航空电子设备-无线电高度表..ppt

1NBAA 20032NBAA 2003 Civil Aviation Flight University of China Radio Altimeter 无线电高度表Radio Altimeter ( RA)3NBAA 2003 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 system 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 frequency-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 received.Radio Altimeter ( RA)4NBAA 2003 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 outputs 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 operate 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)5NBAA 2003 Civil Aviation Flight University of China LRRA TheoryFigure 12.16NBAA 20037NBAA 2003 Civil Aviation Flight 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 frequency 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(=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. 8NBAA 2003 Civil 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 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 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.9NBAA 2003 Civil Aviation Flight University of China LRRA Theory In the altitude processor, the frequency difference 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 difference 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 12.210NBAA 2003 Civil Aviation Flight University of China Radio Altitude Display On B737 airplane, a LRRA indicator is used for the 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 Indicator1234510152025SETPUSHTO TESTABS ALTITUDEHUNDREDS OF FTDH/MDA Selector KnobDecision Height Index Warning FlagDH LightAltitude PointerSelf-test SwitchFigure 12.311NBAA 2003DH LightAltitude PointerSelf-test SwitchDecision Height Index DH/MDA Selector KnobWarning FlagPointer Mask12NBAA 2003 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。