{"id":14536,"date":"2025-06-07T16:34:40","date_gmt":"2025-06-07T11:04:40","guid":{"rendered":"https:\/\/ibexaviation.com\/pilot-training\/?p=14536"},"modified":"2026-07-26T10:00:13","modified_gmt":"2026-07-26T04:30:13","slug":"distance-measuring-equipment","status":"publish","type":"post","link":"https:\/\/ibexaviation.com\/pilot-training\/distance-measuring-equipment\/","title":{"rendered":"Distance Measuring Equipment (DME)"},"content":{"rendered":"<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-1.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<h2>Distance Measuring Equipment (DME) using principle of Secondary Radar<\/h2>\n<p>Distance Measuring Equipment (DME) is a radio navigation system that measures the slant-range distance between an aircraft and a ground station. It works by transmitting UHF radio pulses from the aircraft and measuring the time taken for the ground station to reply. DME helps pilots determine distance for en-route navigation, instrument approaches, and position fixing.<\/p>\n<h3>Distance Measuring Equipment<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-2.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Distance Measuring Equipment works on the principle of secondary radar.<\/li>\n<li>Provides accurate slant range.<\/li>\n<li>Combination of VOR and DME is an ICAO-approved equipment for airways.<\/li>\n<li>TACAN is military equipment equivalent to DME.<\/li>\n<li>Installed in military airfields.<\/li>\n<li>Indications in the aircraft are the same as DME.<\/li>\n<\/ul>\n<h3>Radar Mile<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-3.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Radar Mile is the time taken for a pulse to travel 1 NM and return back to the radar.<\/li>\n<li>Radar Mile can be calculated using the speed of radio waves.<\/li>\n<li><strong>1 Radar Mile = 12.36 Microseconds<\/strong><\/li>\n<li>Distance of the target can be calculated using Radar Mile.<\/li>\n<li><strong>One-way Distance (NM) = Time Taken for Pulse to Return \u00f7 12.36<\/strong><\/li>\n<\/ul>\n<h3>Principle of DME<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-4.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>DME equipment onboard the aircraft sends a pair of interrogation pulses.<\/li>\n<li>Ground transmitter responds after a <strong>50-microsecond delay<\/strong>.<\/li>\n<li>The time difference between the pulses gives the slant range in the aircraft.<\/li>\n<li>Slant range is the straight-line distance of the aircraft from the DME.<\/li>\n<li>Slant range is calculated using the speed of radio waves.<\/li>\n<\/ul>\n<h3>Slant Range and Plan Range<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-5.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Plan range is the range of the aircraft on the ground.<\/li>\n<li>Plan range can be calculated from slant range using Pythagoras&#8217; Theorem.<\/li>\n<li><strong>Plan Range\u00b2 = Slant Range\u00b2 \u2212 Height\u00b2<\/strong><\/li>\n<li>Slant range is considered equal to plan range when the distance exceeds three times the height.<\/li>\n<li>If the aircraft is close to the beacon, slant range must be converted to plan range.<\/li>\n<\/ul>\n<h3>DME Operating Frequencies<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-6.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Frequency range: <strong>962\u20131213 MHz (UHF Band)<\/strong>.<\/li>\n<li>1 MHz spacing between two DME channels.<\/li>\n<li>Accommodates <strong>252 channels<\/strong>.<\/li>\n<li>DME uses two different frequencies for transmission and reception.<\/li>\n<li>The two carrier frequencies are spaced <strong>63 MHz<\/strong> apart.<\/li>\n<li>If the aircraft transmits at <strong>962 MHz<\/strong>, the ground station responds on <strong>1025 MHz<\/strong>.<\/li>\n<\/ul>\n<h3>Echo Protection Unit<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-7.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Transmission frequencies of the aircraft and ground station differ by <strong>63 MHz<\/strong>.<\/li>\n<li>This frequency difference eliminates certain problems.<\/li>\n<li>Prevents aircraft from receiving its own pulses reflected by clouds or the ground.<\/li>\n<li>Prevents repeated self-triggering of the transponder due to cloud reflections.<\/li>\n<li>DME is equipped with an <strong>Echo Protection Unit<\/strong>.<\/li>\n<li>The Echo Protection Unit provides a <strong>50-microsecond delay<\/strong> to avoid ground-reflected waves.<\/li>\n<\/ul>\n<h3>Jittering of Pulses<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-8.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>DME uses the <strong>Random Pulse Repetition Frequency (PRF)<\/strong> technique.<\/li>\n<li>This technique is called <strong>Jittering<\/strong>.<\/li>\n<li>The aircraft transmits pulse pairs at random intervals.<\/li>\n<li>The receiver gates open only to accept the corresponding response.<\/li>\n<li>This prevents confusion between replies intended for different aircraft.<\/li>\n<\/ul>\n<h3>Electronic Gates<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-9.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Twin pulses are transmitted to avoid accepting matching randomised single pulses from ignition systems or other radars.<\/li>\n<li>Electronic gates are established by the aircraft transponder as part of jittering.<\/li>\n<li>These gates open the receiver only for the correct response pulse.<\/li>\n<li>Only responses matching the transmitted PRF are accepted.