The beat frequency has to be amplified and limited for eliminating any amplitude fluctuations, this is done by using an amplifier and a limiter. This is measured by the cycle-counting frequency meter which is calibrated in distance. Frequency—time relationship in FMCW radar The above figure shows the frequency-time relationship in FMCW radar; where solid lines are represents transmitted signals and dashed or dot lines are represents echo signals. The first figure shows the transmitted and echo signal and another one shows the beat frequency.
|Published (Last):||8 June 2015|
|PDF File Size:||20.25 Mb|
|ePub File Size:||19.96 Mb|
|Price:||Free* [*Free Regsitration Required]|
Alexander V. Nebylov1 and Felix J. A crew or a control system needs information about altitude with respect to the ground level over the entire flight. Altimetry is the art of measuring altitude, and an altimeter is a sensor that measures the altitude of a flying vehicle. Normally, an altimeter can also serve as a source of information about the vertical speed of a flying vehicle.
Hence, an altimeter is one of the necessary sensors forming the equipment complement of modern aircraft and spacecraft. For example, radar, laser and acoustic methods are based on the measurement of the time taken by electromagnetic or acoustic waves to travel from an aerospace vehicle to a reflective surface on the Earth or another planet. Alternatively, radioisotope methods may be used to measure radiation backscattering intensity.
For aircraft, a barometric altimeter measures the air pressure at the level in which the aircraft is flying and converts that measurement to the height above sea level according to the standard pressure-altitude relationship described in Chapter 2. At stratospheric heights, one method measures a corona-discharge current that depends on air density, which corresponds to altitude. Also, an inertial sensor based on an accelerometer installed on the gyro-stabilized platform of a standard inertial system can measure vertical acceleration and this may be followed by double integration to determine altitude.
The main topics in this chapter are altimeters using active radar principles Radar Handbook, , that is, radar including laser altimeters, though a brief treatment of radioactivity methods is also given. Various classes of waveform can be used in radar altimetry, the most obvious divisions of all possible waveforms being continuous CW and pulsed waves.
In both contexts, the modulation of a carrier wave is necessary in order to measure the time taken for a signal to reach and return from the ground for conversion to the target range, or altitude.
Pulse altimeters are more popular for altitudes above about feet metres , whereas CW equivalents tend to be preferred for low altitudes. However, this is not a hard-and-fast rule, and 0. Frequency choice depends upon regulations, mission objectives, and other constraints as well as technical possibilities and impossibilities.
If the Earth were a perfectly flat horizontal plane or a smooth sphere, the return 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 receiver from all parts of the surface illuminated by the transmitter. For the radar altimeter to measure distance to the ground accurately, it must distinguish between reflections from points near the vertical and those from points that are more distant. Therefore, a narrow antenna beam pointing vertically down would be desirable.
However, aircraft antennas are limited in size, and antenna beam- width is finite and frequently rather wide. Generally, a reflected signal is formed from a large surface area depending on beam width and flight altitude. Such signals contain data not only on the altitude g H but also on slant ranges i Rwithin the illuminated area BA as is shown in Figure 3. This geometrical features of radar altimetry using both pulse and CW waveforms can be taken into account during signal processing.
Two groups of signal processing methods can be 3 considered: local methods and integral methods. In the local method, only a part of receiving signal that is reflected from the surface area near the perpendicular g H is processed. However, if the integral method is used, the whole signal reflected within the area BA is processed, and different parameters of the complicated received envelope are measured to derive useful information, these depending on which modulation format is employed.
This task becomes more complicated if account is taken of a real antenna pattern that gives different weights to signals from different directions.
However, this antenna pattern is normally known, so that the measured altitude can be easily biased to give the absolute altitude above the surface. In practice, not only is the antenna pattern important but so is the backscattering diagram of the overflown terrain. When flying over different reflecting surfaces water, forest, ploughed field, buildings, etc. In the case where the illuminated surface within area BA Fig. Unfortunately, in most cases a priori data on backscattering diagrams are absent.
That is why, during flights over heterogeneous terrain, mean values of the indicated time delays become random and cannot be taken into account and corrected. Biasing of the altitude indication can also be caused by flight vehicle maneuvers such as rolling and pitching. In this case, axis deviations from the vertical in a rigid antenna can result in changes in the measured average time delay of the reflected signal.
To decrease this altitude hole to a more acceptable level, it is rational to shorten the pulse length in pulse altimeters, or to increase the frequency deviation in CW FM altimeters.
In practice, both CW altimeters and pulse altimeters are frequently designed with two separate antennas for the transmission and reception functions, which allows for further minimization of the altitude hole by improving the isolation and switching time between them.
Such an installation might include a pair of microstrip antennas operating in the C-band, which would typically provide a gain of 10dB over isotropic.
These antennas would be spaced to provide an isolation loss greater than the maximum expected ground return loss at low altitudes: a half-meter spacing between two antennas installed on an aircraft underside would provide about 85dB isolation a Normally, an altimeter has an automatic sensitivity range control that limits receiver sensitivity as a function of altitude, especially at low altitudes, in order to detect ground returns but not antenna leakage.
