Microwave Remote Sensing PDF

Microwave Remote Sensing Microwave remote sensing uses electromagnetic radiation with a wavelength between 1 cm and 1 m (commonly referred to as microwaves) Due to the greater wavelength compared to visible and infrared radiation, microwaves exhibit the important property of penetrating clo...

Microwave Remote Sensing Microwave remote sensing uses electromagnetic radiation with a wavelength between 1 cm and 1 m (commonly referred to as microwaves) Due to the greater wavelength compared to visible and infrared radiation, microwaves exhibit the important property of penetrating clouds, fog, and possible ash or powder coverages (for example, in case of an erupting volcano or a collapsed building) Microwave remote sensing systems are classified into two groups: passive and active. Passive systems collect the radiation that is naturally emitted by the observed surface- microwave radiometer Active microwave sensors provide their own source of microwave radiation to illuminate the target.  A passive microwave sensor detects the naturally emitted microwave energy within its field of view. This emitted energy is related to the temperature and moisture properties of the emitting object or surface. The microwave energy recorded by a passive sensor can be emitted by the atmosphere (1), reflected from the surface (2), emitted from the surface (3), or transmitted from the subsurface (4).  Because the wavelengths are so long, the energy available is quite small compared to optical wavelengths. Thus, the fields of view must be large to detect enough energy to record a signal. Most passive microwave sensors are therefore characterized by low spatial resolution. Applications of passive microwave remote sensing include meteorology, hydrology, and oceanography. By looking "at", or "through" the atmosphere, depending on the wavelength, meteorologists can use passive microwaves to measure atmospheric profiles and to determine water and ozone content in the atmosphere. Hydrologists use passive microwaves to measure soil moisture since microwave emission is influenced by moisture content. Oceanographic applications include mapping sea ice, currents, and surface winds as well as detection of pollutants, such as oil slicks. Active Microwave Remote Sensing Active microwave sensors are generally divided into two distinct categories: imaging and non-imaging. The most common form of imaging active microwave sensors is RADAR. RADAR is an acronym for RAdio Detection And Ranging, which essentially characterizes the function and operation of a radar sensor. The sensor transmits a microwave (radio) signal towards the target and detects the backscattered portion of the signal. The strength of the backscattered signal is measured to discriminate between different targets and the time delay between the transmitted and reflected signals determines the distance (or range) to the target. Non-imaging microwave sensors (altimeters and scatterometers) Radar altimeters transmit short microwave pulses and measure the round trip time delay to targets to determine their distance from the sensor. is used on aircraft for altitude determination and on aircraft and satellites for topographic mapping and sea surface height estimation. Scatterometers are also generally non-imaging sensors and are used to make precise quantitative measurements of the amount of energy backscattered from targets. The amount of energy backscattered is dependent on the surface properties (roughness) and the angle at which the microwave energy strikes the target. Scatterometry measurements over ocean surfaces can be used to estimate wind speeds based on the sea surface roughness. Development in Active Microwave Remote Sensing The first demonstration of the transmission of radio microwaves and reflection from various objects was achieved by Heinrich Hertz in 1886. Shortly after the turn of the century, the first rudimentary radar was developed for ship detection. In the 1920s and 1930s, experimental ground-based pulsed radars were developed for detecting objects at a distance. The first imaging radars used during World War II had rotating sweep displays which were used for detection and positioning of aircrafts and ships. After World War II, side-looking airborne radar (SLAR) was developed for military terrain reconnaissance and surveillance where a strip of the ground parallel to and offset to the side of the aircraft was imaged during flight. In the 1950s, advances in SLAR and the development of higher resolution synthetic aperture radar (SAR) were developed for military purposes. In the 1960s these radars were declassified and began to be used for civilian mapping applications. There are two primary types of SLAR: Real Aperture Radar (brute-force radar)- uses an antenna of fixed length (1-2 m) Synthetic Aperture Radar- also uses a 1-2 m antenna but they are able to synthesize a much larger antenna (≈600 m in length) SAR give very fine resolution from great distance. 11m SAR antenna on orbital platform can be synthesized electronically to have a synthetic length of 15 km. Active Microwave System Component System is mounted on the aircraft Consist of pulse generating device, a transmitter, a duplexer (coordinate transmission and receiving), an antenna, a receiver, a recording device such as high density digital tape/hard disk and a CRT monitor (to check whether the data is being collected or not) Wavelength, frequency and pulse length The pulse of electromagnetic radiation sent out by the transmitter through the antenna is of a specific wavelength and duration As wavelengths are much longer, the microwave energy is measured in centimeters The unusual names associated with the radar wavelengths are an artifact of the secret work on radar remote sensing in World War II when it was compulsory to use an alphabetic descriptor instead of actual wavelength or frequency The shortest radar wavelengths are designated K-bands, but is partially absorbed by water vapour In orbital and sub-orbital platform X- band is the shortest wavelength used Some RADAR systems use more than one frequency and are referred as multiple-frequency radars Band Frequency Wavelength Typical Application Ka 27–40 GHz 1.1–0.8 cm Rarely used for SAR (airport surveillance) K 18–27 GHz 1.7–1.1 cm rarely used (H2O absorption) Ku 12–18 GHz 2.4–1.7 cm rarely used for SAR (satellite