The function of the radio altimeter is to measure and display the vertical distance between the aircraft and the ground directly beneath it. It is important to remember that, whilst it is very accurate, it only measures vertical distance and is incapable of measuring terrain clearance ahead of the aircraft.
Principle of operation
Radio altimeters for civil use operate in the SHF band within the frequency range of 4200 MHz to 4400 MHz. A second frequency range of 1600 MHz to 1700 MHz, in the UHF band, is also reserved for radio altimeter operation, but is not used by civil aircraft.
The principle of operation is to continuously transmit a variable frequency signal in a relatively narrow beam vertically downward.
The signal is reflected from the ground and received at the radio altimeter receiver, located separately from the transmitter. Since it takes a finite time for the signal to travel to the ground and return, and given that the transmitted signal frequency is continuously changing, it follows that the received frequency will differ from the transmitted frequency.
The difference between the received and transmitted frequencies will vary as the aircraft height varies, and the time taken for the signal to travel to the ground and back varies. It is the frequency difference that is used to determine the aircraft height above the ground at any instant, using the speed of propagation of the radio beam and the rate of change of transmitted frequency, which are both known.
The transmitted signal is modulated to sweep over a frequency range of, typically, 100 MHz around 500 times per second. This is a deliberately low sweep rate, designed to avoid height ambiguity which might occur at a
higher rate of frequency change.
The principle components of a radio altimeter system are the transmitter, the receiver and the display unit. The display is incorporated in the ADI displays of EFIS or flight director equipped aircraft, the instrument includes a decision height feature that allows the pilot to set the decision height indel bug on the face of the instrument. When the pointer reaches the set height during a descent a visual and/or aural warning is activated.
The visual warning is usually in the form of a light and the aural warning may be a chime alert or a recorded voice message. In the event of failure of the system due to loss of power, a system or reception fault, a prominent warning flag appears on the face of the instrument. Additionally, the pointer will be obscured on these occasions or when flying above 2500 ft. The pointer will take up a known position when the press-to-test button is depressed. In some displays the instrument scale is logarithmic for heights above 500 ft.
The accuracy of the radio altimeter is given as ±1 ft or ±3% of the indicated height, whichever is the greater. It can be subject to errors due to reflections from parts of the aircraft structure, such as the landing gear, or to leakage of signals between the transmitting and receiving aerials. The positioning of the aerials is therefore very important and every effort is made by the manufacturer to avoid these errors.
It is also conceivable that the receiver might pick up a signal that has been reflected from ground to aircraft more than once, known as a multi-path signal. To a large extent this potentially ambiguous situation is avoided by gain control in the receiver.The principle use of radio altimeter information in large modern aircraft is in conjunction with the automatic landing system and the ground proximity warning system.