Introduction to Microwave Remote sensing

Questions askedModule - III

1. Explain air borne and space borne sensors2. Describe advantages and disadvantages of air

borne sensors3. Describe imaging with microwave radar4. Explain Synthetic Aperture Radar(SAR)5. Define swath, explain the concept of orbital

calendar6. Give advantages and disadvantages of air

borne sensors

Electromagnetic Spectrum

Microwave Remote Sensing

• Microwave remote sensing employing microwave radiation using wavelengths that range from about 1mm to 1m, in frequency interval from 40,000 Hz to 300MHz enables observation in all weather condition without any restriction by cloud or rain

Recap: passive and active RS

• Passive: uses natural energy, either reflected sunlight (solar energy) or emitted thermal or microwave radiation.

• Active: sensor creates its own energy – Transmitted toward Earth or other targets– Interacts with atmosphere and/or surface– Reflects back toward sensor (backscatter)– Advantages: all weathers and all times

RADAR

Passive Active

http://www.ccrs.nrcan.gc.ca/resource/tutor/stereo/chap5/chapter5_3_e.php

RADAR is the most commonly used space-based active sensing system. It is an acronym for RAdio Detection And Ranging.

Widely used active RS systems• RADAR: RAdio Detection And Ranging

– Long-wavelength microwaves (1 – 100 cm)

• LIDAR: LIght Detection And Ranging– Short-wavelength laser light (UV, visible, near IR)

• SONAR: SOund Navigation And Ranging: (very long wave, low Hz)– Sound can not travel through vacuum– Earth and water absorb acoustic energy far less than EMR energy– Seismic survey use small explosions, record the reflected sound– Medical imaging using ultrasound– Sound waves through a water column.– Sound waves are extremely slow (300 m/s in air, 1,530 m/s in sea-water)– Bathymetric sonar (measure water depths and changes in bottom topography )– Imaging sonar or sidescan imaging sonar (imaging the bottom topography and

bottom roughness)

Sending and receiving a pulse of microwave radiation

transmitted pulse

backscattered pulse

antenna

TransmitterDuplexer

• sends and receives

Pulse Generator

CRT Display or Digital Recorder

Receiver

b.

a. antenna

transmitted pulse

backscattered pulse

antenna

TransmitterDuplexer

• sends and receives

Pulse Generator

CRT Display or Digital Recorder

Receiver

b.

a. antenna

University of Kansas11/18/02

Radar Principles

• Radar measures distance by measuring time delay between the transmit and received pulse.– 1 us = 150 m– 1 ns = 15 cm

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Principle of operation of a Radar

Principle of operation of a Radar

Principle of operation of a Radar1. Reflection of electromagnetic waves

The electromagnetic waves are reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction.

2. Electromagnetic energy travels through air at a constant speed, at approximately the speed of light,

3. This constant speed allows the determination of the distance between the reflecting objects (airplanes, ships or cars) and the radar site by measuring the running time of the transmitted pulses.

4. This energy normally travels through space in a straight line, and will vary only slightly because of atmospheric and weather conditions. By using of special radar antennas this energy can be focused into a desired direction. Thus the direction (in azimuth and elevation of the reflecting objects can be measured

Types of radar• Non imaging radar

– Traffic police use handheld Doppler radar system determine the speed by measuring frequency shift between transmitted and return microwave signal

– Plan position indicator (PPI) radars use a rotating antenna to detect targets over a circular area

– Satellite-based radar altimeters (low spatial resolution but high vertical resolution)

• Imaging radar– Usually high spatial resolution, – Consists of a transmitter, a receiver, one or more

antennas, GPS, computers

Two imaging radar systems• In World War II, ground based radar was

used to detect incoming planes and ships (non-imaging radar).

• Non-imaging radar microwave sensors include altimeters and scatterometers

• They take measurements in in one linear dimension

• Radar Altimeters transmits short microwave pulses and measure the time delay to determine the distance from the sensor

• Altimeters look down at nadir below the platform and measure height

• Altimeters are used in aircraft to measure altitude

University of Kansas11/18/02

Active Sensors – Radar Altimeter

• Radar altimeter is a short pulse radar used for accurate height measurements.– Ocean topography.– Glacial ice topography– Sea ice characteristics

• Classification and ice edge• Vegetation

•http://topex-www.jpl.nasa.gov/technology/images/P38232.jpg

Two imaging radar systems• Scatterometers are non-imagery sensors

and are used to measure amount of energy backscattered from the targets

• Backscattered energy depends upon surface properties (roughness) and direction at which microwave energy strikes the target

• Application as tool to characterize different materials and surface types and measurements over ocean surfaces

Two imaging radar systems

• Imaging RADAR was not developed until the 1950s (after World War II). Since then, side-looking airborne radar (SLAR) has been used to get detailed images of enemy sites along the edge of the flight field.

