Chain_Home - Wikiparency

# Chain Home **Chain Home**, usually abbreviated CH, was the codename for the ring of coastal early-warning [[Radar|radar]] stations the Royal Air Force built before and during the Second World War to detect and track approaching aircraft. It was the first early-warning radar network in the world and the first radar system of any kind to reach operational status, feeding the fighter-control system that met German bombers over southern England in the Battle of Britain. Each station paired a set of fixed transmitting towers, which flooded a wide arc of sky with pulses of radio energy, with separate receiving towers that timed the returning echoes for range and compared their relative strength across several aerials to recover bearing and height. The system grew out of a narrow question put to the radio engineer Robert Watson-Watt in late 1934: whether a death ray, then rumoured to be under development in Germany, was physically possible. Watson-Watt's answer was no, but a memorandum on the same subject proposed that radio waves reflected from an aircraft could reveal its position long before it was seen, and on February 26, 1935, a field trial near the BBC's [[Shortwave_radio|shortwave]] transmitter at Daventry proved the idea against a real bomber.[^churchill2015] A chain of stations grew from that single trial to roughly twenty by the outbreak of war in September 1939, stretching along the coast from the English Channel to well north into Scotland.[^bawdseytrust][^subbrit] This article carries no microsim of its own: the pulse-timing arithmetic a Chain Home station used to turn an echo's delay into a range is worked interactively in the [[Radar]] sketch, and the frequency shift a moving aircraft adds to its own echo is explored in the [[Doppler_effect]] sketch. ## Development Radio-based detection of aircraft had been anticipated well before Chain Home: as early as 1922 [[Guglielmo_Marconi|Guglielmo Marconi]] said, in a lecture reported to American radio engineers, that suitable equipment might one day reveal the presence of ships in fog from their reflection of [[Radio_wave|radio waves]], though no one turned the idea into working hardware for another decade.[^marconi1922] On November 10, 1932, the senior minister Stanley Baldwin warned the House of Commons that no defence yet existed against bomber aircraft, reportedly telling members that "the bomber will always get through," and through the early 1930s recurring rumours held that Germany had built, or was building, a radio death ray capable of stopping an aircraft engine or killing its crew at a distance.[^airminded] The government's Committee for the Scientific Survey of Air Defence, chaired by Henry Tizard and generally known as the Tizard Committee, was formed in 1934 to weigh such claims against the plainer fact that no gun or fighter of the day could be aimed at a bomber it could not first see.[^churchill2015] ### Daventry experiment Asked by the Air Ministry to judge whether a death ray was feasible, Robert Watson-Watt calculated that it was not, but his assistant Arnold Wilkins had separately noticed that aircraft flying near BBC shortwave transmitters disturbed reception, and a memorandum titled "Detection and Location of Aircraft by Radio Methods" reached the Air Ministry on February 12, 1935, proposing radio reflection instead.[^churchill2015] Two weeks later, on February 26, 1935, Watson-Watt, Wilkins and the Air Ministry's A. P. Rowe set up a receiver about 10 kilometres (six miles) from the BBC's Daventry transmitter and watched a Handley Page Heyford bomber, flown deliberately through the beam at roughly 1,800 metres (6,000 feet) some 32 kilometres (twenty miles) off, produce a clear beating pattern on a cathode-ray tube: the Daventry experiment, still marked as the moment British radar became a practical possibility rather than a paragraph in a memorandum.[^churchill2015][^iet2015] ### Into production Government funding followed quickly. Research moved to Bawdsey Manor on the Suffolk coast, purchased by the Air Ministry in 1936 with Watson-Watt as its first superintendent, and the first production station came on the air there in May 1937, reaching full operational status on September 24 that year.[^subbrit] Chain Home's builders settled on steel transmitting towers about 110 metres (350 feet) tall paired with wooden receiving towers around 73 metres (240 feet) tall, radiating several hundred kilowatts of pulsed power in the high-frequency and low-[[Very_high_frequency|VHF]] range; the surviving transmitting tower at Stenigot, in Lincolnshire, built in 1939 to the Air Ministry engineer Norman Garnish's design, still stands 110 metres (360 feet) tall and is recorded as the most complete Chain Home transmitter tower left in its original location.