Aircraft altitude is one of the most important parameters of air traffic, compliance with the planned trajectory in the vertical plane directly affects the safety of air travel. Particularly important is information about the exact value of altitude on takeoff and landing stage of aircraft.
Radio altimeter is designed to measure the value of the vertical distance from the aircraft to the earth's surface (Fig. 83). The measured value of the absolute altitude displayed to the pilot through an appropriate indicator or via electronic display.
Fig. 83. Measurement of flight altitude
The principle of measuring absolute altitude lies in radiation of probing signal from the aircraft board and receiving the reflected signal from the earth's surface. With the passage of time measured signal values it is possible to calculate flight altitude: where c - speed of radio waves propagation; t - time of the probing signal from an aircraft to the earth's surface.
To measure the altitude used two methods of construction altimeter, pulse or frequency.
According to the pulse method from the aircraft board emitted radio signal (frequency 845 MHz) in the form of radio pulses. The altitude is measured by the time the signal from the aircraft to the surface. Altimeter built according to the pulse method, measure the height in the range of 500-25000 m. They are not include to the standard set of avionics of aircraft.
However, more widely used frequency altimeter, which measure the passage of the signal. Aircraft radiated signal whose frequency varies linearly in the range 4200 - 4400 MHz. After a while, after re-reflection of the earth's surface, the radio signal is receiving by RAL. After processing the received signal is measured by its frequency, whose value is compared with the frequency of the signal emitted by the transmitter at a time (Fig. 84).
Fig. 84. Measuring of the frequency difference
The resulting difference frequency proportional to the signal in time and altitude of aircraft flight: where f - frequency difference between emitted and received signals; T - period of frequency modulation; Δf - frequency deviation.
Frequency method of measuring the true altitude is effective only at low altitudes – up to 1500 m.
The structure of RAL consists of transmitting and receiving antenna and receiving and transmission module. Antennas are placed underneath of aircraft. On board aircraft usually placed two independent sets of RA (Fig. 85). Example radio altimeter illustrated in Fig. 86.
Fig. 85. Location of RA antennas on B737