Laboratory calibration and field measurement of land surface temperature and emissivity using thermal infrared multiband radiometers

Abstract

Accurate ground measurements of land surface temperature (LST) are necessary for validating satellite LST products. In order to provide reliable data, ground radiometers must be calibrated with reference to an international standard, and radiometric temperatures must be corrected for land surface emissivity. As opposed to water, land surface emissivity is not usually known for many ground covers, so an emissivity value has to be assumed, assigned from spectral emissivity libraries or measured for each land cover and spectral band considered. The aim of this study is to show the laboratory calibration and the methodology for simultaneous field measurements of LST and emissivity employed in the comparison experiment held in the project Fiducial Reference Measurements for validation of Surface Temperature from Satellites (FRM4STS) funded by the European Space Agency. We used multiband CE-312 radiometers (five narrow bands plus one broad band in the 8–13 μm window) to simultaneously retrieve LST and band emissivities by means of the temperature-emissivity separation (TES) method for different ground covers (clover, tarmac, soil, gravel and sand). The TES method requires near-simultaneous measurements of ground-leaving radiances and sky downwelling radiances; the latter being measured in the field using a gold reflectance panel. For each surface cover, TES provided band emissivities in the CE-312 bands and LST from continuous radiance measurements performed over time. As a result of the experiment, we present the laboratory calibration of the CE-312 radiometers carried out against a traceable, reference blackbody, and the LST series and band emissivity values for the ground covers considered, together with a detailed LST uncertainty analysis including the uncertainties associated to the calibration of ground radiometers, the emissivity estimation by means of the TES method, and the sky radiance measurements, among others. According to these results, the total LST uncertainty was estimated as 0.4-0.6 K for the ground covers measured during the experiment.