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ABSTRACT
Synthetic Aperture Radar or SAR is an imaging radar system
that sends a microwave pulse to the surface of the earth and register
the reflections from the earth's surface . On -board processing and
compression of data obtained from the SAR is vital for image formation
.The development of enabling technologies for space-borne SAR
instruments have been a major focus of research and development during
the last few years . At present the SAR systems provides only images
and in future it will have to deliver dedicated information to each
special user.

INTRODUCTION

When a disaster occurs it is very important to grasp the
situation as soon as possible. But it is very difficult to get the
information from the ground because there are a lot of things which
prevent us from getting such important data such as clouds and volcanic
eruptions. While using an optical sensor, large amount of data is shut
out by such barriers. In such cases, Synthetic Aperture Radar or SAR is
a very useful means to collect data even if the observation area is
covered with obstacles or an observation is made at night at night
time because SAR uses microwaves and these are radiated by the sensor
itself. The SAR sensor can be installed in some satellite and the
surface of the earth can be observed.
To support the scientific applications utilizing space-borne
imaging radar systems, a set of radar technologies have been developed
which can dramatically lower the weight, volume, power and data rates
of the radar systems. These smaller and lighter SAR systems can be
readily accommodated in small spacecraft and launch vehicles enabling
significantly reduced total mission cost.
Specific areas of radar technology development include the
antenna, RF electronics, digital electronics and data processing. A
radar technology development plan is recommended to develop and
demonstrate these technologies and integrate them into the radar
missions in a timely manner. It is envisioned that these technology
advances can revolutionize the approach to SAR missions leading to
higher performance systems at significantly reduced mission costs.
The SAR systems are placed on satellites for the imaging
process. Microwave satellites register images in the microwave region
of the electromagnetic spectrum. Two mode of microwave sensors exit-
the active and the passive modes. SAR is an active sensor which carry
on “board an instrument that sends a microwave pulse to the surface of
the earth and register the reflections from the surface of the earth.
One way of collecting images from the space under darkness or
closed cover is to install the SAR on a satellite . As the satellite
moves along its orbit, the SAR looks out sideways from the direction of
travel, acquiring and storing the radar echoes which return from a
strip of earth's surface that was under observation.
The raw data collected by SAR are severely unfocussed and
considerable processing is required to generate a focused image. The
processing has traditionally been done on ground and a downlink with a
high data rate is required. This is a time consuming process as well.
The high data rate of the downlink can be reduced by using a SAR
instrument with on-board processing.


X-BAND SAR INSTRUMENT DEMONSTRATOR
The X-band SAR instrument demonstrator forms the standardized
part or basis for a future Synthetic Aperture Radar (SAR) instrument
with active front- end. SAR is an active sensor. Active sensors carry
on-board an instrument that sends a microwave pulse to the surface of
the earth and register the reflections from the surface of the earth.
Different sensor use different bands in the microwave regions of the
electromagnetic spectrum for collecting data. In the X-band SAR
instrument, the X-band is used for collecting data.

Fig.1. X “ band SAR instrument demonstrator
The demonstrator embraces the active front-end panel, the
central electronics and the Electrical Ground Support Equipment
(EGSE).The active front-end panel consist of the radiators, the T/R
modules, panel control electronics, panel power conditioner,
distribution network and the calibration network. The panel is flight
representative in form, fit and function to lower the development risk
for future SAR instrument applications. The system shall be capable to
change the radar beam within every pulse interval The planar antenna
consist of 30 dual polarized waveguide radiator subarrays which are fed
by the transmit/receive modules. The function of the T/R modules is to
generate frequency modulated microwave pulses . The radiators transmit
these waves to the ground. The T/R modules perform coherent detection
of received signals (analog in form) and transmit the two channel video
signals ( I and Q) to the signal processor.

There are two panel control electronics (PCE) and only one is
active during operation. The PCE generates commands for the T/R modules
on the basis of pre-programmed configuration tables. The PCE acquires
the data received by the T/R modules and sends them to the digital
control electronics (DCE). The DCE forms the part of the central
electronics. The DCE has a timing generator for generating timing
signals for the active array. It also provides for interfacing to the
spacecraft. There is a power converter in the central electronics which
converts a spacecraft voltage of 28V dc to 115V ac and supplies the
panel. On the panel, the ac voltage will be conditioned
for the panel control electronics and the T/R modules. The T/R modules
are connected to a RF ground support equipment. The other parts of the
EGSE are the digital ground support equipment and the master
controller. The master controller will be a computer system which will
control and co-ordinate the whole processes of the system.

