A Presentation On Data Acquisition By Microwave Band

By-

Anamika Debbarma (DC2011BTE0127)

Jorali Longmailai (DC2011BTE0208)

INTRODUCTION TO MICROWAVE

REMOTE SENSING

Electromagnetic radiation in the microwave wavelength region

is used in remote sensing to provide useful information about

the Earth's atmosphere, land and ocean.

Valuable environmental and resource information can be

derived from sensors that operate in the microwave portion of

the electromagnetic spectrum.

It ranges from wavelength of 1mm to 1m. The longest are

about 25,00000 times longer than the shortest waves.

CHARACTERISTICS OF

MICROWAVE ENERGY

Microwaves are capable of penetrating the atmosphere under

virtually all conditions. Depending on the wavelengths

involved, microwave energy can “see through” haze, light rain

and snow, clouds, and smoke.

Microwave reflections or emissions from earth materials bear

no direct relationship to their counterparts in the visible or

thermal portion of the spectrum. For example, surfaces that

appear “rough” in the visible portion of the spectrum may be

“smooth” as seen by microwaves .

MICROWAVE SENSORS

The sensors operating in the microwave region, can be

broadly classified as active & passive sensors.

The microwave sensors can operate during day and night.

Depending on the wavelength, the atmosphere is more

transparent to microwaves than to optical rays, thus providing

an all-weather monitoring capability

Active Microwave Sensors: Active sensors can carry their own source of electromagnetic radiation to illuminate

the target.

The sensors sends short microwave pulses and measures the backscattered energy and the time taken for the round trip travel.

The time measurement gives the distance of the target from the sensor and hence gives the location of the target and the amount of energy backscattered depends on the target properties and hence gives the measure of some of the target characteristics.

Two types of sensors are there depending on the radar source-receiver configuration and analysis techniques namely bistatic radar and monostatic radar.

Separate antennas are used for transmitting and receiving in bistatic radars. On the other hand, same antenna is used for transmitting and receiving in monostaticradars.

Some of the active microwave sensors are Side Looking Airborne Radar (SLAR) & Synthetic Aperture Radar( SAR).

Passive Microwave Sensors: Passive sensors detect the natural radition emanating from the earth.

Passive microwave sensors are generally known as microwave radiometers. The emission could be either from the surface of earth-land ,ocean,ice etc. ,-or from atmospheric constituents ,ie ,the emission of the atmosphere known as atmospheric sounding.

RADAR OPERATION

Imaging radar systems in typical use for remote sensing are

pulsed- the energy that they transmit from their antenna is

confined to a very short interval of time. This outgoing

packet of energy eventually interacts with the landscape and

some of it may be backscattered to return toward the antenna.

In order to keep track of the outgoing and incoming energy

packets, the system uses a pulse repetition frequency which

provides sufficient time for any backscatter from the far

range portion of the scene to return to the antenna before the

next transmitted pulse occurs.

The pulse duration, the time interval during which the

antenna is energized during the transmit phase ,controls the

range –width of the outgoing energy packet.

Single Pulse Time- Space Diagram

Synthetic Aperture Radar

The SAR makes use of the radar principle to form an image by utilizing the

time delay of the backscattered signals. Synthetic aperture radar (SAR)

imaging, microwave pulses are transmitted by an antenna towards the earth

surface. The microwave energy scattered back to the spacecraft.

A radar pulse is transmitted

from the antenna to the groundThe radar pulse is scattered by the

ground targets back to the antenna

In real aperture radar imaging, the ground resolution is limited by the size of the

microwave beam sent out from the antenna. Finer details on the ground can be

resolved by using a narrower beam. The beam width is inversely proportional to

the size of the antenna, i.e. the longer the antenna, the narrower the beam.

The microwave beam sent

out by the antenna

illuminates an area on the

ground (known as the

antenna's "footprint"). In

radar imaging, the recorded

signal strength depends on

the microwave energy

backscattered from the

ground targets inside this

footprint. Increasing the

length of the antenna will

decrease the width of the

footprint.

It is not feasible for a spacecraft to carry a very long antenna which is required for

high resolution imaging of the earth surface. To overcome this limitation, SAR

capitalises on the motion of the space craft to emulate a large antenna .

Imaging geometry for a typical strip-mapping synthetic aperture radar imaging

system. The antenna's footprint sweeps out a strip parallel to the direction of the

satellite's ground track.

SLAR( Side-Looking Airborne Radar)

SLAR imagery is acquired by an antenna array aimed to the

side of the aircraft, so that it forms an image of a strip of land

parallel to, and at some distance from the ground track of the

aircraft.

One of SLAR’s most unique and useful characteristics is its

ability to function during the inclement weather.

SLAR systems are independent of solar illumination,

missions, using SLAR can be schedule at night or during early

morning or evening hours when solar illumination might be

unsatisfactory for acquiring aerial photographs.

GEOMETRY OF THE RADAR IMAGE

The aircraft is viewed head-on, with the radar beam

represented in vertical cross section as the fan-shaped figure at

the side of the aircraft.

The upper edge of the beam forms an angle with a horizontal

line extended from the aircraft; this angle is designated as the

depression angle of the far edge of the image.

The smallest depression angle forms the far –range side of

the image.

The near range region is the edge nearest to the aircraft.

Intermediate regions between the two edges are sometimes

referred to as mid range portions of the image.

RADAR BANDS AND DESIGNATIONS

TRANSMISSION CHARACTERISTICS OF RADAR SIGNALS

The wavelength and the polarization of the energy pulse used

influences the transmission characteristics of the signal of a radar

system.

The letter codes for the various bands were originally selected

arbitrarily to ensure military security.

The wavelength of a radar signal determines the extent to which it is

attenuated and/or dispersed by the atmosphere.

For example-Rain and clouds can affect radar signal returns when the

radar wavelength is 2cm or less. At the same time, the effect of rain is

minimal with wavelengths of operation greater than 4cm. With K- and

X- band radar, rain may attenuate or scatter radar signals significantly.

X Band

POLARIZATION

Electromagnetic energy has two components-electrical and magnetic –

which are planar fields of oscillation that are orthogonal to each other.

Polarization refers to the spatial orientation of the electrical oscillation

plane— it is oriented vertically, horizontally, or at some other angle.

Because radar is an active remote sensing device, the orientation of the

electromagnetic energy that is transmitted can be controlled. Although all

angles are possible, only vertical or horizontal orientations are used. The

orientation of the backscatter which will be received can also be controlled.

This gives four possibilities for a radar system.

HH horizontal transmit and receive(like –polarized or parallel polarized)

VV vertical transmit and receive(like –polarized or parallel polarized)

HV horizontal transmit, vertical receive(cross-polarized )

VH vertical transmit, horizontal receive(cross-polarized)

REFERENCE

Remote Sensing and Image Interpretation-by Lillesand, Kiefer

and Chipman.

Fundamentals of Remote Sensing –by George Joseph.

The Sage Handbook of Remote Sensing-by M Duane

Introduction to Remote Sensing-by James B Campbell

THANK YOU