Vicarious CalVal Facility for Calibration of Satellite Sensors
Vicarious Calibration is a technique that makes use of natural or artificial sites on the surface of the Earth for the post-launch calibration of sensors. These targets are imaged in near-coincident fashion by the sensor to be calibrated and by one or more well-calibrated sensors from satellite or aircraft platforms or on the ground.
State-of-art vicarious CalVal (Calibration/Validation) site is setup at Shadnagar campus of NRSC. The targets are spectrally characterised for spatial uniformity and spectral homogeneity. The Calibration ensures the measurement accuracy of an instrument compared to a known standard; provides corrections required to make the measuring device accurate. Whereas, Validation is a check that an instrument meets the specifications for a specific intended use.
Introduction: 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 borne sensors. On-orbit calibration results are more practical compared to the laboratory calibration as it provides combined effect of ground and atmospheric conditions.
Internationally various research groups under the guidelines of Committee on Earth Observation Satellites (CEOS) have been conducting seasonal field campaigns on standard vicarious sites for calibrating optical and 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 these reasons, an in-house calibration site was set-up at Shadnagar, NRSC with the objectives to establish and maintain the periodic validation of IRS sensors for mid and high/very high resolution sensors. The facility is established with five different natural targets with suitable sizes for characterisng the sensor response. The Facility is also geometrically well calibrated to the accuracy of 30 cm and is supported by Instruments to collect in-situ measurements.
Targets: Five natural target materials selected for the calibration test site are Black soil, Black Stone, Gravel (grey), Red Soil and White Stone covering the reflectance range from 7% to 70%. Out of the five target materials, four (Black soil, black stone, gravel and white stone) are procured/brought from different places, whereas 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 characterised in the laboratory and the samples were selected with the consistent response over the intended wavelengths using an illumination source and spectroradiometer.
During satellite passes over this site, calibrated instruments such as sun photometer, ozonometer and spectroradiometer are used to derive parameters like optical depth, ozone and water vapor which are used as input for the Radiative Transfer Model (6S RT model). The spectral reflectance / radiance measured from satellite and values calculated through this model, can be compared for Very Near Infra Red (VNIR) range of optical sensors for resolutions better than or equal to 30 m.
Additionally, radiometric performance of high resolution data < 5 m is carried out by evaluating Modulation Transfer Function (MTF) using Black and White Stone square targets of 70 m x 70 m 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.
Invariance of the target is very important to characterise on board sensors. Target response from Nov 2014 to Feb 2015 is observed over different calibration experiments for Visible, Near Infra Red (NIR) and Short Wave Infra Red (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.
The research opportunities available with CalVal Site was announced during the recent User Interaction Meet-2016 held at NRSC.