Altitude measurement Chain Home

plotting , reporting tracks manpower intensive operation. image shows receiver station @ raf bawdsey, home of ch development. commanded flight officer wright, on phone. radar operator visible in background, right of centre. communicated plotter, in foreground wearing headphones, via intercom readings made out under attack.

due arrangement of receiver antennas, sensitive area had number of side lobes allowed reception @ multiple vertical angles. typically operator use upper set of antennas @ 215 feet (66 m), had clearest view of horizon. due half-wave interference ground, main lobe antenna directed @ 2.5 degrees above horizontal, sensitive region extending 1 3 degrees. @ ground gain zero, allowed aircraft escape detection flying @ low altitudes. second lobe extended 6 12 degrees, , on. left distinct gap in reception pattern centred @ 5.2 degrees.

this reception pattern provided ch relatively accurate way estimate altitude of target. this, motorized switch in receiver hut used disconnect 4 receiver masts , instead select 2 vertically displaced antennas on 1 mast. when connected radiogoniometer, output on display effected relative signal strength of 2 lobes, rather relative strengths in x , y in horizontal plane. operator swung radiogoniometer looking peak or minimum reception, before, , noted angle.

the number reported operator line-of-sight range target, or slant range, included components of both horizontal distance , altitude. convert real range on ground, plotter used basic trigonometry on right angle triangle; slant range hypotenuse , open angle measurement radiogoniometer. base , opposite sides calculated, revealing distance , altitude. important correction curvature of earth, became significant @ ranges ch worked at. once calculated, allowed range plotted, revealing grid square target, reported chain.

when target first detected @ long range, signal typically did not have enough of return in second lobe perform height finding. became possible aircraft approached station. problem recur target centred in second lobe, , forth. additionally, not possible determine difference between signal being compared between first , second or second , third lobe, caused ambiguity @ short ranges. however, altitude determined long before this, tended not problem in practice.

unfortunately pattern left set of distinct angles reception in both lobes low. address this, second set of receiver antennas installed @ 45 feet (14 m). when 2 lower sets of antennas used, pattern shifted upward, providing strong reception in gaps , @ cost of diminished long-range reception due higher angles.