PROJECT REPORTON

APPLICATION OF MICROWAVE RADIOMETER FOR WEATHER

FORECASTING

SUBMITTED BY:-

AMISHA AGARWAL

093004 , C1(ECE)B.TECH

APEEJAY COLLEGE OF ENGINEERING

UNDER THE GUIDANCE OF

MRS. RANJU MADAN

&

MR. GAJENDRA KUMAR

IMD, LODHI ROAD, NEW DELHI

ACKNOWLEDGMENT

We wish to express our deep sense of gratitude and everlasting indebtedness to our project guide MRS. RANJU MADAN ,Director and MR. GAJENDRA KUMAR for their advice ,constant encouragement & invaluable suggestions . It is pleasure to acknowledge our sincere gratitude to them for giving us much of his valuable time .

We are thankful to MR. RAJINDER SINGH SETHI for their constant guidance suggestions & help during summer training.

We sincerely thankful to lab staff and all other staff members of Indian metrological Department for extending all facilities and help required for completing this work. Finally we wish to express our gratefulness to our friends who helped us directly or indirectly at different stages while preparing this report.

Table of Contents

1.0 Introduction:......................................................................................................3

2.0 Principles of Microwave Radiometry :...............................................................7

2.1 Contents:.........................................................................................................7

2.1.1 Thermal Radiation :...................................................................................7

2.1.2 Antennas :................................................................................................8

2.1.3 Radiometer :.........................................................................................12

2.1.4 Microwave Signatures :..........................................................................13

2.1.5 Conclusions.............................................................................................24

3.0 Technical specifications of different microwave radiometers :........................25

4.0 Types & working of microwave radiometers :.................................................27

5.0 Uses of microwave radiometer for different purposes:...................................38

6.0 REFRENCES AND BIBLIOGRAPHY......................................................................42

1.0 Introduction: A microwave radiometer (MWR) is a radiometer that measures energy emitted at sub-millimetre-to-centimetre wavelengths (at frequencies of 1-1000 GHz) known as microwaves. Their primary application has been onboard spacecraft measuring atmospheric and terrestrial radiation, and they are mostly used for meteorological or oceanographic remote-sensing. Their secondary application is also meteorological, as zenith-pointing surface instruments that view the Earth's atmosphere in a region above the stationary instrument.

Schematic data flow of a differential microwave radiometer By understanding the physical processes associated with energy emission at these wavelengths , scientists can calculate a variety

of surface and atmospheric parameters from these measurements , including air temperature , sea surface temperature , salinity , soil moisture , sea ice , precipitation , the total amount of water vapour and the total amount of liquid water in the atmospheric column directly above or below the instrument. A microwave radiometer is a very sensitive electronic instrument that measures the amplitude (or strength) of microwave signals. Microwaves are electromagnetic waves from the central region of the electromagnetic spectrum, that is, waves having wavelengths shorter than radio waves and longer than light waves and x-rays.

Almost all objects in the universe emit microwaves. In a process known as remote sensing radiometers are used to detect their presence and learn about them. A large antenna is needed to detect weak radiation. In fact, the weaker the radiation strength, the larger the antenna needed to detect it. For example, the sky radiates very weakly and is regarded as a cold radiator.

Water typically radiates more and grass is even warmer (hence radiates still more). These objects can be detected equally

well during daytime and nighttime by using microwave radiometers. Microwave radiometers on spacecraft, with antennas pointed downward, can provide images of thunderstorms, land, forests, surface soil moisture, forest fires, vegetation, rain clouds, oceans, sea ice, and more. These observations help us to better understand our planet and its climate.

Examples of microwave radiometers on meteorological satellites include the Special Sensor Microwave/Imager, Scanning Multichannel Microwave Radiometer and Microwave Sounding Unit. The Microwave Imaging Radiometer with Aperture Synthesis is an interferometer/imaging radiometer capable of resolving soil moisture and salinity over small regions of surface. The Juno spacecraft, scheduled for launch in 2011, will characterize the atmosphere of Jupiter using a suite of microwave radiometers.

2.0 Principles of Microwave Radiometry :

2.1 Contents:

• Thermal Radiation .

• Antennas .

• Radiometer.

• Microwave Signatures.

• Conclusions .