[^histengland] Fifteen stations were on the air by Easter 1939, and by the outbreak of war that September the chain numbered around twenty, from the Isle of Wight to well north into Scotland.[^subbrit][^bawdseytrust] ### Battle of Britain Chain Home's first serious test came in the summer of 1940. Its stations gave Royal Air Force Fighter Command tens of minutes of warning of a raid forming over occupied France, time enough to scramble fighter squadrons to height and position rather than keep standing patrols aloft around the clock, and that warning time is counted among the decisive factors in a battle the RAF could otherwise have lost to simple attrition.[^iet2015] The system had real weaknesses even so: gaps in low-altitude cover let some raids in undetected, and the plotting chain between a receiving station and a fighter controller involved several manual steps that a large enough attack could overwhelm. ### Upgrades In the war's final phase, some Chain Home stations were adapted, under the wartime codename Big Ben, to help fix the launch points of German V-2 rockets from the brief radar signature of a rocket in powered flight; exactly which stations were involved, and when, is not pinned down here.[^bigben] ### ROTOR Chain Home did not long survive the peace in its wartime form. [[Cold_War|Cold War]] tensions in the early 1950s produced ROTOR, a programme that reactivated some 28 former Chain Home sites alongside newly built underground stations; its first stage, budgeted at about 51.5 million pounds, handed 39 stations to Fighter Command by April 1956, after which Chain Home's remaining towers were steadily replaced by rotating radar of a kind Chain Home itself had never used.[^rotor] A handful of towers, Stenigot among them, survive today as scheduled monuments; most of the original chain was scrapped once ROTOR's own stations took over the watch. ## Description Physically, a Chain Home station was two separate installations sharing one compound: a transmitting site with its steel towers, and a receiving site some distance away with its own wooden towers, kept apart so that the receiver's sensitive aerials were not overwhelmed by the transmitter's own signal.[^subbrit] ### Transmitter details The transmitters, duplicated for redundancy, radiated pulses of 300 to 450 kilowatts peak power across a working range of roughly 20 to 60 megahertz depending on the mark of equipment fitted, low enough in frequency that the transmitting aerials had to be strung as [[Dipole_antenna|dipole]] curtains between the steel towers rather than mounted as a compact rotating array, and chosen partly because those frequencies hug the ground rather than skipping off the [[Ionosphere|ionosphere]] the way longer-range shortwave broadcasting does.[^subbrit] Because those aerials could not be steered, a station's beam was better described as a floodlight than a searchlight: it lit a broad arc of sky continuously, and a target's bearing had to be recovered afterward rather than read off wherever a dish happened to be pointing. ### Distance and bearing measurement Range came from the same pulse-echo timing used by any [[Radar|pulsed radar]]: the delay between a transmitted pulse and its echo, multiplied by the speed of light and halved for the round trip, gave a distance any operator watching an oscilloscope trace, fed by a [[Superheterodyne_receiver|superheterodyne receiver]], could read directly. Bearing came from a form of radio direction finding rather than from a narrow beam, comparing the relative strength of an echo across a set of crossed receiving aerials and converting the ratio, by manual calculation, into an angle. ### Altitude measurement Height, unusually for radar of the period, came from a similar comparison rather than a distinct technique: receiving aerials mounted at different heights on the wooden towers picked up a target's echo with different relative strength depending on its elevation angle, and an operator, later helped by a mechanical converter, turned that ratio into an altitude reading. The scheme depended on a target staying within a fairly narrow, well-calibrated height band for each station; one flying unusually low or high could fall outside that band and simply go unmeasured. ### Fruit machine Turning a single station's raw range, bearing and height readings, and those of its neighbours, into one plotted position for several raids at once was demanding enough to need a dedicated electromechanical calculator, nicknamed from its banks of dials the fruit machine; its precise internal mechanism is not pinned down here.