Fig.2. shows a radiator with the 30 radiator subarrays.
A single subarray has two waveguide one for horizontal
polarisation and another for vertical polarisation. A waveguide is a
hollow metallic tube of a rectangular or a circular shape used to guide
an electromagnetic wave. By using a waveguide the no power is lost. At
the rear side of the waveguide is the T/R modules. Connecting the T/R
modules and the waveguides is a thermal plate. The heat generated by
the T/R modules is radiated by the radiator, thus maintaining a good
thermal stability over the operational temperature range of -20oC to
60oC.

Fig. 3 show a single subarray
The fig.4 shows the rear view of a radiator .The PPC, PCE and
the RF fed networks are seen .There is a cross -stiffener for
providing mechanical strength to the whole panel. The cooling loop
shown in the picture is only required for continuous operation on
ground.

Fig.4. Rear view of radiator

ON-BOARD PROCESSING FOR SPACE SAR
Rationale for on-board processing
Image from space under darkness or cloud cover can be obtained
by flying a synthetic aperture radar on a satellite. As the satellite
moves along its orbit ,the SAR looks out sideways from the directions
of travel ,acquiring and storing the radar echoes which return from a
strip of the earth's surface which is under observation.

In contrast to images taken by classical visible and infra-red
camera-like sensors, raw data collected by a SAR are severely
unfocussed and considerable processing is required to generate a
focused image. This processing has traditionally been done on ground
and a downlink with a high data rate is required . A high resolution
SAR instrument combined with one on-board processing unit reduces the
data rate of the downlink. The data rate of a SAR depends on the
product of the no. of echoes per second acquired by SAR .The former may
be reduced by careful system design and latter is determined by system
consideration like the chosen orbit and physical length of antenna and
can only be reduced by data processing. Effective processing is
achieved by using full data set to produce several medium resolution
images, which are then averaged to reduced numbers. This technique is
called multi-looking.
In conclusion , a low data rate combined with reduced noise is
only possible if image is generated onboard.

PROCESSING AND STORAGE SUBSYSTEM
The image formation from the radar echo of the SAR instrument
involves a highly sophisticated processing effort. The main function of
the processing and storage subsystem is to process and store the
information obtained from the SAR instrument. The processing stages
involves-
1. Buffering of the SAR raw data stream in real-time
2. Off-line image processing and compression of the buffered SAR data
3. Mass memory data management and organisation
4. Reformatting and output of compressed data at downlink rate
Raw data buffering : The digital input data stream fed to the
processing and storage subsystem will have a peak data rate of 2.88Gbps
for a SAR instrument with 150MHz bandwidth. This is the maximum data
rate which must be handled by the input of the subsystem. The input
data comes in bursts, which corresponds to the receive echoes of the
radar system. The maximum receive duty cycle of the instrument is
required to be upto 70%. The continuous data stream after the range
extension buffer ,which is realised in the data sorter is upto
2.016Gbps in the worse case. This is the range of data which is
required to be written into the solid state mass memory continuously.
The solid state mass memory is organised in memory modules. The
necessary number of memory modules is determined by the maximum input
data rate of each memory module and by the required total mass memory
capacity.
Off-line SAR data compression: The average orbit duty cycle for the SAR
instrument is specified to be less than 5%. This means that the
instrument is switched off 90% of the time and another 5% is reserved
for downlink of the downlink of the data . The off-line SAR data
compression or processing shall be completed during this time, when the
instrument is switched off. There are three different types of data
compression-
-Data volume reduction of the over sampled data
The SAR instrument is required to operate with a bandwidth
adjusted to the range resolution. This compression operates lossless
and reduces the data volume according to the actual useful data rate.
-Raw data compression with a BAQ type algorithm
The total range of data is target dependent and very high.
Compared to this the instantaneous range is considerably less. This
effect is used for lossy data reduction. If this technique is used on
data in a transform domain, the properties of the instrument and the
SAR processor can be used to achieve even better compression ratios.
This technique can be combined with the data volume reduction of the
over sampled data.
-SAR image processing and compression
The highest compression of SAR data can be achieved when they
are processed to SAR images. Multilooking and very efficient
conventional image compression processes like wavelet compression can
be applied.
Mass memory data management and organisation: The allocation of the SAR
data resulting from different data takes and the header data for each
data set has to be managed.
Reformatting and output of compressed data at downlink rate: The SAR
raw data and the SAR header data have to be read out from the mass
memory, encrypted, packetised and transferred to the data transmission
subsystem.
PROCESSING AND STORAGE ARCHITECTURE
The architecture of the processing and storage subsystem is
shown in fig 5. The digitised raw data enters the subsystem from the
left. The data is assumed to consist of 16 bit complex samples, sampled
at a rate which is higher than (20%)the chirp bandwidth. Hence it is
assumed that the basebanding, demodulation and digitisation have taken
place externally to this subsystem. Digital demodulation could also be
performed within the subsystem. In this case, the input would consist
of 8 bit real samples ,with twice the sampling rate as before. In the
figure, the compressed output exits the subsystem at the right ,
through a number of t parallel channels.

