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Vicarious Calibration Facility
Periodic evaluation of radiometric and geometric performance of the space-borne Optical sensors and validation of satellite derived radiance/reflectance of earth scenes using these sensors are becoming mandatory requirements to ensure availability of consistent and accurate data products to the user community. Moreover, the need for inter-operability of sensors to fill data gaps over different regions using data from multiple sensors calls for inter-comparison through collaboration between different space born sensors. On-orbit calibration results will be more practical compared to the lab calibrations, as it is a combined effect of ground & atmospheric conditions. Internationally various research groups under the guidelines of CEOS (Committee on Earth Observation Satellites) have been conducting seasonal field campaigns on standard vicarious sites for calibrating optical & microwave sensors. These sites are mostly limited to desert areas covering only a limited dynamic range of sensor radiometry and are expensive to reach. Keeping in view of the above reasons, setting up an in-house calibration site at IMGEOS Complex, Shadnagar, NRSC is proposed in the year 2012 with the objectives to establish & maintain for periodic validation of IRS sensors.State of art vicarious CALVAL Site is setup at IMGEOS Complex, Shadnagar campus (Lat/Lon: 17.03/78.18) of NRSC, and it is conforming to CEOS/IVOS criteria followed for internationally acceptable sites. The site is geometrically well calibrated using GAGAN facility and LFDC data. Download Brochure The targets are spectral characterised for spatial uniformity and Spectral homogeneity. As the material procurement progressed started conducting in house experiments for IRS sensors and pruned the radiative transfer code to Site conditions.
Historical Climatic Observations over five years, 2010-2014 for the CALVAL site zone are found to be favourable during the months of January and February for collecting in situ measurements. These Climatic Observations are collected from MODIS (GIOVANNI Web link) Terra & OMI data AWS - NRSC & MOSDAC measurements. Area averaged time series parameters- AOD, Cloud Cover, Water content, ozone, air temperature, relative humidity and rain fall over a period of five years are collected and found to be suitable for carrying experiments during October to April months.
Five natural target materials selected for the calibration test site are Black soil, Black Stone, Gravel (grey), Red Soil and White Stone. Out of the five target materials, four (Black soil, black stone, gravel and white stone) are procured/brought from different places where as Red Soil material is locally available within the site.
It is important to understand the spectral characteristics of each target material sample for qualifying the targets for their spectral response. The material samples provided by the Vendors were spectrally characterized in the laboratory and the samples were selected with the consistent response over the intended wavelengths using an illumination source and ASD spectroradiometer. Figure indicates the targets Red Soil- D, Black Soil - B, Black Stone – A, White Stone – E and Gravel - C)
Invariance of the target is very important to characterize on board sensors. Target response from Nov 2014 to Feb 2015 is observed over different calibration experiments for Visible, NIR and SWIR wave lengths. The mean observations made for various experiments are found to be consistent and the standard deviation observed within the target region found to be in the acceptable limits.
Application / Usability
During satellite passes over this site, calibrated instruments such as sun photometer, ozonometer and spectroradiometer are used to derive optical depth, ozone & water vapor. These parameters are used as input for the 6s RT model.
The spectral reflectance / radiance measured from satellite and values calculated through the 6s RT model, can be compared for VNIR range of optical sensors for resolutions better than or equal to 30m. Additionally, radiometric performance of high resolution data < 5m is carried out by evaluating MTF (Modulation Transfer Function). Using Black and White Stone square targets of 70mx 70m available in the facility Real time Synchronous reflectance measurements were collected on these targets for IRS Sensors along with the atmospheric measurements and off sets were derived using 6S radiative transfer Code. Radiance values are obtained with Spectroradiometer, Ozone was collected and AOD measurements were obtained with Sun Photometer.