• SLAR is usually a real aperture radar. The longer the antenna (but there is limitation), the better the spatial resolution

• Advantage – all weather see through capability and day and night imaging

Two imaging radar systems

• Real aperture radar (RAR)– Aperture means antenna– A fixed length (for example: 1 - 15m)

• Synthetic aperture radar (SAR)– 1m (11m) antenna can be synthesized

electronically into a 600m (15 km) synthetic length.

– Most (air-, space-borne) radar systems now use SAR.

1. RAR: Principle of SLAR

waveform

Early airborne RADAR was Side Looking Airborne Radar SLAR)

Geometry of a side-looking airborne RADAR system. Resolution depended on the size of the antenna Adapted from Lillesand and Kiefer (1987) http://forsys.cfr.washington.edu/JFSP06/radar_overview.htm

Q: Describe imaging with microwave radar

• Radar is a ranging and distance measuring device. It has following components

• Pulse Generators: Which discharges timed pulses of microwave /radio energy

• The pulse of electromagnetic radiation sent out by transmitter through the antenna is of specific wavelength and duration

• Transmitter: Which generates successive short bursts (or pulses of microwave) at regular intervals

Block Diagram of Radar

Q: Describe imaging with microwave radar Refer page 166, Bhatta

• Radar is a ranging and distance measuring device. It has following components

• Pulse Generators: Which discharges timed pulses of microwave /radio energy

• The pulse of electromagnetic radiation sent out by transmitter through the antenna is of specific wavelength and duration

Pulse Generators

Q: Describe imaging with microwave radarRefer page 166, Bhatta

• Duplexer: Which carefully coordinates when microwave energy is transmitted or received

• Directional antenna: Which shapes and focuses each pulse into a stream

• Receiver antenna: Picks up returned pulses and sends to a converter for conversion in to video signals

• Recording device: Which stores the information digitally for later processing

• CRT monitor: Which produces a real time display on it.

System Parameters of RADAR

I. Frequency/wavelengthII. PolarizationIII. Viewing geometryIV. Spatial resolutionV. Speckle (disfigure)

Terrain parameters of RADAR

I. Surface geometryII. Surface roughnessIII. Dielectric properties

Frequency/wavelength• The wavelengths used in Radar is much longer • Microwave energy is measured in centimeters • The usual name are K, Ka, Ku, X, C,S, L and P• They are described by names for the purpose of secrecy• In world war –II, radar was used for tracking aircrafts

and ships• Nowadays used for marine navigation and air traffic

control• Wave lengths of 3cm will be reflected from tree tops

where as wavelength of 24 cm penetrate up to ground• Prime use in displaying topography

Microwaves

Band Designations(common wavelengths Wavelength () Frequency ()shown in parentheses) in cm in GHz_______________________________________________Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5K 1.18 - 1.67 26.5 to 18.0Ku 1.67 - 2.4 18.0 to 12.5X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0C (7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0S (8.0, 9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0P (68.0 cm) 30.0 - 100 1.0 - 0.3

Band Designations(common wavelengths Wavelength () Frequency ()shown in parentheses) in cm in GHz_______________________________________________Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5K 1.18 - 1.67 26.5 to 18.0Ku 1.67 - 2.4 18.0 to 12.5X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0C (7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0S (8.0, 9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0P (68.0 cm) 30.0 - 100 1.0 - 0.3

Polarization in Radar Imaging

Polarization

• Unpolarized energy vibrates in all possible directions perpendicular to the direction of travel.

• The pulse of electromagnetic energy is filtered and sent out by the antenna may be vertically or horizontally polarized.

• The pulse of energy received by the antenna may be vertically or horizontally polarized

• VV, HH – like-polarized imagery

• VH, HV- cross-polarized imagery

Horizontal Polarization in Radar Imaging

Vertical Polarization in Radar Imaging

Polarization in Radar Imaging

• Polarization means orientation of electric field• Polarization has an effect on nature and magnitude

of backscatter • 4 combinations of both transmit and receive

polarizations are;• HH- for horizontal transmit and horizontal receive • VV- for vertical transmit and vertical receive • HV- for horizontal transmit and vertical receive• VH- for vertical transmit and horizontal receive

Viewing Geometry

• In radar system the platform travels forward in flight direction

• Nadir is directly beneath the platform • The microwave beam is transmitted obliquely

at right angle to the direction of flight with a swath

• Swath width : then linear ground distance in the across-track direction which is covered by a sensor on a single overpass