2.1.1 Thermal Radiation :

Radiation in thermal equilibrium

Homogeneous temperature T (K): specific intensity (radiance) Iν (Watt/m2/Hz/ steradian )

determined by Planck Function Bx

c = speed of light, h = Planck constant, kb = Boltzmann constant

(ν = frequency, λ=c/ν wavelength)

For hν<<kbT : Rayleigh-Jeans approximation for Bv(t):

Thermal radiation, general

Iν can be expressed by the brightness temperature TB, and in the approximation of

Rayleigh-Jeans: Tb =TB:

2.1.2 Antennas :

Parameters of lossless antennas

Spectral power density received by an antenna:

or in Rayleigh-Jeans, expressed as antenna temperature

Iν resp. Tb : radiation received from directions

Ae : effective area, antenna axis , related with directivity D (gain):

Effective beam solid angle Ωe, beam efficiency ηb:

Simplest examples (lossless: ηl=1)

1) Isotropic antenna

2) Ideal Pencil-Beam antenna

3) Boxcar antenna:

4) Boxcar antenna with diffuse background:

The smaller Ω0, the more difficult it is to get high beam efficiency ηb →1

5) Still more realistic are Gaussian D functions with some background

ELBARA 1.4 GHz radiometer with conical horn antenna:

• dual-mode excitation with near Gaussian directivity pattern

• dual linear polarization (h, v)

2.1.3 Radiometer :

The signal received from antenna is passed through D-switch and band width filter . the filtered signal of particular bandwidth is passed through isolator , then the signal in isolated form passed through amplifier and power meter . the integrator converts detected signal to the required signal.

Dicke switch and reference load

Fast switching between antenna and reference with synchronize power subtraction to eliminate the (often dominant) receiver noise TN

Radiometric resolution

The radiometric resolution ΔT min is the statistical error. For Dicke radiometers without gain variation with

2.1.4 Microwave Signatures :

Generalized Law of Kirchhoff' on Radiation

Consider the space composed of N volumes at N temperatures, T1 to TN (Planck, 1966, § 43-46). Assumption of Local Thermodynamic Equilibrium (LTE), i.e. Ti and emission controlled locally:

emissivity = absorptivity in each volume

Reciprocal situation

The antenna transmits radiation at frequency ν; the fraction ai (i=1-N) (ai absorptivity) is absorbed in volume i , thus adding heat there. Energy conservation requires:

Real situation

The same antenna receives thermal radiation with the ai weighting

This law is more general than radiative transfer.

This diagram shows the (T1,T2,T3,T4) different thermal radiations on the basis of their different absorptivity and temperature properties at different point.

Example with N=4: Cosmic background (1), atmosphere (2), surface (3), subsurface (4) .

Signatures of Temperature

Object Temperature (K)

Quiet sun 6000 - 100'000 (frequency dependent)

Earth atmosphere (h<20km) 190-320

Earth surface 230-320

Dry snow ≤ 273.15

Wet snow, freezing lake 273.1 - 273.2

Human body 308-313

Living room 290-300

Cosmic background 2.7

High contrast between cosmic background and terrestrial temperatures

Sun often not relevant, due to small solid angle (6.8⋅10-5).

Emissivity Signatures

Fresnel formulas for dielectric half space (simplest possible model)

Transparent atmosphere, homogeneous half space below, plane surface (N=2). Surface

reflectivity (rp, polarization p), absorptivity ap=1-rp,. Let T1 be the brightness temperature

from above, and T2 the temperature des surface; then we have

Tbp=rpT1+(1-rp)T2

= (1-ap)T1+apT2

(vertical polarization) from rh by duality: µ ⇔ ε .

The complex dielectric constant ε=ε'+iε" is a key quantity (non-magnetic media: µ=1).

Fresnel reflectivities rv and rh versus incidence angle θ1 for ε= 5+0.1i. But, Fresnel equations have limited applicability due to surface roughness and inhomogeneities.

Alternatives:

• More complex models

• Empirical signatures (e. g. from ELBARA, satellite sensors)

An example of surface emissivity, measured by satellite at 19 GHz, h pol. Oct. 1992.

Sensor: SMM/I on DMSP Data from C. Prigent

Physical (top) and brightness temperatures (below) at 3.1 and 11 GHz, θ=50° h (dotted) and v (solid) pol. of a bare-soil near Bern (Switzerland) showing 2 freeze-thaw cycles, from Wegmüller, Rem. Sens. Env. 33, 123-135 (1990).

Penetration depth

Determined from absorption coefficient

power penetration depth

where k0 is the wave number in vacuum, and n" is the imaginary part of the refractive index (n=n'+in"). For the electromagnetic field the penetration depth is:

Penetration Depth is a measure of how deep light or any microwave radiation can penetrate into a material. It is defined

as the depth at which the intensity of the radiation inside the material falls to 1/e (about 37%) of its original value at (or more properly, just beneath) the surface.

