Role of water vapor radiometers for in‐flight calibration of the TOPEX microwave radiometer

Abstract

First‐year results are presented for the in‐flight calibration and performance evaluation of the TOPEX microwave radiometer (TMR) based on overflight comparisons with ground‐based water vapor radiometer (WVR) data. Comparisons are made in terms of both TMR brightness temperatures and the retrieved range correction due to tropospheric water vapor (path delay).

Comparisons of TMR brightness temperature measurements with predictions based on WVR data and a calm sea flux model provided early recognition of TMR preflight calibration errors which were not apparent based on ground truth path delay comparisons. Later comparisons, using the final TMR calibration algorithms, are used to constrain the model for the calm sea surface nadir emissivity. The results suggest that the 18‐ to 37‐GHz calm sea flux is enhanced 1–2 K relative to the predictions of a Fresnel model for a plane surface.

Comparisons in the path delay domain illustrate the advantages of using time and space coincident ground measurements for in‐flight calibration and performance monitoring of satellite radiometer measurements. The results demonstrate that elimination of temporal and spatial decorrelation errors reduces path delay comparison residuals to less than 1 cm, in contrast to the 3‐cm scatter produced by the customary radiosonde comparisons.

Keywords:

• TOPEX microwave radiometer
• water vapor radiometer
• wet troposphere range correction
• path delay
• in‐flight calibration
• brightness temperature