Downloaded 389 times
Uploaded byAjab Tanwar
Radio altimeter
This document discusses different types of aircraft altimeters. It describes three main types: pulse type radio altimeter, conventional FMCW altimeter, and constant difference frequency FMCW altimeter. It focuses on explaining the basic principle and operation of conventional FMCW altimeters. The conventional FMCW altimeter measures altitude by varying the carrier frequency linearly over time and determining the change in frequency during the round trip travel time to calculate altitude. The beat frequency detected is directly proportional to the aircraft's altitude.
More Related Content
Radio altimeter
- 2. CONTENTS PURPOSE CLASSIFICATION PRINCIPLE BLOCK DIAGRAM INDICATOR
- 3. PURPOSE It measuresinstantaneous altitude of the aircraft above the ground
- 4. CLASSIFICATION According to themethod of time measurement, altimeters can be classified to three types. i) Pulse type radio altimeter ii) Conventional FMCW altimeter iii) Constant difference frequency FMCW altimeter
- 5. BASIC PRINCIPLE Aircraft altitudeis measured using the basic principle of radio ranging i.e. measuring the elapsed time between transmission of the e.m wave and its reception after reflection from the ground. The altitude is given by half the product of the elapsed time and the speed of light. 2H= C*T Where H = Altitude of the aircraft T = time taken for round trip travel C = speed of light +
- 6. CONVENTIONAL FMCW PRINCIPLE OFOPERATION In a simple continuous wave transmitter/ receiver the time between transmission and reception can not be measured as there is no reference mark to note the time of transmission and reception.
- 7. CONVENTIONAL FMCW In aconventional FMCW altimeter the carrier frequency(4200-4400MHz) is varied linearly at a known rate over a predetermined range. Instead of measuring the time between transmission and reception directly, the change in carrier frequency during the period is measured and divided by rate of change which gives the two way travel time T. To measure the change in carrier freq during the two way travel time the echo is mixed with a sample of the signal being transmitted at that instant and the beat frequency is derived.
- 8. CONVENTIONAL FMCW The beatfreq is proportional to the a/c altitude.
- 9.
- 10. CONVENTIONAL FMCW fc =frequency of carrier wave fh = Beat frequency fm = frequency of triangular modulating wave Δf = maximum deviation of carrier frequency amp t 1/fm
- 11. CONVENTIONAL FMCW Beatfrequency = rate of change of carrier frequency x T where T = The round trip travel time of the transmitted wave Transmitted Radiatedfrequencyfc TimeT fh Δf 1/fm Received
- 12. CONVENTIONAL FMCW Rate ofchange of carrier frequency = Total change ÷ time taken for the change Radiatedfrequencyfc TimeT fh Δf 1/fm ReceivedTransmitted
- 13. Rate of changeof carrier frequency = ∆f/2 ÷ 1/4fm = ∆f/2 x 4fm = 2 ∆f fm CONVENTIONAL FMCWRadiatedfrequencyfc TimeT fh Δf 1/fm ReceivedTransmitted
- 14. CONVENTIONAL FMCW Beat frequencyfh = 2 ∆f fm * T = 2 ∆f fm * 2H/C (where T= 2H/C) = 4 ∆f fm H/C H = fh C / 4 ∆f fm
- 15. CONVENTIONAL FMCW Here theterm 4 ∆f fm /C is a constant value for a given conventional FMCW. So beat frequency fh ∞ H
- 16.
- 17. CONVENTIONAL FMCW ALTIMETER Transmitter MixerAmplifier Limiter Counter Indicator Modulator
- 18. CONSTANT DIFFERENCE FREQUENCYFMCW Transmitter Indicator Reference frequency Mixer Amplifier Discriminato r Loop control Modulator Frequency Gain
- 19. PULSE TYPE RADIOALTIMETER PRINCIPLE OF OPERATION The system is based on the principle of echo range timing i.e. the time taken by a radio pulse to travel out & back is a direct measurement of the distance(Altitude) as the velocity of an e.m wave is constant.
- 20.