Fig.5.Generic architecture for P and S subsystem
The various architecture parameters are:

p=no: of input parameters
q=no: of processing elements in the first MPS
r=no: of processing elements in the second MPS
At the centre of the diagram is located a switch which connects
either the input data lines or one of the agents , located above the
switch, with one of the mass memory banks located below the switch. The
agents generally are the multiprocessor systems (MPS) whose function is
execution of compression algorithms.
One MPS is baseline , shown as the left most agent here, others
are optional. They may be implemented in the event that the memory
capacity of the system is to upscaled.

Fig.6. Switching stages corresponding to different operational modes
of a P and S subsystem
There are three different modes of operation : input mode
processing
mode
output
mode
During input mode, the input data channel consisting of p
parallel subchannels is connected to one of the memory banks. Each
memory bank has p input ports which are used simultaneously.

During processing mode, each agent is connected to either one
or two memory banks. Specifically, an agent can be connected to one
memory bank for data input and to another or the same for data output.
If multiple agents and multiple mass memories are present , the agents
may process their respective data simultaneously.
During output mode, the output formatter is connected to one of
the memory banks. The function of the output formatter is to read data
, which has been compressed, from memory, to generate source packets
of the required format and to output these packets over t parallel
lines. If p is a multiple of t ,p=kt, the t channels of the output
formatter are reconnected to the p channels of a memory bank k times .
This is done in such a way that each memory port is connected to one of
the output lines once and only once.
Most of the modules in this architecture are easily scalable
with respect to different values of p, q, r...that is a new
architecture with different values of these parameters can be built
without redesign of these modules.


TOPAS ARCHITECTURE
TOPAS stands for the Technology Development of a Space-borne
On-Board SAR-Processor and Storage Demonstrator. In TOPAS architecture
there are two agents-a multiprocessor system and a CWIC (constant rate
wavelet based image compressor).This application specific hardware unit
is employed to compress processed SAR images at high data rate. The
compression ratio is user-specified. Due to the high throughput of this
unit, only one module of CWIC is required.
In more powerful versions of TOPAS architecture for 15MHz
bandwidth, the MPS can be scaled to include 6 to 12 processing
elements, increasing the processing speed of the system accordingly.

Fig.7. Architecture as scaled as in TOPAS
Each memory module in the demonstrator has a capacity of
4Gbits. This corresponds to about 24 seconds of raw data intake time
,which is sufficient for demonstration purposes.
After the processing and compression of the data obtained by
the SAR on-board, the data is send to the ground station and
distributed to the customers and interpreting organisations.
ADVANTAGES AND DISADVANTAGES
ADVANTAGES
1. Operational under all weather conditions with the capabilities
for sensing the earth day and night.
2. Provides description of surface texture.
3. Has own source of illumination
4. Cloud and fog cover are not a problem.
5. Vegetation and subsurface penetration capabilities.
DISADVANTAGES
1. Image distortion
2. Coarse resolution
3. Extensive shadowing of areas characterised with relief.


APPLICATIONS
SAR Systems has a wide range of applications such as:
1. Observation of volcanic activities and flood disasters.
2. Land and sea monitoring.
3. Observation of vegetarian growth.
4. Monitoring of ocean currents and traveling icebergs.
5. Detection of oil spills in oceans.


CONCLUSION
Synthetic Aperture Radar is now a well established part of
radar art, both with airborne systems for surveillance and non-
cooperative target identification purposes, and with space-borne
systems for geophysical remote sensing applications over the oceans,
land and polar regions. The capability to operate under all weather
conditions make it an efficient sensor.

BIBLIOGRAPHY
1. R.Zahn,"Innnovative technologies for space-based radars" IEE
Proceedings-Radar Sonar Navigation, vol.150, No:3, June 2003,
pp.104-111.
2. R.Zahn, H.Braumann , "Status of the X-band SAR instrument
demonstrator development", CEOS 99, August 1999.
3. W.Keyedel, "Perspectives and visions for future SAR systems
"IEE Proceedings-Radar Sonar Navigation,vol.150, No:3, June 2003,
pp.97-103.


