Questions askedModule - III • Explain air borne and space borne sensors • Describe advantages and disadvantages of air borne sensors • Describe imaging with microwave radar • Explain Synthetic Aperture Radar(SAR) • Define swath, explain the concept of orbital calendar • Give advantages and disadvantages of air borne sensors
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 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. Passive Active
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)
Radar Principles • Radar measures distance by measuring time delay between the transmit and received pulse. • 1 us = 150 m • 1 ns = 15 cm Radar University of Kansas
Principle of operation of a Radar • Reflection of electromagnetic wavesThe 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. • Electromagnetic energy travels through air at a constant speed, at approximately the speed of light, • 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. • 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
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 University of Kansas
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
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
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 • Frequency/wavelength • Polarization • Viewing geometry • Spatial resolution • Speckle (disfigure)
Terrain parameters of RADAR • Surface geometry • Surface roughness • 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 26.5 K 1.18 - 1.67 26.5 to 18.0 Ku 1.67 - 2.4 18.0 to 12.5 X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0 C (7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0 S (8.0, 9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0 L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0 P (68.0 cm) 30.0 - 100 1.0 - 0.3
Polarization • Unpolarizedenergy 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 horizontallypolarized. • The pulse of energy received by the antenna may be vertically or horizontallypolarized • VV, HH – like-polarized imagery • VH, HV- cross-polarized imagery
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 Azimuth flight direction Look/Range direction Flightline groundtrack Near range Far range
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
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
Synthetic Aperture Radar – Systems and Signal Processing Scattering Mechanisms
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 geometry 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.
Synthetic Aperture Radar. C.C.Tscherning, Nov. 2007
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.
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