求一份英文资料,关于雷达原理方面的
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发布时间：2022-05-14 22:04
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热心网友
时间：2023-07-09 20:08
A typical radar (RAdio Detection and Ranging) measures the strength and round-trip time of the microwave signals that are emitted by a radar antenna and reflected off a distant surface or object. The radar antenna alternately transmits and receives pulses at particular microwave wavelengths (in the range 1 cm to 1 m, which corresponds to a frequency range of about 300 MHz to 30 GHz) and polarizations (waves polarized in a single vertical or horizontal plane). For an imaging radar system, about 1500 high- power pulses per second are transmitted toward the target or imaging area, with each pulse having a pulse ration (pulse width) of typically 10-50 microseconds (us). The pulse normally covers a small band of frequencies, centered on the frequency selected for the radar. Typical bandwidths for an imaging radar are in the range 10 to 200 MHz. At the Earth's surface, the energy in the radar pulse is scattered in all directions, with some reflected back toward the antenna. Thisbackscatter returns to the radar as a weaker radar echo and is received by the antenna in a specific polarization (horizontal or vertical, not necessarily the same as the transmitted pulse). These echoes are converted to digital data and passed to a data recorder for later processing and display as an image. Given that the radar pulse travels at the speed of light, it is relatively straightforward to use the measured time for the roundtrip of a particular pulse to calculate the distance or range to the reflecting object. The chosen pulse bandwidth determines the resolution in the range (cross-track) direction. Higher bandwidth means finer resolution in this dimension.
Radar transmits a pulse Measures reflected echo (backscatter )
Click Here to See Animation
In the case of imaging radar, the radar moves along a flight path and the area illuminated by the radar, or footprint, is moved along the surface in a swath, building the image as it does so.
Building up a radar image using the motion of the platform
The length of the radar antenna determines the resolution in the azimuth (along-track) direction of the image: the longer the antenna, the finer the resolution in this dimension. Synthetic Aperture Radar (SAR) refers to a technique used to synthesize a very long antenna by combining signals (echoes) received by the radar as it moves along its flight track. Aperture means the opening used to collect the reflected energy that is used to form an image. In the case of a camera, this would be the shutter opening; for radar it is the antenna. A synthetic aperture is constructed by moving a real aperture or antenna through a series of positions along the flight track.
Constructing a Synthetic Aperture
As the radar moves, a pulse is transmitted at each position; the return echoes pass through the receiver and are recorded in an 'echo store.' Because the radar is moving relative to the ground, the returned echoes are Doppler-shifted (negatively as the radar approaches a target; positively as it moves away). Comparing the Doppler-shifted frequencies to a reference frequency allows many returned signals to be "focused" on a single point, effectively increasing the length of the antenna that is imaging that particular point. This focusing operation, commonly known as SAR processing, is now done digitally on fast computer systems. The trick in SAR processing is to correctly match the variation in Doppler frequency for each point in the image: this requires very precise knowledge of the relative motion between the platform and the imaged objects (which is the cause of the Doppler variation in the first place).
Synthetic aperture radar is now a mature technique used to generate radar images in which fine detail can be resolved. SARs provide unique capabilities as an imaging tool. Because they provide their own illumination (the radar pulses), they can image at any time of day or night, regardless of sun illumination. And because the radar wavelengths are much longer than those of visible or infrared light, SARs can also "see" through cloudy and sty conditions that visible and infrared instruments cannot.
What is a radar image?
Radar images are composed of many dots, or picture elements. Each pixel (picture element) in the radar image represents the radar backscatter for that area on the ground: darker areas in the image represent low backscatter, brighter areas represent high backscatter. Bright features mean that a large fraction of the radar energy was reflected back to the radar, while dark features imply that very little energy was reflected. Backscatter for a target area at a particular wavelength will vary for a variety of conditions: size of the scatterers in the target area, moisture content of the target area, polarization of the pulses, and observation angles. Backscatter will also differ when different wavelengths are used.
Scientists measure backscatter, also known as radar cross section, in units of area (such as square meters). The backscatter is often related to the size of an object, with objects approximately the size of the wavelength (or larger) appearing bright (i.e. rough) and objects smaller than the wavelength appearing dark (i.e. smooth). Radar scientists typically use a measure of backscatter called normalized radar cross section, which is independent of the image resolution or pixel size. Normalized radar cross section (sigma0.) is measured in decibels (dB). Typical values of sigma0. for natural surfaces range from +5dB (very bright) to -40dB (very dark).
