Imaging RADAR Principles and Applications Lecture 9

Imaging RADAR Principles and Applications Lecture 9
Summer Session 11 August 2011

Active vs. Passive Remote Sensing
Passive: record EM energy that was reflected or emitted from the surface of the earth What we’ve talked about thus far… Active: create their own energy and are not dependent on the sun’s energy or the thermal properties of the earth. This EM energy is: 1. transmitted from the sensor toward the terrain (and is largely unaffected by the atmosphere) 2. interacts with the terrain producing a backscatter of energy 3. is recorded by the remote sensor’s receiver

Active Microwave, Passive Microwave, and LIDAR
Active Microwave: (RADAR) based on the transmission of long wavelength microwave energy through the atmosphere and then recording the amount of energy backscattered from the terrain. Passive Microwave (microwave radiometers): records microwave energy that is naturally emitted from the earth’s surface LIDAR (Light Detection and Ranging): based on the transmission of relatively short wavelength laser light; records the amount of energy backscattered from the terrain

Microwave Radiometers
Land and water surfaces not only emit EM energy that can be detected in thermal IR wavelengths, but also in microwave wavelengths (1 cm to > 1 m) Microwave radiometers have the ability to measure the brightness temperature (TB) of the earth’s surface

Here are examples of sea ice maps from a satellite microwave radiometer system
These are based on multiple channel classification schemes The colder, the longer the wavelength of maximum emission

Lecture Topics Radar definition and radar basics
Measurements made with a radar Real aperture imaging radar or SLAR Synthetic Aperture Radar (SAR) Unique imaging characteristics Image speckle Why do imaging radars see what they see? Spaceborne SAR systems

RADAR – Radio Detection and Ranging
RADAR systems were invented in the 1930s A high powered, radio transmitter/receiver system was developed that would transmit a signal that was reflected from a distant object, and then detected by the receiver Thus, RADAR’s initial function was to detect and determine the range to a target The initial focus of radar systems was to detect ships and airplanes

Radar systems operate in the microwave region of the EM spectrum
We’re talking very long wavelengths here Figure 9.2 from Jensen

Radars typically have wavelengths between .5 cm and 100 cm
In the early days of radar development, the military wanted to keep the wavelengths that radars were being operated at a secret Therefore, they gave different wavelengths specific letter designations Thus, X-band is a 3 cm wavelength C band is a 6 cm wavelength L band is a 24 cm wavelength Up to 1 meter long – that is VERY long You will not find atmospheric particles anywhere close to that size, so atmospheric attenuation is not an issue for radar.

Common Radar Bands Band Frequency Wavelength (most common) X
8 to 12 GHz 2.5 to 4.0 cm (3.0 cm) C 4 to 8 GHz 4 to 8 cm (6.0 cm) L 1 to 2 GHz 15 to 30 cm (24.0 cm) P 0.3 to 1 GHz 30 to 100 cm (65 cm) In reality, the common radar bands span a range of frequencies, hence wavelengths Here we have a table of the four common wavelengths used in imaging radar systems

Wavelength or Frequency?
Earth resource scientists generally describe RADAR systems by their wavelength Engineers describe radar systems by their frequency HOWEVER, since wavelength and frequency are related it really doesn’t matter how they are described as long as you remember: 3x108m/sec = wavelength*frequency OR Wavelength (cm) = 30/frequency (GHz) [approx.]

Key Characteristics of RADAR Systems
Designers select the wavelength and polarization combinations for the RADAR systems (can have multiple wavelengths / polarizations in same system) Radars operate independently of solar illumination conditions – day or night, it doesn’t matter Radars operate independently of cloud cover and most rainfall – only the heaviest downpours will attenuate microwave wavelengths used in imaging radar systems Thus, for monitoring and mapping purposes, radars are all weather/day and night systems

Primary Advantages of RADAR
Active microwave energy penetrates clouds and can be an all-weather remote sensing system. Coverage can be obtained at user-specified times, even at night. Permits imaging at shallow look angles, resulting in different perspectives that cannot always be obtained using aerial photography. Senses in wavelengths outside the visible and infrared regions of the electromagnetic spectrum, providing information on surface roughness, dielectric properties, and moisture content.

