In this research we are aiming at improving the accuracy of satellite-derived data through developing a novel technique for vicarious calibration. The technique uses an optical sensor with enhanced traceability to the SI units. The spectrometer, together with supporting equipment, will be mounted on-board a remote controlled helicopter as an efficient and low cost remote sensing tool. The on-board instruments are controlled using a mini-computer that is connected with a ground-based computer through remote network connection, so real time measurements can be observed and controlled on the ground by the operator. The spectrometer, that is required to be a palm-size detector, is operating in the VIS-NIR spectral range. The technique has the potential to provide accurate and low cost reference surface measurements for large and difficult terrains over a short period of time, which can subsequently be used to calibrate and validate satellite-derived measurements.
The optical sensor, selected to be a photodiode array spectrometer, will be calibrated against a fixed-point blackbody. This blackbody provides a high emissivity and high stability reference source of known spectral radiance over the emitted spectral range. However, the highest fixed-point that is defined by the current International Temperature Scale (ITS-90) is the copper point at 1357.77 K , which is not high enough to provide sufficient optical power at short wavelengths.
It is proposed in this work to calibrate the spectrometer against a eutectic fixed-point, which uses metal carbon alloy rather than pure metals . These fixed-points have the potential to provide a spectral radiance blackbody source with melting points up to 3500 K . However, the thermodynamic temperatures of these fixed-points have not been defined yet (by international agreement). Once the fixed-point temperature has been determined, its spectral emission can be calculated according to Planck’s law. Then all the wavelength channels of the photodiode array spectrometer can be calibrated simultaneously using the eutectic transition. As a side approach, these eutectic fixed-points are strongly nominated to extend the future international temperature scale, accordingly reducing the current uncertainty in high temperature measurements experienced by the end users. Therefore, this will be a double benefit goal of the success of this work.
For the work to be truly valuable for this and future studies, the photodiode array spectrometer should be selected, and the radiometric performance of the spectrometer should be thoroughly evaluated. In general, photodiode array spectrometers suffer from poor stray light rejection, which is an inherent problem in this sort of detectors and a major...