Synthetic Aperture Radar System Seminar Report (1)
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CHAPTER-1 INTRODUCTIONWhen 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 carries 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.
CHAPTER-2 X-BAND SAR INSTRUMENT DEMONSTRATORThe 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 panels consist of the radiators, the T/R modules, panel control electronics, panel power conditioner, distribution network and the3
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 sub arrays 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.
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
Fig.4. Rear view of radiator 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
CHAPTER-3 ON-BOARD PROCESSING FOR SPACE SARRationale for on-board processingImage 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.7
PROCESSING AND STORAGE SUBSYSTEMThe 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 organization 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 up to 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 organized 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 consi