in which the same single radar receiver is controlled by a plurality of video signals and 65 orresponding range gates, developed from reflections from a plurality of aircraft in such approach zone, it being remembered, that the beams from the azimuth and elevation antennas, 10, 11, respectively, scan through such 70 approach zone on a time sharing. basis, that the period of azimuth and elevation scan each being in the order of 1/4 of.one second. For this. purpose, as illustrated in Figure 6, a plurality of range tracking units are provided, there being one. 75 range tracking'unit per control channel and each of such
[5]9 range tracking units develops a corresponding range gate. Also, there is provided a plurality of range gated automatic gain control channels 31, each of which have supplied thereto a range gate from a corresponding range tracking unit 34. Each stage 31 and each stage 34 is supplied with video signals developed at the output of the detector stage 34. The output of the range gated automatic gain control stages 31 each comprise a gain gate, and all of the gain gates developed in the plurality of units 31, are applied to a common IF strip 28 in the manner inustrated in connection with Figure 4. While Figure 6 illustrates the units 31 duplicated, it is observed, that in practice, the input video amplifier 1680 and the mixer stage 1661 and amplifler stage 1667, are common to all of such units to minimize the number of circuit components. While the particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claiins is to cover all such changes and modifications as fall within the true spirit and scope of this invention. We
