Views: 3 Author: Site Editor Publish Time: 2023-02-02 Origin: Site
A radar altimeter (RA),also known as a radio altimeter (RALT),electronic altimeter,reflection altimeter,or low-range radio altimeter (LRRA),measures altitude above terrain beneath an aircraft or spacecraft by timing how long it takes a radio beam to travel to ground,reflect and return to the aircraft.This type of altimeter provides the distance between the antenna and the ground directly below it,in contrast to a barometric altimeter,which provides the distance above a defined vertical datum,usually mean sea level.
As the name implies,radar (radio detection and ranging) is the basic principle of the system.The system transmits radio waves down to the ground and measures how long it takes for them to be reflected back to the aircraft.The height above the ground is calculated from the travel time of radio waves and the speed of light.Radar altimeters required a simple system to measure time-of-flight that could be displayed using conventional instruments other than the cathode ray tubes commonly used by early radar systems.To do this,the transmitter sends a frequency modulated signal whose frequency varies with time,rising and falling between two frequency limits Fmin and Fmax for a given time T.In the first unit this is done using an LC tank with a tuning capacitor driven by a small motor.The output is mixed with the RF carrier signal and sent out to the transmitting antenna.Since it takes some time for the signal to reach the ground and return,the frequency of the received signal is slightly delayed relative to the signal sent at the time.The difference between these two frequencies can be extracted in the mixer because the difference between the two signals is due to the delay in reaching the ground and returning,and the resulting output frequency encodes height.The output is usually on the order of hundreds of cycles per second,not megacycles,and can easily be displayed on an analog instrument.This technique is known as frequency modulated continuous wave radar.Radar altimeters typically operate in E-band,Ka-band, or for more advanced sea-level measurements,S-band.Radar altimeters also provide a reliable and accurate means of measuring water surface altitude when flying long distance trajectories.These are critical for use when operating in and out of oil rigs.The altitude specified by the device is not the altitude indicated by the standard barometric altimeter.Radar altimeters measure absolute altitude altitude above ground (AGL). Absolute altitude is sometimes called altitude because it is the altitude above the base terrain.As of 2010,all commercial radar altimeters use linear frequency modulation-continuous wave (LFM-CW or FM-CW).As of 2010, approximately 25,000 aircraft in the U.S.had at least one radio altimeter.
Appleton's ionospheric measurements
During this same period, the physics community raged on about the nature of radio propagation.Guglielmo Marconi's successful transatlantic transmission seemed impossible.Studies of radio signals have shown that they travel in straight lines, at least over long distances,so broadcasts from Cornwall should be lost in space rather than received in Newfoundland.In 1902,Oliver Heaviside in Great Britain and Arthur Kennelly in the United States independently postulated the existence of an ionosphere in the upper atmosphere that reflected signals back to the ground for reception.This is called the Heaviside layer.While this is an attractive idea, direct evidence is lacking.In 1924, Edward Appleton and Miles Barnett demonstrated the existence of such layers in a series of experiments conducted in collaboration with the BBC.After the scheduled transmission for the day ended,the BBC transmitter in Bournemouth sent out a signal that slowly increased in frequency.This was picked up by Appleton's receiver in Oxford where two signals emerged.One is a direct signal from the space station,known as ground waves,while the other is received in time after reaching the Heaviside layer and back again, known as sky waves.The trick is how to accurately measure the distance the sky wave travels to prove that it is indeed in the sky.This is what changing the frequency is for. Since the terrestrial signal travels a shorter distance,it is closer in time and therefore closer to the frequency it was sent at that time.Sky waves,which have to travel longer distances,are delayed and thus are at a frequency some time ago.By mixing the two in a mixer,a third signal is produced,which has its own unique frequency,encoding the difference of the two inputs.Since the difference is caused by the longer path in this case,the resulting frequency directly reveals the path length.Although more technically challenging,this is ultimately the same basic technique that Bell uses to measure the distance to a reflector in a wire.
Everitt and Newhouse
In 1929,Ohio State University professor William Littell Everitt began considering Appleton's basic technology as the basis for an altimeter system.He assigned the work to two seniors,Russell Conwell Newhouse and M.W.Havel.Their experimental system was more similar to Bell's earlier work,using changes in frequency to measure distance to the end of a wire.In 1929, the two used it as the basis for their joint graduation thesis.Everitt disclosed the concept to the U.S.Patent Office, but did not apply for a patent at the time. He then sought development funding from the Daniel Guggenheim Aviation Promotion Fund.Foundation Secretary Jimmy Doolittle approached Vannevar Bush of Bell Labs for a judgement.Bush was skeptical of the system's development at the time,but nonetheless suggested that the foundation fund the development of a working model.This enabled Newhouse to collaborate with J.D.Corley to build an experimental machine that formed the basis of his 1930 master's thesis.The device was taken to Wright Airport to be tested by renowned aircraft navigation expert Albert Francis Helgenberger.Hegenberger found the system worked as advertised,but said it would have to work at higher frequencies to be practical.