Sensor calibration

Why is sensor calibration important? Sensors drift over time and this results in uncertainties To deduce accurate information from imaging spectroscopy data

Calibration categories 3 types of calibration categories Lab calibration In-flight calibration Vicarious calibration

Lab calibration Ideally before and after each mission Seismic block 7 m 6 m 12 m Lab calibration Ideally before and after each mission In practice before and after flight season Labour and cost intensive (especially for pushbroom sensors) Low humidity and T-controlled circumstances Calibration on seismic block

Lab calibration (cont’d) 3 types of lab calibration Geometric Spectral Radiometric

Geometric calibration To determine the spatial response function of each detector element Using e.g. high precision tilt stage to direct a collimated beam into the FOV and scanning the spatial response for each detector element across track

Lab calibration (cont’d) 3 types of lab calibration Geometric Spectral Radiometric

Spectral calibration To determine the centre wavelength and width of the spectral response function of each detector element (row) Row to wavelenght is a linear function Using a wavelenght standard (e.g. Mylar) with discrete emission or absorption features or a monochromator (expensive!)

Lab calibration (cont’d) 3 types of lab calibration Geometric Spectral Radiometric

Radiometric calibration To convert DN to radiance Using a radiance standard (e.g. integrating sphere) with known radiance which is Stable Uniform Lambertian

Radiometric calibration (cont’d) ADS Integrating Sphere Field Spectrometer with Fiber Optics FOV QTH Lamps for various radiance levels APEX Ø1,60 m

Radiometric calibration (cont’d) Known radiance of the standard is compared to the detector response (DN) after spectral calibration and dark signal subtraction Linear function assumed

Calibration categories 3 types of calibration categories Lab calibration In-flight calibration Vicarious calibration

In-flight calibration Operational conditions differ from lab conditions Not a full calibration but a check of the lab calibration using an on-board lamp during operation Less labour and cost intensive than lab calibration Before and after data acquisition

Calibration categories 3 types of calibration categories Lab calibration In-flight calibration Vicarious calibration

Vicarious calibration Means to validate the sensor’s performance By simultaneous data acquisition and in-situ measurements of stable, homogeneous, Lambertian targets Atmospheric input parameters required to link airborne/spaceborne data with in-situ data