Detection, jamming and counter-jamming Chain Home

1 detection, jamming , counter-jamming

1.1 detection
1.2 anti-jamming technologies
1.3 first attempts, halting followup
1.4 spoofing jammers, jitter
1.5 klein heidelberg

detection, jamming , counter-jamming
early detection

from may august 1939 lz130 graf zeppelin ii made flights along britain s north sea coast investigate 100-metre-high radio towers being erected portsmouth scapa flow. lz130 performed series of radiometric tests , took photographs. german sources report 12 m chain home signals detected , suspected radar; however, chief investigator not able prove suspicions. other sources said report different results.

during battle of france, germans observed 12 m pulse signals on western front without being able recognize origin , purpose. in mid-june 1940, deutsche versuchsanstalt für luftfahrt (dvl, german aeronautic research institute) set special group under direction of professor von handel , found out signals originated installations on coast of english channel.

their suspicions proven in aftermath of battle of dunkirk, when british forced abandon mobile gun laying radar (gl mk. i) station in normandy. wolfgang martini s team of specialists able determine operation of system. gl rather crude system of limited effectiveness, , led germans have dim view of british radar systems. however, effective system requires more radar, plotting , reporting equally important, , part of system developed in chain home. german s failure realize value of system whole has been pointed 1 of great failings during war.

anti-jamming technologies

the british had been aware germans determine purpose of system , attempt interfere it, , had designed in variety of features , methods in order address of these issues first stations being built. obvious of these ch s ability operate on different frequencies, added allow stations avoid sort of continuous-broadcast interference on operating frequency. additionally, interference rejection unit, or ifru, allowed output of intermediate stages of amplifiers clipped in attempt finely tune receiver station s own signals , reject broadband signals.

more complex system built ch displays, implemented in order remove spurious signals unsynchronized jamming pulses. consisted of 2 layers of phosphor in crt screen, quick-reacting layer of zinc sulphide below, , slower afterglow layer of zinc cadmium sulphide on top. during normal operation bright blue signal zinc sulphide visible, , signal activate yellow zinc cadmium sulphide layer, causing averaged signal displayed in yellow. filter out jamming pulses, yellow plastic sheet placed in front of display, rendering blue display invisible , revealing dimmer yellow averaged signal. reason many radars war through 1960s have yellow displays.

another method use range-only measurements multiple ch stations produce fixes on individual targets, chapman method . aid task, second display installed fed y-axis signal distant ch station on telephone lines. system never required.

first attempts, halting followup

when jamming first attempted germans handled in more clever fashion had been anticipated. observation transmissions of individual stations spread out in time, in order avoid mutual interference, exploited. system designed send spurious broadband pulses on chosen ch station s time slot. ch operator avoid signal changing time slot slightly, jamming not received. however, caused station s signals start overlapping s time slot, station attempt same cure, affecting station in network, , forth.

a series of such jammers set in france starting in july 1940, , concentrated single station in calais affected ch time. however, timing of these attempts extremely ill-considered. british developed operational methods counteract jamming, , these had eliminated effect of jamming opening of battle of britain on 10 july. germans on way develop more sophisticated jamming systems, these not ready operation until september. meant ch system able operate unmolested throughout battle, , led well-publicized successes.

by opening of battle in july german luftwaffe operational units aware of ch, , had been informed dvl not expect remain undetected, in clouds. however, luftwaffe did little address , treated entire topic level of disdain. own radars superior ch in many ways, yet in actions had proven marginally useful. during air battle of heligoland bight in 1939, german freya radar detected raid while still hour away target, yet had no way report of fighter units intercept it. getting information radar pilots in useful form appeared difficult problem, , germans believed british have same problems , radar have little real effect.

some desultory effort put attacking ch stations, during opening stages of battle. however, british engineers able return these units service, or in cases pretend in order fool germans thinking attacks failed. pattern of these attacks became clear, raf began counter them increasing effectiveness. junkers ju 87 dive bombers subjected catastrophic losses , had withdrawn battle. germans gave trying attack ch directly on reasonable scale.

thus, ch allowed operate throughout battle largely unhindered. although communications indeed serious problem, precisely problem dowding system had been set address, @ great expense. result every british fighter twice effective, or more, german counterpart. raids met 100% of fighters dispatched engaging targets, while german aircraft returned home on half time having never seen enemy. reason churchill credits chain home winning battle.

spoofing jammers, jitter

this second jamming system activated @ cap gris nez in september, using system triggered signal in response reception of pulse ch. meant system responded ch station if moved time slot. these systems, known garmisch-partenkirchen used during operation donnerkeil in 1941. further improvements basic concept allowed multiple returns generated, appearing multiple aircraft on ch display.

although relatively sophisticated, ch operators adapted these new jammers periodically changing pulse repetition frequency (prf) of station s transmitter. caused synchronized jamming signals briefly go out of synch station, , blips jammers jitter on screen, allowing them visually distinguished. intentional jitter anti-jamming unit , ijaj, performed automatically , randomly, making impossible german jammers match changes.

another upgrade helped reject unsynchronized pulses, supplanting two-layer display. device, anti-jamming black-out unit, ajbo, fed y-axis signal delay , brightness control of crt. short pulses appeared , disappeared muted, disappearing display. similar techniques using acoustic delay lines, both jamming reduction , filtering out noise, became common on many radar units during war.

klein heidelberg

the germans made use of ch own passive radar system, known klein heidelberg. used ch s transmissions source, , series of receivers along channel coast. comparing time of arrival of signals selected aircraft, range , direction determined accuracy. since system sent out no signals of own, allies not aware of until overran stations in 1944. of stations had been built when overrun.

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