{"id":18852509,"date":"2026-09-01T11:12:44","date_gmt":"2026-09-01T09:12:44","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/terrain-following-radar-low-level-flight-explained\/"},"modified":"2026-09-01T11:13:08","modified_gmt":"2026-09-01T09:13:08","slug":"terrain-following-radar-low-level-flight-explained","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/fr\/terrain-following-radar-low-level-flight-explained\/","title":{"rendered":"Terrain-Following Radar: Flying at 200 Feet in the Dark"},"content":{"rendered":"<style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\r\n<p>The U-2 shootdown over Sverdlovsk in 1960 settled an argument. Altitude was no longer sanctuary. If you wanted to reach a defended target, you would have to go under the radar rather than over it, at night, in weather, at a few hundred feet.<\/p><p>Which raises an immediate problem. A pilot cannot see a ridge in cloud at 500 knots, and by the time he could, the aircraft would already be inside its own turning circle.<\/p><p>The answer was to let a radar look ahead and fly the aeroplane over the landscape automatically. It worked, it has killed people, and it is quietly being replaced by something that does not transmit at all.<\/p>\r\n<div style=\"background:#f5f5f5;padding:20px 22px;margin:26px 0;border-left:4px solid #5C91FF\"><p style=\"margin:0 0 10px;font-weight:700;font-size:17px;color:#222\">Informations cl\u00e9s<\/p><table style=\"width:100%;border-collapse:collapse;font-size:15px;line-height:1.6\"><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>The distinction<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">Terrain following changes height vertically; terrain avoidance turns laterally around obstacles<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>First production system<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">Ferranti, for the BAC TSR.2, built from 1959 and first flown in a Canberra testbed in 1962<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>First US operational automatic TFR<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">Texas Instruments AN\/APQ-110 in the General Dynamics F-111A and E<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>F-111E clearance settings<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">200, 300, 400, 500, 750 and 1,000 ft on a six-position knob<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>Ride settings<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">Hard 0 g, medium 0.5 g, soft 0.75 g of commanded negative g<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>Fail-safe trigger<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">Radar altitude below 68% of set clearance, loss of any data-good input, or an internal fault<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>Fail-safe response<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">Up to +3.8 g initially, reaching limit pitch in about two seconds<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>Lowest published clearances<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">Tornado IDS about 200 ft, F-111 200 ft, MC-130H 250 ft<\/td><\/tr><tr><td style=\"padding:6px 14px 6px 0;color:#555;white-space:normal;vertical-align:top;min-width:130px\"><strong>The passive successor<\/strong><\/td><td style=\"padding:6px 0;white-space:normal\">TERPROM terrain-referenced navigation, using stored elevation data instead of transmitting<\/td><\/tr><\/table><\/div>\r\n<h2 style=\"padding-top:22px\">The electronic ski-toe<\/h2><p>The principle is more elegant than the hardware suggests, and the clearest description of it comes from the engineer who led the Ferranti programme for the TSR.2.<\/p>\r\n<div style=\"background:#f8f9fa;border-left:4px solid #1565c0;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7;display:flex;gap:20px;align-items:flex-start\"><div><em>&ldquo;The required flight vector, to maintain a safe low clearance height, was determined by sliding an electronic ski-toe, extending from 1,500 ft to 20,000 ft in front of the aircraft, over the radar derived ground profile. The electronic ski-toe was effectively pivoted at the nose of the aircraft with the flat part of the ski set at the required clearance height below the aircraft.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Bill Blain<\/strong> &mdash; Radar systems engineer, Ferranti Defence Systems, who led the TSR.2 terrain-following radar development<\/div><\/div><\/div>\r\n<p>Imagine a ski held ahead of the aircraft, its flat section sitting at the height you want to maintain, its upturned toe reaching out to the horizon. Slide it along the ground profile the radar is drawing. Wherever the terrain pushes the ski up, the aircraft must climb; wherever it falls away, the aircraft may descend.<\/p>\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img data-opt-id=1692735123  fetchpriority=\"high\" decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/ml5psubhxdln.i.optimole.com\/cb:QU55.d034\/w:auto\/h:auto\/q:mauto\/ig:avif\/https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/terrain-following-radar-ride-command-diagram.png\" alt=\"Diagram of terrain-following radar ride command logic\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The template in schematic form: the zero command line for the selected clearance, terrain returns including radar-shadowed sections shown dotted, and the resulting climb or descend command. Diagram: Wikimedia Commons<\/figcaption><\/figure>\r\n<p>The template was a physical thing in the cockpit sense. The F-111E preflight checklist has the crew verify that the zero command line moves properly and produces a smooth curve across all three ride settings and every clearance setting, because both knobs reshape the ski.