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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.
