Synthetic Aperture Radar System

Seminar Report ’03 System

Synthetic Aperture Radar

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.

Dept. of AEI

1

MESCE Kuttippuram

Seminar Report ’03 System

Synthetic Aperture Radar

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 onboard processing.

Dept. of AEI

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Seminar Report ’03 System

Synthetic Aperture Radar

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

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Seminar Report ’03 System

Synthetic Aperture Radar

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 coordinate the whole processes of the system.

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Seminar Report ’03 System

Synthetic Aperture Radar

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.

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Seminar Report ’03 System

Synthetic Aperture Radar 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.

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Seminar Report ’03 System

Synthetic Aperture Radar

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 involves1. 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

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Seminar Report ’03 System

Synthetic Aperture Radar

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

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Seminar Report ’03 System

Synthetic Aperture Radar

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

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Seminar Report ’03 System

Synthetic Aperture Radar

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

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Seminar Report ’03 System

Synthetic Aperture Radar

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.

Dept. of AEI

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Seminar Report ’03 System

Synthetic Aperture Radar

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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Seminar Report ’03 System

Synthetic Aperture Radar

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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Seminar Report ’03 System

Synthetic Aperture Radar

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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Seminar Report ’03 System

Synthetic Aperture Radar

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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Seminar Report ’03 System

Synthetic Aperture Radar

BIBLIOGRAPHY 1.

R.Zahn,"Innnovative

Proceedings-Radar

Sonar

technologies

for

space-based

Navigation,

vol.150,

No:3,

radars" June

IEE 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.

Dept. of AEI

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MESCE Kuttippuram

Seminar Report ’03 System

Synthetic Aperture Radar

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.

Dept. of AEI

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Seminar Report ’03 System

Synthetic Aperture Radar

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

6.

TOPAS ARCHITECTURE

7.

ADVANTAGES AND DISADVANTAGES

13

8.

APPLICATIONS

14

9.

CONCLUSION

15

10.

BIBLIOGRAPHY

16

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Seminar Report ’03 System

Synthetic Aperture Radar

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.

Dept. of AEI

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