What is a Radio Altimeter?
A Radio Altimeter (RADALT) is an electronic instrument used to measure the distance between an aircraft and the terrain directly below it. Also known as a radar altimeter, the device analyzes the time and frequency signatures of reflected radio waves between 4,250 and 4,350 MHz to provide crucial military and commercial flight navigation data.
Invented by Lloyd Espenschied nearly 100 years ago, the radio altimeter gained widespread acceptance during World War II and has since become an indispensable element of flight control. In addition to providing ongoing altitude data to the cockpit, an aircraft radio altimeter is requisite for instrument landings, ground proximity warning systems (GPWS), and the low-altitude, low-visibility flight operations that are instrumental to military aviation.
VIAVI provides the most comprehensive radio altimeter test equipment available for both military and commercial applications with direct connect or antenna RF output testing and fully dynamic altitude simulation. Customizable takeoff, departure, approach, and landing profile legs can be created and recalled for any scenario, thereby ensuring the safe operation of this vital equipment by replicating in-flight conditions.
Who Does VIAVI Support Through Our RADALT Offerings?
Radio altimeters are now obligatory for commercial aviation, with most airliners equipped with redundant RADALT equipment. During flight, elevation data between takeoff and 2,500 feet above ground level (AGL) is monitored continuously by radio altimeters. Much of the advanced flight technology familiar to casual passengers, including auto-landing in inclement weather and the flare maneuvers that help pilots to facilitate soft landings, are only made possible through the use of radio altimeters.
In military applications, situational awareness gains heightened importance as terrain, water, and other obstacles can represent immediate threats or tactical decision points. This is especially true for the low altitude flight paths utilized to avoid radar detection. During combat, highly accurate radar altimeters can also be used to trigger automatic pilot safety algorithms designed to prevent grounding.
The numerous safety benefits of the radio altimeter in aircraft are readily apparent and supported by historical data. In recognition of this fact, the FAA has extended requirements for radio altimeter deployment to helicopters used for commercial or general aviation. As this mission-critical equipment continues to propagate the industry, a versatile radio altimeter tester provides the logical complement with the capability to proactively diagnose issues, improve reliability, and avoid costly and unnecessary delays, equipment pullouts, or altimeter servicing.
What Types of RADALT Does VIAVI Test?
The VIAVI ALT-8000 FMCW/CDF and Pulse Radio Altimeter Flight Line Test Set is the industry leader for commercial applications. With linear altitude simulation up to +5500 ft. for both pulse and frequency-modulated continuous wave (FMCW) applications, the ALT-8000 is compatible with a wide variety of commercially available radio altimeters, including the most popular offerings from Honeywell, Rockwell Collins, and other leading manufacturers.
Military applications for radio altimeter systems continue to push the envelope for performance and accuracy. Advanced mission parameters require sustained low altitude flight paths, rapid ascent and descent, and integrated forward-looking, terrain following radar to supplement downward-facing RADALT equipment.
The ALT-8015 FCMW/CDF and Military Radio Altimeter Test Set supports the demanding accuracy requirements for a growing number of military applications, with high-resolution programmable flight simulation profiles at linear altitude rates of up to 120,000 feet/min. Highly advanced new VIAVI test solutions are continually being developed to encompass an even greater percentage of military radio altimeter test applications.
Types of Altimeters
The radio altimeter is elementally woven into modern air travel, although additional altimeter types remain in use for aviation and other purposes. The radio altimeters supported by VIAVI test solutions include the distinct subcategories of pulse and FMCW altimeters.
As the name suggests, the pulse method directly interprets roundtrip time of a reflected radio pulse as a function of distance. The interchangeable monikers of radio altimeter and radar altimeter may be attributable to this similarity with conventional pulse radar operation. The FMCW method, which interprets the difference between send and receive frequencies to determine altitude, has become an increasingly prevalent radio altimeter option for commercial and military aviation, as the digital signal processing technology has continued to improve.
The barometric altimeter predated the radio altimeter as an essential flight control and safety device and remains in use today for determining altitude both above and below 2,500 feet. The primary difference between radio altimeter and barometric altimeter technology is that the latter is based on air pressure rather than direct altitude measurement.
Because the atmospheric pressure drops proportionally to altitude increase, the difference between the observed pressure and the known pressure at the airfield elevation can be used to determine the altitude of the aircraft. Weather conditions can also influence air pressure, so this must be carefully considered to obtain accurate barometric altimeter readings.
