Weather radar has made many improvements in the last 10 There are more improvements on the All of the radars of the past and present work off the same basic principle: the radar equation The basic concept of weather radar works off of the idea of a reflection of The radar sends out a signal， as seen to the right， and the signal is then reflected back to the The stronger that the reflected signal is， the larger the For more basic information on weather radar， click here for a video from the Franklin Institute Science M 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 There is only one value that is missing， but it can be solved for Below is the list of variables， what they are， and how they are Pr: Average power returned to the radar from a The radar sends up to 25 pulses and then measures the average power that is received in those The radar uses multiple pulses since the power returned by a meteorological target varies from pulse to This is an unknown value of the radar， but it is one that is directly Pt: Peak power transmitted by the This is a known value of the It is important to know because the average power returned is directly related to the transmitted G: Antenna gain of the This is a known value of the This is a measure of the antenna's ability to focus outgoing energy into the The power received from a given target is directly related to the square of the antenna : Angular beamwidth of the This is a known value of the Through the Probert-Jones equation it can be learned that the return power is directly related to the square of the angular The problem becomes that the assumption of the equation is that precipitation fills the beam for radars with beams wider than two It is also an invalid assumption for any weather radar at long The lower resolution at great distances is called the aspect ratio H: Pulse Length of the This is a known value of the The power received from a meteorological target is directly related to the pulse K: This is a physical This is a known value of the This constant relies on the dielectric constant of This is an assumption that has to be made， but also can cause some The dielectric constant of water is near one， meaning it has a good The problem occurs when you have meteorological targets that do not share that Some examples of this are snow and dry hail since their constants are around L: This is the loss factor of the This is a value that is calculated to compensate for attenuation by precipitation， atmospheric gases， and receiver detection The attenuation by precipitation is a function of precipitation intensity and For atmospheric gases， it is a function of elevation angle， range， and Since all of this accounts for a 2dB loss， all signals are strengthened by 2 dB : This is the wavelength of the transmitted This is a known value of the The amount of power returned from a precipitation target is inversely since the short wavelengths are subject to significant The longer the wavelength， the less attenuation caused by Z: This is the reflectivity factor of the This is the value that is solved for mathematically by the The number of drops and the size of the drops affect this This value can cause problems because the radar cannot determine the size of the 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 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" R: This is the target range of the This value can be calculated by measuring the time it takes the signal to The range is important since the average power return from a target is inversely related to the square of its range from the The radar has to normalize the power returned to compensate for the range Using a relationship between Z and R， an estimate of rainfall can be A base equation that can be used to do this is Z=200*R^ This equation can be modified at the user's request to a better fitting equation for the day or the For more information on the basic physics of Radar， check out the Cassini Radar Web P 回答者：coleyinhe2 - 魔法师 五级 5-26 23:18===========Nobody can be credited with "inventing" The idea had been around for a long time--a spotlight that could cut through But the problem was that it was too advanced for the technology of the It wasn't until the early 20th century that a radar system was first One of the biggest advocators of radar technology was Robert Watson-Watt， a British Great Britain made a big effort to develop radar in the years leading up to World War T Some people credit them with being pioneers in the As it was， the early warning radar system (called "Chain Home") that they built around the British Isles warned them of all aerial This gave the outnumbered Royal Air Force the edge they needed to defeat the German Luftwaffe during the Battle of B While radar development was pushed because of wartime concerns， the idea first came about as an anti-collision After the Titanic ran into an iceburg and sank in 1912， people were interested in ways to make such happenings In 1934 a large-scale Air Defence exercise was held to test the defences of Great Britain and mock raids were carried out on L Even though the routes and targets were known in advance， well over half the bombers reached their targets without Prime Minister Baldwin's statement "The bomber will always get through" seemed 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 Dr HE Wimperis， the first Director of Scientific Research for the Air Ministry， and his assistant Mr AP Rowe arranged for Air Marshall Dowding to visit the Army site on the Romney Marshes to see a On the morning of the test the experiment was completely wrecked by a milk cart rattling Rowe was so concerned by this failure that he gathered up all the Air Ministry files on the subject of Air D 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 He suggested that the best advisors obtainable should review the whole situation to see whether any new initiatives could be On 12th November Wimperis put this proposal to the Secretary of State and a Committee was set up under Henry TThe 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 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 Watson-Watt passed the letter to AF 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 This was reported to the first meeting of the Tizard Committee on 28th January and Rowe was instructed to get quantitative estimates for Wilkins made further calculations from which Watson-Watt wrote a memorandum proposing a system of radio-location using a pulse/echo The Committee gave this a very favourable reception and Wimperis asked Dowding for £10，000 to investigate this new method of Dowding， though very interested， said he must have a simple practical demonstration to show feasibility before committing scarce funds to the 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 Detection was achieved at up to 8 miles and the £10，000 was A site at Orfordness was chosen to do the detection experiments over the Aerials mounted on three pairs of 75ft wooden lattice masts were installed and detection ranges of 17 miles were These were rapidly increased to 40 miles by J 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 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 Construction of 5 stations was authorised and the one at Bawdsey was in operation by August The others followed shortly Plots were to be telephoned to a central operations room and combined with data from the Royal Observer Corps and the radio direction-finding 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 Their results convinced him that effective interceptions could be obtained against mass raids by day， but not against dispersed attackers at He therefore pressed for equipment to go into fighters for them to find and engage targets when positioned within a few 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 To get a transmitter into an aircraft and reduce the size of the aerial a much lower wavelength was 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 From then on Air Interception (AI) and Air to Surface Vessel (ASV) equipments were 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 A 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