Secondary Advantages of RADAR
May penetrate vegetation, sand, and surface layers of snow. Has its own illumination, and the angle of illumination can be controlled. Enables resolution to be independent of distance to the object, with the size of a resolution cell being as small as 1 x 1 m. Images can be produced from different types of polarized energy (HH, HV, VV, VH). May operate simultaneously in several wavelengths (frequencies) and thus has multi-frequency potential. Can measure ocean wave properties, even from orbital altitudes. Can produce overlapping images suitable for stereoscopic viewing and radargrammetry. Supports interferometric operation using two antennas for 3-D mapping, and analysis of incident-angle signatures of objects.

Key Components of a Radar System
Microwave Transmitter – electronic device used to generate the microwave EM energy transmitted by the radar Microwave Receiver – electronic device used to detect the microwave pulse that is reflected by the area being imaged by the radar Antenna – electronic component through which microwave pulses are transmitted and received

Transmitter / Receiver
Microwave Transmitter / Receiver Target Antenna Microwave EM energy pulse transmitted by the radar Microwave EM energy pulse reflected from a target that will be detected by the radar

Transmitter / Receiver
Microwave Transmitter / Receiver Target 1. Transmitted pulse travels to the target Antenna 2. The target reflects the pulse, and the reflected pulse travels back to the microwave antenna / receiver 3. The radar measures the time (t) between when the pulse was transmitted and when the reflected signal reaches the receiver 4. The distance, R, from the antenna to the target is calculated as ct / 2, where c is the speed of light

Radar Nomenclature • Nadir • azimuth flight direction
• range (near and far) • depression angle () look angles (f) • incidence angle () • altitude above-ground-level, H • polarization Jensen, 2008

• The aircraft travels in a straight line that is called the azimuth flight direction. • Pulses of active microwave electromagnetic energy illuminate strips of the terrain at right angles (orthogonal) to the aircraft’s direction of travel, which is called the range or look direction. • The terrain illuminated nearest the aircraft in the line of sight is called the near-range. The farthest point of terrain illuminated by the pulse of energy is called the far-range. Near-range and far-range are sort of analogous to how far off-nadir your sensor is looking

The range or look direction for any radar image is the direction of the radar illumination that is at right angles to the direction the aircraft or spacecraft is traveling. • Generally, objects that trend (or strike) in a direction that is orthogonal (perpendicular) to the range or look direction are enhanced much more than those objects in the terrain that lie parallel to the look direction. Consequently, linear features that appear dark or are imperceptible in a radar image using one look direction may appear bright in another radar image with a different look direction.

Range Direction: A – range direction B – ground range C – slant range
A – range direction B – ground range C – slant range A – far range B – near range

The depression angle (g) is the angle between a horizontal plane extending out from the aircraft fuselage and the electromagnetic pulse of energy from the antenna to a specific point on the ground. • The depression angle within a strip of illuminated terrain varies from the near-range depression angle to the far-range depression angle. Summaries of radar systems often only report the average depression angle.

The incident angle (q) is the angle between the radar pulse of EMR and a line perpendicular to the Earth’s surface where it makes contact. The incident angle best describes the relationship between the radar beam and surface slope. • The incident angle is assumed to be the complement of the depression angle.

Radar systems control the polarization of both the transmitted and received microwave EM energy
Radars send and receive polarized energy. The transmitted pulse of electromagnetic energy interacts with the terrain and some of it is back-scattered at the speed of light toward the aircraft or spacecraft where it once again must pass through a filter – horizontal or vertical. Polarization combinations include: HH, VV, HV, and VH. Two of the types are vertical send, and vertical receive. Figure 9.6 from Jensen

• send vertically polarized energy and receive only vertically
It is possible to: • send vertically polarized energy and receive only vertically polarized energy (designated VV), • send horizontal and receive horizontally polarized energy (HH), • send horizontal and receive vertically polarized energy (HV), or • send vertical and receive horizontally polarized energy (VH). • HH and VV configurations produce like-polarized radar imagery. • HV and VH configurations produce cross-polarized

Like polarized on the left, cross-polarized on the bottom right.
Right is a northern Arizona basalt lava flow... Much easier to delineate in the HH polarization (top) than in the HV (bottom) Jensen, 2008

