synthetic aperture radar s

8/3/2019 Synthetic Aperture Radar s

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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 readilyaccommodated 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.


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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 movesalong 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.


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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).Theactive 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


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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 thebasis 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.


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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 60

oC.

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.


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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 combinedwith 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


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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 data3. 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


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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.


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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 externallyto 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.


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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.


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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


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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.


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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.


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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.


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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.


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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.


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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.


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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.


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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


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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 seminar.

I also thank all the other faculty members of AEI department and my

friends for their help and support.

synthetic aperture radar s

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