Modern technology continues to provide additional platforms for altimeter operation. However, many newer methods are currently used only for unregulated applications such as mountaineering and hobbyist drone operation. This includes GPS or Global Navigation Satellite System (GNSS) altimeters as well as laser altimeters that utilize reflected infrared light waves rather than radio waves.
How Do Radio Altimeters Work?
The basic operating principles behind pulse and FMCW radio altimeters are based on the constant speed of light (300,000 km/sec), which enables accurate altitude calculations to be performed based only on the roundtrip time of the pulse or the frequency shift of the radio waves. Each radio altimeter system includes a receiver transmitter as well as discrete transmit (TX) and receive (RX) RADALT antennas. A signal is sent from the transmitter to the transmit antenna, reflected off the ground, and then returns to the receive antenna.
The ultra-high speed of the transmitted signal allows an aircraft moving at Mach 1 or greater speed to send and receive signals from the same relative position on the aircraft without introducing significant radio altimeter errors. Using a pulse altimeter, the altitude is determined based on a simple calculation of pulse round trip duration times one half the speed of light. Most radio altimeters will operate within a narrow band of their overall 4,250 - 4,350 MHz range, remaining accurate to within ± 2 feet at low altitude.
FMCW altimeters rely on this consistent radio frequency transmission to interpret the proportional shift of the returning signal. Each 40 Hz increment between the transmitted signal frequency and that observed by the receiving antenna is equivalent to one additional foot of altitude. The potential impact of even a minute variation in frequency on navigation and safety highlights the need for certified, precise, and portable radio altimeter test equipment with the ability to simulate all expected airborne conditions.
Why Test Your RADALT?
Despite the inherent criticality of their function, radio altimeters in aircraft testing is not formally mandated, leaving many pilots and operators with only “Go/No-Go” operational status to ascertain readiness. Even with the robust construction and quality standards observed by manufacturers, radio altimeter errors can lead to potentially hazardous or catastrophic consequences. An undetected malfunction can induce an incorrect approach angle, unsafe obstacle clearance height, or early flare activation, among other potential hazards.
Despite continuous technology upgrades, radio altimeter accuracy is further challenged by unpredictable inputs, including variable ground conditions, thermal noise, and available bandwidth. Safety and performance concerns can be exacerbated by sources of radio altimeter interference leading to aviation industry lobbying of the FCC to restrict nearby frequency bands to limit flexible use applications such as 5G communication.
VIAVI radio altimeter test sets have created a new and unique test solution category, introducing altimeter test capabilities far beyond what was previously available. With a direct-connect option supplemented by convenient radio altimeter system testing over supplied antenna couplers, flight safety can now be quickly and cost-effectively ensured using a lightweight and user-friendly altimeter tester. This comprehensive verification and analysis effectively isolates faulty RADALT antenna, cable, or radio components thus avoiding costly and time-consuming troubleshooting efforts.
Radio Altimeter Test Sets from VIAVI
The altimeters deployed on commercial airliners for over fifty years underscore the utility of advanced radio altimeter test equipment. Along with the unparalleled capabilities of the VIAVI radio altimeter test set offerings, the world’s only RF-based portable radio altimeter test solution also provides unmatched precision and usability.
The easily programmable touchscreen interface of the ALT-8000 radio altimeter test set allows for intuitive simulation profile creation, storage, and recall with pulse and FMCW linear altitude simulation accurate to ±1.5 ft. RADALT and RF type can be detected automatically prior to testing, saving valuable configuration time.
Power and frequency measurement for all radio altimeters under test remains accurate to within ±2 dB and ±10 MHz, respectively. A split-screen test view makes it easy to monitor and compare UUT and simulator values in real-time during test execution, and an easily enacted self-test routine is included to further guarantee reliable test performance.
Demanding military applications benefit from the ruggedness and versatility of the ALT-8015 radio altimeter test set. Unique military flight profiles are accommodated by the flexible altitude and duration options for each discrete leg of the simulation profile. Rapid ascent and descent rates can be simulated precisely, with a linear altitude range extending to 8,000 ft. AGL for both pulse and FMCW military radio altimeter types. The accuracy and reliability of the ALT-8015 is further supported through available delay calibration and RADALT sensitivity testing features.