A useful rule-of-thumb in analyzing radar images is that the higher or brighter the backscatter on the image, the rougher the surface being imaged. Flat surfaces that reflect little or no microwave energy back towards the radar will always appear dark in radar images. Vegetation is usually moderately rough on the scale of most radar wavelengths and appears as grey or light grey in a radar image. Surfaces inclined towards the radar will have a stronger backscatter than surfaces which slope away from the radar and will tend to appear brighter in a radar image. Some areas not illuminated by the radar, like the back slope of mountains, are in shadow, and will appear dark. When city streets or buildings are lined up in such a way that the incoming radar pulses are able to bounce off the streets and then bounce again off the buildings (called a double- bounce) and directly back towards the radar they appear very bright (white) in radar images. Roads and freeways are flat surfaces so appear dark. Buildings which do not line up so that the radar pulses are reflected straight back will appear light grey, like very rough surfaces.
Imaging different types of surface with radar
Backscatter is also sensitive to the target's electrical properties, including water content. Wetter objects will appear bright, and drier targets will appear dark. The exception to this is a smooth body of water, which will act as a flat surface and reflect incoming pulses away from a target; these bodies will appear dark.
Backscatter will also vary depending on the use of different polarization. Some SARs can transmit pulses in either horizontal (H) or vertical (V) polarization and receive in either H or V, with the resultant combinations of HH (Horizontal transmit, Horizontal receive), VV, HV, or VH. Additionally, some SARs can measure the phase of the incoming pulse (one wavelength = 2pi in phase) and therefore measure the phase difference (in degrees) in the return of the HH and VV signals. This difference can be thought of as a difference in the roundtrip times of HH and VV signals and is frequently the result of structural characteristics of the scatterers. These SARs can also measure the correlation coefficient for the HH and VV returns, which can be considered as a measure of how alike (between 0/not alike and 1/alike) the HH and VV scatterers are.
Different observations angles also affect backscatter. Track angle will affect backscatter from very linear features: urban areas, fences, rows of crops, ocean waves, fault lines. The angle of the radar wave at the Earth's surface (called the incidence angle) will also cause a variation in the backscatter: low incidence angles (perpendicular to the surface) will result in high backscatter; backscatter will decrease with increasing incidence angles.
Radar backscatter is a function of incidence angle, (theta)i
NASA/JPL's Radar Program
NASA/JPL's radar program began with the SEASAT synthetic aperture radar (SAR) in 1978. SEASAT was a single frequency (L-band with lambda ~ 24 cm or 9.4 inches), single polarization, fixed-look angle radar. The Shuttle Imaging Radar-A (SIR-A), flown on the Space Shuttle in 1981, was also an L- band radar with a fixed look angle. SIR-B (1984) added a multi-look angle capability to the L-band, single polarization radar. SIR-C/X-SAR is a joint venture of NASA, the German Space Agency (DARA), and the Italian Space Agency (ASI). SIR-C/X-SAR provided increased capability over Seasat, SIR-A, and SIR-B by acquiring images at three microwave wavelengths (lambda), L- band (lambda ~ 24 cm or 9.4 inches) quad-polarization; C-band (lambda ~ 6 cm or 2.4 inches) quad- polarization; and X-band (lambda ~ 3 cm) with VV polarization. SIR-C/X-SAR also has a variable look angle, and can image at incidence angles between 20 and 65 degrees. SIR-C/X-SAR flew on the shuttle in April and in October of 1994, providing radar data for two seasons. Typical image sizes for SIR-C data procts are 50kmx100km, with resolution between10 and 25 meters in both dimensions.
Parallel to the development of spaceborne imaging radars, NASA/JPL have built and operated a series of airborne imaging radar systems. NASA/JPL currently maintain and operate an airborne SAR system, known as AIRSAR/TOPSAR, which flies on a NASA DC-8 jet. In one mode of operation, this system is capable of simultaneously collecting all four polarizations (HH,HV, VH and VV) for three frequencies: L- band (lambda ~ 24 cm); C-band (lambda ~ 6 cm) ; and P-band (lambda ~ 68 cm). In another mode of operation, the AIRSAR/TOPSAR system collects all four polarizations (HH,HV, VH and VV) for two frequencies: L- band (lambda ~ 24 cm); and P-band (lambda ~ 68 cm), while operating as an interferometer at C-band to simultaneously generate topographic height data. AIRSAR/TOPSAR also has an along-track interferometer mode which is used to measure current speeds. Typical image sizes for AIRSAR/TOPSAR procts are 12kmx12km, with 10 meter resolution in both dimensions. Topographic map procts generated by the TOPSAR system have been shown to have a height accuracy of1 m in relatively flat areas, and 5 m height accuracy in mountainous areas.
JPL are studying designs for a free-flying multi- parameter imaging radar system like the one flown ring the SIR-C/X-SAR missions. JPL are also studying a global mapping mission (TOPSAT) which will use radar interferometry to generate high quality topographic maps over the whole world and monitor changes in topography in areas prone to earthquakes and volcanic activity.
