Department of Electrical Engineering and Automation

MetEOC4 (2020-2023)

Metrology Research Institute is involved in project MetEOC4 - "Metrology to establish an SI-traceable climate observing system".

The overall goal of this project is to build on the outputs of previous projects (by EMRP/EMPIR and projects funded by parties, such as the EU and ESA) to create the metrology tools and framework needed to underpin a global climate observing system. This project is the follow-up of the EMPIR MetEOC3 where Metrology Research Institute (MRI) was contributing to develop a gonioreflectometer for the measurement bidirectional reflectance distribution function (BRDF) of sand of reference sites.

One of the objectives of the current project is to upgrade and validate the open-source radiative transfer software Eradiate (originally specified in the MetEOC-3 project and operationally implemented through a project with ESA). Radiative Transfer (RT) models are used to simulate the interaction of light with the atmosphere and with complex real-world surfaces. Eradiate was developed to implement RT-model simulation and validation using modern, evolved software and computational technologies. To validate the new models and code so that it can be trusted and applicable for EO product calibrations, it requires a thorough validation against SI-traceable reference samples. 

This task will compare the new software’s RT models against actual measurements of appropriately designed, produced, and characterized artificial targets. An artificial target was developed using Eradiate by simulating its scattering distribution using various surface geometries. The base material of the artificial target was found by measuring various material types using Aalto’s 3D gonioreflectometer. The final target design is shown in Figure 1.

A device wiht holes in a plastic holder
Figure 1 Artificial target possessing an anisotropic scattering distribution that has distinct reflective lobes. The target is constructed from three plates, with the top plate machined to have a 12 x 12 array of uniform holes.

The target was measured for its BRDF using Aalto’s SI-traceable 3D instrument. Figure 2 shows the measured BRDF of the artificial target recorded at 500 nm using the 3D instrument. The figure shows that the target has an anisotropic BRDF showing district reflective lobes at increments of 45° in viewing azimuth angle.

plots
Figure 2 BRDF of the artificial target measured at 500 nm. The target possesses distinct reflective lobes as a function of the viewing azimuth angle.

The RT code simulated multi-angular reflectances that was compared with the measured multi-angular reflectances (BRDF measurements) of the 3D instrument. The comparison showed good results compared with previous attempts at RT code validation. Eradiate and the 3D instrument agreed well within their uncertainties in both in-plane and out-of-plane geometries. Thus, the RT simulation software was capable of simulating BRDF with an uncertainty of 2 % comparable to an SI-traceability facility.

References

[1]  D. Lanevski, A. Bialek, E. Woolliams, F. Manoocheri, N. Fox and E. Ikonen, “Gonioreflectometric Properties of the Sand from RadCalNet Gobabeb Test Site,” in International Conference on New Developments and Applications in Optical Radiometry, 2021. 
[2]  V. Leroy, R. Aschan, P. Woolliams, S. Schunke, F. Manoocheri and Y. Govaerts, “An SI-traceable protocol for the validation of radiative transfer model-based reflectance simulation,” Submitted for publication in 2024. 
[3]  V. Leroy, Y. Nollet, S. Schunke, N. Misk and Y. Govaerts, “Eradiate radiative transfer model,” 2023. [Online]. Available: https://github.com/eradiate/eradiate. [Accessed 1 March 2024].

Contact persons: Farshid Manoocheri and Robin Aschan

More information can be found at the project website: http://www.meteoc.org/

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