Measurements made with a simple radar
Range to the target (distance) Intensity of the returned pulse Spatial resolution Azimuth resolution Range resolution Range to the target is important because if you are flying at a constant altitude (above sea level), you can discriminate the height rather precisely of different objects

Measuring distance with radar
Range to Target = (ct) / 2 where c = speed of light (3 x 108 m sec -1) t = time for the radar pulse to travel to the target and back

Spatial Resolution (1) To determine the spatial resolution at any point in a RADAR image, it is necessary to compute the resolution in two dimensions: the range and azimuth resolutions. Range = across track (length) Azimuth = along track (width) The shorter the pulse length, the finer the range resolution. Pulse length is a function of the speed of light (c) multiplied by the duration of the transmission (t).

Spatial Resolution (2) We must also compute the width of the resolution element in the direction the aircraft or spacecraft is flying — the azimuth direction. Azimuth resolution (Ra) is determined by computing the width of the terrain strip that is illuminated by the radar beam. A shorter wavelength pulse will result in improved azimuth resolution (just like range resolution). BUT!! The shorter the wavelength, the poorer the atmospheric and vegetation penetration capability.

Side-Looking Airborne Radar (SLAR) Geometry
Size of the antenna is inversely proportional to the size of the angular beam width. Smaller the angular beam width, the higher the azimuth resolution. Therefore, the size of the antenna determines azimuth resolution. This is only important (in the context of this lecture) insomuch as it provides the rationale for the development of SAR – synthetic aperture radar. It doesn’t make sense to fly around with enormously long antennae trying to get fine azimuth resolution.

In real aperture radar systems, fine range resolution can be achieved by having a short transmitted pulse On the other hand, along track or azimuth resolution is restricted by beam width (antenna length) Synthetic aperture radars (SARs) were invented to overcome azimuth resolution restrictions encountered in SLARS

Synthetic Aperture Radar (SAR)
Engineers have developed procedures to synthesize a very long antenna electronically SAR technology basically makes a relatively small antenna work like it is much larger This is done by taking advantage of the aircraft’s motion Doing so allows for much finer spatial resolution in the azimuth direction even at large distances from the earth’s surface

Unique Characteristics of Radar Imagery
Slant vs. Ground Range Geometry Relief displacement Radar foreshortening Radar layover Radar shadowing

Slant vs. Ground Range Geometry
Radar imagery has a different geometry than that produced by most conventional remote sensor systems 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. Jensen, 2008

Image analysts must convert slant range to ground range
Field A and field B are the same size in the real world. One field is in the near-range, close to the aircraft and one is in the far-range. Field A gets compressed relative to field b in the slant range display we can convert the slant range to true ground-range so that the features are in their proper planimetric (x,y) position relative to one another in the final radar image. Jensen, 2008

RADAR Relief Displacement, Image Foreshortening, and Shadowing
Geometric distortions exist in almost all radar imagery, including: shadowing foreshortening layover Relief Displacement Jensen, 2008

RADAR Shadows - 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. Jensen, 2008

Radar Shadowing

Radar shadow Radar shadow

Here you can see that the placement of shadows will differ according to the look direction of the sensor. Figure 9.4 from Jensen

Relief Displacement Horizontal displacement of an object that occurs due to an object’s height or elevation The higher the object, the closer it is to the radar antenna and therefore the sooner it is detected by the radar Tops of objects are therefore recorded before the bottoms of objects causing displacement. This displacement is in the direction toward the radar antenna.

Relief Displacement The tree appears to lean toward the radar

Foreshortening Jensen, 2008
The higher the object above the terrain, the more foreshortening that will take place. notice the relative orientations of a, b, and c are maintained, but distorted. Jensen, 2008

RADAR Relief Displacement: Foreshortening
All terrain that has a slope inclined toward the RADAR will appear compressed or foreshortened relative to slopes inclined away from the radar. Radar foreshortening refers to the compression of the range dimension of an elevated object towards the direction the radar is looking Jensen, 2008

RADAR Foreshortening is Influenced by:
• object height: The greater the height of the object above local datum (known elevation), the greater the foreshortening. • depression angle (or incident angle): The greater the depression angle (g) or smaller the incident angle (q), the greater the foreshortening. • location of objects in the across-track range: Features in the near-range portion of the swath are generally foreshortened more than identical features in the far-range. Foreshortening causes features to appear to have steeper slopes than they actually have in the near-range of the radar image and to have shallower slopes than they actually have in the image far-range. Jensen, 2008

RADAR Layover Layover is an extreme form of relief displacement
It occurs in hilly and mountainous regions, where the tops of mountains appear closer to the RADAR than the bottoms This distortion cannot be corrected even when the surface topography is known. Great care must be exercised when interpreting radar images of mountainous areas where the thresholds for image layover exist.

