1、,Civil Aviation Flight University of China,Radio Altimeter,無線電高度表,Radio Altimeter ( RA),Civil Aviation Flight University of China,The radio altimeter system provides accurate terrain clearance altitude information displayed in the flight compartment, for use by the flight crew. The radio altimeter s

2、ystem consists of radio altimeter Receiver/Transmitter (R/T) unit, radio altimeter antenna, and display. A radio altimeter is called radio rather than radar because its transmissions are not pulsed as radar transmissions are. The transmission is a continuous wave with constant amplitude and a freque

3、ncy-modulated carrier of 4300MHz (C-band). The rate of frequency modulation is 100 times per second. The radio altitude is computed by determining the difference between the frequency of the reflected signal and the frequency of the signal being transmitted at the instant the reflected signal is rec

4、eived.,Radio Altimeter ( RA),Civil Aviation Flight University of China,Each R/T unit transmits RF signal to the ground through a dedicated transmit antenna. The reflected RF signals are received by a dedicated receive antenna and routed to the R/T unit for altitude computation. Radio altitude output

5、s from the receiver/transmitter units are transmitted to the flight instrument system display, as well as to the using systems autopilot flight director system, EICAS, ground proximity warning system and central warning system. The radio altimeter is called low range because it is not intended to op

6、erate at airplane altitudes above ground greater than 2500 feet. Its principal usage is during the final approach and takeoff. By the abbreviation LRRA, we mean “Low Range Radio Altimeter”.,Radio Altimeter ( RA),Civil Aviation Flight University of China,LRRA Theory,Figure 12.1,Civil Aviation Flight

7、University of China,LRRA Theory,If at time t1, frequency f1 is transmitted to the ground, the same frequency f1 will be received, after reflection, at time t2. The time difference T= t2- t1 is then the travel time of the wave to the ground and back to the airplane. During this time travel, the frequ

8、ency being currently transmitted has changed. At the time of reception of the reflected frequency f1, the current transmitted frequency is now f2,. Consequently, during the time interval T, the transmitter frequency has changed from f1 to f2. The frequency difference for given altitude is equal to F

9、(=f2-f1 ) which is proportionate to T. It is supposed that the airplane altitude is 300 ft, the time difference t(=t3-t1)then will be the travel time of wave to the ground and back to the airplane. And the frequency difference for 300ft will be equal to f(=f3-f1 ) which is proportionate to t.,Civil

10、Aviation Flight University of China,LRRA Theory,Therefore. (Given altitude/300ft)=T/t=F/f consequently. Given altitude=300ft(F/f) In fact, there are two 300ft reference signal generators in the radio altimeter system to produce the 300ft reference signal. The 300ft signal is a signal whose frequency

11、 is equal to a frequency difference f that would result from a 300ft altitude. The altitude processor and the monitor processor use 300ft reference signal for computation to assure output data integrity. The altitude processor receives its signal from the 300ft reference signal generator 1, whereas

12、the monitor processor receives its signal from the 300ft reference signal generator 2. Both processors perform identical computation function. The results are then compared in the monitor processor.,Civil Aviation Flight University of China,LRRA Theory,In the altitude processor, the frequency differ

13、ence F received from the signal mixer is compared with a 300ft reference signal f for the purpose of establishing their mutual ratio F/f. The value of the difference frequency F is a measure of the altitude, based on the slope of the modulation wave. The 300ft reference signal is the frequency diffe

14、rence f associated with a simulated precision altitude of 300 feet, also based on the slope of the modulation wave. Their ratio multiplied by 300ft is the measured altitude.,Figure 12.2,Civil Aviation Flight University of China,Radio Altitude Display,On B737 airplane, a LRRA indicator is used for th

15、e radio altitude display. But on B757 airplane the radio altitude and the decision height are displayed on the Electronic Attitude Director Indicator (EADI).,LRRA System Indicator,Figure 12.3,DH Light,Altitude Pointer,Self-test Switch,Decision Height Index,DH/MDA,Selector Knob,Warning Flag,Pointer M

16、ask,Civil Aviation Flight University of China,Radio Altitude Display,The LRRA indicator provides an analog display of radio altitude from minus 10 to 2500 feet. A pointer mask conceals the altitude pointer under certain conditions. A warning flag in view signifies that the altitude displayed is inva

17、lid. A Decision Height (DH) index, positioned by the rotation of the DH/MDA selector knob sets the reference altitude at which the DH light on the ADI illuminates. A self-test switch allows the initiation of a complete test of the indicator and R/T unit.,Civil Aviation Flight University of China,Rad

18、io Altitude Display,The output signal from the LRRA R/T unit drives the indicator pointer on the scale. If the measured altitude exceeds 2500 feet, the altimeter pointer is positioned behind the pointer mask.,Civil Aviation Flight University of China,Radio Altitude Display,The radio altitude and the decision height are displayed in the right-hand