Radars and Meteorology

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• Radars and Meteorology
• Shri S.H. Damle
• Indian Institute of Tropical Meteorology
• Pune

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Radar Meteorology

• Historical Back Ground
• Basic concept of Radar
• Nikola Tesla 1900
• Article in Century Magazine

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• Some british German Patents on Detection
  Ranging of Remote metallic objects by Radio Waves
  1900-1906
• First practical Demonstration of Ranging using
  FM-CW transmitter.
• Dec 1924
• Appleton Kings College London
• Barnet - Cambridge University

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• They observed the reflections from ionospheric
  layers beyond 100 km The Appleton Layer
• Use of Pulsed Techniques
• Breit Tuve Carneigy Institute 1925, with NRL
  collaborations.
• Again Inospheric Echoes 150 km away.

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• Work in another direction was being perused by a
  young engineer turned Meteorologist (Sir) Watson.
  Watt 1915
• Study of e-m radiation by lightening in
  thunderstorms
• Objective Timely thunderstorm warnings to world
  War I aviations.

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• 1935 World War II scenario
• British govt. committee on scientific survey of
  Air Defence (CSSAD)
• Consultations with sir Watson Watt Wilkins
• Proposal firmed up 27 Feb 1935
• Successful Demonstration of detection ranging
  of aircraft July 1935.

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• This early war time effort firmly established
  that RADAR was a tool of Aviation.
• Since it evolved through the effort of
  meteorologists meteorology was clubbed as part of
  civil Aviation. Even in India as in many other
  countries Dept. of meteorology was a part of Dept
  of civil Aviation

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• 1940 Dr. J.W. Ryde work on a 10 cm Radar Most
  probably related to precipitation detection but
  no direct record of the period wartime secrecy.
• 1946 Rydes publication on estimation of
  attenuation and echoing properties of clouds
  rain.
• Thus 1940-46 may be marked as the Birth of Radar
  Meteorology.

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• Early Radars deployed in RADAR Meteorology
• S C Band 2700 MHz / 5600 MHz
• Most of these were pulsed incoherent Radars the
  Tx source being high power Magnetrons
• These were suitable for receiving echoes form
  precipitation/detecting, cyclonic/severe weather
  system mostly Intensify/reflectivity estimations.

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• Quantitative Estimation of wind fields could only
  be done by scan to scan tracking
• Reflectivity Mapping DVIP
• The advent of Klystron Technology then led to
  development of Doppler weather Radars which could
  directly measure the average wind speeds in the
  cyclonic systems
• The advances in digital/computational Technology
  then gave further impetus to these Radar system
  developments.

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• The Polarization diversity Radar An effort to
  improve precipitation measurements by weather
  Radars
• Hydrometeors/Raindrops tend to elongated as
  they fall from height.
• The scattering x-sections in the two
  polarizations is therefore different.
• The differential reflectivity in the two
  polarizations give a better handle on rain rate
  estimation.
• Typical dual polarization radar requires
  polarization switching n a pulse to pulse basis
  requiring advanced high power switching
  technology.

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The Clear air Radar Wind profiler

• Gradients in refractive index fluctuations
  leading to e-m back scatter
• The average wind carries along these
  irregularities and in turn they become tracers of
  mean wind.
• A Radar operating at wavelength ? is most
  sensitive to scale sizes of these irregularities
  of ?/2 or multiples of ?/2.

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• The atmospheric Radar Eqn volume Target Eqn
  The signal power

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• Note
• Dependence on 1/R2
• Proportional to Ae PT power Aperture product
• Proportional to - The Radar pulse length
• ? volume Reflectivity of atmospheric Target

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The Noise Power

• In Practice
• One integrates nc pulses assuming the signal
  remain coherent during the period - typically
  upto few seconds in troposphere.
• Invariably use spectral processing to detect the
  signal and use DFFT techniques with P points DFT
• Integrate certain number (ni) of spectra
  incoherently the delectability is then defined
  as (S/N)dt

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Coding Decoding

• Coding offers dual advantages
• Good/High resolution
• High average power
• A long single pulse made up of segment of pulses
• Binary phase coding is one convenient form of
  coding technique which is suitable for digital
  implementation

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• The carrier phase is altered either as 0o or 180o
  according to a binary code
• Complementary code sequences are popular in
  profiler applications
• If A B are two complementary codes then they
  possess the property that the range side lobes of
  autocorrelation function of A are in opposite
  sign to the autocorrelation function of B.

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• Thus if the complementary sequences A B are
  transmitted one after the other and on receive
  side their autocorrelation functions are added
  the range side lobes disappear in the receiver
  output leaving in the receiver output a single
  peak at signal location.

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• Once we know A B are complementary codes, then
  
• are also complementary.
• Examples A AB -
• B - AB -
• Use of m baud complementary code pair sequence,
  subsequent decoding addition on receive side
  thus provides a (S/N) improvement by a factor m.

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• This is because all the target returned energy
  which was distributed in range side lobes is
  recovered this is as if Transmit power is
  increased m times compared to a single pulse
  (code) transmission.
• In the Pune profiler a 8 baud code pair sequence
  (baud length 2 microsecond) is used in the higher
  height mode of operation. The code pair is -
  -
• - ---
• which can be generated from the basic pair
  -

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Receiver system Hardware consideration

• Since Cn2 in atmosphere could vary by more than
  70 dB ( 7 orders of magnitude) a high dynamic
  range receiver is required.
• The signal dynamic range is to be achieved
  without saturation of any stage because of
  background noise.
• RF video gain is to be adjusted such that the
  lowest expected signal level at Rx input is
  amplified upto atleast one bit level of the
  ADC.
• This will ensure full utilization of the DSPG

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I-Q Imbalance
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The basic vector wind computation

• Vradial(east) ucos? wsin?
• Vradial (north) vcos? wsin?
• W (zenith) w
• U V being Zonal and Meridional component of the
  wind.
• W Zenith beam estimate of vertical velocity.

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Quality controls on data

• Range tracking temporal continuity
• consensus averaging
• Need to know beam position angle ? accurately.
• If ugtgtw
• Vre ucos? therefore
• Thus for 1 accuracy of radial wind
• Is approximately .170 for ?750

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Similar systems abroad

• NOAA Network at 449 MHz Typical specs
  identical with Pune profiler-Summary performance

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• Thank You