Correcting Geolocation Errors for Microwave Instruments Aboard NOAA Satellites

Microwave (MW) satellite data are widely used as input in numerical weather prediction models and also in other applications such as climate monitoring and re-analysis. MW satellite data are prone to different problems, including geolocation errors. These data do not have a fine spatial resolution like visible and infrared data; therefore, the accuracy of their geolocation cannot be easily determined using the normal methods such as superimposing coastlines on the satellite images. Currently, no geolocation correction is performed on data from MW instruments aboard the satellites in the National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellite program. However, geolocation error can be a significant source of bias in the satellite measurements. In this paper, we investigated and corrected the geolocation errors of the observations from the Advanced Microwave Sounding Unit (AMSU)-A aboard NOAA-15 to NOAA-19, AMSU-B aboard NOAA-15 to NOAA-17, and Microwave Humidity Sounder (MHS) aboard NOAA-18 and NOAA-19. We used the difference between ascending and descending observations along the coastlines to quantify the geolocation errors in terms of the satellite attitudes (Euler angles), i.e., pitch, roll, and yaw. Then, new geographical coordinates and scan/local zenith angles were calculated using new attitudes. The results show that NOAA-15 AMSU-A2 instrument has a mounting error of about 1.2 $^{\circ}$ cross-track, and $-0.5^{\circ}$ along-track, NOAA-16 AMSU-A1 and -A2 instruments have a mounting error of about $-0.5^{\circ}$ along-track, and NOAA-18 AMSU-A2 instrument has a mounting error of more than $-1^{\circ}$ along-track.

I. Introduction

Microwave (MW) satellite data play a very important role in weather forecasting and also in climate monitoring and assessment. MW data are widely used to derive a variety of hydrological products, including precipitation [1]–[4], precipitable water [5], [6], snow cover [1], tropospheric humidity [7], cloud liquid water [8], deep convective clouds [9], and land surface emissivity [10]. The primary motivation of this paper was to develop corrections to geolocation of MW sounders that will contribute to the creation of more accurate time series for the development of a long-term climate data record from MW satellite data [11]. Geolocation errors are one of the main sources of uncertainty in the MW satellite data and have serious effects on intercalibrating, validating, and retrieving geophysical variables from satellite data.