Chapter 15 Radio Altimeter RA Radar Altimeter Overview
- Slides: 8
Chapter 15 Radio Altimeter RA
Radar Altimeter Overview The radar altimeter measures absolute altitude, the altitude the aircraft is above the ground, AGL (Above Ground Level). The radar altimeter system sends out a radio wave, and that radio wave bounces off the ground and is received. AGL is computed by taking the speed of the radio wave and the amount of time for it takes for it to be returned. Unlike radar, the signal is not pulsed, but FM and is transmitted at 4300 MHz at 350 milliwatts. Radar Altimeters operate between -20 and 2500 feet. The radar altimeter is primarily used for Decision Height Alerts and also GPWS (Ground Proximity Warning System), Gear Warning, “Rising Runway” (for Category II or IIIa approaches) symbol on the flight director and is used in helicopters during vertical descent in darkness as well as autopilot, autothrottle and flight data recorder.
Radar Altimeter Operation Textbook page 106 The radar altimeter produces a radio frequency carrier with a resting frequency of 4300 MHz. As it transmits, the frequency shifts 50 times per second off the resting frequency.
Radio Altimeter Operation The airplane is shown over a point on the ground. At this instant, the radar altimeter is transmitting the carrier at its lowest frequency (A). The signal travels to the ground and is reflected back to the airplane at the same frequency (A). The returning signal reaches the airplane after an interval of time, the number of microseconds it takes for the round trip.
Radar Altimeter Operation Because time has passed, the radar altimeter is now transmitting on a higher frequency (B). The system has 2 frequencies to process: A, the reflection from the ground and B the higher frequency. 3 quantities are now known: the amount the carrier frequency is changing, elapsed time for the signal to make the round trip and the speed of the signal (which is the speed of light).
Radar Altimeter Displays
Radar Altimeter Antennas The radar altimeter antennas are installed on the bottom of the aircraft.
Review Q & A Chapter 15 15. 1 What is the main application of the radar altimeter? Answer: Decision Height 15. 2 The radar altimeter measures ____ altitude. Answer: Absolute. 15. 3 The altitude indicated by a radar altimeter is AGL. What do the letters mean? Answer: Above Ground Level. 15. 4 What are 3 major components of a radar altimeter? Answer: Display, receiver transmitter, and antenna. 15. 5 What is the resting carrier frequency of a radar altimeter? Answer: 4300 MHz. 15. 6 What categories of approach require a radar altimeter? Answer: Category II and IIIa 15. 7 The carrier of a radar altimeter is frequency modulated, meaning it moves ____and ____ in frequency. Answer: Up, down. 15. 8 One factor measuring radio altitude is the difference in frequency between the ____and____ wave. Answer: Reflected and higher frequency. 15. 9 When are radar altimeters useful in helicopter operations? Answer: For vertical descents in darkness.
