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15 Décembre 2020
The performance of a radar system can be judged by the following: (1) the maximum range at which it can see a target of a specified size, (2) the accuracy of its measurement of target location in range and angle, (3) its ability to distinguish one target from another, (4) its ability to detect the desired target echo when masked by large clutter echoes, unintentional interfering signals from other 'friendly' transmitters, or intentional radiation from hostile jamming (if a military radar), (5) its ability to recognize the type of target, and (6) its availability (ability to operate when needed), reliability, and maintainability. Some of the major factors that affect performance are discussed in this section.
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The maximum range of a radar system depends in large part on the average power of its transmitter and the physical size of its antenna. (In technical terms, this is called the power-aperture product.) There are practical limits to each. As noted before, some radar systems have an average power of roughly one megawatt. Phased-array radars about 100 feet (30 metres) in diameter are not uncommon; some are much larger. There are specialized radars with (fixed) antennas, such as some HF over-the-horizon radars and the U.S. Space Surveillance System (SPASUR), that extend more than one mile (1.6 km). Raya pro 3 0.
The sensitivity of a radar receiver is determined by the unavoidable noise that appears at its input. At microwave radar frequencies, the noise that limits detectability is usually generated by the receiver itself (i.e., by the random motion of electrons at the input of the receiver) rather than by external noise that enters the receiver via the antenna. A radar engineer often employs a transistor amplifier as the first stage of the receiver even though lower noise can be obtained with more sophisticated (and more complex) devices. This is an example of the application of the basic engineering principle that the 'best' performance that can be obtained might not necessarily be the solution that best meets the needs of the user.
The receiver is designed to enhance the desired signals and to reduce the noise and other undesired signals that interfere with detection. A designer attempts to maximize the detectability of weak signals by using what radar engineers call a 'matched filter,' which is a filter that maximizes the signal-to-noise ratio at the receiver output. Windows media player for macbook. Raw power 1 0 2. The matched filter has a precise mathematical formulation that depends on the shape of the input signal and the character of the receiver noise. A suitable approximation to the matched filter for the ordinary pulse radar, however, is one whose bandwidth in hertz is the reciprocal of the pulse width in seconds.
The size of a target as 'seen' by radar is not always related to the physical size of the object. The measure of the target size as observed by radar is called the radar cross section and is given in units of area (square metres). It is possible for two targets with the same physical cross-sectional area to differ considerably in radar size, or radar cross section. For example, a flat plate 1 squaremetre in area will produce a radar cross section of about 1,000 square metres at a frequency of 3 GHz when viewed perpendicular to the surface. A cone-sphere (an object resembling an ice-cream cone) when viewed in the direction of the cone rather than the sphere could have a radar cross section of about 0.001 square metre even though its projected area is also 1 square metre. In theory, the radar cross section has little to do with the size of the cone or the cone angle. Thus, the flat plate and the cone-sphere can have radar cross sections that differ by a million to one even though their physical projected areas are the same.
The sphere is an unusual target in that its radar cross section is the same as its physical cross-sectional area (when its circumference is large compared with the radar wavelength). That is to say, a sphere with a projected area of 1 square metre has a radar cross section of 1 square metre.
Commercial aircraft might have radar cross sections from about 10 to 100 square metres, except when viewed broadside, where the cross sections are much larger. Most air-traffic-control radars are required to detect aircraft with a radar cross section as low as 2 square metres, since some small general-aviation aircraft can be of this value. For comparison, the radar cross section of a man has been measured at microwave frequencies to be about 1 square metre. A bird can have a cross section of 0.01 to 0.001 square metre. Although this is a small value, a bird can be readily detected at ranges of several tens of kilometres by long-range radar. In general, many birds can be detected by radar, so special measures must usually be taken to ensure that their echoes do not interfere with the detection of desired targets.
The radar cross section of an aircraft and that of most other targets of practical interest fluctuate rapidly as the aspect of the target changes with respect to the radar unit. It would not be unusual for a slight change in aspect to cause the radar cross section to change by a factor of 10 to 1,000.
Echoes from land, sea, rain, snow, hail, birds, insects, auroras, and meteors are of interest to those who observe and study the environment, but they are a nuisance to those who want to detect aircraft, ships, missiles, or other similar targets. Clutter echoes can seriously limit the capability of a radar system; thus, a significant part of radar design is devoted to minimizing the effects of clutter without reducing the echoes from desired targets. The Doppler frequency shift is the usual means by which moving targets are distinguished from the clutter of stationary objects. Detection of targets in rain is less of a problem at the lower frequencies, since the radar echo from rain decreases rapidly with decreasing frequency and the average cross section of aircraft is relatively independent of frequency in the microwave region. Ip scanner pro 3 45 download free. Because raindrops are more or less spherical (symmetrical) and aircraft are asymmetrical, the use of circular polarization can enhance the detection of aircraft in rain. With circular polarization, the electric field rotates at the radar frequency. Because of this, the electromagnetic energy reflected by the rain and the aircraft will be affected differently, which thereby makes it easier to distinguish between the two. (In fair weather most radars use linear polarization; i.e., the direction of the electric field is fixed.)