TABLE OF CONTENTS
1. INTRODUCTION
1
2. X-BAND SAR INSTRUMENT DEMONSTRATOR 3
3. ON-BOARD PROCESSING FOR SPACE SAR 6
4. PROCESSING AND STORAGE SUBSYSTEM 7
5. PROCESSING AND STORAGE ARCHITECTURE 9
6. TOPAS ARCHITECTURE
12
7. ADVANTAGES AND DISADVANTAGES 13
8. APPLICATIONS
14
9. CONCLUSION
15
10. BIBLIOGRAPHY
16


ACKNOWLEDGEMENT
I extend my sincere gratitude towards Prof. P.Sukumaran Head of
Department for giving us his invaluable knowledge and wonderful
technical guidance.
I express my thanks to Mr. Muhammed Kutty our group tutor and
also to our staff advisor Ms. Biji Paul for their kind co-operation and
guidance for preparing and presenting this seminars.
I also thank all the other faculty members of AEI department
and my friends for their help and support.

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A technical seminar report on
Synthetic aperture radar
Synthetic-aperture radar (SAR)
It is a form of radar in which multiple radar images are processed to yield higher-resolution images than would be possible by conventional means. Either a single antenna mounted on a moving platform (such as an airplane or spacecraft) is used to illuminate a target scene or many low-directivity small stationary antennae are scattered over an area near the target area. The many echo waveforms received at the different antenna positions are post-processed to resolve the target. SAR can only be implemented by moving one or more antennae over relatively immobile targets, by placing multiple stationary antennae over a relatively large area, or combinations thereof. SAR has seen wide applications in remote sensing and mapping.

The surface of Venus, as imaged by the Magellan probe using SAR
Basic operation
In a typical SAR application, a single radar antenna is attached to the side of an aircraft. A single pulse from the antenna will be rather broad (several degrees) because diffraction requires a large antenna to produce a narrow beam. The pulse will also be broad in the vertical direction; often it will illuminate the terrain from directly beneath the aircraft out to the horizon. If the terrain is approximately flat, the time at which echoes return allows points at different distances to be distinguished. Distinguishing points along the track of the aircraft is difficult with a small antenna. However, if the amplitude and phase of the signal returning from a given piece of ground are recorded, and if the aircraft emits a series of pulses as it travels, then the results from these pulses can be combined. Effectively, the series of observations can be combined just as if they had all been made simultaneously from a very large antenna; this process creates a synthetic aperture much larger than the length of the antenna (and much longer than the aircraft itself).
Combining the series of observations requires significant computational resources. It is often done at a ground station after the observation is complete, using Fourier transform techniques. The high computing speed now available allows SAR processing to be done in real time onboard SAR aircraft. The result is a map of radar reflectivity (including both amplitude and phase). The phase information is, in the simplest applications, discarded. The amplitude information contains information about ground cover, in much the same way that a black-and-white picture does. Interpretation is not simple, but a large body of experimental results has been accumulated by flying test flights over known terrain.
Image resolution of SAR is mainly proportional to the radio signal bandwidth used and, to a lesser extent, on the system precision and the particular techniques used in post-processing. Early satellites provided a resolution in the tens of meters. More recent airborne systems provide resolutions to about 10 cm, ultra-wideband systems (developed and produced in the last decade) provide resolutions of a few millimeters, and experimental terahertz SAR has provided sub-millimeter resolution in the laboratory.

