Traceability and consistency of COSMIC radio occultation in comparison with NOAA-20 CrIS infrared sounder observations

Abstract

The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellite system for the Radio Occultation (RO) mission provides advances in meteorology, ionospheric research, climatology, and space weather by utilizing the readily available Global Navigation Satellite System (GNSS) signals in conjunction with GNSS receivers in low Earth orbiting (LEO) satellites. COSMIC was launched in 2006 with six satellites in a constellation known as FORMOSAT-3 in low inclination orbits to provide global coverage. RO relies on the calculation of GNSS signal time delay in carrier phase due to the atmosphere in the L1 and L2 signals transmitted between the GNSS and receiving satellites in the LEO orbit, from which the bending angle, refractivity, and atmospheric profiles can be retrieved. Since the Atomic Frequency Standard (AFS) based GNSS signal is International System of Units (SI) traceable, is actively maintained, and the precise orbit of both the GNSS and the LEO satellites can be determined accurately, RO data from COSMIC have been recognized as stable references for data assimilation (DA) in Numerical Weather Prediction (NWP) models. Currently, NWP customers are eager to obtain similar data from COSMIC2 which will be launched in the next few months to mitigate the aging COSMIC constellation and diminishing number of ROs. Meanwhile, the calibration of the hyperspectral sounders such as Cross-track Infrared Sounder (CrIS) on NOAA-20 relies on a high quality onboard blackbody which is also traceable to SI through prelaunch characterization relating to the laboratory blackbody with traceable calibration to NIST, and hyperspectral sounders have been recognized as onorbit calibration references for other broad- or narrow-band infrared (IR) observations. In this paper we analyze the traceability of both systems in their raw measurements as well as retrieved geophysical variables. Comparisons are also made in spectral radiance/brightness temperature derived from the two systems. The objective is to gain a better understanding of the different paths of traceability to SI and ensure the consistency of the products for numerical weather prediction and other applications. This study directly supports the COSMIC2 verification and validation, as well as postlaunch calibration/validation of NOAA-20 CrIS.