A methodology for obtaining on-orbit SI-traceable spectral radiance measurements in the thermal infrared

A methodology to achieve spectral thermal radiance measurements fromspace with demonstrable on-orbit traceability to the International System ofUnits (SI) is described. This technique results in measurements of infraredspectral radiance R(υ )˜ , with spectral index υ˜ in cm−1, with a relativecombined uncertainty uc[R(υ )˜ ] of 0.0015 (k = 1) for the averagemid-infrared radiance emitted by the Earth. This combined uncertainty,expressed in brightness temperature units, is equivalent to ±0.1 K at 250 Kat 750 cm−1. This measurement goal is achieved by utilizing a new methodfor infrared scale realization combined with an instrument design optimizedto minimize component uncertainties and admit tests of radiometricperformance. The SI traceability of the instrument scale is established byevaluation against source-based and detector-based infrared scales in definedlaboratory protocols before launch. A novel strategy is executed to ensurefidelity of on-orbit calibration to the pre-launch scale. This strategy foron-orbit validation relies on the overdetermination of instrument calibration.The pre-launch calibration against scales derived from physicallyindependent paths to the base SI units provides the foundation for a criticalanalysis of the overdetermined on-orbit calibration to establish anSI-traceable estimate of the combined measurement uncertainty. Redundantcalibration sources and built-in diagnostic tests to assess componentmeasurement uncertainties verify the SI traceability of the instrumentcalibration over the mission lifetime. This measurement strategy can berealized by a practical instrument, a prototype Fourier-transformspectrometer under development for deployment on a small satellite. Themeasurement record resulting from the methodology described here meetsthe observational requirements for climate monitoring and climate modeltesting and improvement.