NASA's AirSAR instrument is attached to the side of a DC-8
Before rapid computers were available, the processing stage was done using holographic techniques. This was one of the first effective analogue optical computer systems. A scale hologram interference pattern was produced directly from the analogue radar data (for example 1:1,000,000 for 0.6 meters radar). Then laser light with the same scale (in the example 0.6 micrometers) passing through the hologram would produce a terrain projection. This works because SAR is fundamentally very similar to holography with microwaves instead of light.
The invention of SAR by Carl A. Wiley
Carl A. Wiley, a mathematician at Goodyear Aircraft Corporation 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. During the 1950s and 1960s Goodyear Aircraft (later Goodyear Aerospace) introduced numerous advancements in SAR technology
The SAR algorithm, in its simplest form, does the following:
A three-dimensional array (a volume) is defined which will represent the volume of space within which targets exist. Each element of the array is a cubical voxel representing the probability (a "density") of a reflective surface being at that location in space. (Note that two-dimensional SARs are also possible”showing only a top-down view of the target area).
Initially, the SAR algorithm 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 that voxel to the antenna(e) used to capture that waveform is calculated. That distance represents a time delay into the waveform. 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. Note that there are several optional approaches here, depending on the precision of the waveform timing, among other things. For example, if phase cannot be accurately known, then only the envelope magnitude (with the help of a Hilbert transform) of the waveform sample might be added to the voxel. If polarization and phase are known in the waveform, and are accurate enough, then these values might be added to a more complex voxel that holds such measurements separately.
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. Voxels whose density is below that threshold are ignored. Note that the threshold level chosen must at least be higher than the peak energy of any single wave”otherwise that wave peak would appear as a sphere (or ellipse, in the case of multistatic operation) of false "density" across the entire volume. 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. Optionally, added visual quality can sometimes be had by use of a surface detection algorithm like marching cubes.
More complex operation
The basic design of a synthetic-aperture radar system can be enhanced to collect more information. Most of these methods use the same basic principle of combining many pulses to form a synthetic aperture, but may involve additional antennae or significant additional processing.
Multistatic operation
SAR requires that echo captures be taken at multiple antenna positions. The more captures taken (at different antenna locations) the more reliable the target characterization.
Multiple captures can be obtained by moving a single antenna to different locations, by placing multiple stationary antennae at different locations, or combinations thereof.
The advantage of a single moving antenna is that it can be easily placed in any number of positions to provide any number of monostatic waveforms. For example, an antenna mounted on an airplane takes many captures per second as the plane travels.
The principal advantages of multiple static antennae are that a moving target can be characterized (assuming the capture electronics are fast enough), that no vehicle or motion machinery is necessary, and that antenna positions need not be derived from other, sometimes unreliable, information. (One problem with SAR aboard an airplane is knowing precise antenna positions as the plane travels).
For multiple static antennae, all combinations of monostatic and multistatic radar waveform captures are possible. Note, however, that it is not advantageous to capture a waveform for each of both transmission directions for a given pair of antennae, because those waveforms will be identical. When multiple static antennae are used, the total number of unique echo waveforms that can be captured is
where N is the number of unique antenna positions.
Polarimetry

Radar waves have a polarization. Different materials reflect radar waves with different intensities, but anisotropic materials such as grass often reflect different polarizations with different intensities. Some materials will also convert one polarization into another. By emitting a mixture of polarizations and using receiving antennae with a specific polarization, several different images can be collected from the same series of pulses. Frequently three such RX-TX polarizations (HH-pol, VV-pol, VH-pol) are used as the three color channels in a synthesized image. This is what has been done in the picture at left. Interpretation of the resulting colors requires significant testing of known materials.

SAR image of Death Valley colored using polarimetry
New developments in polarimetry also include utilizing the changes in the random polarization returns of some surfaces (such as grass or sand), between two images of the same location at different points in time to determine where changes not visible to optical systems occurred. Examples include subterranean tunneling, or paths of vehicles driving through the area being imaged. Enhanced SAR sea oil slick observation it has been developed by appropriate physical modelling and use of fully-polarimetric and dual-polarimetric measurements.
Interferometry