| hertz | Hz | cycles per second | 100 Hz | 1 Hz |
| kilohertz | kHz | one thousand hertz | 103 Hz | 1,000 Hz |
| megahertz | MHz | one million hertz | 106 Hz | 1,000,000 Hz |
| gigahertz | GHz | one billion hertz | 109 Hz | 1,000,000,000 Hz |
| terahertz | THz | one trillion hertz | 1012 Hz | 1,000,000,000,000 Hz |
| Band | Frequency | Wavelength | Notes |
|---|---|---|---|
| S | 2.455 GHz | 4.827 in 12.261 cm | obsolete |
| X | 9.41 GHz | 1.254 in 3.186 cm | Europe |
| X | 9.90 GHz | 1.192 in 3.028 cm | Europe |
| X | 10.525 GHz | 1.121 in 2.848 cm | USA |
| Ku | 13.450 GHz | 0.878 in 2.229 cm | Europe Middle East |
| K | 24.125 GHz | 0.4892 in 1.243 cm | USA, Australia, Europe |
| K | 24.150 GHz | 0.4897 in 1.241 cm | USA |
| Ka | 33.4 - 36.0 GHz | 0.353 - 0.328 in 8.976 - 8.328 mm | USA, Australia, Europe |
| IR -- Infrared | 331.6 THz | 904 nm | Laser Radar |
| Frequency Band Designations |
Military Radar Bands
Military radar band nomenclature, L, S, C, X, Ku, and K bands originated during World War II as a secret code so scientists and engineers could talk about frequencies without divulging them. Snagit 4 1 9 – screen capture utility. After the war the codes were declassified and Ka band and millimeter (mm) were added. Military radar band nomenclature is widely used today in radar, satellite and terrestrial communications, and military electronic countermeasure applications.
| Radar Band | Frequency | Notes |
|---|---|---|
| HF | 3 - 30 MHz | High Frequency |
| VHF | 30 - 300 MHz | Very High Frequency |
| UHF | 300 - 1000 MHz | Ultra High Frequency |
| L | 1 - 2 GHz | |
| S | 2 - 4 GHz | |
| C | 4 - 8 GHz | |
| X | 8 - 12 GHz | |
| Ku | 12 - 18 GHz | |
| K | 18 - 27 GHz | |
| Ka | 27 - 40 GHz | |
| mm | 40 - 300 GHz | millimeter wavelength |
Military HF, VHF, UHF same as Radio Band HF, VHF, UHF respectively.
ITU Radar Bands
The International Telecommunications Union (ITU) specifies bands designated for radar systems. The ITU radar bands are sub-bands of military designations.
| ITU Band | Frequency |
|---|---|
| VHF | 138 - 144 MHz 216 - 225 MHz |
| UHF | 420 - 450 MHz 890 - 942 MHz |
| L | 1.215 - 1.400 GHz |
| S | 2.3 - 2.5 GHz 2.7 - 3.7 GHz |
| C | 5.250 - 5.925 GHz |
| X | 8.500 - 10.680 GHz |
| Ku | 13.4 - 14.0 GHz 15.7 - 17.7 GHz |
| K | 24.05 - 24.25 GHz |
| Ka | 33.4 - 36.0 GHz |
VHF -- Very High Frequency
UHF -- Ultra High Frequency
Radio Bands
Radio band designations are summarized below. Note that the radio band chart includes wavelength. In the early days of radio it was easier to measure wavelength than frequency. https://cooldfiles259.weebly.com/orion-markup-2-80-download-free.html.
| Band | Nomenclature | Frequency | Wavelength |
|---|---|---|---|
| ELF | Extremely Low Frequency | 3 - 30 Hz | 100,000 - 10,000 km |
| SLF | Super Low Frequency | 30 - 300 Hz | 10,000 - 1,000 km |
| ULF | Ultra Low Frequency | 300 - 3000 Hz | 1,000 - 100 km |
| VLF | Very Low Frequency | 3 - 30 kHz | 100 - 10 km |
| LF | Low Frequency | 30 - 300 kHz | 10 - 1 km |
| MF | Medium Frequency | 300 - 3000 kHz | 1 km - 100 m |
| HF | High Frequency | 3 - 30 MHz | 100 - 10 m |
| VHF | Very High Frequency | 30 - 300 MHz | 10 - 1 m |
| UHF | Ultra High Frequency | 300 - 3000 MHz | 1 m - 10 cm |
| SHF | Super High Frequency | 3 - 30 GHz | 10 - 1 cm |
| EHF | Extremely High Frequency | 30 - 300 GHz | 1 cm - 1 mm |
ECM Bands
The electronic countermeasures (ECM) industry has it's own band designations.
|
| Sound Waves |
Sound waves are air pressure waves that travel at 765 mph at sea level, not like electromagnetic radio or radar waves that travel at the speed of light. Sound is a pressure wave of vibrating air molecules, and does not exits in the vacuum of outer space.
Most people at best can hear sound waves between 20 and 20,000 Hertz, the audio band. Sound, pressure waves, can extend as high as 10 MHz, however above 160 kHz propagation range greatly decreases due to absorption by atmospheric gases, air.
|
| Frequency Range | Use | Band |
|---|---|---|
| 0 - 20 Hz | Elephants, Whales | infrasound |
| 20 - 20,000 Hz | Humans, Animals, Fish, SONAR | audio |
| 10 - 30 kHz | Rodents | audio - ultrasound |
| 20 - 75 kHz | Insects | audio - ultrasound |
| 20 - 160 kHz | Bats, Dolphins | ultrasound |
| 100 kHz - 2 MHz | Structures Test | ultrasound |
| 1 - 10 MHz | Medical Applications | ultrasound |
Chocolat 3 1. AM radio broadcast electromagnetic waves from 0.5 - 1.6 MHz
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