Radio Altimeter provides accurate height of aircraft above ground level using Frequency Modulated Continuous Wave (FMCW) transmission
In addition to providing height when aircraft is between 50 – 2500 Feet AGL, it also provides height information to Auto Land system and GPWS
Since the minimum pulse width possible by pulse radar is half micro second, pulse radars are unsuitable for height measurement less than 250 feet
Therefore, Frequency Modulated Continuous Wave on SHF band on 4200-4400 MHz frequencies
Radio Altimeter accuracy is the higher of 3% of the indicated height or 1 Feet and the final display accuracy is 5 Feet
Radio altimeter provides a vertical beam width of 30° providing correct indications until 30° pitch up and down
Lateral beam width of radio altimeter is 60° providing correct indications up to 60° bank angle
Principle of Radio Altimeter
Radio Altimeter transmits a Frequency Modulated Continuous Wave on a frequency of 4300 MHz that is continuously varied by 50 MHz (4250 – 4350)
Frequency of transmitted wave is varied as a pre-determined saw-tooth pattern repeated 300 times per second
Radio Altimeter compares the frequency of received frequency with the transmitted frequency at any instant and converts it to height Above Ground Level
If the frequency difference is constant, the aircraft is at constant height, while it varies during climb or descent
At the minimum and minimum frequencies in sawtooth pattern, the frequency change reverses causing an ambiguity in height measurement
This ambiguity in position of received wave affects the height measurement which is automatically corrected by the receiver
Indications of Radio Altimeter
Radio Altimeter has a circular dial with a logarithmic graduated scale providing more accuracy at lower heights
Failure warning function masks the needle above 2500 feet above ground level or unserviceability of transmitter or power supply failure
Press to Test (PTT) can be used to test the system that swings pointer to 1000 ft
PTT button can be used to set the decision height on ILS to warn pilot on descending below the selected decision height
Decision Height selection provides both visual warning is in the form of a low altitude light and an audio tone on the headset
Installation Delay due to Residual Height
Radalt indicates negative height on ground due to aircraft installation delay which is a combination of residual height and cable length
Residual height varies with aircraft size, landing attitude, oleo extension and height of wheels in bogey under carriage
Cable length delay is due to the length of cable from avionics bay to landing gear in large aircrafts