Rather than discarding the phase data, information can be extracted from it. If two observations of the same terrain from very similar positions are available, aperture synthesis can be performed to provide the resolution performance which would be given by a RADAR system with dimensions equal to the separation of the two measurements. This technique is called Interferometric SAR or InSAR.
If the two samples are obtained simultaneously (perhaps by placing two antennas on the same aircraft, some distance apart), then any phase difference will contain information about the angle from which the radar echo returned. Combining this with the distance information, one can determine the position in three dimensions of the image pixel. In other words, one can extract terrain altitude as well as radar reflectivity, producing a digital elevation model (DEM) with a single airplane pass. One aircraft application at the Canada Centre for Remote Sensing produced digital elevation maps with a resolution of 5 m and altitude errors also on the order of 5 m. Interferometry was used to map many regions of the Earth's surface with unprecedented accuracy using data from the Shuttle Radar Topography Mission.
If the two samples are separated in time, perhaps from two different flights over the same terrain, then there are two possible sources of phase shift. The first is terrain altitude, as discussed above. The second is terrain motion: if the terrain has shifted between observations, it will return a different phase. The amount of shift required to cause a significant phase difference is on the order of the wavelength used. This means that if the terrain shifts by centimeters, it can be seen in the resulting image (A digital elevation map must be available in order to separate the two kinds of phase difference; a third pass may be necessary in order to produce one).
This second method offers a powerful tool in geology and geography. Glacier flow can be mapped with two passes. Maps showing the land deformation after a minor earthquake or after a volcanic eruption (showing the shrinkage of the whole volcano by several centimeters) have been published.
Differential interferometry
Differential interferometry (D-InSAR) 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, which guarantees minimal distortion of the image of the target surface. In principle, 3 images of the ground area with similar image acquisition geometry is often adequate for D-InSar. The principle for detecting ground movement is quite simple. One interferogram is created from the first two images; this is also called the reference interferogram or topographical interferogram. A second interferogram is created that captures topography + distortion. Subtracting the latter from the reference interferogram can reveal differential fringes, indicating movement. The described 3 image D-InSAR generation technique is called 3-pass or double-difference method.
Differential fringes which remain as fringes in the differential interferogram are a result of SAR range changes of any displaced point on the ground from one interferogram to the next. In the differential interferogram, each fringe is directly proportional to the SAR wavelength, which is about 5.6 cm for ERS and RADARSAT single phase cycle. Surface displacement away from the satellite look direction causes an increase in path (translating to phase) difference. Since the signal travels from the SAR antenna to target and back again, the measured displacement is twice the unit of wavelength. This means in differential interferometry one fringe cycle -pi to +pi or one wavelength corresponds to a displacement relative to SAR antenna of only half wavelength (2.8 cm). There are various publications on measuring subsidence movement, slope stability analysis, landslide, glacier movement, etc tooling D-InSAR. Further advancement to this technique whereby differential interferometry from satellite SAR ascending pass and descending pass can be used to estimate 3-D ground movement. Research in this area has shown accurate measurements of 3-D ground movement with accuracies comparable to GPS based measurements can be achieved.
Ultra-wideband SAR
Conventional radar systems emit bursts of radio energy with a fairly narrow range of frequencies. A narrow-band channel, by definition, does not allow rapid changes in modulation. Since it is the change in a received signal that reveals the time of arrival of the signal (obviously an unchanging signal would reveal nothing about "when" it reflected from the target), a signal with only a slow change in modulation cannot reveal the distance to the target as well as can a signal with a quick change in modulation.
Ultra-wideband (UWB) refers to any radio transmission that uses a very large bandwidth “ which is the same as saying it uses very rapid changes in modulation. Although there is no set bandwidth value that qualifies a signal as "UWB", systems using bandwidths greater than a sizable portion of the center frequency (typically about ten percent, or so) are most often called "UWB" systems. A typical UWB system might use a bandwidth of one-third to one-half of its center frequency. For example, some systems use a bandwidth of about 1 GHz centered around 3 GHz.
There are as many ways to increase the bandwidth of a signal as there are forms of modulation “ it is simply a matter of increasing the rate of that modulation. However, the two most common methods used in UWB radar, including SAR, are very short pulses and high-bandwidth chirping. A general description of chirping appears elsewhere in this article. The bandwidth of a chirped system can be as narrow or as wide as the designers desire. Pulse-based UWB systems, being the more common method associated with the term "UWB radar", are described here.
A pulse-based radar system transmits very short pulses of electromagnetic energy, typically only a few waves or less. A very short pulse is, of course, a very rapidly changing signal, and thus occupies a very wide bandwidth. This allows far more accurate measurement of distance, and thus resolution.
The main disadvantage of pulse-based UWB SAR is that the transmitting and receiving front-end electronics are difficult to design for high-power applications. Specifically, the transmit duty cycle is so exceptionally low and pulse time so exceptionally short, that the electronics must be capable of extremely high instantaneous power to rival the average power of conventional radars. (Although it is true that UWB provides a notable gain in channel capacity over a narrow band signal because of the relationship of bandwidth in the Shannon“Hartley theorem and because the low receive duty cycle receives less noise, increasing the signal-to-noise ratio, there is still a notable disparity in page link budget because conventional radar might be several orders of magnitude more powerful than a typical pulse-based radar.) So pulse-based UWB SAR is typically used in applications requiring average power levels in the microwatt or milliwatt range, and thus is used for scanning smaller, nearer target areas (several tens of meters), or in cases where lengthy integration (over a span of minutes) of the received signal is possible. Note, however, that this limitation is solved in chirped UWB radar systems.
The principle advantages of UWB radar are better resolution (a few millimeters using commercial off-the-shelf electronics) and more spectral information of target reflectivity.
Doppler-beam sharpening

Doppler Beam Sharpening commonly refers to the method of processing unfocused real-beam phase history to achieve better resolution than could be achieved by processing the real beam without it. Because the real aperture of the RADAR antenna is so small (compared to the wavelength in use), the RADAR energy spreads over a wide area (usually many degrees wide in a direction orthogonal (at right angles) to the direction of the platform (aircraft). Doppler-beam sharpening takes advantage of the motion of the platform in that targets ahead of the platform return a Doppler upshifted signal (slightly higher in frequency) and targets behind the platform return a Doppler downshifted signal (slightly lower in frequency).
The amount of shift varies with the angle forward or backward from the ortho-normal direction. By knowing the speed of the platform, target signal return is placed in a specific angle "bin" that changes over time. Signals are integrated over time and thus the RADAR "beam" is synthetically reduced to a much smaller aperture “ or more accurately (and based on the ability to distinguish smaller Doppler shifts) the system can have hundreds of very "tight" beams concurrently. This technique dramatically improves angular resolution; however, it is far more difficult to take advantage of this technique for range resolution. (See Pulse-doppler radar).
Chirped (pulse-compressed) radars