When microwave radiation is incident on the surface of a material, it may be (partly) reflected from that surface and there will be a field containing energy transmitted into the material. This electromagnetic field interacts with the atoms and electrons inside the material. Depending on the nature of the material, the electromagnetic field might travel very far into the material, or may die out very quickly. For a given material, penetration depth will generally be a function of wavelength

Measurement e.g. with emission (in transmission) experiments:

The above diagram shows emission (in transmission) experiment for measuring the penetration depth of any microwave radiation .

(Field) penetration depth in units of the vacuum wavelength

`

The above graph platted between ε'-1 and ε" shows the penetration depth of microwave in units of the vacuum wavelength .

Water shows a Debye Relaxation spectrum of ε=ε'+iε" as determined by the dissipative interaction of the water molecules in the liquid state.

Vegetation :Empirical formula for green vegetation (density near 1g/cm3):

md dry-mass fraction and εsw is (0.9% salinity) water dielectric constant.

Imaginary (left) and real (right) dielectric constant of leaves versus frequency at T≅20C for md=0.5 (triangles and lower curves) and md=0 extrapolated values (bullets and upper curves).

Atmospheric Transmissivity .

Transmissivity spectrum in vertical direction of standard winter (solid) and summer (dashed) atmosphere dominated by spectral lines of water vapor and oxygen.

2.1.5 Conclusions

• Atmospheric windows with high transmissivity allow remote sensing of the earth surface .

• Passive sensing in Rayleigh-Jeans Approximation gives linear temperature averages of the sensed regions (Kirchhoff's Law) Feasibility for temperature sounding .

• Emissivities can dominate Tb thanks to the large contrast between cold cosmic background and terrestrial temperature.

• Key quantity is the complex dielectric constant. • Water and ice have special signatures that are favourable

for sensing soil moisture, vegetation, snow, frost, clouds, etc.

3.0 Technical specifications of different microwave radiometers :

Technical Specifications

MP3008 (USA based) RPG/HATPRO (German based)

Temperature Sensing channel

51-59GHz ( No of channels: 14)

51.26-58.0 GHz ( No of channels: 7)

Humidity sensing channel

22-30GHz(No of channels:21)

22.4-31.4GHz (No of channels:7)

Sample Time 10Sec, Variable >1 Sec, User selectable

Humidity 0-100%Accuracy + 2 % 5% RMSResolutionTemperatureRange -40o to 60oC 0-800 KAccuracy + 0.5 o C 0.25 -1o C RMSResolution 0.2 + 0.002* ITkBB-

TskyI2 0CFrequency of Operation

K Band & V Band K Band & V Band

Operating Temperature

-40o to 50oC -40oC to 45oC

Operating Voltage

85-264 V at ( 50-60 Hz)

90-230 V at ( 50-60 Hz)

Scanning mode

Azimuthal & Elevational

Azimuthal & Elevational

Angular coverage

All sky with optional azimuth positioned

All sky within 6 minutes azimuth positioned option.

Operating Environment

All weather

Height Coverage

10 Km 0-10km

It should be capable of taking cloud base temperatureTechnology Used

Frequency Agile Filter bank

Can operate during rain

Can not operate during rain

Here we are comparing the two mwr

MP3008 (USA based)

RPG/HATPRO (German based)

Here both of these are differentiated on the basis of different parameters . on the basis of temp sensing we have 14 channels in USA based system while 7 on the basis of German based system while on the basis of humidity these are 21 & 7 respectively sample time taken by mp3008 is around 100ms with the humidity of 0-100% & having an accuracy of +-2%,while RMS having time of >1 ms & with the accuracy of about 5%rms.

While both of these are using the k & v band for there operations & there is a small difference in the operating temp for USA based system its -40 to 50 while for the other type its goes to -40 to 45,while the voltage ranges from 85-264V & 90-230V respectively.

Both of these having the azimuthal & elevational type of scanning modes which covers all sky with optional azimuth positioned . USA bases system has a height coverage of

10km while the other system has 0-10kms.one of the important differences between these is that USA based system can operate during the rain while the either system can’t.

4.0 Types & working of microwave radiometers :

Stepped Frequency Microwave Radiometer (SFMR) from Pro Sensing Inc.

