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Radar Meteorology

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Radar Meteorology RADAR METEOROLOGY P.S.Biju [email protected] Courtesy: Presentations of Dr.D.Pradhan, Scientist-G, DDGM(UI),New Delhi, Shri.S.B.Thampi,DDGM,Chennai & Dr.Y.K.Reddi, Scientist-F,MCHyderabad Chapter 1: Introduction RADAR is an acronym for Radio Detection and Ranging. Similar principle is Light Detection and Ranging (LIDAR) used in ceilometers. So many other similar principles are there with Detection and Ranging (DAR) having the same equation for range measurement. Radar principle is explained in the following figure: The similar principle LIDAR is illustrated below: 1 Range of Radar Radar is an electronic device which is capable of transmitting an electromagnetic signal, receiving back an echo from a target, and determining various things about the target from the characteristics of the received signal. Range is the distance of the target given by the values of c and t , which is explained as h = ct/2 . Milestones of weather radar • 1842 : Doppler effect • 1888: Electromagnetic waves discovered by Hertz • 1922 : Detection of ships by radio waves by Marconi • 1947: The first weather radar in Washington D.C. • 1990: Introduction of Doppler weather radar • 2000: Doppler weather radars in India 2 Electromagnetic wave A wave propagation containing mutually perpendicular electric and magnetic fields perpendicular to the direction of propagation. Light wave is an example of electromagnetic wave Polarisation of radar signal The direction of propagation of electric field in an electromagnetic wave is known as polarisation. Hence an electromagnetic wave used in radar is either horizontally or vertically polarised. S-band Doppler weather radar of IMD is horizontally polarised and C-band is dual polarised (both horizontal and vertical). 3 Wavelength ( λλλ)and frequency( ννν) Length of one wave is known as wavelength Time taken to travel one complete wavelength is known as period (T) . Number of wavelengths travelled in one second is known as frequency( ν) . Hence T = 1/ ν and Velocity C= λ/T = νλ Electromagnetic spectrum The arrangement of electromagnetic wave according the order of wavelength is known as electromagnetic spectrum. Radar signal uses wavelength in the microwave region ( 1mm to 1 m) in the following bands. 4 IMD utilised S (10 cm), C ( 5 cm) and X ( 3 cm) bands in DWR, Polarised radar and Multimet radar respectively. 5 X-band become obsolete in IMD ? Attenuation of radar signal while passing through a medium is inversely proportional to wavelength as per the following equation: X-band radars are not suitable for the tracking of clouds, cyclones etc due its smaller wavelength gives more attenuation while passing through it. Hence Cyclone detection radars and Doppler weather radars at coastal stations uses S –band only. Doppler effect Doppler effect observed in sound was described by Christian Andreas Doppler that the sound waves from a source coming closer to a standing person have a higher frequency while the sound waves from a source going away from a standing person have a lower frequency. The approach of Doppler in sound waves proved to be valid for light waves also. Light waves from a source coming closer to an observer have a higher frequency (lower wavelength-Blue shift) while the light waves from a source 6 going away from an observer have a lower frequency (Larger wavelength-Red shift). Doppler effect in Radar In Doppler weather radar (DWR) this principle is adopted by considering radar as observer and the moving target as the source of light ( In fact the original source is also radar, but the scattered light is reflected is from the target. Hence for the radar (observer) the source is the target) Doppler shift in frequency ( ∆ν) is given by the basic equation; Where V is the velocity of target. Hence Doppler weather radar will give both range and velocity of the target. Distinguish between conventional radar and DWR Conventional radar 1. Gives only position of a target 2. Analog technology and mostly black and white pictures 3. No provision for unattended operation Doppler weather radar 1. DWR gives both position and velocity of a target 2. Automatic control and Mostly unattended operation 3. User friendly colour images 4. Large number of products for various applications like aviation, hydrology, weather forecasting etc 7 Block diagram of a Radar Transmitter: This part requires high power for the transmission of electromagnetic signal upto 500 Km range. The basic component is a Radio Frequency generator (RF Generator). The generated RF frequency signal is amplified to high power electromagnetic signals by the one of the following transmitters: 