A radio altimeter - alternatively termed a radar altimeter - is an airborne electronic instrument that measures the vertical distance between the aircraft and the terrain directly beneath it. By timing the interval between the transmission of a radio signal from the aircraft and the reception of its reflection, the device delivers accurate height readings above the Earth's surface. These precise measurements are pivotal during approach, landing, and climb-out phases, when knowing the exact clearance below the aircraft is vital for safe flight operations.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a radio altimeter in aviation?
A radio altimeter measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft. This type of altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined vertical datum, usually mean sea level.
The International Telecommunication Union defines radio altimeters as radionavigation equipment on board an aircraft used to determine the height of the aircraft above the Earth's surface or another surface. A radio altimeter is an airborne electronic device that measures vertical distance between aircraft and ground. The altimeter is a self-contained on-board aid used to measure true height of the aircraft.
How does a radio altimeter work?
A radio altimeter works by transmitting a radio signal to the ground and measuring the time it takes for the signal to return. The instrument times the elapsed interval between the outgoing pulse and the reflected echo. Because the speed of the radio wave is constant, this interval is a direct function of aircraft height.
Pulse systems send short bursts and count the delay, whereas an FMCW altimeter transmits a variable-frequency signal downward, then compares the frequency shift between the outgoing and returning waves to calculate altitude. The result is a precise reading, displayed to the crew as a numerical height above ground, updated continuously during approach and landing.
A radio altimeter is a device that offers an accurate reading of the aircraft’s altitude. By calculating the time taken by a pulse to return back to the aircraft after reflection, the altimeter offers an exact reading. While we were unmoving at the airport terminal, the dial accurately depicted our elevation above ground.
What is the frequency of a radio altimeter?
Radio altimeters transmit high-frequency radio signals in the 4.2-4.4 GHz band (13.8-14.5 GHz). The centre frequency is approximately 4300 MHz (14,170 MHz) with a frequency stability of typically up to 25 MHz ( 82 MHz) over a temperature variation of 55°C (67°F) to +70°C (+158°F), and this wavelength corresponds to roughly 7 cm (2.8 in). A second frequency range from 1600 MHz to 1700 MHz is reserved for radio altimeter operation but is not used by civil aircraft.
Radio altimeter and 5G interference arises because the FCC approved the use of the previously reserved 3.7-3.98 GHz band (3.7-3.98 GHz) for 5G systems deployment. The 4.2-4.4 GHz band is separated by only 220 megahertz (0.22 GHz) from these C-band telecommunication systems, so 5G spurious emissions land within the 4.2-4.4 GHz band directly. The FAA determined that aircraft will encounter 5G C-band signals at power levels shown to create interference, and FAA Airworthiness Directives 2021-23-12 and 2021-23-13 provide guidance on this risk. To mitigate the problem, the FAA now requires reduced power limits near airports for 5G emitters and a downward tilt requirement for 5G base stations. AT&T and Verizon reduced power levels near airports.
What is the operating range of a low altitude radio altimeter?
Low-altitude radio altimeters used in precision approaches have an operating range of 0 to 5000 ft (0 to 1524 m). Most radio altimeters in planes measure from about -20 ft (-6 m) to 2,500 ft (762 m), while typical pulse radio altimeters have a reported altitude range from -6 m (-20 ft) to 2,500 m (8,200 ft). The ALA-52B radio altimeter exemplifies this class with an operating range of -20 to +8,000 ft (-6.1 to 2,438 m). These instruments operate in the 1540-1660 MHz band and provide a continuous indication of height above the earth's surface up to the stated maximum height.
How accurate is a radio altimeter?
A radio altimeter accuracy is 3% of the indicated height. The normal error is less than one meter, a precision well-suited for low-altitude approach, landing, and climb-out phases. Because radio waves are reflected by the sea, the radar altimeter performs reliably over water as over the ocean AGL happens to be MSL.
The ALA-52B provides accurate digital height measurements above terrain during approach, landing, and climb-out phases. Honeywell proved the ALA-52B can withstand 5G interference after electromagnetic interference testing performed on behalf of the FAA.
What are the limitations of a radio altimeter?
The limitations of a radio altimeter are outlined below.
- Radio altimeters have a limitation of 2500 ft
- Radar altimeters below it
- Radar altimeters may be susceptible to interference either
- Unintended signals can potentially lead to inaccurate or misleading altitude readings.