<\/li>\n<\/ul>\n<h2>Operating Modes and Features of DME<\/h2>\n<h3>Search, Track and Memory Modes of DME<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-10.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<h3>Search Mode<\/h3>\n<ul>\n<li>Active during initial contact.<\/li>\n<li>Aircraft transmits <strong>150 pulse pairs per second<\/strong>.<\/li>\n<li>After approximately 100 responses are received, transmission is reduced to <strong>60 pulse pairs per second<\/strong> until lock-on.<\/li>\n<\/ul>\n<h3>Tracking Mode<\/h3>\n<ul>\n<li>Activated after contact with the ground station is established.<\/li>\n<li>Aircraft transmits <strong>25 pulse pairs per second<\/strong>.<\/li>\n<li>Uses the lock-and-follow technique with electronic gates.<\/li>\n<li>The electronic gates continuously adjust according to the changing distance from the station.<\/li>\n<\/ul>\n<h3>Memory Mode<\/h3>\n<ul>\n<li>Operates during temporary loss of DME responses.<\/li>\n<li>DME display shows calculated distances based on ground speed.<\/li>\n<li>Airborne equipment retains memory for approximately <strong>10 seconds<\/strong>.<\/li>\n<\/ul>\n<h3>Beacon Saturation<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-11.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Assumes <strong>95%<\/strong> of aircraft are in tracking mode and <strong>5%<\/strong> are in search mode.<\/li>\n<li>Average requirement is <strong>27 pulse pairs per second<\/strong> per aircraft.<\/li>\n<li>Ground station average output is <strong>2700 pulse pairs per second<\/strong>.<\/li>\n<li>Can accommodate approximately <strong>100 aircraft<\/strong>.<\/li>\n<li>Beacon saturation occurs when the ground station exceeds <strong>2700 pulse pairs per second<\/strong>.<\/li>\n<li>Occurs when more than 100 aircraft interrogate the beacon.<\/li>\n<li>The ground station reduces receiver gain, excluding distant aircraft.<\/li>\n<\/ul>\n<h3>Associated VOR\/DME<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-12.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>VOR and DME are selected using the same frequency selection in the aircraft.<\/li>\n<li><strong>Associated Terminal VOR-DME<\/strong> stations are located within <strong>100 ft (30 m)<\/strong>.<\/li>\n<li><strong>Associated Route VOR-DME<\/strong> stations are located within <strong>2000 ft (600 m)<\/strong>.<\/li>\n<li>Non-associated VOR\/DME may be located at the same place.<\/li>\n<li>Non-associated VOR\/DME stations separated by 1 NM have the third ident letter <strong>Z<\/strong>.<\/li>\n<li><strong>Example:<\/strong> VOR BBB may have DME BBZ.<\/li>\n<li>Non-associated VOR\/DME stations more than 6 NM apart have different identifiers.<\/li>\n<\/ul>\n<h3>Accuracy of DME<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-13.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>DME accuracy is based on a <strong>95% probability<\/strong>.<\/li>\n<li><strong>Narrow Spectrum DME (DME\/N):<\/strong> \u00b11.25% of range or \u00b10.25 NM.<\/li>\n<li><strong>Precision DME (DME\/P):<\/strong> \u00b10.20 NM.<\/li>\n<li>Ground speed computation becomes less accurate when directly overhead the station.<\/li>\n<li>Ground speed indication is valid only when flying directly toward or away from the station.<\/li>\n<li>ILS\/DME distance is measured from the runway threshold.<\/li>\n<li>This distance is accurate only during the approach.<\/li>\n<\/ul>\n<h3>Designated Operational Coverage (DOC)<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r14-distance-measuring-equipment-14.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Maximum range is based on the line-of-sight equation.<\/li>\n<li>Practical range is approximately <strong>250 NM<\/strong>.<\/li>\n<li>Designated Operational Coverage (DOC) protects DME from range errors caused by co-channel interference.<\/li>\n<li><strong>DOC 20\/240<\/strong> indicates coverage of <strong>20 NM at 24,000 ft<\/strong>.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Distance Measuring Equipment (DME) using principle of Secondary Radar Distance Measuring Equipment (DME) is a radio navigation system that measures the slant-range distance between an aircraft and a ground station. It works by transmitting UHF radio pulses from the aircraft and measuring the time taken for the ground station to reply. DME helps pilots determine distance for en-route navigation, instrument approaches, and position fixing. Distance Measuring Equipment Distance Measuring Equipment works on the principle of secondary radar. Provides accurate slant range. Combination of VOR and DME is an ICAO-approved equipment&hellip;<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"disable_featured_image":false,"footnotes":""},"categories":[326],"tags":[],"class_list":["post-14536","post","type-post","status-publish","format-standard","hentry","category-cpl-atpl-radio-navigation"],"_links":{"self":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14536","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/comments?post=14536"}],"version-history":[{"count":2,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14536\/revisions"}],"predecessor-version":[{"id":16609,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14536\/revisions\/16609"}],"wp:attachment":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/media?parent=14536"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/categories?post=14536"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/tags?post=14536"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}