<\/p>\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/vSyDslyADv4\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n<p>The original General Dynamics training film for the F-111 system is above, and it is the best surviving explanation of how the equipment was actually operated.<\/p><h2 style=\"padding-top:22px\">Hard, medium and soft<\/h2><p>The ride switch is the part crews remember. On the <a href=\"https:\/\/migflug.com\/afterburner\/fr\/aeronef\/f-111-aardvark\/\">F-111<\/a> it selects how much negative g the system is allowed to command as it pushes over a crest: zero on hard, half a g on medium, three quarters on soft. Hard keeps the aircraft pinned closest to the terrain and is thoroughly unpleasant. Soft computes an earlier, gentler anticipatory command and costs you exposure.<\/p><p>Crucially, the flight manual is explicit that the fail-safe fly-up signal is not affected by the ride switch. You can choose how roughly you are flown. You cannot choose whether the system saves you.<\/p>\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img data-opt-id=447139495  fetchpriority=\"high\" decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/ml5psubhxdln.i.optimole.com\/cb:D-Ec.d035\/w:auto\/h:auto\/q:mauto\/ig:avif\/https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/raaf-general-dynamics-f-111c-aardvark.jpg\" alt=\"Royal Australian Air Force General Dynamics F-111C Aardvark\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">A RAAF F-111C. The Aardvark carried the first operational automatic terrain-following radar in US service, the Texas Instruments AN\/APQ-110. Photo: Wikimedia Commons<\/figcaption><\/figure>\r\n<p>Nor is the negative-g limit arbitrary. It exists because the radar cannot see behind a crest. Push over too aggressively and the aircraft arrives in a valley the system has not yet drawn, with a second hill waiting in the shadow of the first. Blain put the TSR.2 limit at half a g for exactly that reason, and the F-111 numbers bracket the same value.<\/p><h2 style=\"padding-top:22px\">When it stops working<\/h2><p>Two failure modes are built into the physics rather than the engineering.<\/p><p>The first is low-reflectivity ground. Over calm water, dry lake beds, smooth sand or snow, very little energy comes back. The F-111 manual states plainly that terrain following will then be commanded by the radar altimeter, which looks only downward and has no forward-looking capability. On rising ground that is a serious deficiency, and the manual quantifies it: the aircraft will fly below set clearance by 87 feet for every degree of slope.<\/p><p>The second is that the system knows when it is blind, and its response is violent. Any internal fault, any loss of roll or air-data or radar-altimeter validity, or a radar altitude below 68 per cent of the selected clearance, triggers an automatic fly-up. The initial response may reach 3.8 g, hitting limit pitch attitude in about two seconds, with pitch trim disabled and the stick centred.<\/p>\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/lV0P-zsUHqM\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n<p>Edwards Air Force Base footage of B-1B low-altitude terrain following is above.<\/p><h2 style=\"padding-top:22px\">The accident that shows the trust problem<\/h2><p>On 30 November 1992 a <a href=\"https:\/\/migflug.com\/afterburner\/fr\/aeronef\/b-1-lancer\/\">B-1B<\/a> from Dyess Air Force Base flew into a ridgeline 36 miles south-south-west of Van Horn, Texas, on a night low-level sortie. All four crew were killed. The aircraft struck roughly 300 feet below the crest of a 6,500-foot ridge.<\/p><p>The automatic terrain-following system had generated a fly-up thirteen seconds before impact. Just before the crash, the crew manually interrupted it.<\/p><p>That is the central tension of the whole technology. A system aggressive enough to save you will also cry wolf, and a crew that has been thrown around by unnecessary fly-ups will eventually reach for the override.<\/p>\r\n<div style=\"background:#f8f9fa;border-left:4px solid #d32f2f;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7;display:flex;gap:20px;align-items:flex-start\"><div><em>&ldquo;The Accident Investigation Board President found by clear and convincing evidence that the accident was caused by a loss of situational awareness on the part of the entire Flight Deck Crew that placed the aircraft in a low energy climb situation with respect to the surrounding mountainous terrain.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>USAF Aircraft Accident Investigation Board<\/strong> &mdash; Executive Summary, MC-130H 87-00127, Tirana, Albania, 31 March 2005<\/div><\/div><\/div>\r\n<p>The Albanian accident, which killed nine, followed a similar shape: the crew missed the start-climb point for a 5,500-foot saddle, switched the terrain-following system on and immediately received an obstacle warning, and stalled about 200 feet above the terrain. The board also noted the crew&rsquo;s focus on terrain masking rather than terrain avoidance.<\/p><h2 style=\"padding-top:22px\">Who flew it<\/h2><p>By February 1964 the USAF already had terrain-following radar in the B-52H and the F-105, and Texas Instruments held subcontracts for the RF-4C and the F-111. The contemporary trade press was unusually candid about why: proof of Soviet ability to knock down high-flying aircraft had forced the United States to build capability for low-level penetration.