To inquire about the availability of imaging radar data from the SIR-C, SIR-B, SIR-A or Seasat missions, or the airborne AIRSAR/TOPSAR system, please contact:
Radar Data Center
Mail Stop 300 - 233
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
Fax: (818) 393 2640
Other Contact Information
To learn more about NASA/JPL's Imaging Radar Program, if you are an Internet user, please refer to World Wide Web server site at URL: http://southport.jpl.nasa.gov/
热心网友
时间：2023-07-09 20:09
The basic concept of weather radar works off of the idea of a reflection of energy. The radar sends out a signal, as seen to the right, and the signal is then reflected back to the radar. The stronger that the reflected signal is, the larger the particle. For more basic information on weather radar, click here for a video from the Franklin Institute Science Muesum.
Image at right is courtesy COMET.
At the heart of the principle of radar is the radar equation .
Pr=PtG^2Θ^2H∏^3K^2L/1024(In2)λ^2 * Z/R^2
This equation involves variables that are either known or are directly measured. There is only one value that is missing, but it can be solved for mathematically. Below is the list of variables, what they are, and how they are measured.
Pr: Average power returned to the radar from a target. The radar sends up to 25 pulses and then measures the average power that is received in those returns. The radar uses multiple pulses since the power returned by a meteorological target varies from pulse to pulse. This is an unknown value of the radar, but it is one that is directly calculated.
Pt: Peak power transmitted by the radar. This is a known value of the radar. It is important to know because the average power returned is directly related to the transmitted power.
G: Antenna gain of the radar. This is a known value of the radar. This is a measure of the antenna's ability to focus outgoing energy into the beam. The power received from a given target is directly related to the square of the antenna gain.
: Angular beamwidth of the radar. This is a known value of the radar. Through the Probert-Jones equation it can be learned that the return power is directly related to the square of the angular beamwidth. The problem becomes that the assumption of the equation is that precipitation fills the beam for radars with beams wider than two degrees. It is also an invalid assumption for any weather radar at long distances. The lower resolution at great distances is called the aspect ratio problem.
H: Pulse Length of the radar. This is a known value of the radar. The power received from a meteorological target is directly related to the pulse length.
K: This is a physical constant. This is a known value of the radar. This constant relies on the dielectric constant of water. This is an assumption that has to be made, but also can cause some problems. The dielectric constant of water is near one, meaning it has a good reflectivity. The problem occurs when you have meteorological targets that do not share that reflectivity. Some examples of this are snow and dry hail since their constants are around 0.2.
L: This is the loss factor of the radar. This is a value that is calculated to compensate for attenuation by precipitation, atmospheric gases, and receiver detection limitations. The attenuation by precipitation is a function of precipitation intensity and wavelength. For atmospheric gases, it is a function of elevation angle, range, and wavelength. Since all of this accounts for a 2dB loss, all signals are strengthened by 2 dB.
: This is the wavelength of the transmitted energy. This is a known value of the radar. The amount of power returned from a precipitation target is inversely since the short wavelengths are subject to significant attenuation. The longer the wavelength, the less attenuation caused by precipitate.
Z: This is the reflectivity factor of the precipitate. This is the value that is solved for mathematically by the radar. The number of drops and the size of the drops affect this value. This value can cause problems because the radar cannot determine the size of the precipitate. The size is important since the reflectivity factor of a precipitation target is determined by raising each drop diameter in the sample volume to the sixth power and then summing all those values together. A ¼" drop reflects the same amount of energy as 64 1/8" drops even though there is 729 times more liquid in the 1/8" drops.
R: This is the target range of the precipitate. This value can be calculated by measuring the time it takes the signal to return. The range is important since the average power return from a target is inversely related to the square of its range from the radar. The radar has to normalize the power returned to compensate for the range attenuation.
Using a relationship between Z and R, an estimate of rainfall can be achieved. A base equation that can be used to do this is Z=200*R^1.6. This equation can be modified at the user's request to a better fitting equation for the day or the area.
For more information on the basic physics of Radar, check out the Cassini Radar Web Page.
回答者：coleyinhe2 - 魔法师 五级 5-26 23:18
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Nobody can be credited with "inventing" radar. The idea had been around for a long time--a spotlight that could cut through fog. But the problem was that it was too advanced for the technology of the time. It wasn't until the early 20th century that a radar system was first built. One of the biggest advocators of radar technology was Robert Watson-Watt, a British scientist.