The Stepped Frequency Microwave Radiometer (SFMR) developed by Pro Sensing has been used by the National Oceanic and Atmospheric Administration (NOAA) aboard their hurricane reconnaissance aircraft to measure over-ocean wind speed and rain rate in hurricanes and tropical storms. The SFMR is a compact, airborne radiometer designed to measure surface brightness temperature in 6 frequency bands spanning 4.5 to 7 GHz.

Calibrated values of brightness temperature generated by SFMR are reported in real time to a wind speed retrieval algorithm, developed in cooperation with NOAA's Hurricane Research Division. This algorithm generates a real time measure of surface level wind speed and rain rate in hurricanes and tropical storms.

Microwave Radiometer from Optimare

Microwave Radiometer (MWR) is a six-channel across-track scanning microwave spectrometer which is capable of detecting and mapping oil layers exceeding a thickness of 0.05 millimeters Furthermore, this system is capable of measuring and mapping oil layer thickness in the range from 0.05 to 3 millimeters.

APPLICATION

Detection and mapping of very thick areas oil spills (exceeding a layer thickness of 0.05 mm).

Remote measurement of oil spill thickness in the range from 0.05 to 3 millimeters.

Quantification of oil volume. Airborne support of oil spill response actions even above the

overcast layer and in case of fog, drizzle and rain.

Microwave Radiometer from MICROCURE

EIT’s MicroCureTM measurement system is comprised of two separate stand-alone items; the data collection radiometer which is placed in the ultraviolet (UV) environment to be measured, and a Data Reader which reads and displays the measurement results from the radiometer. Read The UV integrating radiometer is a microprocessor-based, electro-optical instrument that measures and accumulates the total UV energy that is applied to the measurement surface of the instrument. The MicroCureTM radiometer measures the total amount of UV energy that would be impinged on a work piece passing through the curing system and the peak UV irradiance. The radiometer combines very small physical size and adaptability to address a variety of demanding physical and thermal measurement environments.

Features

Small Size: 1.3” x 0.95” x 0.25” Lightweight: 0.33 ounces Measures total energy density in Joules/cm2

Measures peak power density in Watts/cm2 Battery powered High sample rate: 2000 Samples per second Automatic operation High temperature resistance

Applications· UV systems requiring small size and automatic operation· Small container curing (bottles, cans, etc.)· Three-dimensional objects· Web printing· Very small conveyorized and batch applications(semiconductor printing, small part bonders, etc.)· Statistical Process Control measurements

60 GHz Microwave Scanning Radiometer from BEST Boulder Environmental Sciences and Technology

Compact, low cost 60 GHz microwave scanning radiometer at oxygen absorption line for boundary layer air temperature profile measurements. Center frequency: 60 GHz IF Frequency band: 2GHz (DSB Mode)

Center frequency: 60 GHz IF Frequency band: 2GHz (DSB Mode) Receiver noise temperature: 800 K Radiometer temperature resolution: 0.03 K @ 1sec

integration time Corrugated horn antenna with low side lobes level < -30 dB Antenna beam width: 6.5 degrees Mirror spinning rate: 2 revolutions per second Dimensions and weight:

o Microwave unit with scanner: 700 x 120 x 180 mm, 9 kg

o Power supply: 340 x 230 x 130 mm, 7.5 kg Power consumption: 50 W Power requirements: 120-220 V, 1 A, 50-60 Hz

LWP-90-DP150 Microwave Radiometer from Radiometer Physics GmbH

The LWP-90-DP150 instrument is a stand-alone system for automated weather-station use under nearly all environmental conditions. The total (integrated) amount liquid water in the vertical column above the radiometer is retrieved from the two-channel brightness temperature measurements.

Read More brightness temperature measurements. There is only very limited information on water vapor (humidity) due to the lack of an water vapor line frequency.

Optical resolution: 2.0°nbsp;HPBW at 90 and 150 GHz (same field of view)

Radiometric resolution: 0.2 K RMS at 1.0 s integration time Absolute system stability: < 1.0 K Receiver and antenna thermal stabilization: < 0.02 K Environmental temperature range for operation: -30 °C to +45

°C Automatic rain-detection system (including a dew-blower) Integrated PC on-board GPS clock Additional external meteorological sensors: barometric

pressure, humidity, temperature Stand alone operation Data storage and backup on-board, power-failure safe-guard

Liquid Water Path 90 GHz Upgrade Radiometer from Radiometer Physics GmbH

This LWP-U90 radiometer is an upgraded version of the above LWP two-channel radiometer. By including a high frequency channel at 90 GHz the sensitivity to thin cloud (small LWP) is

significantly improved: Instead of approximately 30 g/m^2 the three channel version has a detection limit of 5 to 10 g/m^2.