A common technique for many radar systems (usually also found in SAR systems) is to "chirp" the signal. In a "chirped" radar, the pulse is allowed to be much longer. A longer pulse allows more energy to be emitted, and hence received, but usually hinders range resolution. But in a chirped radar, this longer pulse also has a frequency shift during the pulse (hence the chirp or frequency shift). When the "chirped" signal is returned, it must be correlated with the sent pulse. Classically, in analog systems, it is passed to a dispersive delay line (often a SAW device) that has the property of varying velocity of propagation based on frequency. This technique "compresses" the pulse in time “ thus having the effect of a much shorter pulse (improved range resolution) while having the benefit of longer pulse length (much more signal returned). Newer systems use digital pulse correlation to find the pulse return in the signal
Data collection
Highly accurate data can be collected by aircraft overflying the terrain in question. In the 1980s, as a prototype for instruments to be flown on the NASA Space Shuttles, NASA operated a synthetic-aperture radar on a NASA Convair 990. However, in 1986, this plane caught fire on takeoff. In 1988, NASA rebuilt a C, L, and P-band SAR to fly on the NASA DC-8 aircraft. Called AIRSAR, it flew missions at sites around the world until 2004. Another such aircraft, the Convair 580, was flown by the Canada Center for Remote Sensing until about 1996 when it was handed over to Environment Canada due to budgetary reasons. Most land-surveying applications are now carried out by satellite observation. Satellites such as ERS-1/2, JERS-1, Envisat ASAR, and RADARSAT-1 were launched explicitly to carry out this sort of observation. Their capabilities differ, particularly in their support for interferometry, but all have collected tremendous amounts of valuable data. The Space Shuttle has also carried synthetic-aperture radar equipment during the SIR-A and SIR-B missions during the 1980s, as well as the Shuttle Radar Laboratory (SRL) missions in 1994 and the Shuttle Radar Topography Mission in 2000.
A model of a German SAR-Lupe reconnaissance satellite inside a Cosmos-3M rocket.
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.
Synthetic-aperture radar was first used by NASA on JPL's Seasat oceanographic satellite in 1978 (this mission also carried an altimeter and a scatterometer); it was later developed more extensively on the Spaceborne Imaging Radar (SIR) missions on the space shuttle in 1981, 1984 and 1994. The Cassini mission to Saturn is currently using SAR to map the surface of the planet's major moon Titan, whose surface is partly hidden from direct optical inspection by atmospheric haze.
The Mineseeker Project ([1]) is designing a system for determining whether regions contain landmines based on a blimp carrying ultra-wideband synthetic-aperture radar. Initial trials show promise; the radar is able to detect even buried plastic mines.
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.
The National Reconnaissance Office maintains a fleet of (now declassified) Synthetic Aperture Radar satellites commonly designated as Lacrosse or Onyx.
In February 2009, the Sentinel R1 surveillance aircraft entered service in the RAF, equipped with the SAR-based Airborne Stand-Off Radar (ASTOR) system.
The German Armed Forces' (Bundeswehr) military SAR-Lupe reconnaissance satellite system has been fully operational since July 22, 2008.
CONCLUSIONS
The combination of advanced microstrip patch antenna designs, quadrant steering and tuning architecture, and newly emerging Ferro tunable materials promises to enable the development of large-scale, flexible, steerable and frequency-agile antenna arrays, for use in aperture synthesis, scanning radar, adaptive telecommunications, and a host ofrelated aerospace and commercial applications. During the second half of this three-year contract, Q or Tek plans to further optimize the basic design, and implement it more fully on new materials as they become available

