In order to understand how a radio altimeter works, consider a radio pulse emanating from a radio beacon to a flat surface. Ref: the configuration when the trailing edge of the pulse arrives at the flat surface. Radio altimeter calculations us the Pythagorean theorem :. Note that the time delay between transmission and reflected wave is too short to be able to use a single antenna for both functions.
Two antennae are therefore required, which must be physically separated in order to avoid interference. Another type of aviation radio altimeter is the frequency-modulated radio altimeter.
This type takes advantage of the fact that the reflected signal will be received at a different frequency to the transmitted signal. The rate of change in signal frequency is constant, meaning that altitude can be calculated as it will be proportional to the frequency difference between transmitter and receiver.
To make this calculation, the time of flight of radio waves from a known point to another is again used. Altitude calculated using GNSS is, however, not accurate or reliable enough to obviate the use of a backup system, such as a barometric altimeter - unless some method of augmentation is used. Errors in height calculation using GNSS are typically in the region of 5 meters. Although useful for many Unmanned Aerial Vehicles UAV during flight when a GNSS may provide sufficient accuracy for general navigation , GNSS is not accurate enough to provide height information for precision manoeuvres such as low altitude flight or landing.
This type of altimeter works by using electromagnetic waves within the visible range of the spectrum instead of radio waves. Operation requires a signal from the transmit antenna to be directed to the ground. When the signal hits the ground it is reflected back to the receive antenna.
The system then performs a time calculation to determine the distance between the aircraft and ground, as the altitude of the aircraft is proportional to the time required for the transmitted signal to make the round trip.
Similar to FMCW, pulse radar altimeters provide the aircraft with accurate, independent and absolute measurement of the minimum distance to the earth surface below that aircraft. In aviation pulse altimeters are more popular for altitudes above about feet whereas FMCW radar altimeters have been widely used for short range measurements benefiting from their high ranging resolution.
Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions and automatic landings, allowing the autopilot to know when to begin the flare maneuver. Radar altimeters give data to the flight computer which manages the autothrottle function. A radar altimeter also functions as part of terrain avoidance warning system providing predictive forward looking capability on the flight deck, and if necessary a warning, when an aircraft descends beneath a certain altitude or too close to the ground.
Another usage of radar altimeter technology is terrain aided navigation, which is an alternative to GPS that could be used for INS Inertial Navigation System alignment or simply to provide redundancy as a backup system. It is based on estimating the position of a moving platform by utilizing the terrain height profile under the platform and a stored terrain elevation map.
The radar altimeter measurements of heights above ground and the barometric altimeter measurement above mean sea level are used to construct a ground profile of the platform throughout the flight.
Then, the constructed terrain profile is compared with a stored map data to achieve a profile fit, thus the aim is to decide on the position of the vehicle and to correct the INS. Radar altimeters are also used in military air platforms to fly quite low over the land and the sea to avoid radar detection and targeting by anti-aircraft guns or surface-to-air missiles. Meteksan Defence gained a great deal of knowledge and experience after starting to study this technology for the first time to meet the needs of Turkish Defence Industry.
Both products are low probability of intercept LPI sensors with low output RF power, high bandwidth and power management characteristics. The products features start up, initiated and periodic built-in self-test capabilities and designed to used in cruise missiles as navigation aids for keeping the missile on a prescribed flight path and at certain flight altitudes.
High performance of these advanced equipments has successfully proven in low level terrain tracking and sea skimming missiles. What is the difference between radio altimeter and radar altimeter? Does radar altimeter work over water? Which radar is used as airborne altimeter? What is the main application of the radar altimeter? How does radar altimeter work? How does a plane know its altitude? How high can you fly unpressurized? Who was the pilot of Flight ?
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