Read More The total (integrated) amounts of water vapor and liquid water in the vertical column above the radiometer are retrieved from the two-channel brightness temperature mesurements.

A variety of retrieval algorithms (custom designed or global standard algorithms) can be selected.

Optical resolution: 3.5° HPBW at 23.8 GHz Radiometric resolution: 0.2 K RMS at 1.0 s integration time Absolute system stability: <1.0 K Receiver and antenna thermal stabilization: < 0.02 K Environmental temperature range for operation: -30 °C to +45

°C Automatic rain-detection system (including a dew-blower) Integrated PC on-board Full internal retrieval algorithm, retrieval data and raw data

output GPS clock Additional external meteorological sensors: barometric

pressure, humidity, temperature.

Dual Polarization rain Radiometer (DP-RR) from Radiometer Physics GmbH

The RPG DP-RR instrument is a stand-alone system for automated weather-station use under nearly all environmental conditions. With orthogonal polarization information and brightness temperatures at 6, 10, 19, and 36 GHz this instrument is dedicated to the observation of raining clouds. Polarization effects on non-spherical hydrometeors can be observed with this kind of instrument.

With orthogonal polarization information and brightness temperatures at 6, 10, 19, and 36 GHz this instrument is dedicated to the observation of raining clouds. Polarization effects on non-spherical hydrometeors can be observed with this kind of instrument.

Radiometric resolution: 0.2 K RMS at 1.0 s integration time Absolut system stability: < 1.0 K Environmental temperature range for operation: –30 °C to +45

°C Integrated PC on-board Stand alone operation Data storage and backup on-board, power-failure safe-guard Low maintenance (3 months interval for absolute calibration

using LN)

Advanced Microwave Radiometer from Aviso User Service

The AMR measures water vapor content in the atmosphere so that we can determine how it impacts radar signal propagation. Its measurements also can be used directly for studying other atmospheric phenomena, particularly rain.

Read More The AMR is a passive receiver that collects radiation reflected by the oceans at frequencies of 18.7, 23.8 and 34.0GHz. Radiation measured by the radiometer depends on surface winds, ocean temperature, salinity, foam, absorption by water vapor and clouds, and various other factors. To determine atmospheric water vapor content accurately, we need to eliminate sea surface and cloud contributions from the signal received by the radiometer. That is why the AMR uses different frequencies, each of which is more sensitive than the others to one of these contributions. The main 23.8-GHz frequency is used to measure water vapor; the 34-GHz channel provides the correction for non-rain bearing clouds; and the 18.7-GHz channel is highly sensitive to wind-driven variations in the sea surface. By combining measurements acquired at each of these frequencies, we can extract the water vapor signal

Function

The AMR measures water vapor content in the atmosphere so that we can determine how it impacts radar signal propagation. Its measurements also can be used directly for studying other atmospheric phenomena, particularly rain.

5.0 Uses of microwave radiometer for different purposes:

Advanced Water Vapor Radiometer (AWVR), a very stable ground-based microwave radiometer for measurements of the water vapor contribution to the atmospheric refractive index at microwave frequencies. A key application is measurement of "path delay" in Deep Space Network communication links.

Microwave Temperature Profiler (MTP), a "forward looking" airborne instrument that has flown on numerous aircraft, for measurements of atmospheric temperature profiles. MTP is a very important data provider for studies of chemistry and dynamics in the upper troposphere and lower stratosphere.

Passive-Active L/S band system (PALS), an airborne instrument for measurement of ocean surface salinity, a precursor and demonstrator for the ESSP Aquarius mission. PALS includes both a passive radiometer and a radar scatterometer in one integrated system.

High Altitude MMIC Sounding Radiometer (HAMSR), a "nadir looking" airborne instrument for profiling temperature and humidity.

The Juno MWR (Microwave Radiometer) will be built for the Juno mission to Jupiter, expected to launch in 2012. MWR is presently in the preliminary design phase. The Juno MWR will be the second microwave instrument to explore the planets since the first observations from Mariner 2 of Venus in 1962, which confirmed that the high temperature inferred from radio measurements indeed reflected surface and deep atmospheric conditions rather than a hot ionosphere. The Jet Propulsion Laboratory builds the MWR.

