Assimilation
Level 1b Algorithm Theoretical Basis Document (ATBD)
Describes the theoretical bases of the algorithms used to convert the raw detector output (data numbers) from the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU) and Humidity Sounder Brazil (HSB) to physical radiance units and, in the case of AIRS, perform in-orbit spectral calibrations. The description of the algorithms which convert the level 1b measurements to geophysical units is covered in the Level 2 ATBD.
The Level 1B ATBD for AIRS is divided into three parts:
AIRS visible/near-infrared (VIS/NIR) channels
Level 2 Algorithm Theoretical Basis Document (ATBD)
Describes the theoretical basis of the AIRS Level 2 Products Algorithm. Many products are presented in one document because of the basic structure and approach of the Level 2 Products Algorithm. In order to achieve the basic requirement of temperature profile accuracy of 1K in 1 km thick tropospheric layers, a multi-spectral simultaneous retrieval of both the atmospheric thermodynamic state and atmospheric composition is attempted. Hence the Level 2 Products refer to the basic thermodynamic variables and trace gas abundance that control the outgoing infrared radiance.