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PRESENTED BY
LAVANYA.N

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[attachment=9623]
INTRODUCTION
• SAR IS AN IMAGING RADAR SYSTEM
• SAR SENDS MICRO WAVE PULSES TO THE SURFACE OF THE EARTH
• SAR SYSTEM PLACED ON SATELLITES FOR THE IMAGING PROCESS
X-BAND SAR INSTRUMENT DEMONSTRATOR
A RADIATOR WITH THE 30 RADIATOR SUBARRAYS
A SINGLE SUBARRAY
PROCESSING AND STORAGE SUBSYSTEM
• BUFFERING OF THE SAR RAW DATA STREAM IN REAL TIME
• OFF LINE IMAGE PROCESSING AND COMPRESSION OF THE BUFFERED SAR DATA
• MASS MEMORY DATA MANAGEMENT AND ORGANISATION
• REFORMATTING AND OUTPUT OF COMPRESSED DATA AT DOWNLINK RATE
PROCESSING AND STORAGE ARCHITECTURE
TOPAS ARCHITECTURE
ADVANTAGES
• OPERATIONAL UNDER ALL WEATHER CONDITIONS WITH THE CAPABILITIES FOR SENSING THE EARTH DAY AND NIGHT
• PROVIDES DESCRIPTION OF SURFACE TEXTURE
• HAS OWN SOURCE OF ILLUMINATION
• CLOUD AND FOG COVER ARE NOT A PROBLEM
• VEGETATION AND SUBSURFACE PENETRATION CAPABILITIES
DISADVANTAGES
• IMAGE DISTORTION
• COARSE RESOLUTION
• EXTENSIVE SHADOWING OF AREAS CHARACTERISED WITH RELIEF
APPLICATIONS
• OBSERVATION OF VOLCANIC ACTIVITIES AND FLOOD DISASTERS
• LAND AND SEA MONITORING
• OBSERVATION OF VEGETARIAN GROWTH
• MONITORING OF OCEAN CURRENTS AND TRAVELING ICEBERGS
• DETECTION OF OIL SPILLS IN OCEANS
CONCLUSION
• SAR IS A WELL ESTABLISHED PART OF RADAR ART
• THE CAPABILITY TO OPERATE UNDER ALL WEATHER CONDITIONS

WHAT ARE THE APPLICATIONS OF SYNTHETIC APERTURE RADAR IN OCEAN OBSERVATIONS

PRESENTED BY,
NAFEENA NAZEER

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[attachment=14498]
What is SAR system ?
Synthetic Aperture Radar (SAR) is one of the most useful of the remote sensing strategies used by research scientists and operation specialists in a variety of fields.
SAR is an active sensor.
It is originated as an advanced form of side-looking airborne radar SLAR.
SAR System
SAR is an imaging radar system that sends a microwave pulse to the surface of the earth and register the reflection from the earth surface.
SAR sensor can collect data even if the observation area is covered with obstacles or an observation is made at night time.
How Can SAR’s Help Map Natural Disasters and Hazards?
Synthetic Aperture Radar is a space or airborne technology that provides high-resolution monitoring for environmental and military purposes in all weather or day/night conditions
With SAR it is possible to obtain detailed information quickly that may be of great use in combating such natural disasters as wildfires, oil spills, ice and seas, etc.
STRUCTURE OF SAR SYSTEM
It consist of
-> Active front end pane
-> Central electronics
-> Electrical ground support
equipment (EGSE)
STRUCTURE OF SAR SYSTEM
The active front-end panel consist of
-> Radiators
->T/R modules
->Panel control electronics
->Panel power conditioner
->Distribution network
->Calibration network
Synthetic Aperture Radar System
Synthetic Aperture Radar System
Synthetic Aperture Radar System
SYNTHETIC APERTURE RADAR SYSTEM
PROCESSING AND STORAGE SUBSYSTEM
Process and store the information obtained from SAR instrument.
The processing stage involves:-
1.Buffering of the SAR raw data
stream in real-time.
2.Off-line image processing &
compression of the buffered SAR
data.
Contd..
3.Mass memory data management and organisation.
4.Reformatting and output of compressed data at downlink rate.
PROCESSING OF IMAGES
APPLICATIONS
Observation of volcanic activities and flood disasters.
Land and Sea monitoring.
Observation of vegetarian growth.
Monitoring of ocean currents and travelling icebergs.
Detection of oil spills in oceans.
ADVANTAGES
Opertional under all weather conditions with the capabilities for sensing the earth day & night.
Provides description of surface texture.
Has own source of illumination.
Cloud & fog cover are not a problem.
Vegetation & subsurface penetration capabilities.
DISADVANTAGES
Image distortion.
Coarse resolution.
Extensive shadowing of areas characterised with relief.
CONCLUSION
SAR have the capability to operate under all weather conditions .
SAR is well established part radar art
>with airborne systems for survelliance and non-cooperative target identification purpose.
>with space-borne systems for geophysical remote sensing applications over oceans,land & polar regions.
Contd..
SAR instruments have been a major focus of research and development during the last few years.
At present it provides only images & in future it will have to deliver dedicated information to each special users.

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