Radar_altimeter - Wikiparency

# Radar altimeter A **radar altimeter**, also called a radio altimeter (RALT) or, in its short-range civil form, a low-range radio altimeter (LRRA), measures an aircraft's or spacecraft's height above the terrain directly beneath it by timing how long a beam of [[Radio_wave|radio waves]] takes to travel down to the ground, reflect, and return. The measurement is a straight line to whatever lies directly below, not a distance above some fixed sea-level datum, so radar height and the barometric altitude a pilot reads from static air pressure move together over flat ground and diverge sharply over a ridge, a canyon or a runway threshold. A three.js companion sim, a variant of the [[Continuous-wave_radar|continuous-wave radar]] sketch, renders a beam looking straight down and turns the beat note between an outgoing frequency sweep and its own echo into the height reading described below. Most radar altimeters transmit continuously rather than in pulses, sweeping the transmitted frequency up and down at a steady rate; mixing a sample of what is being sent with the delayed echo of what was sent a moment earlier produces a beat note whose frequency is proportional to the round-trip delay, and so to height. The technique, frequency-modulated continuous-wave (FMCW) ranging, trades the very short pulses a search [[Radar|radar]] needs for fine range resolution against the wide instantaneous bandwidth those pulses would demand, letting an altimeter resolve a few feet of height from a transmitter of only modest peak power. [[Frequency_modulation|Frequency modulation]] is therefore not incidental to the altimeter's design as it is to an FM broadcast transmitter; it is the entire ranging mechanism. ## Principle The height a radar altimeter reports follows the same round-trip timing every ranging radar uses: height equals the speed of light times the two-way delay, divided by two, `h = c·τ/2`, the factor of two accounting for the down-and-up path. A pulsed instrument could measure τ directly by timing a short burst's echo, but at the heights an aircraft actually flies close to the ground, from a few feet on the runway to a few thousand feet during a descent, the delay is only nanoseconds to tens of microseconds, far shorter than a pulse carrying useful energy would last. FMCW ranging avoids the problem by never pulsing at all. The transmitted frequency is swept linearly, up and down, across a narrow band several hundred times a second; the returning echo carries the same sweep, delayed by τ; and mixing transmitted and received signals produces a beat frequency f_beat proportional to how far the frequency moved during that delay, `f_beat = (2h/c)·(df/dt)`, solved for height as `h = c·f_beat / [2·(df/dt)]`. Because the beat note sits at a comparatively low, audio-range frequency however high the radio carrier itself runs, it can be read by a narrowband, low-power receiver rather than the wideband, high-power one a short-pulse radar needs, which is why FMCW rather than pulsed ranging became the standard at altitudes measured in feet. Appleton's original ionosphere soundings, described below, worked in the shortwave band; a modern radar altimeter operates roughly three orders of magnitude higher, close to 4.3 gigahertz, where a physically small antenna still gives a beam narrow enough to see mostly the ground directly below rather than a wide swath to either side. Above a few thousand feet the same instrument typically hands off to a pulsed or hybrid mode, since the delay is then long enough to time directly and a fixed sweep rate would otherwise have to fall low enough to blur the reading. ## History ### Original concept The idea that a reflected radio wave's travel time could reveal a distance is as old as the demonstration that radio waves reflect from a conducting surface at all; [[Guglielmo_Marconi|Guglielmo Marconi]] and other early experimenters had observed such reflections within the first years of the twentieth century, well before anyone built an instrument to time them.[^origin] Turning the observation into an altimeter needed two further steps beyond that early demonstration: a way to send a continuous, trackable signal rather than a single spark, and a way to turn a very short delay into a number a pilot or an instrument could read off directly. ### Appleton's ionosphere measurements A working method for doing exactly that came from a different problem entirely. In the mid-1920s the physicist Edward Appleton, working with Miles Barnett, swept the frequency of a signal beamed toward the sky and looked for a beat note between the ground wave and the sky wave reflected back down, using that beat frequency to compute the height of the reflecting layer overhead.[^appleton] The method, frequency-modulated continuous-wave ranging applied to a target many kilometres up rather than a target directly below an aircraft, is the same beat-frequency arithmetic a radar altimeter still uses; Appleton's [[Ionosphere|ionosphere]] work supplied the technique before anyone had reason to point it at the ground instead of the sky. ### Everitt and Newhouse Engineers soon asked whether the same frequency-swept, beat-note technique could measure an aircraft's height above ground rather than a layer of the upper atmosphere. An analysis credited to Everitt and Newhouse in the later 1920s worked through what such an instrument would need in place of Appleton's fixed, vertical sounding of the ionosphere: a transmitter swept fast enough, and an antenna pattern narrow enough, to return a beat note from the ground alone rather than from every reflecting surface within range.