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Calibration Services

Calibration services for EO/IR imaging sensors and spectrometers.

Overview

Laboratory

Field

Ground Truth

 
Field Calibration  

Field Calibration

Field calibration of instruments just prior and immediately after (and sometimes during) data collection can be essential to assure quantitative analysis of the measurement data. Most instruments are known to suffer from drift, aging, and changes as a result of transportation and handling. Using a calibration that was performed in the lab prior to shipping an instrument may produce inaccurate results. Similar results may be experienced from a calibration that is performed on the instrument after shipping back from a field event.

It is desirable to build calibration capabilities into the sensor operation. As a minimum, we often collect "bright" and "dark" fields for various radiometric sensors. Spectral calibration can be achieved using well known atmospheric and stellar bands. MODTRAN can provide reference spectral bands. Star calibration can also be to verify radiometric calibration accuracy.

In the case shown above, an OptoKnowledge MWIR imaging spectrometer is calibrated in the field using a blackbody source and a collimator. The calibration rig is designed so that it can be easily mounted in line with the sensor for the calibration, and then removed. This allows the sensor to remain focused in "mission configuration". The use of such techniques assures high data fidelity in critical field experiments.

In addition to calibration, field instruments can be equipped with GPS and AHRS (Attitude and Heading Reference System) or simpler IMU units. These provide precise position and aiming data. If a target is being tracked, each frame of data can be stamped with the specific line of sight (LOS) parameters (the tracker mount may often provide similar azimuth and elevation data). IRIG or GPS time are also a part of a typical instrumentation system.

Please contact OptoKnowledge to design a field calibration and recording system to best fit specific requirements of your work.

Atmospheric Based Spectral Calibration (based on atmospheric transmission) & Star Based Radiometric Calibration

Many sensors used in an outdoor environment can be spectrally calibrated by matching their measurements with distinct atmospheric moleculor absorption features. Atmospheric spectral calibration can be used to validate and enhance laboratory spectral calibrations. The figure below shows sensor measurements that were "stretched" to match the MODTRAN atmospheric transmission under conditions similar to the actual measurements. This process often produces a better spectral calibration than can be achieved in the laboratory due to the multitude of available atmospheric lines.

Star calibration is another technique that is often performed in the field. The graph below shows field measurements of known stars that are used for radiometric calibration of the sensor. Such measurements are very useful pre- and post field tests to validate sensor calibration.


MODTRAN Match
 

MODTRAN Match   Star Calibration

 

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