[^fruitmachine] The plotted positions then travelled by telephone to Fighter Command's filter rooms, where controllers reconciled overlapping reports from adjacent stations into a single raid track before anything reached a fighter controller. ## Detection, jamming and counter-jamming Germany's air intelligence services were aware of Chain Home's transmissions well before the war, but historians generally judge that Luftwaffe planning underestimated both the network's technical effectiveness and how tightly Britain had integrated it into fighter control; a German wartime assessment specific enough to cite directly is not pinned down here.[^germanassess] A floodlight system like Chain Home is inherently awkward to jam in the way a narrow rotating beam can be jammed, since there is no single direction for a jammer to aim at, and this passive robustness mattered to the system's wartime record independently of any deliberate counter-jamming feature built into it. The most sophisticated German response was not jamming at all but eavesdropping. From about 1942, a system codenamed Klein Heidelberg, built and operated from sites on the French and later the Dutch coast, listened passively for a Chain Home pulse scattered back off a British or Allied aircraft and combined the geometry of transmitter, target and receiver, a [[Bistatic_radar|bistatic]] arrangement, to plot the aircraft's position while radiating no signal of its own to give itself away. British intelligence did not identify Klein Heidelberg until late 1944, and the countermeasure it prompted, deliberately jittering each station's [[Pulse-repetition_frequency|pulse timing]] so that Klein Heidelberg's own timing assumptions no longer held, arrived only in the war's final months, by which time Allied forces were already overrunning the sites that used it.[^kleinheidelberg] ## Comparison with other systems Chain Home's architecture looks unusual next to the radar that followed it, and much of that is a direct consequence of being first. Its floodlight transmission and its direction-finding style of bearing measurement, worked out because no one yet knew how to build a directional microwave beam at any useful power, gave way within a few years to sets that scanned a narrow beam mechanically and read bearing directly off the beam's own pointing angle on a [[Plan_position_indicator|plan-position indicator]] rather than by comparing signal strengths after the fact. That later approach needed a wavelength short enough to focus into a narrow beam from an aerial small enough to rotate, which is precisely what the [[Cavity_magnetron|cavity magnetron]], invented after Chain Home was already operational, went on to provide; Chain Home itself used conventional oscillator valves at metre wavelengths and never a magnetron of any kind, trading the fine bearing accuracy a rotating microwave beam gives for the sheer power and simplicity of lighting the whole sky at once. ## Chain Home sites Bawdsey, on the Suffolk coast, was Chain Home's birthplace: research moved there in 1936, and the station that grew up around Bawdsey Manor came on the air in May 1937, reaching full operational status on September 24 of that year, the first in the chain.[^subbrit] By Easter 1939 fifteen stations covered the most exposed stretches of coast, and by the outbreak of war that September the chain numbered around twenty, running from the English Channel to well north into Scotland.[^subbrit][^bawdseytrust] Most of Chain Home's original steel and timber towers were dismantled once postwar programmes such as ROTOR took over the early-warning role with rotating radar, but a few survive. The transmitting tower at Stenigot, built in 1939 to Norman Garnish's design, still stands 110 metres (360 feet) tall and is recorded as the most complete Chain Home transmitter tower left in its original location, though it has lost its topmost platforms.