Post-Launch Calibration of the Jason-3's Advanced Microwave Radiometer

Jason-3, launched on 17 January 2016, is the latest in a series of international satellite mission that extends the ocean surface topography measurements started by the Topex/Poseidon mission launched in 1992, followed by Jason-1 in 2001 and Jason-2 in 2008. The Advanced Microwave Radiometer (AMR) is one of the primary instruments on-board the Jason-3 satellite that determines radar travel time delay, also known as path delay (PD), caused by atmospheric water vapor and liquid water content in the atmosphere using three frequency channels (23.8, 18.7 and 34.0 GHz). Once the PD measurements are known, corrections can be applied to altimeter’s radar signals in removing the effects of water in the atmosphere.

The mission goal is to maintain the radiometer PD measurements to 1 mm/year, that requires the radiometric measurements be stable to better than 0.1K over the course of the mission. This study provides an overview of the Jason-3/AMR’s instrument and post-launch calibration performance during the cal/val phase of the mission. Results are presented from the stability monitoring in the radiometric measurements using various calibration methodologies, including, 1) vicarious cold reference calibration, 2) Amazon hot reference calibration, 3) model and radiative transfer based calibration, and 4) cold sky calibration. These methods were used to develop a calibration coefficient time series that finds the best fit to the external references weighted by their respective uncertainties. The data and method for deriving the coefficients are discussed. The accuracy and stability of the post-launch re-calibration are evaluated, and it has been shown that how the long-term calibration is stabilized to < 0.1 K in the radiometer data record. An independent assessment of PD measurements is also carried out against ECMWF model observations, ensuring the stability in the PD measurements remains within the mission goal of 1 mm/year.

One unique aspect of the Jason-3 calibration and validation period is the presence of Jason-2 orbiting ~80 seconds ahead of the Jason-3 satellite on the same ground track. Jason-2 carries an identical copy of the AMR.  This so called tandem phase of the Jason-3 mission is the first time two identical radiometers have co-flown in space, allowing an unprecedented assessment of the achieved inter-calibration between the two systems and an analysis of the inter-sensor calibration uncertainty. This is directly relevant to future proposed missions that seek to use identical co-flying radiometers to measure the temporal evolution of dynamic events, such as the NASA TEMPEST and TROPICS missions. An analysis of the achieved inter-sensor calibration and assessment of the uncertainty will be presented.