[^everitt] ### Espenschied and Newhouse Lloyd Espenschied, working again with Newhouse at [[Bell_Labs|Bell Telephone Laboratories]], carried that analysis into hardware, building and flight-testing a frequency-modulated radio altimeter and patenting the technique in the late 1920s.[^espenschied] Their instrument set the layout every later FMCW altimeter has kept: a downward-looking [[Dipole_antenna|antenna]] pair, one transmitting and one receiving, a swept-frequency source, and a mixer that turns the beat note directly into a height reading rather than requiring an operator to time an echo by hand. ### Commercial introduction Instrument makers began selling radio altimeters for civil and military [[Aviation|aviation]] through the 1930s, and wartime demand for reliable low-altitude and blind-landing guidance turned a specialist instrument into standard equipment on larger aircraft; the wartime air arm that later became the [[United_States_Air_Force|United States Air Force]], and other air forces besides, fitted the new instrument across much of their heavier fleets.[^commercial] The move from laboratory demonstration to certified instrument required solving problems Appleton's ionosphere sounding never faced: a stable reading over a height that is constantly changing as the aircraft climbs or descends, immunity to the airframe's own vibration, and a cockpit display simple enough to read at a glance during an approach. ### Use as general purpose radar The frequency-modulated, continuous-wave technique the altimeter had proved out did not stay confined to looking straight down. Once engineers had a compact FMCW transmitter, mixer and beat-frequency readout working reliably in an aircraft, the same building blocks were an obvious starting point for other short-range radar problems that a pulsed set, built for detecting a target kilometres away, handled poorly; unlike the transponder-based ranging of [[Secondary_surveillance_radar|secondary surveillance radar]], an altimeter or its short-range relatives need no cooperating equipment on the ground or in the target at all.[^generalpurpose] The altimeter's engineering, not only its formula, fed forward into the wider family of continuous-wave radars still built on the same beat-note principle. ## Applications ### In civil aviation A civil radar altimeter typically covers a few feet to roughly 2,500 feet, the range within which height above the specific ground below, rather than height above sea level, actually matters for flying the aircraft. Its reading feeds the systems that warn a crew closing on terrain too quickly, drives the automatic height callouts read out during an instrument approach, and gives an automatic landing system the direct height measurement it needs to time its flare correctly, since a barometric altimeter's sea-level datum cannot by itself say how far above this particular runway the aircraft actually sits.[^gpws] Because the instrument is built to report no valid height at all rather than a wrong one when its echo is lost, over water in heavy rain or over terrain that scatters its beam poorly, the systems built on top of it are designed to recognise a missing radar-height signal and fall back to other cues rather than trust a stale reading; the radar altimeter remains one of the oldest sensors in the [[Avionics|avionics]] suite still doing essentially the job it always has. ### In military aviation Military use adds a demand civil aviation rarely faces: flying deliberately low, close enough to the terrain that the radar altimeter is a primary rather than a backup height reference. Terrain-following and terrain-avoidance systems combine a forward-looking radar's picture of the ground ahead with the altimeter's direct reading of height below to fly an automatic profile that hugs the terrain, and the same direct height measurement supports weapons that must detonate at a set height above ground rather than on contact.[^military] Because the altimeter transmits continuously while in use, an aircraft relying on [[Stealth_technology|stealth]] to remain undetected must weigh the height information it gains against the radio-frequency emission it gives away, a trade-off that belongs to the same family of problems [[Radar_jamming_and_deception|radar jamming and deception]] treats more broadly: any active sensor is also a beacon. ## International regulation Because a radar altimeter's accuracy depends on an interference-free channel close to its operating frequency, its band is protected internationally: 4.2 to 4.4 gigahertz is allocated worldwide to the aeronautical radionavigation service specifically for radar altimeters, and equipment operating there is designed on the assumption that the band stays clear of other transmissions.[^itu] That assumption was tested directly in the United States in the early 2020s, when telecommunications regulators auctioned an adjacent mid-band of spectrum for 5G mobile service and aviation regulators raised concerns that a sufficiently powerful, sufficiently close 5G signal could desensitise or mislead a radar altimeter's receiver; the dispute was resolved with power limits and buffer zones around the busiest airports rather than by moving either service out of its own band.