<\/p>\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7;display:flex;gap:20px;align-items:flex-start\"><div><em>&ldquo;Terrain following refers to vertical change in flight direction, going up or down to maintain a constant altitude above the ground. Terrain avoidance implies ability to turn left or right to avoid obstructions or to take advantage of the hills that protect a valley.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>John F. Mason and Harold C. Hood<\/strong> &mdash; Electronics magazine, Attack Planes Hug Hostile Terrain with New Radar, 21 February 1964<\/div><\/div><\/div>\r\n<p>Le <a href=\"https:\/\/migflug.com\/afterburner\/fr\/aeronef\/panavia-tornado\/\">Panavia Tornade<\/a> carried two separate nose radars, the smaller one dedicated to terrain following, coupled to the autopilot for automatic flight at around 200 feet. Both radars were built by Texas Instruments, making them the only major items in the baseline aircraft not manufactured in Europe.<\/p>\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img data-opt-id=173948981  decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/ml5psubhxdln.i.optimole.com\/cb:QU55.d034\/w:auto\/h:auto\/q:mauto\/ig:avif\/https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/raf-tornado-gr4-low-level-training-sortie.jpg\" alt=\"RAF Tornado GR4 on a low-level training sortie\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">A No. IX(B) Squadron Tornado GR4 low over north-west England. The Tornado&rsquo;s dedicated terrain-following radar allowed coupled automatic flight at around 200 feet. Photo: UK MOD, Open Government Licence<\/figcaption><\/figure>\r\n<p>Le <a href=\"https:\/\/migflug.com\/afterburner\/fr\/aeronef\/f-15e-strike-eagle\/\">F-15E<\/a> took a different route, carrying the function in the LANTIRN navigation pod rather than building it into the airframe. The Soviet Su-24 used a dedicated set named Relyef alongside its main scanners. The MC-130H Combat Talon, officially cleared as low as 250 feet in adverse weather, used a redundant dual-band forward-looking radar.<\/p>\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/TPAfhMikIRA\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n<h2 style=\"padding-top:22px\">Why it is going away<\/h2><p>A terrain-following radar has one unavoidable characteristic: it transmits. It points energy forward, at low altitude, in the direction it is flying, which is precisely where anyone who wishes to intercept it is standing.<\/p><p>The clearest evidence of how seriously that was taken is the engineering response. The B-1B&rsquo;s AN\/APQ-164 uses a fixed electronically scanned antenna canted downward specifically for reduced radar observability, and its manufacturer advertises low probability of intercept terrain-following modes. Low-probability-of-intercept modes only exist because ordinary ones are interceptable.<\/p><p>The modern answer removes the transmission entirely. TERPROM, conceived in 1977 as a private venture at British Aerospace, compares radar-altimeter readings against a stored digital elevation model to work out where the aircraft is and what is ahead, without emitting anything forward. It has been fitted to the F-16, Harrier, Jaguar, Tornado, A-10, Typhoon and C-130 among others, and it delivers a limited but entirely passive terrain-following capability.<\/p><p>The F-35 has no terrain-following radar at all. Its Auto-GCAS uses onboard digital terrain data to determine whether a collision is imminent and, at the last instant, commands a roll to wings level and a 5 g pull. The system was fielded from June 2019 and won the Collier Trophy.<\/p><p>Which makes a neat closing comparison. In 1967 an analogue computer sliding an electronic ski over a radar picture would save your life with 3.8 g. In 2019 a digital elevation map does the same job with 5 g, tells nobody you are coming, and never needs to see the hill at all.<\/p><p><em>Sources: T.O. 1F-111E-1 General Dynamics F-111E flight manual; Bill Blain, TSR2 Terrain Following Radar Development, recollections; Electronics magazine, 21 February 1964; USAF Aircraft Accident Investigation Board report, MC-130H 87-00127; Aviation Safety Network wikibase 48343; USAF and AFSOC fact sheets; Northrop Grumman AN\/APQ-164 material; Lockheed Martin Auto-GCAS releases.<\/em><\/p>\r\n<div style=\"background:#f0f4ff;border-left:4px solid #5C91FF;padding:16px 20px;margin:32px 0 8px\"><p style=\"margin:0 0 8px;font-weight:600;color:#333\">Articles similaires<\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/fr\/saab-37-viggen-chasseur-routier-base-routiere\/\">Le Saab Viggen\u00a0: le chasseur con\u00e7u pour s\u2019envoler des routes<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/fr\/compressor-stall-jet-engine-surge-explained\/\">Compressor Stall: Why a Jet Engine Coughs Fire<\/a><\/p><\/div>","protected":false},"excerpt":{"rendered":"<p>How strike aircraft fly at a few hundred feet at night and in cloud: the electronic ski-toe, the ride switch, the 3.8 g fail-safe, and why the whole technology is being replaced by something that does not transmit.<\/p>","protected":false},"author":23,"featured_media":18851878,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[665],"tags":[],"class_list":["post-18852509","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aviation-world"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.3 - 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