They comprise microwave, millimeter-wave, laser and radioactive altimeters, though these delineations are rather rough because each waveband is very broad. For example, the frequency band 4. This frequency band is high enough to result in reasonably small-sized antennas able to produce a 40o to 50o beam but is sufficiently low so that rain attenuation and backscatter have no significant range limiting effects. Detailed information on other frequencies of operation will be provided in particular cases.
Another attribute is the waveform, as was mentioned in section 3. Radar altimeters also differ in their application and functionality. Obviously, it is possible to distinguish altimeters for aircraft and spacecraft, military and civil applications, altimeters as sensors measuring motion parameters for navigation purposes, altimeters as sensors for remote earth surface sensing, low range radio altimeters LRRA and high range altimeters.
The basic function of an aircraft radar altimeter is to provide terrain clearance directly beneath the aircraft, particularly in mountainous areas and during bad-weather landings. Additional functions include the measurement of vertical rate of climb or descent and selectable low altitude warnings. Radar altimeters are also essential parts of many blind- landing and automatic navigation systems.
In civil aviation, they are designed to support automatic landing, flare and touchdown computations. Another application is for map-matching Kayton and Fried, , also called terrain- contour navigation, which is a type of terrain reference navigation TRN Collinson, Here, the profile of the terrain is measured by using the readings of both a baro-inertial altimeter calibrated for altitude above sea level MSL , and a radar altimeter measuring height above the terrain.
An on-board computer calculates the autocorrelation function between the measured profile and each of many stored profiles on possible parallel paths that can be taken by the vehicle. Finally, the aircraft radar altimeter is a key sensor and component of the ground proximity warning system treated in Chapter 4. However, in addition there are some purely military applications. For example, radar altimeters are used in bombs, missiles, and shells as proximity fuses to cause detonation or to initiate other functions at set altitudes Kayton and Fried, In particular, they have been applied to measuring the shapes of the geoid and the heights of waves and tides over the oceans.
Continuous monitoring of sea level and the measurement of terrain relief can be accomplished from orbiting satellites. Spacecraft radar altimeters can also provide topographic information on other planets.
One of the main attributes of any altimeter is measurement accuracy. Depending on the altimeter function and the class of flying vehicle, measurement accuracy requirements for altitude and vertical speed can be essentially different. Therefore, values for acceptable errors should be set individually for partial cases. Altimeters designed for terrain correlation for navigational purposes must process an extremely small ground illumination spot size at high altitudes in order to provide the required altitude resolution.
Altitude marking radars are generally low altitude altimeters designed specifically to provide mark signals at specific altitudes for the initiation of automatic operations such as fuse triggering at a given distance to a target, or parachute opening upon return from space for an automatic landing. Performance characteristics and even tasks that are desirable and expedient are in part dictated by the level of engineering possible at the time of manufacture.
Altimeters built during the early s weighed 7 kg or more and transmitted about W of peak pulse 6 power, whilst at the end of s radar altimeters typically weighed 2 to 5 kg, exhibited a 0. Nowadays, even these figures can be improved upon. The altimeter is an integral part of an aerospace vehicle navigational system and is used in vehicle control, whilst in remote sensing applications it is a kind of useful load, the aerospace vehicle itself being basically a carrier for remote sensors.
The wide class of altimeters designed for remote sensing of terrain is beyond the scope of this book, and will only be touched upon where necessary.
Figure 3. For example, if a spacecraft altimeter is required to measure altitudes up to km, its PRF should be less than By contrast, if an aircraft altimeter is designed to measure a maximum altitude of m, the PRF can be under Classical radar theory rigorously proves that to achieve high accuracy in measuring target range altitude in this case , the waveform should be as wideband as possible, so enabling the accurate measurement of the relevant time delay. This actually means that the use of wideband waveforms with time-bandwidth products B.
A large modulation index is needed for practical reasons. Practical systems introduce reverse FM on the receive signal using digital signal processing before the Fast Fourier Transform process is used to produce the spectrum. This is repeated with several different demodulation values. Range is found by identifying the receive spectrum where width is minimum. Practical systems also process receive samples for several cycles of the FM in order to reduce the influence of sampling artifacts. Configurations[ edit ] Block diagram of a simple continuous-wave radar module: Many manufacturers offer such transceiver modules and rename them as "Doppler radar sensors" There are two different antenna configurations used with continuous-wave radar: monostatic radar , and bistatic radar. Monostatic[ edit ] The radar receive antenna is located nearby the radar transmit antenna in monostatic radar.
Alexander V. Nebylov1 and Felix J. A crew or a control system needs information about altitude with respect to the ground level over the entire flight. Altimetry is the art of measuring altitude, and an altimeter is a sensor that measures the altitude of a flying vehicle.
ALT-8000 FMCW/Pulse Radio Altimeter Flight Line Test Set
It is primarily used to determine height above ground and is therefore used to measure ground distance in avionics for flying objects. It transmits a frequency-modulated signal via a radar antenna, which propagates in space. If the electromagnetic wave hits an obstacle, the radar wave is either reflected or attenuated, depending on the material properties. With a FMCW radar sensor the height above ground can be determined. The main obstacle is the ground itself. It reflects the radar wave and in a receiving antenna in the FMCW radar sensor the reflected wave is detected, converted into an electrical signal and on the basis of the transit time, that the signal needs for the distance covered, a frequency difference proportional between transmitted and received signal can be determined. This frequency difference correlates with the distance.