{"id":14538,"date":"2025-06-07T16:34:58","date_gmt":"2025-06-07T11:04:58","guid":{"rendered":"https:\/\/ibexaviation.com\/pilot-training\/?p=14538"},"modified":"2026-07-26T10:00:53","modified_gmt":"2026-07-26T04:30:53","slug":"radio-altimeter","status":"publish","type":"post","link":"https:\/\/ibexaviation.com\/pilot-training\/radio-altimeter\/","title":{"rendered":"Radio Altimeter (Radalt)"},"content":{"rendered":"<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-1.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<h2>Measurement of Height above Ground using Radio Altimeter<\/h2>\n<p>Radio Altimeters measure an aircraft&#8217;s height above the ground (Above Ground Level, AGL) using radio waves. It works by transmitting radio signals toward the ground and calculating altitude from the time taken for the reflected signal to return. Radio altimeters are essential during take-off, landing, low-level flight, and for systems such as terrain awareness and automatic landing.<\/p>\n<h3>Radio Altimeter<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-2.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Radio Altimeter provides the accurate height of the aircraft above ground level (AGL).<\/li>\n<li>It operates using the <strong>Frequency Modulated Continuous Wave (FMCW)<\/strong> principle.<\/li>\n<li>Used when the aircraft is close to the ground, such as during approach and landing.<\/li>\n<li>Radio altitude information is supplied to the <strong>Autoland System<\/strong>.<\/li>\n<li>The <strong>Ground Proximity Warning System (GPWS)<\/strong> uses radio altitude to determine height and rate of descent.<\/li>\n<\/ul>\n<h3>Frequency Modulated Continuous Wave (FMCW)<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-3.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Radio Altimeter operates in the <strong>SHF band<\/strong> between <strong>4200\u20134400 MHz<\/strong>.<\/li>\n<li>The minimum pulse width achievable by pulse radar is approximately <strong>0.5 microsecond<\/strong>.<\/li>\n<li>Pulse radar is unsuitable for measuring heights below approximately <strong>250 ft<\/strong>.<\/li>\n<li>Therefore, <strong>Frequency Modulated Continuous Wave (FMCW)<\/strong> is used.<\/li>\n<li>Radio Altimeter normally operates between <strong>50 and 2500 ft AGL<\/strong>.<\/li>\n<\/ul>\n<h3>Beam Width Limitations of Radio Altimeter<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-4.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Accuracy is <strong>\u00b13% of indicated height or \u00b11 ft<\/strong>, whichever is greater.<\/li>\n<li>Final display accuracy is approximately <strong>\u00b15 ft<\/strong>.<\/li>\n<li>The vertical beam width is <strong>30\u00b0<\/strong>.<\/li>\n<li>Altitude indications remain accurate up to <strong>\u00b130\u00b0 pitch<\/strong>.<\/li>\n<li>The lateral beam width is <strong>60\u00b0<\/strong>.<\/li>\n<li>Correct indications are available up to <strong>60\u00b0 bank angle<\/strong>.<\/li>\n<\/ul>\n<h3>Saw-Tooth Pattern of FMCW<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-5.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Radio Altimeter continuously transmits a <strong>Frequency Modulated Continuous Wave<\/strong>.<\/li>\n<li>The transmitted frequency is centered around <strong>4300 MHz<\/strong> and varies by approximately <strong>50 MHz (4250\u20134350 MHz)<\/strong>.<\/li>\n<li>The frequency variation follows a <strong>saw-tooth waveform<\/strong>.<\/li>\n<li>This modulation cycle repeats approximately <strong>300 times per second<\/strong>.<\/li>\n<\/ul>\n<h3>Principle of Radio Altimeter<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-6.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The transmitted frequency changes continuously in a predetermined pattern.<\/li>\n<li>The receiver antenna receives the reflected signal from the ground.<\/li>\n<li>The receiver compares the transmitted and received frequencies.<\/li>\n<li>The frequency difference is converted into aircraft height above ground.<\/li>\n<li>If the frequency difference remains constant, the aircraft is maintaining constant height.<\/li>\n<li>If the frequency difference changes, the aircraft is climbing or descending.<\/li>\n<\/ul>\n<h2>Operation of Radio Altimeter<\/h2>\n<h3>Incorrect Reading in FMCW Waveform<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-7.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>A dip occurs due to reversal of frequency change in the saw-tooth waveform.<\/li>\n<li>This dip appears at the maximum and minimum frequency values.<\/li>\n<li>The frequency reversal can introduce small height measurement errors.<\/li>\n<li>The receiver automatically compensates for these errors.<\/li>\n<\/ul>\n<h3>Display of Radio Altimeter<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-8.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The basic display consists of a stand alone indicator.<\/li>\n<li>It is generally a circular dial with a logarithmic graduated scale.<\/li>\n<li>The display provides greater accuracy at lower heights.<\/li>\n<li>The Radio Altimeter includes a failure warning function that masks the indicator needle.<\/li>\n<li>The needle is masked under the following conditions:\n<ul>\n<li>Aircraft climbs above <strong>2500 ft AGL<\/strong>.<\/li>\n<li>Transmitter equipment becomes unserviceable.<\/li>\n<li>Power supply failure to the Radio Altimeter.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<h3>Press-to-Test and Height Selector<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-9.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The Radio Altimeter has a spring-loaded rotating control button.<\/li>\n<li>Pressing the button performs a system test.<\/li>\n<li>During the test, the indicator needle moves to approximately <strong>1000 ft<\/strong>.<\/li>\n<li>The rotating knob is also used to select the <strong>Decision Height (DH)<\/strong>.<\/li>\n<li>The system warns the pilot when descending below the selected Decision Height.<\/li>\n<li>A visual warning is provided by a <strong>Low Altitude<\/strong> light.<\/li>\n<li>An audio warning tone is also heard through the headset.<\/li>\n<\/ul>\n<h3>Installation Delay Due to Residual Height<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r15-radio-altimeter-10.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Radio Altimeter may indicate a small negative height while the aircraft is on the ground.<\/li>\n<li>This is caused by installation delay, which is a combination of residual height and cable length.<\/li>\n<li>Residual height varies with:\n<ul>\n<li>Aircraft size.<\/li>\n<li>Landing attitude.<\/li>\n<li>Oleo strut extension.<\/li>\n<li>Height of the wheels in a bogie landing gear.<\/li>\n<\/ul>\n<\/li>\n<li>Cable length delay results from the distance between the avionics bay and the antenna installation, particularly in large aircraft.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Measurement of Height above Ground using Radio Altimeter Radio Altimeters measure an aircraft&#8217;s height above the ground (Above Ground Level, AGL) using radio waves. It works by transmitting radio signals toward the ground and calculating altitude from the time taken for the reflected signal to return. Radio altimeters are essential during take-off, landing, low-level flight, and for systems such as terrain awareness and automatic landing. Radio Altimeter Radio Altimeter provides the accurate height of the aircraft above ground level (AGL). It operates using the Frequency Modulated Continuous Wave (FMCW) principle.&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-14538","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\/14538","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=14538"}],"version-history":[{"count":2,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14538\/revisions"}],"predecessor-version":[{"id":16611,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14538\/revisions\/16611"}],"wp:attachment":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/media?parent=14538"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/categories?post=14538"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/tags?post=14538"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}