Synthetic Aperture Radar

Agenda…

Introduction Motivation and applications Basic principle Algorithm Existing spectral estimation approaches Typical operation Image appearance History Relationship to phased arrays Data collection Data distribution

Introduction Synthetic Aperture Radar is a form of radar that is used to create 2 or 3dimensional

images of objects, such as landscapes. Uses the motion of the radar antenna and mounted on Moving Platform target in the time taken for the radar pulses to return to the antenna creates the large

"synthetic“ antenna aperture. To create a SAR image, successive pulses of radio waves are transmitted to

"illuminate" a target scene the echo of each pulse is received and recorded. the antenna location relative to the target changes with time. Current (2010) airborne systems provide resolutions of about 10 cm, ultra wideband

systems provide resolution of about 10 cm Ultra wideband systems provide resolutions of a few millimetres and experimental

terahertz SAR has provided submillimetre resolution in the laboratory.

Motivation and applications

The properties of SAR can be described as having high resolution capability SAR also have a great day and night imaging capability SAR images have wide applications in remote sensing and mapping of the surfaces of

both the Earth and other planets. Some of the other important applications of SAR are topography, oceanography,

glaciology, geology

Basic Principle

Electromagnetic waves are sequentially transmitted and reflected echoes are collected, digitized and stored by the radar antenna for later processing.

As transmission and reception occur at different time, they map to different positions.

combination of the received signals builds a virtual aperture that is much longer than the physical antenna length.

The range direction is parallel to flight track and perpendicular to azimuth direction.

The 3D processing is done in two steps: generation of 2D (azimuth range) high resolution images digital elevation model (DEM)

In addition to this, multiple baselines can be used to extend 3D imaging to the time dimension

Algorithm A three dimensional array (a volume) of scene elements is defined which will represent the volume of

space within which targets exist Initially, the SAR gives each voxel a density of zero. Then, for each captured waveform, the entire volume is iterated. For a given waveform and voxel, the distance from the position represented by the voxel to the

antenna(e) used to capture that waveform is calculated. The sample value at that position in the waveform is then added to the voxel's density value. This represents a possible echo from a target at that position. After all waveforms have been iterated over all voxels, the basic SAR processing is complete. What remains, in the simplest approach, is to decide what voxel density value represents a solid

object. Thus to detect a point on a target, there must be at least two different antenna echoes from that point Consequently, there is a need for large numbers of antenna positions to properly characterize a target. The voxels that passed the threshold criteria are visualized in 2D or 3D.

Existing spectral estimation approaches Synthetic Aperture Radar determines the 3D reflectivity from measured SAR data. SAR image stack are sampled version of the Fourier transform of reflectivity in elevation direction. Thus the spectral estimation techniques are used to improve the resolution, reduce speckle1. Nonparametric methods - FFT : - FFT is one such method which is used in majority of the spectral estimation algorithms

and there are many fast algorithms for computing the multidimensional discrete Fourier transform - A branch of finite multidimensional linear algebra, is used to identify similarities and

differences among various FFT algorithm variants and also to create new variants. - The FFT implementation is essentially a realization of the mapping of the

mathematical framework, through generation of the variants and executing matrix operations.

Advantages of FFT method : it is easier to identify mapping between computing structures and

mathematical expressions and thus better than conventional methods.

- reducing the computational effort and improve their implementation time.

Disadvantages of FFT method : - FFT cannot separate sinusoids closer in frequency. Also if the

periodicity of the data does not match FFT, edge effects are seen

Other methods are Capon Method and Apes Method

Parametric subspace decomposition methods

Eigenvalue Method- This subspace decomposition method separates the eigenvectors of the

autocovariance matrix into those corresponding to signals and to clutter. Advantages :- Shows features of image more accurately Disadvantages : - High computational complexity.

Modes1. Stripmap mode airborne SAR2. Spotlight mode SAR3. Scan mode SAR

Polarimetry

- Radar waves have a polarization. - but anisotropic materials such as grass often reflect

different polarizations with different intensities.- Interpretation of the resulting colors requires significant testing of known materials.- Examples include subterranean tunneling or paths of

vehicles driving through the area being imaged.- SAR polarimetry is a technique used for deriving qualitative

and Quantitative physical information for land, snow and ice, ocean and urban Applications

Fig . SAR image of Death Valleycolored using polarimetry

Interferometry- Rather than discarding the phase data, information can be extracted from it- If the two samples are obtained simultaneously then any phase difference will contain information about the angle from which the radar echo returned. - Differential interferometry Differential interferometry (DInSAR) requires taking at least two images with addition of a DEM. The DEM can be either produced by GPS measurements or could be generated by interferometry as long as the time between acquisition of the image pairs is short.

Typical application Of SAR- In a typical SAR application, a single radar antenna is attached to an aircraft- Resolution in the range dimension of the image is accomplished by creating pulses which define very short time intervals, either by emitting short pulses.- The total signal is that from a beamwidth sized patch of the ground.- The process can be thought of as combining the series of spatially distributed observations as if all had been made simultaneously with an antenna as long as the beamwidth and focused on that particular point.- Image resolution of SAR in its range coordinate (expressed in image pixels per distance unit) is mainly proportional to the radio bandwidth of whatever type of pulse is used.-

Image appearance

- The two dimensions of a radar image are range and cross range- Because slant ranges to level terrain vary in vertical angle, each elevation of such terrain appears as a curved surface- Fine resolution radar images of small areas can appear most nearly like familiar optical ones- An important fact that applies to SARs but not to real aperture radars is that the direction of overlay of any scene point is not directly toward the radar, but toward that point of the SAR's current path direction that is nearest to the target point.- Objects in motion within a SAR scene alter the Doppler frequencies of the returns.- These considerations, along with the speckle structure due to coherence, take some getting used to in order to correctly interpret SAR images.

Relationship to phased arrays

- A technique closely related to SAR uses an array (referred to as a "phased array") of real antenna elements spatially distributed over either one or two dimensions perpendicular to the radar range Dimension.- In comparison, a SAR's (commonly) single physical antenna element gathers signals at different positions at different times.- Core of these both : distance that radar waves travel to and back from each scene element consist of some integer number of wavelengths plus some fraction of a "final“ wavelength.- Every wave scattered from any point in the scene has a circular curvature about that point as a center.

Data collection- Synthetic aperture radar was first used by NASA on JPL's Seasat oceanographic satellite in 1978- In the 1980s, as a prototype for instruments to be flown on the NASA Space Shuttles- The Venera 15 and Venera 16 followed later by the Magellan space probe mapped the surface of Venus over several years using synthetic aperture radar.- The Mineseeker Project is designing a system for determining whether regions contain landmines based on a blimp carrying ultrawideband synthetic aperture radar.- SAR has been used in radio astronomy for many years to simulate a large radio telescope by combining observations taken from multiple locations using a mobile antenna.

Data distribution- The Alaska Satellite Facility provides production, archiving and distribution to the scientific community of SAR data products- CSTARS downlinks and processes SAR data (as well as other data) from a variety of satellites

A model of a German SARLupereconnaissance satelliteinside a Cosmos3Mrocket.