The radio altimeter (RAD ALT) system provides the pilot with accurate indication of the aircraft’s height above the terrain, usually in the range of zero to 2500 feet AGL (Above Ground Level). A radio altimeter system consists of a receiver-transmitter, indicator, and transmit and receive antennas. (Some systems employ a single antenna for both the receive and transmit function.) Because the system operates at radar frequencies (C band, 4.3 GHz), radio altimeters are sometimes called radar altimeters. Radio altimeters differ from barometric altimeter systems in that they measure height above terrain, rather than barometric altitude.
Most radio altimeters use an FM modulated system to determine altitude, although some use a more traditional radar pulse system. The receiver-transmitter (RT) outputs a signal to the transmit antenna, which travels to the ground, and is reflected back to the receive antenna. Processing circuits in the RT compare the transmitted and received signals, which differ in timing due to the distance traveled to the ground and back. The indicator in the cockpit displays the calculated height above ground. The indicator commonly has a Decision Height (DH) control, allowing the pilot to set a height above ground at which a visual or audible alert is activated. In addition to the DH alerting capability, most radio altimeters RTs also provide up to six discrete outputs indicating when the descending aircraft passes through pre-set altitudes.
Radio altimeter outputs can interface with other aircraft components and systems (such as autopilot and ground proximity warning systems). Faulty operation of the radio altimeter can affect these interconnected systems resulting in fault conditions, flags and warnings. Proper servicing and calibration of radio altimeters is essential to reliable operation of these aircraft systems. Servicing radio altimeters requires specialized equipment to accurately simulate altitudes and signal strengths on the test bench. Investing in such equipment enables us to test, repair and calibrate a wide range of radio altimeter components.
A properly calibrated radio altimeter indicates zero feet at the moment the aircraft contacts ground. This requires taking into account the height of the antennas above ground (at touchdown), the length and propagation delays of the antenna cables, and the inherent delays in the RT. The sum of all these delay factors is termed the Aircraft Installation Delay (AID). The RT usually employs external strapping to select the correct AID for that particular aircraft installation, using specified antenna cable lengths. Strapping connections and antenna lengths must match the installation requirements of the aircraft, or the altitude will not accurately indicate zero at touchdown. After touchdown, the weight of the aircraft may cause the radio altimeter to read slightly below ground level. (The less common pulse-type systems usually have a zero foot calibration adjustment accessible through the case of the RT.)
An improperly aligned RT can result in spurious altitude outputs above 2500 feet AGL, creating nuisance flags and warnings. Some radio altimeters may produce spurious or inaccurate altitude outputs or flag conditions when the aircraft passes over unusually abrupt terrain changes or unusual surfaces (such as water). Improperly sealed, painted or bonded antennas can also adversely affect radio altimeter outputs and sensitivity. When the radio altimeter system operates unreliably, antenna bonding and cable integrity should be assessed, if the RT and indicator bench test normally. Radio altimeter antennas should only be refinished or repainted to manufacturers’ specifications.