Radar Remote sensingRadar Remote sensing

Dr. P. K. ManiBidhan Chandra Krishi Viswavidyalaya

E-mail: pabitramani@gmail.com Website: www.bckv.edu.in

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What is RADAR?

• Radio Detection and Ranging

• Radar is a ranging instrument

• (range) distances inferred from time elapsed between transmission of a signal and reception of the returned signal

Types of radar:• Nonimaging radar

– Traffic police use hand held 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, such as NEXRDA

– 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

The most common form of imaging active microwave sensors is RADAR

Non-imaging microwave sensors include Altimeters and scatterometers.

imaging radars (side-looking) used to acquire images (~10m - 1km)

altimeters (nadir-looking) to derive surface height variationsscatterometers to derive reflectivity as a function of

incident angle, illumination direction, polarisation, etc

Non-Imaging Radar

To provide a polar-coordinate map-like display of targets, NRL originated the radar PLAN-POSITION INDICATOR (PPI)-the well-known radar scope with the round face and the sweeping hand-between 1939 and 1940. The PPI is now universally used by military and commercial interests around the world for the display of radar information for such functions as air and surface detection, navigation, air traffic control, air intercept, and object identification

Two imaging radar systemsIn World War II, ground based radar was used to detect incoming

planes and ships (non-imaging radar).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

• 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.

Advantages• All time / all weather capability • Information on surface roughness at the “human” scale • Centimeters rather than microns • Penetration of soil : function of the dielectric constant • Rule of thumb is that for dry soils, penetration depth (cm) = 10• For hyper-arid environments, radar can penetrate 3-5 m

Disadvantages• Very costly

•  Imagery is complex and typically hard to interpret

•  Little to no information on composition of the surface materials

Imaging Radar - Advantages• Active system (works day or night).

– There is also passive microwave imaging (radiometer) mode. This senses surface radio-emission, which can be converted to radiant temperatures.

• Not affected by cloud cover or haze if > 2 cm. It operates independent of weather conditions. Water clouds have a significant effect on radar with wavelength < 2 cm.

• Unaffected by rain > 4 cm.• Can penetrate well-sorted dry sand in hyper-arid regions

to a depth of about 2 m.

Active and Passive Radar Imaging Systems

Active radar systems transmit short bursts or 'pulses' of electromagnetic energy in the direction of interest and record the origin and strength of the backscatter received from objects within the system's field of view. Passive radar systems sense low level microwave radiation given off by all objects in the natural envt.

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Component of RADAR

• A Radar system has three primary functions:

- It transmits microwave (radio) signals towards a scene

- It receives the portion of the transmitted energy backscattered from the scene

- It observes the strength (detection) and the time delay (ranging) of the return signals.

• Radar is an active remote sensing system & can operate day/night

How Radar WorksMicrowave energy pulses (A) are emitted at regular intervals and focused by the antenna into a radar beam (B) directed downwards and to the side. The radar beam illuminates the surface obliquely at a right angle to the motion of the platform. Objects on the ground reflect the microwave energy depending on factors such as roughness and attitude. The antenna receives this reflected (or backscattered) energy (C).

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Principle of ranging and imaging in Side-looking Airborne Radar (SLAR)

Tree is less reflective of radar waves than the house, a weaker response is recorded in the graph

Tree is less reflective of radar waves than the house, a weaker response is recorded in the graph

By electronically measuring the return time of signal echoes, the range or distance, between the transmitter and reflecting objects, may be determined.

Since the energy propagates in air at approximately the velocity of light c, the slant range, SR, to any given object is given by, SR= ct/2the factor 2 enters into the equation because the time is measured for the pulse to travel both the distance to and from the target

How Radar WorksBy measuring the time delay between the transmission of a pulse and the reception of the backscattered "echo" from different targets, their distance from the radar and thus their location can be determined. As the sensor platform moves forward, recording and processing of the backscattered signals builds up a two-dimensional image of the surface.

Radar Geometry• In airborne and spaceborne radar imaging systems, the platform travels forward in the flight direction (A)

with the nadir (B) directly beneath the platform. The microwave beam is transmitted obliquely at right angles to the direction of flight illuminating a swath (C) which is offset from nadir. Range (D) refers to the across-track dimension perpendicular

to the flight direction, while

azimuth (E) refers to the

along-track dimension

parallel to the flight

direction.

Near Range is the portion of the image swath closest to the nadir track Far Range is the portion of the swath farthest from the nadir track.Depression or Grazing Angle is the angle between the horizontal and a radar ray path.Slant Range Distance is the radial line of sight distance between the radar and each target on the surface.Ground Range Distance is the true horizontal distance along the ground corresponding to each point measured in slant range.

