All remote sensing sensors recieve electromagnetic radiation. The sensor then converts the radiation into electronic signal. However, electronic signal is meaningless unless it is related to a physical unit. Simply speaking, calibration of a remote sensing sensor is the procedure to establish the conversion parameter that converts output of the sensor to a measurable physical unit.
For example, the output reading of a spectroradiometer has the units radiance or irradiance at a specifically measured wavelength. The spectroradiometer must be calibrated in radiance, irradiance, and wavelength properly.
In the case of an airborne hyperspectrometer (hyper-spectroradiometer, or imaging spectrometer), the sensor measures not only the radiance but also the spatial aspects of the image. This then requires characterization of the spatial resolving and positioning capability of the instrument. Other calibrations may be required due to specific optical system used. (e.g. wisk broom scanning, push broom scanning, single-facet scanning, or multi-facet scanning optics, etc.)
The GER group along with its affiliate organizations maintains fully equipped optical laboratories for complete calibration and characterization of all GER produced sensors.
The following is a list of calibrations performed on GER products.
Ground Units
(1) Wavelength calibration: All GER field spectrometers are calibrated to its published specification. Equipment used in this calibration are: fully automated monochrometer, spectral line source (e.g. Argon, Mercury, Krypton, etc.), narrow band filters and other known band targets.
GER proprietary software is used to generate calibration parameters from all calibration measurements.
(2)Radiometric calibration: All GER field spectrometers are calibrated by NIST taceable, automated integrating sphere calibration standard (Optronic Laboratories, Inc., OL Series 462).
(3)Field of View calibratrion: The optical field of view for a field spectrometer determines what is being measured. All GER field spectrometers are calibrated to give a clearly defined field of view.
(4)Customer specific calibration: GER will calibrate for any specific requirement if desired by the customer. For example: optical element reflectivity, transmission measurement of custom parts.
Airborne Systems
(1)Wavelenght calibration: Calibration is done by GER group proprietary Windows based software. This software controls the scanning of each detector/band with narrow band signal from a monochrometer. Hundreds of bands can be done automatically without human intervention. Full report of spectral response curves is generated automatically.
(2)Radiometeric Calibration: A NIST traceable, automated integrating sphere calibration standard (Optronic Laboratories, Inc., OL Series 462), is used at the system focal point for this work. GER proprietary software is used.
(3)Spatial calibration: Precision spatial targets are used at the system focal point for this work. System MTF (Modulation Transfer Function) can be calculated. GER proprietary software is used.
(4)Other system characterizations: Signal to Noise Ratio (SNR), Noise Equivalent Radiance (NER), Noise Equivalent Temperature Difference (NEDT, for thermal sensor), electronic linearity and bandwidth, and many other system characters can be measured.
(5)Customer specific calibration: GER can meet customer specific calibraton requirements. For example, GER will install a complete calibration facility at the customer's location with full hardware, software, training, and documentation support.