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-
v Buffering
of the SAR raw data stream in real-time
v Off-line
image processing and compression of the buffered SAR data
v Mass
memory data management and organisation
v Reformatting
and output of compressed data at downlink rate
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.
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 nighttime because SAR uses microwaves and the sensor itself
radiates these. The SAR sensor can be installed in some satellite and the
surface of the earth can be observed. 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.
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.
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.
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.
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.
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.
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