What Is SAR..?

Synthetic Aperture Radar (SAR) is a remote sensing technology that uses radar to create high-resolution images and gather information about objects or terrain on the Earth's surface, even in adverse weather conditions or during nighttime. SAR systems are commonly used in various applications, including Earth observation, environmental monitoring, agriculture, forestry, disaster management, and military reconnaissance.

Here are some key characteristics and features of Synthetic Aperture Radar:

Radar Technology: SAR systems operate by emitting radio waves (microwaves) towards the Earth's surface and then recording the reflections (echoes) that bounce back from objects on the ground. These echoes are used to create images.
Aperture Synthesis: The "synthetic aperture" in SAR refers to the creation of a large effective antenna aperture by moving or electronically steering the radar antenna during data collection. This process allows SAR systems to achieve high-resolution imaging without requiring an enormous physical antenna.
High-Resolution Imaging: SAR can provide high-resolution images with pixel sizes in the range of a few centimeters to several meters, depending on the specific system and parameters used.
All-Weather Capability: SAR is not affected by weather conditions, such as clouds, rain, or fog, which can hinder optical and infrared imaging. This all-weather capability makes SAR especially valuable for monitoring and surveillance in adverse conditions.
Day and Night Operation: SAR can operate 24/7 since it does not rely on sunlight or natural illumination. This feature is particularly important for continuous monitoring and surveillance.
Various Operating Frequencies: SAR systems can operate at different microwave frequencies, such as X-band, C-band, L-band, and others, each with its own advantages and trade-offs in terms of resolution and penetration through various materials.
Polarimetry: Some SAR systems are polarimetric, meaning they transmit and receive radar waves with different polarization states. This can provide additional information about the surface properties of objects.
Interferometry: SAR interferometry (InSAR) is a technique that uses phase differences between multiple SAR images of the same area taken at different times to measure ground deformations, such as subsidence or uplift.
Applications: SAR is used in a wide range of applications, including land cover mapping, agriculture monitoring, forest management, disaster monitoring (e.g., detecting changes after earthquakes or floods), ice and glacier monitoring, and defense and intelligence gathering.
In summary, Synthetic Aperture Radar is a versatile remote sensing technology that provides valuable information about the Earth's surface and is especially useful in situations where optical imagery may be limited due to adverse weather or lighting conditions.

How does it work..?

Under normal conditions, a two-dimensional image is generated by synthetic aperture radar. The two dimensions in a SAR image are referred to as azimuth and range.

In order to achieve a fine azimuth resolution, a large antenna is required to bring the transmitted and received energy into focus to a sharp beam. The azimuth resolution is defined by the sharpness of the beam. It is the capability of synthetic aperture radar to generate a relatively fine azimuth resolution which separates synthetic aperture radar from other radar systems.

Compared to optical systems, synthetic aperture radars are in much lower frequency. An antenna length of several hundred meters is often needed. This makes it impractical for the antenna to be carried by an airborne platform. Nevertheless, data could be collected and processed by airborne radar as if it came from a long antenna. The distance that the aircraft flies in synthesizing the antenna is called the synthetic aperture. It creates an aperture that is larger than the physical size of the antenna, and thus the resolution of the image is higher. A fine resolution can be achieved through a relatively long synthetic aperture.

Data

Sea Sat

1978

The June 1978 launch of Seasat was the first civilian application of synthetic aperture radar, and it provided a powerful new tool to scientists studying the earth. Prior to Seasat, civilian image acquisition of the earth was via Landsat cameras, using visible light and providing resolutions in the tens of meters.

The history of synthetic-aperture radar begins in 1951, with the invention of the technology by mathematician Carl A. Wiley, and its development in the following decade. Initially developed for military use, the technology has since been applied in the field of planetary science . Carl A. Wiley,[1] a mathematician at Goodyear Aircraft Company in Litchfield Park, Arizona, invented synthetic-aperture radar in June 1951 while working on a correlation guidance system for the Atlas ICBM program.

In early 1952, Wiley, together with Fred Heisley and Bill Welty, constructed a concept validation system known as DOUSER ("Doppler Unbeamed Search Radar").

During the 1950s and 1960s, Goodyear Aircraft (later Goodyear Aerospace) introduced numerous advancements in SAR technology, many with the help from Don Beckerleg.

Independently of Wiley's work, experimental trials in early 1952 by Sherwin and others at the University of Illinois' Control Systems Laboratory showed results that they pointed out "could provide the basis for r adar systems with greatly improved angular resolution" and might even lead to systems capable of focusing at all ranges simultaneously.

References

Kirscht, Martin, and Carsten Rinke. "3D Reconstruction of Buildings and Vegetation from Synthetic Aperture Radar (SAR) Images." MVA. 1998.
Introduction to Airborne RADAR", G. W. Stimson, Chapter 1 (13 pp).
Synthetic Aperture Radar Imaging Using Spectral Estimation Techniques. Shivakumar Ramakrishnan, Vincent Demarcus, Jerome Le Ny, Neal Patwari, Joel Gussy.

Tomographic SAR. Gianfranco Fornaro. National Research Council (CNR). Institute for Electromagnetic Sensing of the Environment (IREA) Via Diocleziano, 328, I-80124 Napoli, ITALY
Oliver, C. and Quegan, S. Understanding Synthetic Aperture Radar Images. Artech House, Boston, 1998.
University of Michigan."Science Engineering & Sustainability: Bridge monitoring with satellite data SAR"

Applications

A picture is worth a thousand words; in the case of satellite imagery, considerably more so. However, sometimes these pictures are worth much less or nothing at all, since traditional imagery faces capture obstacles such as cloud cover and nightfall. Essentially, traditional Earth observation satellites need an unobstructed and illuminated view of the Earth in order to capture meaningful images. These limitations have created a need for new types of spaceborne technologies like Synthetic Aperture Radar (SAR) satellites.

Since SAR instruments don't depend on the Sun's energy to collect surface data, SAR satellites can operate just as well during the day or night. Additionally, SAR signals can penetrate through clouds to “see” the covered surface underneath, allowing satellites to have a full view of the Earth’s surface regardless of atmospheric or lighting conditions. SAR can also “see” through other types of cover such as smoke, vegetation, snow, or sand, depending on the satellite’s designated operating band (which indicates the sensor’s associated frequency and wavelength). SAR bands are helpful in categorizing the penetration strength and thus the potential applications of a satellite, such as Germany’s TanDEM-X (low-penetration X-band), Canada’s RCM (moderate-penetration C-band), and Japan’s ALOS-2 (high-penetration L-band).

Below are just a few examples of how SAR is being used for such purposes:

Agriculture: Differences in surface roughness are indicative of field ploughing, soil tillage, and crop harvesting.

Floods: Differences in surface reflection can help distinguish heavy flooding, light flooding, urban areas, and permanent bodies of water.

Land subsidence: Differences in measurements over time can reveal displacements of land, such as sinking ground caused by the extraction of underground natural resources.

Snow cover: Differences in surface reflection can help forecast snowmelt by distinguishing wet snow, dry snow, and snow-free areas.

Wildfires: Penetration through thick smoke can provide more accurate and timely information about the extent of a forest fire and can help quantify vegetation loss.

Wetlands: Penetration through wetland areas can reveal flooded vegetation where land is covered by shallow water.

What Is SAR..?

Here are some key characteristics and features of Synthetic Aperture Radar:

How does it work..?

Data

Sea Sat

References

Applications

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