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SAR: Basics

RADAR or RAdio Dectection and Ranging  is believed to have started at the beginning of the 20th century whose credits go to German inventor Christian Huelsmeyer who had developed the system to develop the system to detect distant metallic objects, OR British enginner Robert Watson Watt who had object detecting system far up to 30km. The technology emerged during World War II and the developement of which has then had developed further to be kept on the airplanes by 1940s. Its applications grew even in the areas of Earth Observation.

The advantage of the RADAR system always remained in its utilizations with disregards of weather or any time of the day or night. The surface was interacted differently according to radar signals with more information about the surface. Side Looking Airborne Radar (SLAR) systems developed around in 1950s, where the radar sensor mounted on the platform moved in the straight line at altitude H and the radar system points to the nadir at the look angle a. The area illuminated at the ground is called antenna footprint; illuminated by the short microwaves pulses of pulse length t. The size S of the footprint is dtermined by the system wavelength and the side length of the antenna L and slant height R;

 S ≈  wavelength/L*R

The two dimensional image are distinguished from the arrival time. Objects at different ranges can be distinguished only if their range separation is half their transmitted pulse length. The range resolution is given by; Pr = (c* t)/2 where c is the speed of the light. The resolution refers to the system's ability to differentiate the two objects at different slant distances.

The ground based resolution is given by Pg = Pr/ sin(a); which refers to the improvement with the increase in a and not constant across the  swath; howver the ground resolution decreases with increase in a. The azimuth resolution in SLAR is dependent upon the footprint in SLAR system which is limited by the side length L. The dependence to R makes it impractical in space borne satellites which requires increase in the length of the antenna. Howver, maximum antenna length is unreasonable; for which Synthetic Aperture Principle was developed in 1952. SLAR system is equally popular in ground based and airborne applications.

SAR works with creating linger synthesized antenna called synthetic aperture from the shorter antenna while moving along. This allows high resolution imaging even in the spaceborne platoforms. The object on the ground is imaged by consecutinve radar pulses consecutively, which when later post processed results the image to have acquired from the single longer antenna. Its resolution is higher than SLAR images.

Geometric distortions in SAR data include foreshortening, layover and shadow which are due to oblique observation. In radar image, the tall objects such as mountains wouls appear to be leaning; this is called foreshortening. In the layover, the tops of mountain are imaged ahead of the base. And the shadow is increased due to larger a. 

The other radiometric properties within the SARimage include speckle (salt and pepper noise) which occurs due to scattering events or interference. Speckle is multiplicative noise i.e. not constant within the image. Many filtering methods have been developed to address it. 

Radar Cross Section (RCS) is the rato between the inicdent and received signal intensity of microwave signals, which is recorded by SAR. The RCS is influenced by surface roughness and dielectric properties of the imaged object. The senor wavelength determines penetration, for instance; C and L band being able to penetrate deeper in to the vegetation and sometimes in the density of the canopy. Similarly, the roughness is also determined according to wavelength of the sensor. 

In SAR, the orientation of the plane of the oscillation of the propagating signal can be controlled. This is called as polarization and the sensors can transmit the signals at different polarization. HH, for instance refers to as Horizontal Polarization - transmit and Horizontal Polarization - receive. Current senors provide the abilities in providing different polarization capabililies bringing HH, HV, VV, VH polarized imagery. The visibility is enhanced by HH, VV and HV is combined as the single RGB image. 

Seasat, first earth observing satellite

The first SAR was NASA's Seasat satellite, launched on June 28 1978, had HH polzrized L-band SAR which had objectives to study oceans conditions. Seasat was considered was successful and after ERS-1 in 1991, more and more SAR sensors have been launched; however, making data from different sensors not being incompatible.  

The SAR system uses frequencies from 1 to  90 GHz whose names have been developed from World War II, during the period where microwave remote sensing was being developed heavily. Certain frequencies withhin are further divided naming the bands and their applications differ according to their penetrable capabilites. C-band sensors have been popular form the past three decades. Further developments are likely to be sesen in L and P based sensors.

SAR data types include:

SAR RAW Data: They are the raw observed information made by sensor.  They are refereed as L0 data. 

SLC (Single Look Complex Image): It includes the information with amplitude and phase informaton stored. They are refered as L1 data. 

Detected Amplitude Images: They are fully focused image. They might be in various geocoding stages. They are categoried as L1 detedted images. 

Polarimetric Products: NASA JPL run airborne remote sensing provides two data types: Compressed Strokes Matrix and Pauli Decomposition Matrix

Level 2 products: data products that are have been projected to the ground and transformed into physical varaibles. 

SAR Data: Most systems operate on free and open data policy. 

The process of correcting the angle a is called Radiometric Terrain Correction (RTC) and Geometric Terrain Correction (GTC) includes the removing the geometric image distortions. 

The SAR applications include on Interferometric SAR and change dtection.

 This animation, comprised of images acquired by Envisat’s Advanced Synthetic Aperture Radar (ASAR) between 30 May and 9 June 2008, highlights the rapidly dwindling strip of ice that is protecting thousands of kilometres of the ice shelf from further break-up. This is the first ever-documented episode to occur in winter.
Source: Commons

The above contents has been the attempt to prepare the excerpt from the book " The SAR Handbook" Ch-2 while trying to learn. The chapter has the very minute details and information for every avid individual interested in SAR. The contents written above doesnot fully capture the whole chapter for sure.


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