Radar Geometry

Radar Nomenclature

• nadir: point on the ground vertically beneath the center of camera lens• azimuth (or flight) direction• look (or range) direction: direction in which pulses of microwave energy are transmitted• range (near, middle, and far)• depression angle ()• incidence angle ()• altitude above-ground-level, H• polarization

Radar Nomenclature

• nadir: point on the ground vertically beneath the center of camera lens• azimuth (or flight) direction• look (or range) direction: direction in which pulses of microwave energy are transmitted• range (near, middle, and far)• depression angle ()• incidence angle ()• altitude above-ground-level, H• polarization

Azimuth flight direction

Flightline groundtrack

Look/Range direction

Far range

Near range

Spatial resolution of Radar System

Speckle

• Speckle are a form of noise which degrades the quality of image and makes interpretation more difficult

• Using SAR, we can get high spatial resolution in the azimuth dimension (direction). But the coherently recording returned echoes (SAR) also causes speckle noise.

• By modeling the noise, we can remove them

Speckle

• Grainy salt-and-pepper pattern in radar imagery

– Caused by coherent nature of the radar wave, which causes random constructive and destructive interference, and hence random bright and dark areas in a radar image

• Reduced by multiple looks

– processing separate portions of an aperture and recombining these portions so that interference does not occur

a.

b.

c.

1 - Look radar image

4 - Look radar image

16 - Look radar image

a.

b.

c.

1 - Look radar image

4 - Look radar image

16 - Look radar image

Surface Roughness

• The surface roughness of a feature controls surface scattering

• The way in which the energy interacts with the particular surface and this is dominant factor on tones seen on a radar image

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Scattering Mechanisms

Synthetic Aperture Radar – Systems and Signal Processing

• Add description from Bhatta on Scattering and surface roughness

Dielectric Properties

• Dielectric substance is highly resistant to the flow of electric energy

• Presence or absence of moisture affects the electrical properties of an object. These change in properties influence the change in absorption, transmission and reflection of microwave energy

Slant-range vs. Ground-range geometryRadar imagery has a different geometry than that produced by most conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements. • Uncorrected radar imagery is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene. • It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image.

Radar imagery has a different geometry than that produced by most conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements. • Uncorrected radar imagery is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene. • It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image.

C.C.Tscherning, Nov. 2007 46

Synthetic Aperture Radar.

Synthetic Aperture Radar (SAR) removes the need for a long antenna

2. SAR

A major advance in radar remote sensing has been the improvement in azimuth resolution through the development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object. Doppler principles are then used to monitor the returns from all these additional microwave pulses to synthesize the azimuth resolution to become one very narrow beam.

A major advance in radar remote sensing has been the improvement in azimuth resolution through the development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object. Doppler principles are then used to monitor the returns from all these additional microwave pulses to synthesize the azimuth resolution to become one very narrow beam.

Azimuth resolution is constant = D/2, it is independent of the slant range distance, , and the platform altitude. So the same SAR system in a aircraft and in a spacecraft should have the same resolution. There is no other remote sensing system with this capability.

• Synthetic Aperture Radars were developed as a means of overcoming the limitations of real aperture radars.

• These systems achieve good azimuth resolution that is independent of the slant range to the target, yet use small antennae and relatively long wavelengths to do it

• SAR PrincipleA synthetic aperture is produced by using the forward motion of the radar. As it passes a given scatterer, many pulses are reflected in sequence. By recording and then combining these individuals signals, a "synthetic aperture" is created in the computer providing a much improved azimuth resolution

Real Aperture Radar•A narrow beam of energy is directed perpendicularly to the flight path of the carrier platform (aircraft or spacecraft). •A pulse of energy is transmitted from the radar antenna, and the relative intensity of the reflections is used to produce an image of a narrow strip of terrain.•Reflections from larger ranges arrive back at the radar after proportionately larger time, which becomes the range direction in the image. •When the next pulse is transmitted, the radar will have moved forward a small distance and a slightly different strip of terrain will be imaged.

Real Aperture Radar

RAR

• These sequential strips of terrain will then be recorded side by side to build up the azimuth direction. The image consists of the two dimensional data array.

• In this figure, the strip of terrain to be imaged is from point A to point B. Point A being nearest to the nadir point is said to lie at near range and point B, being furthest, is said to lie at far range.

RAR

• The distance between A and B defines the swath width.

• The distance between any point within the swath and the radar is called its slant range.

• Ground range for any point within the swath is its distance from the nadir point (point on the ground directly underneath the radar).