- With foreshortening, the relative positions of a, b, and c were maintained – layover is much more severe and cannot be corrected Jensen, 2008

Example of radar layover in a Seasat satellite image
Example of radar layover in a Seasat satellite image. The top of the mountain covers the glacial valley because of layover Image from

Constructive and Destructive Interference
If two pulses of microwave EM energy transmitted by a radar intersect after they have been reflected from a surface, the two waves can merge into a single wave that is detected by the radar

Constructive Interference
If the two waves are “in phase” (the peak amplitudes match) then there is constructive interference, and the resulting wave adds the energy of the two waves together –creating a single wave with twice the amplitude

Destructive Interference
If the two waves are “out of phase” (the peak amplitudes are offset) then there is destructive interference – the energy from one wave cancels out that from the other, creating a flat wave with zero amplitude

Partial Destructive Interference
When EM waves are slightly out of phase, partial destructive interference occurs

RADAR Image Speckle Areas with similar land or water cover can have a very “salt and pepper” appearance on radar imagery because of constructive and destructive interference between reflected microwave EM waves – This phenomena is called radar image speckle.

Image speckle The speckle can be reduced by processing separate portions of an aperture and recombining these portions so that interference does not occur. This process, called multiple looks, produces a more pleasing appearance, and in some cases may aid in interpretation of the image but at a cost of degraded resolution.

Radar Backscatter -  Radar backscatter is the amount of energy received from the area of interest by a radar relative to the energy received from a metal target with a specified area energy from study area = _________________________ energy from calibrated target Radar backscatter is typically expressed in logarithmic units – decibels (dB)  (dB) = 10 log ()

Factors affecting RADAR backscatter
Surface characteristics Surface roughness Surface dielectric constant Structural complexity RADAR Characteristics Wavelength Polarization Incidence angle

Types of Backscattering/Surface Reflectance
Specular reflection or scattering No return Diffuse reflection or scattering Lots of return In order for a RADAR system to detect a signal, it’s transmitted signal has to be reflected or backscattered back to the detector… this will not happen with a perfectly smooth surface

Microwave scattering as a function of surface roughness is wavelength dependent.
This illustrates the fact that scattering is dependent on wavelength It takes more roughness for a surface to be a diffuse scatterer at longer wavelengths than at shorter wavelengths

Variation in microwave backscatter from a rough surface (grass field) as a function of wavelength – As the wavelength gets longer, the backscattering coefficient drops In these images, I want you to focus on the dark areas in the L-band images These are grass fields These fields appear smooth are dark at the longer wavelength L-band (24 cm), but are rougher (and brighter) at the shorter X and C band wavelengths (e.g., 3 and 6 cm wavelengths)

Radar backscattering is dependent on the relative height or roughness of the surface
Figures from mgddf/chap5/f5-4f.gif

Microwave scattering is dependent on incidence angle
Figure from mgddf/chap5/f5-4f.gif

Dielectric Constant The dielectric constant is a measure of the electrical conductivity of a material To some degree, dielectric constants are dependent on microwave wavelength and polarization The most significant parameter influencing a material’s dielectric constant is moisture content Degree of back-scattering by an object or surface is proportional to the dielectric constant of the material - The moister, the higher the dielectric constant; the higher the dielectric constant, the higher the scattering/reflectance

Dielectric Constants of Common Materials
Water – 80 Soil – 3 to 6 Vegetation – 1 to 3 For most terrestrial materials, the moisture content determines the amount of backscattered radar energy Wet materials – high reflection Dry material – low reflection Note the role of moisture is the opposite of that in visible and infrared remote sensing. The moisture content is so important because basically the amount of water in a soil determines how deep the radar can penetrate. A moister soil will not allow as much radar to penetrate, meaning that there is more energy available to be returned to the sensor. - The moister, the higher the dielectric constant; the higher the dielectric constant, the higher the scattering/reflectance… so, the moister, the more reflective  IN MICROWAVE REGIONS