- Interference can cause misleading RA data
- Interference can cause loss of RA data
- RTCA and Manufacturer Testing indicate loss of RA data or misleading RA data may occur
- Different RA models have different levels of susceptibility
- Different RA models have different susceptibility levels
- Some manufacturers extend the limit up to 6000 ft
- RTCA and Manufacturer Testing found loss of RA data may occur
Radio altimeters have a limitation of 2500 ft. They cannot see terrain directly ahead of the aircraft but only below it. Radio altimeters may be susceptible to interference either within their band or within nearby frequency bands which can cause misleading RA data. Other electronic devices emit signals that overlap with the radio waves used by the altimeter, and this overlap leads to inaccurate altitude readings. Pulse systems had relatively low accuracy, particularly at higher altitudes, because propagation time delay determines altitude.
5G C-Band signals bleed into the RA band, and 5G fundamental emissions overcome altimeter receiver filtering. Spurious emissions from 5G enter the radar altimeter band, and interference entering the receiver negatively impacts performance. FAA flight evaluations confirmed that aircraft in the United States will encounter 5G C-Band signals at power levels shown to create interference. High-power emission sources like ground-based surveillance radars are generally far from the band, but a guard band of 220 MHz separation is needed. FAA requires reduction in spurious emission limits, and implementation of a downward tilt requirement is needed for 5G C-Band. Airworthiness Directives for transport category aircraft prohibit certain low-visibility landing operations in presence of 5G C-Band interference, and aircraft not hardened against 5G effects will not be permitted to perform low-visibility landing operations anywhere.
What is the difference between radio altimeter and radar altimeter?
Radio altimeter and radar altimeter are often synonymous but their application is context-dependent on the industry. Both technologically refer to equipment used to measure true altitude above terrain through electromagnetic waves, but the terms may differ in naming conventions and usage context.
The difference between radio altimeter and radar altimeter is explained in the table below.
| Attribute | Radio Altimeter | Radar Altimeter |
|---|---|---|
| Usage | Used in civil aviation terminology | Used in military, aerospace, and UAV contexts |
| Working Principle | Transmits radio waves and receives ground reflections | Based on radar ranging technology |
| Operational Altitude | Typically effective from 0 to 2,500 feet | Wider range, including ultra-low altitudes and high-speed platforms |
| Application | Used in autoland systems and low-altitude obstacle avoidance | Supports terrain-following, target tracking, and rapid distance measurement in complex environments |
| Actual Difference | Different terms for the same technology | A terminological distinction, not a technical one |
What is the difference between radio altimeter and DH?
A radio altimeter is the sensor that supplies the height reading itself whereas a decision height (DH) is your height above the touchdown zone elevation (TDZE) and uses the runway as its baseline. For CAT I approaches, a radio altimeter is not required. Minimums are taken from the barometric altimeter and are expressed as a decision altitude (DA) referenced to mean sea level. For CAT II and CAT III approaches, the DH is always assessed by reference to the radio altimeter, never to the barometric altimeter, and the minima are expressed as DH, not DA. The radio-altimeter decision height (RA-DH) is therefore the published minimum for those approaches. The radio altimeter warning occurs as the aircraft descends through the selected RA-DH, alerting the crew that the decision point has been reached.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What are the types of radio altimeters?
The types of radio altimeters are outlined below.
- Pulsed-type Radio Altimeter: The pulsed-type radio altimeter uses the radar principle in measuring the absolute height of an airplane above the terrain below. A pulse of radio frequency energy is transmitted towards the earth and the time which elapses between the transmitted pulse and the received pulse is measured.
- FM Radio Altimeter: A Frequency Modulated altimeter is designed to measure altitude accurately at low altitudes and has a small fixed error.
As of 2010, all commercial radar altimeters use linear frequency-modulated continuous-wave (LFMCW or FMCW) altimeters. Most civil/commercial Rad Alts use linear FMCW modulation. Military rad alts use PSK pulsed modulation, PSK CW modulation, and pulsed CW modulation. Some civil/commercial Rad Alts use pulsed CW. FreeFlight Systems TERRAIN SERIES radar altimeters are engineered to resist 5G C-Band interference and meet AD 2023-11-07.
Where is the radio altimeter located?