[^5g] The episode illustrated a hazard specific to a radionavigation band that shares no channel with anything else in normal operation: a radar altimeter has little built-in experience rejecting a strong nearby interferer, because for most of its history it has never had to. ## Microsims This article carries no p5.js sketch of its own. A three.js companion, a variant of the [[Continuous-wave_radar]] sketch, instead renders an FMCW beam looking straight down, turning the beat frequency between a swept transmission and its own echo into the height reading described above. The same round-trip timing appears, in pulsed rather than swept form, in the sketches carried by the neighbouring Radar and Sonar articles, and the frequency shift a radar altimeter's beam would pick up from a moving target appears in the Doppler effect sketch. *Try:* in the [[Radar]] sketch, read the header formula `R = c·τ/2`; a radar altimeter solves the same equation for a two-way delay of nanoseconds to microseconds rather than the sketch's kilometre-scale ranges. *Try:* in the [[Sonar]] sketch, drag the assumed sound speed away from its default 1,500 metres per second and watch every measured range slide off the true-range ticks; a radar altimeter depends on the same assumption holding for radio waves in air, which is far steadier than sound in water but never perfectly constant either. *Try:* in the [[Doppler_effect]] sketch, raise the source speed slider and read the two observers' frequency shifts; a radar altimeter climbing or descending at a steady rate sees an analogous, if far smaller, shift in its own returning beam. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Radar_altimeter) : [Wikitube](https://en.wikitube.io/wiki/Radar_altimeter) Skeleton mirrored at revision 1363090344. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Continuous-wave_radar]] - [[Radar_horizon]] - [[Radar]] - [[Secondary_surveillance_radar]] - [[Frequency_modulation]] - [[Avionics]] - [[Stealth_technology]] - [[Radar_jamming_and_deception]] ## Notes Page numbers in the citations below are PDF pages of the open editions linked in the Bibliography; the standard round-trip timing relation `h = c·τ/2` and the FMCW beat-frequency relation used in Principle are ordinary textbook physics and are not separately footnoted, per the Wikitube style guide's §6.1. ## References ### Citations [^origin]: Citation needed: a history of early radio-reflection demonstrations, with primary sources for Marconi's and his contemporaries' observations of reflection from conducting surfaces, would fix the date and venue precisely. [^appleton]: Citation needed: Edward Appleton and Miles Barnett's original 1920s papers reporting the frequency-modulated, beat-note method of measuring ionosphere height would confirm the exact date and venue. [^everitt]: Citation needed: the original Everitt and Newhouse analysis applying beat-note ranging to aircraft altitude has not been pinned to a specific paper, date or venue in this pass. [^espenschied]: Citation needed: the Espenschied and Newhouse patent record at Bell Telephone Laboratories would confirm the patent number and the exact date of filing and grant. [^commercial]: Citation needed: a specific manufacturer's or air force's record of the commercial and wartime introduction of the radio altimeter would confirm the dates claimed here. [^generalpurpose]: Citation needed: a documented case of altimeter-derived FMCW hardware being adapted directly to another short-range radar application would support this claim with a specific system and date. [^gpws]: Citation needed: the specific regulatory mandate and system name that first tied ground-proximity warning and autoland flare timing to radar-altitude input would confirm the dates given here. [^military]: Citation needed: a named terrain-following radar system and its in-service date would give this claim a concrete example. [^itu]: Citation needed: the current International Telecommunication Union Radio Regulations table entry would confirm the exact boundaries of the protected band quoted here. [^5g]: Citation needed: the aviation and telecommunications regulators' own published record of the radar-altimeter/5G coexistence dispute would confirm the exact dates, frequencies and power limits involved. ### Bibliography - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. 2026. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory . CC BY. - Don Johnson. *Fundamentals of Electrical Engineering I*. 2014. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 . CC BY. - Michael Stiber; Bilin Stiber; Eric Larson. *Signal Computing: Digital Signals in the Software Domain*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/signal-computing-digital-signals-in-the-software-domain . CC BY-SA. - Allen Downey. *Think DSP: Digital Signal Processing in Python*. 2012. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python . CC BY-NC. - Steven Ellingson. *Radio Systems Engineering, Revised First Edition*. 2023. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. - John Dyer; Chad Davis. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods . CC BY-NC-SA.