Vegetation and Structural Complexity
Vegetation is a complex scattering medium. Not all microwave energy is scattered back to the radar from the vegetation itself, Some is transmitted through the vegetation and is scattered by the ground. However, the level of scattering & transmission are dependent on wavelength polarization vegetation structure In surfaces covered by vegetation, you need to understand that there can be multiple sources of scattering and attenuation Let us assume that you have a short vegetation canopy, without any woody material, e.g., a grass field

Vegetation and Structural Complexity
The radar return from this situation can come from three sources: Direct scattering from the vegetation Direct scattering from the ground Multiple scattering between the ground and the canopy (Also, the canopy might absorb some of the microwave energy, so you have to account for attenuation by the canopy) This is sort of like the “at sensor radiance” specifically for microwave and radar.

Surface factors that influence RADAR scattering from vegetated surfaces
Changes in soil moisture Changes in canopy moisture Differences in canopy structure/biomass Presence or absence of water on top of soil (e.g., surface inundation or flooding in wetland ecosystems) When you think about it, there are actually a limited number of scene or surface factors that are going to cause differences in backscatter, and hence image intensity H2O is a big deal – and to illustrate this point further, let’s look at a few examples

Monitoring Soil Moisture Variation
Consider 3 cases for radar backscattering from wetlands with non-woody vegetation Low Soil Moisture High Soil Moisture Flooding or inundation of the soil surface with water There is a natural wet and dry season in this region, so many of the wetlands are only flooded for part of the year during the wet season There are three radar scattering scenarios to consider

Radar backscatter from a wetland with low soil moisture
Results in moderate radar backscatter Low direct scattering from canopy Low multi-path scattering Moderate scattering from soil

Radar backscatter from a wetland with high soil moisture
Results in high radar backscatter Low direct scattering from canopy Low multi-path scattering High scattering from soil

Radar backscatter from a wetland that is flooded
Results in low radar backscatter Low direct scattering from canopy Low multi-path scattering No scattering from soil Low  moderate High  high Super high  NO SCATTERING  not a linear process

Areas that have exposed, but moist soils during the dry season
The changes in radar backscatter are due to variations in flooding and soil moisture Areas that have exposed, but moist soils during the dry season Regions that are flooded during the wet season

Primary Spaceborne SAR Systems
Seasat SIR-C ERS-1/ERS-2 JERS-1 Radarsat Envisat

Res. (resolution) – range x azimuth  - wavelength (cm)
Lifetime Res. (m)  Pol. I Swath Seasat Jul-Sep 1978 25 x 25 23.5 HH 23 100 km ERS-1/2 July 1991 to present 26 x 30 5.6 VV JERS-1 Feb 1992 to Oct 1998 18 by 18 39 75 km SIR-C/X-SAR April/Oct 1994 10 to 30 3.0 5.8 quad 15 to 55 15 to 90 km Radarsat Nov 1995 to present 10 to 100 10 to 59 50 to 500 km Envisat/ ASAR Spring 2002 25 VV, Res. (resolution) – range x azimuth  - wavelength (cm) Pol. (Polarization) – HH – horizontal transmit/horizontal receive; VV – transmit/vertical receive, quad pol – four polarizations, HH, VV, VH (vertical transmit/horizontal receive), HV (horizontal transmit/vertical receive)

SIR-C/X-SAR Images of a Portion of Rondonia, Brazil, Obtained on April 10, 1994
Jensen, 2008

Shuttle Imaging Radar (SIR-C) Image of Los Angeles
Cardinal effect?  Jensen, 2008

Aerial Photography and RADAR Imagery of the Pentagon in Washington, DC
Jensen, 2008

Shuttle Radar Topography mission  took images around the world… used RADAR data to make 3D representation of earth Jensen, 2008

Intermap X-band Star 3i Orthorectified Image of
Bachelor Mountain, CA & Derived Digital Elevation Model Jensen, 2008

Imaging RADAR Principles and Applications Lecture 9

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