Abstract
The soil moisture active passive (SMAP) microwave radiometer is a fully-polarimetric <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> -band radiometer flown on the SMAP satellite in a 6 AM /6 PM sun-synchronous orbit at 685-km altitude. After the SMAP L1B_TB data product version 4 was released in 2018, the radiometer has undergone further calibration and validation. The goal is to reduce the difference between antenna temperature of ascending and descending orbits during the eclipse, and to reduce the dips in the calibration drift over the cold sky (CS) during the eclipse seasons in 2017 and 2018. The postlaunch calibration algorithm has been revisited by retrieving all of the calibration parameters simultaneously with two different options for the hot calibration source (the global ocean, or the radiometer internal reference load). The performance of the two options are compared here. The option with the radiometer internal reference load has been chosen by the SMAP science team for data release version 5. In addition, a correction offset is applied to the input signal to account for offsets during the early-mission stages with the SMAP synthetic aperture radar transmitter operating alongside the radiometer.
Highlights
T HE soil moisture active passive (SMAP) mission was launched on January 31, 2015 in a 6 AM/ 6 PM sun-synchronous orbit at 685 km altitude with the Manuscript received April 27, 2021; revised July 14, 2021; accepted October 1, 2021
The radiometer uses a combination of noise-diodes and Dicke-loads for internal calibration with a design similar to that used by the Jason, soil moisture and ocean salinity (SMOS) and Aquarius series radiometers [2]–[4]
The data product version 4 satisfies the soil moisture science requirements, two calibration issues remain: there is a maximum of 0.7 K difference in the calibration drift over the global ocean between ascending and descending orbits, and dips of 0.4 K magnitude in the calibration drift over cold sky (CS) in 2017 and 2018 but not in the previous years
Summary
T HE soil moisture active passive (SMAP) mission was launched on January 31, 2015 in a 6 AM/ 6 PM (descending/ascending) sun-synchronous orbit at 685 km altitude with the Manuscript received April 27, 2021; revised July 14, 2021; accepted October 1, 2021. The data product version 4 satisfies the soil moisture science requirements, two calibration issues remain: there is a maximum of 0.7 K difference in the calibration drift over the global ocean between ascending and descending orbits, and dips of 0.4 K magnitude in the calibration drift over CS in 2017 and 2018 but not in the previous years. These were reported after the release of data version 4 and appeared during the summer eclipse seasons (i.e., from May to August). The results will be discussed, and conclusions will be made for the data product version 5
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