The radio-altimeter( transmit/receive pair (Tx/Rx)) is usually located in the forward fuselage, but the radio-altimeter antennas are commonly positioned on the bottom of the aircraft. This belly installation keeps the narrow beam pointing vertically at the ground from take-off to flare, giving an unobstructed view of the terrain below.
What are the components of a radio altimeter?
The components of a radio altimeter system are the transmitter, the receiver and the display unit. Transmitter and receiver are the main components of the radio altimeter; the transmitter is located separately from the receiver. The altimeter system provides a display unit for altitude indication, showing absolute altitude.
A single-antenna FM radio altimeter comprises a transmitter, a receiver and a processing block. The antenna, transmitter and receiver circuitry is included in a hermetically sealed housing. Digital Signal Processor (DSP) performs altitude computation and all other Input/Output functions of the altimeter. The DSP is designed to be remotely located from the antenna, transmitter and receiver hermetic assembly.
A radio altimeter system includes discrete transmit (TX) and receive (RX) antennas. A circulator connects to the antenna via a coax cable. The difference in two frequencies is extracted in a frequency mixer. Reflected signal is received at the receiver after the frequency mixer extracts the difference in two frequencies.
The altimeter system provides a coax cable for remote location of RF components. The transmitter includes a first oscillator for producing a clock signal. The frequency mixer extracts the difference of two frequencies. A separate TX antenna introduces TX leakage signal into the receiver, causing interference.
What causes radio altimeter failure?
A radio altimeter failure springs from three principal areas: the antenna-cabling run, the internal unit, and the atmosphere outside the aircraft. Moisture causes corrosion between the antenna and the fuselage, while the cabling itself becomes nicked or damaged during routine maintenance or vibration making connectors intermittent. If the unit works on the bench but fails at altitude, this most often indicates an issue with the antenna or with the cabling to the antenna.
Inside the aircraft, the cause of the altimeter failure could be a leak at the connector that links the static circuit with the cabin differential pressure indicator. This leak feeds erroneous pressure data and produces a mushing error that makes the indicated height lag or drift. Electromagnetic interference caused by nearby electrical equipment or power systems masks the reflected radio signal and creates the same mushing error or a complete dropout.
Outside the aircraft, external RF interference lasting only tens or hundreds of milliseconds is highly likely to present as a common-mode failure across all radar altimeters onboard, and the introduction of 5G networks operating within the aeronautical GHz band is expected to introduce harmful interference with radar altimeters.
If the radio altimeter fails, the MOM advised operators of B737s and Boeing Business Jets to monitor their primary flight instruments and not to use the autopilot/throttle systems when approach and landing, because radio altimeter failure causes loss of autopilot/autothrottle functionality, incorrect flare altitude setting, or the autothrottle to go idle incorrectly. Flight crews must inform ATC when they are unable to determine the aircraft's actual altitude and must exercise extreme caution when flying in proximity to obstructions or terrain.
How is radio altimeter calibration performed?
Radio altimeter calibration is performed on the ground through an automated system that utilizes a calibration switch. The installer invokes the calibration mode by depressing a switch for a short period of time, which causes the radio altimeter's transmitter section to initiate a calibration radio altimeter signal. This calibration signal is transmitted out through the radio altimeter's transmit antenna, reflected off the ground, and received by the altimeter's receiver section, guaranteeing the radio altimeter indicates zero altitude on the ground.
Current radio altimeter calibration technology requires adjusting the length of the cable connecting the radio altimeter to its antenna during installation. The antenna cable length is adjusted to fine-tune the system and eliminate any delay that affects accuracy. Specialized test sets like the ALT 8000 and EA 6061 support this process by generating signals that mimic ground returns at various altitudes, simulating different altitude scenarios. These test sets provide variable delay and calibrated propagation loss, measure RADALT receiver sensitivity, and can calibrate the delay of test cables. Through these combined procedures, performance verification and calibration of radio altimeters are completed quickly and reliably at manufacturing sites, calibration centers, and flight-line avionics depots.
What aircraft systems use radio altimeter data?
Aircraft systems that use radio altimeter data include Ground Proximity Warning Systems (GPWS), Enhanced Ground Proximity Warning Systems (EGPWS), and Terrain Awareness and Warning Systems (TAWS). These systems issue audible and visual alerts when the aircraft approaches dangerously close to the ground, obstacles, or steep terrain.