altimetry) High resolution SAR (urban monitoring,; ice and snow, little penetration into vegetation X 8–12 GHz 3.8–2.4 cm cover; fast coherence decay in vegetated areas) SAR Workhorse (global mapping; change detection; monitoring of areas with low to C 4–8 GHz 7.5–3.8 cm moderate penetration; higher coherence); ice, ocean maritime navigation Little but increasing use for SAR-based Earth observation; agriculture monitoring (NISAR S 2–4 GHz 15–7.5 cm will carry an S-band channel; expends C-band applications to higher vegetation density) Medium resolution SAR (geophysical L 1–2 GHz 30–15 cm monitoring; biomass and vegetation mapping; high penetration, InSAR) Biomass. Vegetation mapping and P 0.3–1 GHz 100–30 cm assessment. Experimental SAR. Antenna is mounted beneath and parallel to the aircraft fuselage. Azimuth Direction- The aircraft travel in a straight line and is referred as azimuth flight direction Pulses of energy illuminates strips of the terrain at right angle to the aircraft’s direction of travel- range or look direction The pulse of energy illuminate a certain part on the terrain The terrain illuminated nearest to the aircraft in the line of sight is called near range The farthest point of terrain is called the far-range Depression Angle- angle between a horizontal plane extending out from the aircraft fuselage and the line- of-sight to a specific point on the ground Near and Far-range depression angle. Look Angle- angle between the vertical from the antenna to the ground and radar line- of-sight Incident angle- angle between radar pulse of energy and line perpendicular to the Earth’s surface where it makes contact In case of flat terrain, the incident angle is assumed to be the compliment of the depression angle Polarization- Unpolarized light travels in all direction Radar send and receive polarized energy Examples of horizontal (black) The pulse of energy is filtered and vertical (red) polarizations so that its electrical wave of a plane electromagnetic wave vibrations are only in a single plane that is perpendicular to the direction of travel The pulse of electromagnetic energy sent out by the antenna may be vertically or horizontally polarized  Many radars are designed to transmit microwave radiation that is either horizontally polarized (H) or vertically polarized (V). A transmitted wave of either polarization can generate a backscattered wave with a variety of polarizations. It is the analysis of these transmit and receive polarization combinations that constitutes the science of radar polarimetry. Any polarization on either transmission or reception can be synthesized by using H and V components with a well-defined relationship between them. For this reason, systems that transmit and receive both of these linear polarizations are commonly used. There can be four combinations of polarizations: HH - for horizontal transmit and horizontal receive VV - for vertical transmit and vertical receive HV - for horizontal transmit and vertical receive, and VH - for vertical transmit and horizontal receive. The first two polarization combinations are referred to as "like-polarized" because the transmit and receive polarizations are the same. The last two combinations are referred to as "cross-polarized" because the transmit and receive polarizations are orthogonal to one another. Radar system can have different levels of polarization complexity: single polarized - HH or VV or HV or VH dual polarized - HH and HV, VV and VH, or HH and VV four polarizations - HH, VV, HV, and VH A quadrature polarized (i.e. polarimetric) radar uses these four polarizations, and measures the phase difference between the channels as well as the magnitudes. Some dual polarized radars also measure the phase difference between channels, as this phase plays an important role in polarimetric information extraction. Slant Range and Ground Range Geometry Radar imagery has different geometry than other remote sensing system Range Resolution Range and azimuth resolution Radar measures the distance to the objects by sending out and receiving pulses of energy The range resolution in the across-track direction is proportional to the length of microwave pulse The shorter the pulse length the finer is the range resolution (pulse length is function of speed of light multiplied by the duration of transmission). The pulse length must travel to the target and back to the sensor, thus it is divided by 2 to measure the slant range resolution Using mathematical conversion, it can be further computed into ground range resolution  Azimuth Resolution- accuracy for length and width of the resolution element- is determined by computing the width of the terrain strip that is illuminated by the radar beam Real aperture produce a lobe shaped beam which is narrower in the near range and spreads out in the far range The angular beam width is directly proportional to the wavelength of the transmitted pulse of the energy The longer the wavelength the wider is the beam width and shorter the wavelength the narrower is the beam width In real aperture radar a shorter wavelength pulse give improved azimuth resolution The beam width is also inversely proportional to antenna length (L) 𝑆 ×𝜆 𝑅𝑎 = 𝐿 S= Slant range distance  In case of SAR azimuth resolution is given by SARa= L/2 The coherent nature of SAR signal produces speckle in the image, to remove these the image is usually processed using several looks Thus equation is adjusted by the equation SARa= N (L/2) Eg- 12 m antenna with 4 looks will have 24 m azimuth resolution  Speckle- is a grainy salt and pepper pattern in radar imagery due to coherent nature of radar wave which causes random constructive and destructive interference- causing random bright and dark areas in a radar image Relief Displacement The horizontal displacement of an object in the image caused by the object’s elevation Elevation induced distortions in radar imagery are referred as foreshortening and layover  All terrain that has slope inclined toward the radar will appear compressed or foreshortened relative to the sloped inclined away from the radar Foreshortening is affected by: object height Depression angle Location of the object in the across track range Image layover is extreme case of image foreshortening Shadow occurs when high object reflect all of its energy on the foreslope of the object and produces a black shadow for the backslope