[^histengland] Bawdsey itself, the system's birthplace, is preserved today as a heritage site rather than as an active installation. ## Microsims This article has no microsim of its own. Its practical importance is in the pulse-echo timing and direction-finding arithmetic a Chain Home operator worked by hand, and that arithmetic is what the portal's interactive sketches demonstrate directly: a pulsed transmitter's ranging problem, the frequency shift a moving target adds to its echo, and the resolution limit a pulse's own length imposes on separating two close targets. All three are shown on neighbouring articles rather than here. *Try:* in the [[Radar]] sketch, drag the pulse-repetition frequency and watch the dashed unambiguous-range ring move, the same round-trip timing a Chain Home receiver read off an oscilloscope trace by hand. *Try:* in the [[Doppler_effect]] sketch, raise the source speed and compare the two observer readouts; the split between them is information Chain Home's own receivers had no way to extract. *Try:* in the [[Sonar]] sketch, drag the ping length and watch a close pair of targets merge into one echo, the same resolution limit a Chain Home pulse's length imposed on separating two aircraft flying close together. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Chain_Home) : [Wikitube](https://en.wikitube.io/wiki/Chain_Home) Skeleton mirrored at revision 1374194580. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radar]] - [[Cavity_magnetron]] - [[Bistatic_radar]] - [[Dipole_antenna]] - [[Plan_position_indicator]] - [[Cold_War]] ## Notes This article carries no microsim of its own; the sketches referenced under Microsims belong to neighbouring articles and are described here only in general terms, never with Chain Home-specific numbers borrowed from them. Where a claim below rests on a specialist secondary account rather than a primary Air Ministry or RAF record, the footnote says so rather than dressing it up as more certain than it is. ## References The pulse-echo timing, direction-finding and height-finding principles described above are standard radar-engineering material and are not separately footnoted; every date, name, place and figure below is sourced independently, never to Wikipedia, which is this article's pair and never its source. [^churchill2015]: "'An Extraordinary Advantage': Winston Churchill, Robert Watson-Watt and the Development of Radar." *Finest Hour* 195, International Churchill Society, 2015. https://winstonchurchill.org/publications/finest-hour/finest-hour-195/an-extraordinary-advantagewinston-churchill-robert-watson-watt-and-the-development-of-radar/ . [^iet2015]: "28 February 2015 - 80th Anniversary of the Birth of British Radar." IET Archives Blog, Institution of Engineering and Technology, February 19, 2015. https://ietarchivesblog.org/2015/02/19/28-february-2015-80th-anniversary-of-the-birth-of-british-radar/ . [^airminded]: Holman, Brett. "The Bomber Will Always Get Through." Airminded, November 10, 2007. https://airminded.org/2007/11/10/the-bomber-will-always-get-through/ . [^bawdseytrust]: "The Daventry Experiment." Bawdsey Radar Trust. https://www.bawdseyradar.org.uk/the-daventry-experiment/ . [^subbrit]: "Bawdsey Chain Home Radar Station." Subterranea Britannica. https://www.subbrit.org.uk/sites/bawdsey-chain-home-radar-station/ . [^histengland]: "Chain Home Transmitter Tower, former RAF Stenigot, Donington on Bain." List entry 1259301, Historic England. https://historicengland.org.uk/listing/the-list/list-entry/1259301 . [^kleinheidelberg]: "Klein-Heidelberg." cdvandt.org, Communications and radar history archive. https://www.cdvandt.org/k-h.htm . [^rotor]: "The ROTOR Project." TheTimeChamber.co.uk. https://www.thetimechamber.co.uk/beta/sites/military/rotor-radar-stations . [^marconi1922]: "Marconi Radar." Marconi Radar History archive, citing Marconi's 1922 remarks to the Institute of Radio Engineers. https://marconiradarhistory.pbworks.com/w/page/29024389/MARCONI%20RADAR . [^bigben]: Citation needed: a primary Air Ministry or RAF record identifying which Chain Home stations were adapted, and precisely when, to help track V-2 launch points under the wartime codename Big Ben. [^germanassess]: Citation needed: a specific, citable German wartime intelligence assessment of Chain Home, rather than the general historical judgment that its importance was underestimated. [^fruitmachine]: Citation needed: a primary technical manual or Air Ministry record describing the internal mechanism of the calculating device nicknamed the fruit machine. ## Further reading None of this portal's assigned open textbooks treats the Chain Home system, or the history of pulsed early-warning radar, in any depth; they are modern signal-processing and radio-engineering texts rather than radar histories. The independent sources footnoted above are offered as further reading in their place. ## External links This article has no live sketch of its own to link. The interactive sketches referenced under Microsims are published from their own articles, [[Radar]], [[Doppler_effect]] and [[Sonar]].