The primary goal of the Juno Microwave Radiometer is to probe the deep atmosphere of Jupiter at radio wavelengths ranging from 1.3 cm to 50 cm using six separate radiometers to measure the planet's thermal emissions. The MWR experiment will provide answers to two key questions: How did Jupiter form? and How deep is the atmospheric circulation that was measured from the Galileo Probe down to 22 bars of pressure, and at the cloud top level from imaging data returned by other missions?

The first question will be addressed by the determination of the water abundance in the deep atmosphere. The MWR will obtain measurements of ammonia and water in the Jupiter atmosphere, which are the principle absorbers in the microwave region, by scanning Jupiter along the orbital track as the spacecraft spins. These observations will allow scientists to determine whether the water abundance on Jupiter is three times that of the sun or nine times that of the sun.

The Juno MWR avoids the synchrotron emission from Jupiter's magnetosphere by using shorter wavelengths and achieves high accuracy to measure water abundance in the deep atmosphere by using "relative limb darkening," a parameter that depends on the emission angle of the radiation. The vertical profile of water abundance is obtained by using multiple frequencies, much like the retrieval of temperature profiles on earth with multi-spectral infrared measurements from orbiting weather satellites.

The MWR uses three antennae mounted on the spacecraft body, which sweep across the planet as the spacecraft spins to measure the radiation at six different wavelengths along the orbital track. Successive orbits will map the planet longitudinally. The six different wavelengths observed by the MWR, combined with the emission angle dependence will provide a good idea of the atmospheric temperature profile to ~ the 200 bar pressure level on Jupiter. The latitudinal dependence of the temperature profile and depth will enable inference of the circulation of Jupiter's deep atmosphere to a much greater depth than that obtained by the Galileo probe.

Microwave radiometer working at airport : A newly acquired microwave radiometer began its operation at the airport in late May this year. The instrument measures the microwave radiation emitted from the oxygen molecules and water vapor in the air and, with the use of statistical methods, determines the temperature and humidity profiles of the atmosphere from the ground up to 10 km aloft. Such profiles are also obtainable from the conventional upper-air measurements using balloons, which are available normally twice a day. For the radiometer, measurements are made once every several minutes. This greatly enhances the monitoring of upper-air temperatures and humidity, and facilitates the forecasting of windshear, thunderstorms and low-visibility weather.

Figure 2 shows an example where atmospheric instability index (K index) is derived from the radiometer data for forecasting the occurrence of thunderstorms. During the daytime of 29 May 2008, the atmosphere became more humid and unstable. This was indicated by an increase in the instability index (red dots) derived from the data that rose to relatively high level (about 35). This favours the development of thunderstorms. In fact, lightning activity appeared that night (blue dots) and persisted until the morning of the next day. In this period, the instability index remained at rather high level. It is not until around noon time of 30 May that the instability index dropped to lower values (about 30) and the lightning activity abated.

Fig.1: The microwave radiometer operating at the airport

Fig.2: The time series of instability index (K index) as derived from the radiometer, and the number of lightning flashes recorded within 20 km from the radiometer.

Advanced Microwave Radiometer (AMR) onboard Jason 2 measures water vapor content in the atmosphere so that we can determine how it impacts radar signal propagation. Its measurements also can be used directly for studying other atmospheric phenomena, particularly rain. The AMR is a passive receiver that collects radiation reflected by the oceans at frequencies of 18.7, 23.8, and 34 GHz. Radiation measured by the radiometer depends on surface winds, ocean temperature, salinity, foam, absorption by water vapor and clouds, and various other factors. To determine atmospheric water vapor content accurately, we need to eliminate sea surface and cloud contributions from the signal received by the radiometer. That is why the AMR uses different frequencies, each of which is more sensitive than the others to one of these contributions. The main 23.8-GHz frequency is used to measure water vapor; the 34-GHz channel provides the correction for non-rain-bearing clouds; and the 18.7-GHz channel is highly sensitive to wind-driven variations in the sea surface. By combining measurements acquired at each of these frequencies, we can extract the water vapor signal.

Scientists and engineers are implementing new instrument designs for future applications. These designs will be driven by strict requirements on radiometric stability over a wide range of frequencies.

6.0 REFRENCES AND BIBLIOGRAPHY

1. STANDARD BRIEF – INDIAN MEROLOGICAL DEPARTMENT, NEW DELHI.

2. IMD OFFICIAL WEBSITE , www.imd.gov.in

3. Microwave Radiometers and Combined Sensors by FRANK S. MARZANO.

4. WKIPEDIA—THE FREE ENCYCLOPEDIA

5. LABMANUALS, at IMD LODHI COLONY