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Radar Height Finder and Altimeter
Published in Habibur Rahman, Fundamental Principles of Radar, 2019
A radar altimeter is, according to the International Telecommunications Union, defined as “the radio navigation equipment, on board an aircraft or spacecraft, used to determine the height of the aircraft or the spacecraft above the earth's surface or another surface.” Thus, radar altimeters are used on aircraft or spacecraft to measure altitude of the aircraft or the spacecraft above the earth's surface or another surface. The altimeter transmits electromagnetic energy down to the ground and measures the time taken by the return echo after being reflected from the ground. The altitude is then calculated from the knowledge of the travel time and the velocity of propagation of the wave. Radar altimeters also provide a reliable and accurate method of measuring height above water, when flying long sea tracks.
Atmosphere
Published in Mohammad H. Sadraey, Aircraft Performance, 2017
A radar altimeter uses electromagnetic waves to measure the distance of an aircraft (or other aerospace vehicle) above the ground. Radar altimeters are often used in aircraft during bad-weather landings. Radar altimeters are much more accurate and more expensive than pressure altimeters. They are an essential part of many blind-landing and navigation systems and are used over mountains to indicate terrain clearance. Special types are used in surveying for quick determination of profiles. Radar altimeters have been in use on various spacecraft, starting with Skylab in 1973, to measure the shape of the geoid and heights of waves and tides over the oceans. The altimeter measures height by determining the time required for a radio wave to travel to and from a target. If the Earth were a perfectly flat horizontal plane, the signal would come only from the closest point and would be a true measure of altitude. However, the Earth is not smooth, and energy is scattered back to the radar from all parts of the surface illuminated by the transmitter.
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Published in Pamela S. Tsang, Michael A. Vidulich, Principles and Practice of Aviation Psychology, 2002
A radar altimeter measures ground clearance under the aircraft, providing a direct, accurate, and instantaneous indication of altitude AGL. Its drawback is that it samples only the ground under the plane, not in front along its intended flight path. The sampling cone is about 60 degrees in solid angle, extending out from directly beneath the aircraft so that at 100 feet altitude AGL, the radar altimeter indicates the distance to the closest object or elevation on the ground within a circle with a radius of about 50 feet. If the plane is flying over the flat and level ocean, then the radar altimeter is predictive of altitude AGL ahead and provides a useful source of information to the pilot about ground clearance. However, if the terrain is irregular, the radar altimeter cannot predict clearance over individual ridges until the aircraft actually gets over each protrusion. By then, the plane has either collided with or cleared the ridge.
Retarded state-multiplicative stochastic systems - Robust H ∞ and H 2 vertex-dependent filtering
Published in International Journal of Control, 2022
In order to demonstrate the application of the our theory to practical control engineering, we consider the problem of altitude estimation with measurements from a RADAR altimeter and a baro altimeter (Gershon et al., 2005). We bring a short description of the problem at hand for convenience. The barometric altitude measurement is based on a static pressure measurement. As a result of various sources of error, (e.g. initial reference error, static pressure measurement bias, or temperature measurement errors) the baro altimeter is corrupted with a bias error (see Chapter 11, Gershon et al., 2005) up to 1000 ft together with a small white noise component. Denoting the true altitude above ground by h, we have the following approximate model for the altitude hold loop which is commanded by the altitude command where is the time constant of the command response, b represents the baro altitude measurement bias and is a standard zero-mean white noise with intensity , that is: The RADAR altimeter measures the height above ground level without bias, however, its output is corrupted by a broad band measurement noise, the intensity of which increases with height due to a lower SNR (signal to noise ratio) effect at higher altitude.
Applied geophysics for cover thickness mapping in the southern Thomson Orogen
Published in Australian Journal of Earth Sciences, 2018
I. C. Roach, C. B. Folkes, J. Goodwin, J. Holzschuh, W. Jiang, A. A. McPherson, A. J. Meixner
Both methods were performed on high quality regional airborne magnetic line-data acquired with flight-line spacings of 400 m or less, detailed in Table 1. Flight-line data are preferred over the derivative gridded data for two reasons. First, flight-line data are usually recorded with a radar altimeter channel so that the aircraft height above ground is known. The actual aircraft height may vary by tens of metres from the nominal flight height, leading to significant uncertainty in the depth estimates from gridded data. Second, the sample spacing along the lines (»7 m) is less than the cell size of the derivative grids (generally 80 m for a survey acquired at 400 m flight-line spacing). The closely spaced sampling along flight-line data ensures that the highest frequency anomalies from magnetic sources at the surface are adequately sampled, meaning that there is no loss of depth resolution that may occur for the same magnetic sources from the gridded data.
Remote sensing data quality model: from data sources to lifecycle phases
Published in International Journal of Image and Data Fusion, 2019
Árpad Barsi, Zsófia Kugler, Attila Juhász, György Szabó, Carlo Batini, Hussein Abdulmuttalib, Guoman Huang, Huanfeng Shen
Most common passive sensors are typically the optical sensors that are either single channel, multichannel, hyperspectral or thermal depending on their sensitivity to the electromagnetic spectra (Table 2). Other sensors emit their own energy to register backscatter of the radiation like RADAR systems using the microwave range of the spectra. It usually produces elevation measurements like InSAR technology or performs nadir single measurement like RADAR Altimeter. Furthermore, laser beam can be also used to range distance to target with laser scanning technology. Besides, microwave emission of surfaces or objects can be measured with passive microwave sensors like scatterometers and microwave satellites.