Great Britain made a big effort to develop radar in the years leading up to World War Two. Some people credit them with being pioneers in the field. As it was, the early warning radar system (called "Chain Home") that they built around the British Isles warned them of all aerial invasions. This gave the outnumbered Royal Air Force the edge they needed to defeat the German Luftwaffe ring the Battle of Britain.
While radar development was pushed because of wartime concerns, the idea first came about as an anti-collision system. After the Titanic ran into an iceburg and sank in 1912, people were interested in ways to make such happenings avoidable.
In 1934 a large-scale Air Defence exercise was held to test the defences of Great Britain and mock raids were carried out on London. Even though the routes and targets were known in advance, well over half the bombers reached their targets without opposition. Prime Minister Baldwin's statement "The bomber will always get through" seemed true.
To give time for their guns to engage enemy aircraft as they came over, the Army was experimenting with the sound detection of aircraft by using massive concrete acoustic mirrors with microphones at their focal points.
Dr H.E. Wimperis, the first Director of Scientific Research for the Air Ministry, and his assistant Mr A.P. Rowe arranged for Air Marshall Dowding to visit the Army site on the Romney Marshes to see a demonstration. On the morning of the test the experiment was completely wrecked by a milk cart rattling by. Rowe was so concerned by this failure that he gathered up all the Air Ministry files on the subject of Air Defence. He was so appalled that he wrote formally to Wimperis to say that if we were involved in a major war we would loose it unless something new could be discovered to change the situation. He suggested that the best advisors obtainable should review the whole situation to see whether any new initiatives could be found. On 12th November Wimperis put this proposal to the Secretary of State and a Committee was set up under Henry Tizard.
The idea of using rays to kill or disable people or machines was very popular, so to start things off Wimperis got Professor Hill to estimate the radio energy needed to cause damage to humans. He sent this to Mr Watson-Watt, Superintendent of the Radio Research Station at Slough for his views on the possibility of developing a radio "Death Ray" to melt metal or incapacitate an aircraft pilot. Watson-Watt passed the letter to A.F. Wilkins who reported that there was no possibility of achieving these destructive effects at a distance but that energy reflected from aircraft should be detectable at useful ranges. This was reported to the first meeting of the Tizard Committee on 28th January and Rowe was instructed to get quantitative estimates for detection.
Wilkins made further calculations from which Watson-Watt wrote a memoranm proposing a system of radio-location using a pulse/echo technique. The Committee gave this a very favourable reception and Wimperis asked Dowding for £10,000 to investigate this new method of detection. Dowding, though very interested, said he must have a simple practical demonstration to show feasibility before committing scarce funds to the project.
For this demonstration Watson-Watt and Wilkins decided to make use of transmissions from the powerful BBC short-wave station at Daventry and measure the power reflected from a Heyford bomber flying up and down at various ranges. Detection was achieved at up to 8 miles and the £10,000 was granted.
A site at Orfordness was chosen to do the detection experiments over the sea. Aerials mounted on three pairs of 75ft wooden lattice masts were installed and detection ranges of 17 miles were obtained. These were rapidly increased to 40 miles by July. Work was done to show how map position and height might be determined and Watson-Watt submitted proposals for a chain of stations to be erected round the coast to provide warning of attack and to tell fighters where to engage the attackers. He suggested that a full-scale station should be built at once, to be followed, if successful, by a group of stations to cover the Thames Estuary and then by a final chain covering the South and East coasts. Construction of 5 stations was authorised and the one at Bawdsey was in operation by August 1936. The others followed shortly after. Plots were to be telephoned to a central operations room and combined with data from the Royal Observer Corps and the radio direction-finding system.
In February 1936 Bawdsey Manor became the centre for the expanding research team and Tizard inspired the RAF at Biggin Hill to investigate fighter control and interception techniques. Their results convinced him that effective interceptions could be obtained against mass raids by day, but not against dispersed attackers at night. He therefore pressed for equipment to go into fighters for them to find and engage targets when positioned within a few miles. Initial tests using a large television transmitter on the ground operating on a wavelength of 6 metres and a receiver in a Heyford Bomber with an aerial between its wheels gave detection ranges of over 10 miles. To get a transmitter into an aircraft and rece the size of the aerial a much lower wavelength was required. Bowen installed a crude equipment operating at 1 metre in an Anson and in the autumn of 1937 aircraft were detected and also Naval ships several miles away in appalling weather.
From then on Air Interception (AI) and Air to Surface Vessel (ASV) equipments were developed. Further Air Defence Trials showed that better detection of low flying aircraft was needed and Chain Low (CHL) stations were evolved from Coastal Defence (CD) equipments which had been developed for the Army. Gun laying equipments (GL) were developed and also equipments to improve navigation (GEE) and bombing (OBOE) and (H2S). These are dealt with in the following sections.
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时间：2023-07-09 20:09