Incidence Angle is the angle

between the radar beam and

ground surface

Look Angle is the angle

at which the radar "looks“

at the surface, or the

angle between vertical

and a ray path

Backscatter

• The portion of the outgoing radar signal that the target redirects directly back towards the radar antenna.

• When a radar system transmits a pulse of energy to the ground (A), it scatters off the ground in all directions (C). A portion of the scattered energy is directed back toward the radar receiver (B), and this portion is referred to as "backscatter".

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Range resolution (across track): RAR

τ

i.e. A-B is < PL/2 cannot resolve A & BDependence of range resolution on pulse length

Pulse of length PL (duration of the pulse transmission) has been transmitted towards buildings A and B

Note that the slant range distance (the direct sensor to target distance) between the buildings is less than PL/2

For a SLAR system to image separately two ground features that are close to each other in the range direction, it is necessary for all parts of the two objects reflected signals to be received separately by the antenna. Any time overlap between the signals from two objects will cause their images to be blurred together.

Because of this propagation of wavefront, pulse has had time to travel to B and have its echo returns to A while the end of the pulse at A continues to be reflected. Consequently, the two signals are overlapped and will be imaged as one large object extending from building A to building B. If the slant range distance betweenA and B were anything greater than Pl/2, the two signals would be received separately, resulting in two separate image responses.

Although the slant-range resolution of an SLR system does not change with distance from the aircraft, the corresponding ground-range resolution does. As shown in Figure 8.6, the ground resolution in the range direction varies inversely with the cosine of the depression angle. This means that the ground-range resolution becomes smaller with increases in the slant-rangedistance.Accounting for the depression angle effect, the ground resolution in the range direction Rr is found from

where τ is the pulse duration.

Range (or across-track) Resolution

cos2

ctRr

• t.c called pulse length. It seems the short pulse length will lead fine range resolution.

• However, the shorter the pulse length, the less the total amount of energy that illuminates the target.

t.c/2 t.c/2

Azimuth (or along-track) Resolution

L

SRa

L

SRa

L = antenna length S = slant range = height H/sinλ = wavelength

L sinγ

H

As shown in Figure 8.7, the resolution of an SLR system in the azimuth direction, Ra, is determined by the angular beam width β of the antenna and the ground range GR. As the antenna beam "fans out" with increasing distance from the aircraft, the azimuth resolution deteriorates. Objects at points A and B in Figure 8.6 would be resolved (imaged separately) at GR1

but not at GR2. That is, at distance GR1 , A and B result in separate return signals. At GR2, distance, A and B would be in the beam simultaneously and would not be resolved.Azimuth resolution Ra is given by

A given SLAR system has a 1.8-mrad antenna beamwidth. Determine the azimuth resolution of the system at ranges of 6 and 12 km.

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Azimuth resolution: SAR

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The Radar Equation

Relates characteristics of the radar, the target, and the received signal

The geometry of scattering from an isolated radar target (scatterer) is shown.When a power Pt is transmitted by an antenna with gain Gt , the power per unit solid angle in the direction of the scatterer is Pt Gt, where the value of Gt in that direction is used.

Pr = Pt Gt Gr λ2σ

(4π)3 R4

Reeves, (1979)

G = Gt = Gr

Pr = Pt G2 λ

2σ (4π)3 R4

Pt= transmitted power

Pr= received power

Gt= gain of transmitted antenna

Gr= gain of receiver antenna

R= distance between target and sensor

λ= wavelength of radiation

σ = scattering cross-section

The Radar Equation

Amount of backscatter per unit areahttp://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/parameters_affecting.htm

sin8h

Intermediate wrong

•Peake and Oliver (1971) – surface height variation h

Penetration of the radar signal

• Can penetrate vegetative cover and soil surface

• Depth of penetration is assessed by the skin depth – the depth to which the strength of a signal is reduced.

• Skin depth increases with increasing wavelength and in the absence of moisture

Penetration of the radar signal

• Optimum penetration is in arid and long wavelength radiation

• Penetration also related to surface roughness and incidence angle. The steeper the incidence angle the greater the penetration.

• There is no clear defined way to assess penetration

Polarization

• Denotes the orientation of the field of EM energy emitted and received by the antenna.

• Radar systems can be configures to transmit and receive either horizontally or vertically.

• Unless otherwise specified, an imaging radar transmits and receives horizontal polarized EM waves.

