Imaging RADAR Principles and Applications Lecture 9

Imaging RADAR Principles and ApplicationsLecture 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

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 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

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) Common Radar Bands

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

Primary Advantages of RADAR • Active microwave energy penetrates clouds and can be an all-weatherremote 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

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

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

http://www.ccrs.nrcan.gc.ca/glossary/index_e.php?id=2830 • 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.

http://www.ccrs.nrcan.gc.ca/glossary/index_e.php?id=2830 The range or look directionfor 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: http://www.ccrs.nrcan.gc.ca/glossary/index_e.php?id=2830 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 Polarization: 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. Figure 9.6 from Jensen

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 radar imagery.

Jensen, 2008

Measurements made with a simple radar • Range to the target (distance) • Intensity of the returned pulse • Spatial resolution • Azimuth resolution • Range resolution

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 resolutionat 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 lengthis 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.

On the other hand, along track or azimuth resolution is restricted by beam width (antenna length) In real aperture radar systems, fine range resolution can be achieved by having a short transmitted pulse 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 radargrammetricmeasurements • • 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 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 • - Shadowsin 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

RadarShadowing

Radar shadow Radar shadow

Figure 9.4 from Jensen

ReliefDisplacement • 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

Foreshortening Jensen, 2008

RADAR Relief Displacement: Foreshortening All terrain that has a slope inclined toward the RADARwill appear compressed or foreshortened relative to slopes inclined away from the radar. 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

Imaging RADAR Principles and Applications Lecture 9