Radio altimeter data feeds into Traffic Collision Avoidance Systems (TCAS) and Automatic Landing Systems (Autoland). TCAS uses radio altitude to compute collision avoidance parameters, while Autoland uses the data to monitor the approach and trigger various alarms during Category III automatic landings. The TCAS processor uses pressure altitude, radar altitude, and discrete aircraft status inputs from the own aircraft to logic the collision avoidance control parameters.
Radio altimeter data is provided to the flight computer's autothrottle, instrument system, Search and Rescue (SAR) autopilot modes, hover autopilot modes, and windshear detection and alerting algorithms. It serves as input for Instrument Landing System (ILS) approaches, Required Navigation Performance (RNP) procedures with Authorization Required (AR), and manual flight control guidance system operations. It is used for Enhanced Flight Vision System (EFVS) touchdown, alert lamps or EFIS display, visual and audio alert messages on flight-deck speakers and intercom, and aural callouts.
The inertial navigation system and the inertial guidance system both rely on exact elevation information. The former uses it to perform a touchdown and the latter to run a fine flare. The ground closeness alarming scheme, GPWS, receives precise height-above-terrain scale from the radio measuring device and can air an opportune GLIDE-SLOPE aural alarm. Because the wireless measuring device gives exact altitude information, the autoland scheme is able to function in the rigorous circumstances of Category III approaches where touchdown happens with nearly no outdoor precision.
What are the radio altimeter requirements?
A radio altimeter must meet the protection criteria of ITU-R M.2059-0 Annex 3 to avoid harmful interference. The FAA developed interference tolerance requirements that are used across the affected fleet which are described in the policy statement - Demonstration of Radio Altimeter Tolerant Aircraft. An airplane is deemed tolerant when it demonstrates the tolerances using an FAA-approved method whereas a non-tolerant airplane does not demonstrate these tolerances. The required antenna isolation for maximum altitude is a minimum of 80 dB.
Regulatory mandates differ by aircraft category. After April 24, 2017, no person operates a rotorcraft unless it is equipped with an operable FAA-approved radio altimeter or an FAA-approved device that incorporates a radio altimeter, unless otherwise authorized in the certificate holder's approved minimum equipment list. The Administrator authorizes deviations from this requirement through a Letter of Deviation Authority. Airworthiness directives AD 2021-23-12 and AD 2023-10-02 apply to transport and commuter category airplanes equipped with a radio altimeter, while AD 2021-23-13 applies to helicopters so equipped. Operators of foreign-registered airplanes are urged to voluntarily comply with U.S. actions.
Standard operating procedures must include radio-altimeter use procedures. Flight crews call indications below obstacle-clearance requirements during approach, and the call is made at 2,500 feet (762 meters). Flight crews and pilots must apply ICAO corrections for low temperatures to all published altitudes: minimum en route altitude (MEA), minimum safe altitude (MSA), transition route altitude, procedure turn altitude, final approach fix (FAF) altitude, step-down altitudes and MDA(H) during a non-precision approach, outer marker (OM) crossing altitude during an ILS approach, and initial approach 1,000 feet (304.8 meters).
Cost is driven by technical specifications and certification. An example unit, the Ultra 4503-100, meets MIL-STD-461E, employs Adaptive Power Control to minimize transmit power, provides outputs in RS-422, ARINC-429 and RS-232 formats, measures 3.5 in (8.89 cm), 3.0 in (7.62 cm), and 1.75 in (4.445 cm), weighs 1.2 lbs (0.54 kg), and requires 80 dB antenna isolation. While the FAA does not maintain a public list of tolerant altimeters, each model must undergo individual FAA approval, and operators bear the expense of procurement, installation, and any necessary avionics integration to comply with directives.
What is the history of the radio altimeter?
The history of the radio begins in 1924. The American Lloyd Espenschied invented the first radio altimeter in 1924, yet the device was not sold until 1937. It took fourteen years before Bell Labs was able to put the device in a form adaptable for aircraft use, and in 1938 the FM radio altimeter was first demonstrated in New York by Bell Labs. Radio altimeters were first used in October 1938 - the German Junkers Ju-87 Stuka dive bomber carried one for automatic pullouts. During World War II the radio altimeter gained widespread acceptance, and the cavity magnetron invented in Britain was sent to America in the Tizard Mission of September 1940. Espenschied eventually became the director of high-frequency transmission advancement at Bell Telephone Laboratories, where the underlying concept had originated in studies of long-distance telephony.