Polarization

• Some systems produce combinations– HH-image or the like-polarized mode– HV-image or the cross-polarized mode

• Comparing the two images, the interpreter can identify features that tend to depolarize the signal.

• Example: bright HV image vs dark HH image

Polarization

• Causes of depolarization is related to physical and electrical properties (rough surface with respect to wavelength)

• Volume scattering from an inhomogeneous medium (occurs when the radar penetrates the ground)

Radar Shadow

• Shadows in radar images can enhance the geomorphology and texture of the terrain. Shadows can also obscure the most important features in a radar image,

such as the information behind tall buildings or land use in deep valleys. If certain conditions are met, any feature protruding above the local datum can cause the incident pulse of microwave energy to reflect all of its energy on the foreslope of the object and produce a black shadow for the backslope

• Unlike airphotos, where light may be scattered into the shadow area and then recorded on film, there is no information within the radar shadow area. It is black.

Radar Image Geometry - Shadow

Radar Image Geometry - Shadow

Shadow is more of a problem at far range

Radar Image Geometry - Layover

Layover occurs when the radar beam reaches the top of a tall feature before it reaches the base. The top of the feature is displaced towards the radar sensor and is displaced from its true ground position - it 'lays over' the base. The visual effect on the image is similar to that of foreshortening.

Foreshortening• Even if there is no layover, radar returns from facing steep

slopes will make the terrain look steeper than it is. This is known as ‘foreshortening’. Features which show layover in the near range will show foreshortening in the far range.

Foreshortening occurs because radar measure distance in the slant-range direction such that the slope A-B appears as compressed in the image (A'B') and slope C-D is severely compressed (C'D')

Radar Signal PolarizationPolarization of the radar signal is the orientation of the the electromagnetic field and is a factor in the way in which the radar signal interacts with ground objects and the resulting energy reflected back. Most radar imaging sensors are designed to transmit microwave radiation either horizontally polarized (H) or vertically polarized (V), and receive either the horizontally or vertically polarized backscattered energy.

Penetration abilityinto subsurface

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Nicobar Islands

December 2004 tsunami flooding in red

SIR-C Image of Vesuvius and Naples, Italy

• Mt. Vesuvius, one of the best known volcanoes in the world primarily for the eruption that buried the Roman city of Pompeii in AD 79, is shown in the center of this radar image. The central cone of Vesuvius is the dark purple feature in the center of the volcano. This cone is surrounded on the northern and eastern sides by the old crater rim, called Mt. Somma. Recent lava flows are the pale yellow areas on the southern and western sides of the cone. It shows an area 100 kilometers by 55 kilometers (62 miles by 34 miles.)

Shuttle Imagery Radar-C, April and Sept. 1994, 10 days each. X-, C-, L- bands multipolarization

(HH, VV, HV, VH), 10-30 m resolution

SIR-C image of Nile Paleochannel, Sudan

• The top image is a photograph taken with color infrared film from Space Shuttle Columbia in November 1995. The radar image at the bottom is a SIR-C/X-SAR image. The thick, white band in the top right of the radar image is an ancient channel of the Nile that is now buried under layers of sand. This channel cannot be seen in the photograph and its existence was not known before this radar image was processed. The area to the left in both images shows how the Nile is forced to flow through a chaotic set of fractures that causes the river to break up into smaller channels, suggesting that the Nile has only recently established this course. Each image is about 50 kilometers by 19 kilometers.

• Red = Chv; Green = Lhv; Blue = Lhh

Nov. 2002 Oil spill in Spain• A damaged oil tanker off the northwest coast of Spain split in half on November 19, 2002, creating a series of large oil slicks. The image shows the oil slick with RADARSAT data. Black areas indicate the location of the slick on November 18. The land is shown using Landsat falsecolor

Archeology of Angor, Cambodia

• The city houses an ancient complex of more than 60 temples dating to the 9th to 15th centuries. Today the Angkor complex is hidden beneath a dense rainforest canopy, making it difficult for researchers on the ground. The principal complex, Angkor Wat, is the bright square just left of the center of the image. It is surrounded by a reservoir that appears in this image as a thick black line. The larger bright square above Angkor Wat is another temple complex called Angkor Thom. Archeologists studying this image believe the blue-purple area slightly north of Angkor Thom may be previously undiscovered structures. In the lower right is a bright rectangle surrounded by a dark reservoir, which houses the temple complex Chau Srei Vibol.

• Image is 55 x 85km.

• Red=L hh, Green =L hv, and Blue =C hv.