Synthetic aperture radar (SAR) is a unique and well established approach for remote sensing providing high-resolution digital images independent of daylight or cloud coverage. Space borne SAR systems such as the current RISAT – 1, RADARSAT – 2 and Sentinel systems continuously yield data from extensive area of the globe with a revisit time in order of days. Airborne systems, on the other hand, map on a more local scale but can offer lower revisit times with higher resolutions. The utilizations for the acquired SAR data range from mapping of land use(forest, water, agriculture, urban) to change detection based on time series, monitoring of sea ice, marine surveillance, traffic monitoring, soil moisture and biomass estimation, and quick response mappings to support humanitarian aid in crisis situations. A SAR system is not simply a tool for remote feature detection, but it is also a measurement instrument to record reflectivity. As for any other measurement instrument, system calibration is hence necessary to ensure that different SAR acquisitions are comparable to each other. Calibration in SAR is a cornerstone for scientific applications as well as for verifiably reliable products for a growing commercial market. The SAR calibration activities are usually split into geometric and radiometric calibration.In geometric calibration, the SAR image co-ordinate system is compared with a reference co-ordinate system, and corrections are determined and applied to ensure that SAR images are geo-located with respect to a known co-ordinate system. Radiometric calibration, on the other hand, is not concerned with the location but the magnitude on a known radiometric scale to achieve comparability across different SAR images. Both types of calibration depend on measurements standards such as corner reflector, which have an accurately known location and reflectivity. In this paper RISAT – 1 data have been used for processing of radiometric calibration. In recent past many techniques have been developed to greatly improve the correction of systematic effects in SAR, but the fundamental problems can be solved through radiometric calibration. Radiometric Calibration of a SAR system is a very challenging task and includes continuous monitoring of the signal path and compensations if any deviations are detected. Advancement in SAR design such as interferometry, polarimetry and phased array antennas, these modes have to be characterised during calibration process. The basic process of radiometric calibration is still the same and involves two steps ie, radiometric normalization and Radiometric Calibration.An improvement in radiometric SAR system calibration will ensure verifiable data and allow a consistent exchange of SAR data across different SAR systems.
Calibration can be carried out with either transponders or passive targets. Transponders are active devices with their own power and carrying out the calibration process with transponders is difficult. They are very costly and are very bulk. Other way is by the use of passive target devices such as reflectors. They are comparatively low cost and are very simple in design and construction. Due to these advantages Corner reflectors (commonly called as target points) are widely used in the calibration process. Some of the commonly used passive targets are Trihedral Corner reflectors, flat metal plate, square corner reflectors etc.
RISAT – 1
India’s first SAR based satellite is Radar Imaging SATellite (RISAT – 1) operating at C-band with active antenna. Launched on 26th of April 2012 the satellite is in operation from 1st of May 2012. The satellite is placed at an altitude of 536Km (sun-synchronous orbit) and has a repetivity of 24 days. Resolutions of up to 1m x 0.7m can be obtained with the onboard SAR which is operating at 5.35GHz. RISAT- 1 is first of its kind in the world to operate in hybrid circular polarimetric mode. Some of the data acquiring modes of RISAT – 1 are High Resolution Spotlight (HRS) Fine Resolution Mode (FSR), Medium Resolution Mode (MRS) and Coarse Resolution Mode (CRS).
Characteristics for deployment of the Standard Targets
The corner reflectors are to be deployed on ground with very minimal interference from its surroundings. The site selection is based on parameters such as
1. Surface Roughness: The surface should be of plain terrain with very less or no hilly area. The selected area should not have any power stations, high tension wires. The rms height fluctuations of the area should also be less.
2. Reflectivity: Reflectivity also becomes a factor to be considered before selecting the site for deployment. The selected site should be very dry and the vegetation level should be very low. With this we have to assure the SAR is free from reflectivity from ground.
3. Dimensions of the selected area: The size of the deployment site should be very large to include both main lobe and the side lobes of the target and we should ensure at least 10 pixels of data in both azimuth and range directions.