Transcript Radar & Meteorology

Radars and Meteorology
Shri S.H. Damle
Indian Institute of Tropical Meteorology
Pune
Radar & Meteorology
Historical Back Ground
Basic concept of Radar
Nikola Tesla – 1900
Article in Century Magazine
 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
 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.
 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.
 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.
 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
– 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
– Ryde’s publication on
estimation of attenuation and echoing
properties of clouds & rain.
 Thus 1940-46 may be marked as the
Birth of Radar Meteorology.
 1940
 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.
 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.
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.

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 λ is
most sensitive to scale sizes of these
irregularities of λ/2 or multiples of λ/2.

The atmospheric Radar Eqn – volume Target
Eqn : The signal power
 t Pt Gt  R 2c  

Pr 
.
.
. r . Ae
2
2


4R
 1 6 ln 2  4R
4Ae

Gt 
,   
2

Ae
Th u s
Pr 
 t Pt r Ae
2
6 4R ln 2
.c .
Note:
2
 Dependence on 1/R
 Proportional to Ae PT : power
Aperture product
 Proportional to The
Radar
c
pulse length
 η:
volume Reflectivity of
atmospheric Target
The Noise Power
N  Pn  kTs B  k ( r TB  Tn ) B
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
i)
Integrate certain number (ni) of spectra ‘incoherently’
the delectability is then defined as (S/N)dt
 r t Pr Ae(c )
nc.P
S
..
ni
   2
m
 N  dt R 64ln 2k ( rTB  Ts) B
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
 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.
 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.
AB
 Once
we know A & B are
complementary codes, then AB & AB
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.
 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 ++ & +-
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
I-Q Imbalance
The basic vector wind computation
Vradial(east) = ucosθ + wsinθ
Vradial (north) = vcosθ + wsinθ
W (zenith)
=w
U &V being Zonal and Meridional
component of the wind.
W Zenith beam estimate of vertical velocity.
Quality controls on data
 Range
tracking; temporal continuity:
consensus averaging
 Need to know beam position angle θ
accurately.
 If u>>w
Vr
  tand
Vre = ucosθ therefore
Vr
d
Thus for 1% accuracy of radial wind
Is approximately .170 for θ~750
Similar systems abroad

NOAA Network at 449 MHz: Typical specs – identical with Pune
profiler-Summary performance