Analysis of pseudo-noise for infrared sounder instruments in geostationary orbit

Increasing user and service needs for Numerical Weather Prediction impose an Infrared Sounding (IRS) mission on Meteosat Third Generation platforms. A major objective of the IRS mission is to provide a vertical sounding capacity in the vicinity of clouds, above clouds and below semi-transparent clouds. Thus, cloud contamination has to be severely controlled at instrument or at system level. Pseudo-noise is defined as the measurement error generated by scene heterogeneity. Spectrally correlated, it affects the information content of the vertical sounding and thus the quality of high-level products. An IRS pseudo-noise estimation model has been developed in view of supporting the consolidation of observational requirements during the detailed instrument design and system specification process. Geophysical inputs to the model are a spatial atmosphere distribution and the corresponding high-resolution radiance spectra. Major instrument inputs are a PSF model and an ISRF model. The paper presents a mathematical specification of pseudo-noise components and their quantification upon current design assumptions for the two considered instrument concepts: dispersive and Fourier transform spectrometer. The pseudo-noise budget is established for meaningful geophysical scenarios, and the associated observational requirements are derived and discussed, providing useful orders of magnitude for further iteration of the trade-off between instrument specifications and system requirements.