Tangent point position and atmosphere composition in limb scanning instruments

TL;DR

This paper investigates how the tangent point positions and atmospheric composition in limb scanning instruments are affected by ray bending due to air refractivity, using numerical solutions and atmospheric data for calibration.

Contribution

It introduces a method to relate nadir angles, atmospheric properties, and tangent point positions in limb scanning, accounting for Earth's shape and atmospheric variations.

Findings

Tangent point positions vary with nadir angles and atmospheric conditions.

Differences identified between CAIRT and MIPAS instrument configurations.

Numerical solutions to the Eikonal equation improve line-of-sight modeling.

Abstract

In the forward model for limb-scanning instruments, ray tracing must be accounted for because variations in air refractivity cause the lines of sight to bend from straight paths into curves. The tangent point of a line of sight is defined as the minimum height point. The lines of sight can be adjusted by varying the nadir angles of the instrument, which must be calibrated to account for Earth's ellipticity and the actual atmospheric conditions sampled by these paths. In this study, we investigate the relationship between the nadir angles, atmospheric properties, and tangent point positions, using a configuration relevant to the CAIRT instrument. Atmospheric data from reanalysis databases are utilized, and the lines of sight are determined by numerically solving the Eikonal equation. Our findings are compared to those of a previous study conducted for the MIPAS instrument, highlighting key differences.