1. Magnetron 2. Klystron 3. Solid state transmitters Magnetron has Lighter weight, Easy to carry and 200 MW or more power. But its frequencies are not purer, which is essential for Doppler weather radar. Conventional radars used magnetron as the transmitter Klystron has Heavier weight, Purer frequencies, Wave forms can be controlled and generate power of more than 200 MW. Doppler weather radar uses Klystron as the transmitter. Solid state transmitters have power only up to 50 W, but desirable power can be achieved by making an array of a large numbers of transmitters. But seldom used for meteorological purposes. Modulator : Modulator is the ON/OFF switch of the Radar Transmitter. When and which duration it should transmit will be decided by the modulator. It also decides the correct wave form of the transmitted signal. Master clock and PRF: Master clock controls the entire radar system. It determines how often the radar will transmit signal into space. The rate at 8 which the radar transmits is called Pulse Repetition Frequency (PRF). Usually its value is between 200Hz to 3000 Hz. The duration of transmitter signal names either pulse duration or pulse length. Typical value of Pulse duration is 0.1 to 10 µs. DWR of IMD uses two Pulse widths 1 µs and 2 µs. Antenna: Antenna is a device for radiating and receiving of EM waves. It can be isotropic or non isotropic. An antenna that sends the radiation equally in all directions is called isotropic antenna. It is similar to the light of a candle except the bottom portion. Radar antennas are more like flash lights Main parameters in the selection of an antenna are: Wave length Diameter of reflector ( small as a foot to 30 ft) Gain Gain is the ratio of power received at a point in space on the centre of the beam axis to the power received at the same point from an isotropic antenna. As shown above, gain has no unit. But logarithm of gain multiplied by 10 has a unit called deciBell. Typical gain is 20 dB to 45 dB. 9 Ideal antenna would direct all of the radar energy into a single direction and this is practically impossible. Practically radar signal have a bright spot called the main lobe and also having energy off to the side of the main lobe called side lobes. Radar signal also have energy behind the antenna called back lobes. Relation between gain and beam width: Beam width is the angular distance across the antenna beam at the point where the power is reduced to one half of the peak power which exists along the centre axis of the antenna beam pattern k2 depends on the kind and shape of the antenna and for circular reflector k2=1 10 deci Bell (dB) unit For example the output power of Klystron is about 800kW. It can be expressed in dB as: In dBm as Half power in dB 10 log (1/2) = - 3 dB, i.e. the power reduced to half means power is reduced by 3 dB For example power reduced from 8W to 4W 10 log (8)=9 dB and 10 log (4) = 6 dB. Clearly the reduction is 3 dB 10 log (1/4)=-6 dB i.e. the power reduced to one-fourth means power reduced by 6 dB For example power reduced from 8W to 2W 10 log (8)=9 dB and 10 log (2) = 3 dB. Clearly the reduction is 6 11 Wave guide Regular wires and coaxial cables cause so much loss of signals that they are not useful at radar frequencies Wave guide is a conductor connecting transmitter/receiver and antenna .Wave guide is usually a hollow, rectangular, metal conductor whose interior dimensions depend upon of the wavelength of the signal being carried. T/R Switch or Duplexer Most of the radars transmit power from 1000 W to more than 1 MW. At the same time it is capable of receiving powers as small as 10 -10 W or less. If transmitter sent power in to the receiver it would burn up quickly. An automatic switch known as T/R switch or Duplexer is added in the radar system to protect the receiver from the high power of the transmitter. When the transmitter is turned on , the duplexer acts to direct the strong pulse of energy to the antenna and as soon as the transmitter stops sending a signal, 12 the duplexer switches to connect antenna with the receiver and transmitter will be disconnected from the antenna. Receiver Receivers detect and amplify the very weak signal received by the antenna. Most of the radars used super heterodyne type receivers where the high frequency received signal is mixed with a reference signal and converts it into a much lower frequency (typically 30 to 60 Hz).,which can be easily processed. Co-axial cables can be used to connect receivers with displays since frequency and distance are less. 13 Display The earliest and easiest display is to put the radar data in to an oscilloscope where horizontal axis is time and vertical axis is signal strength.
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