**1. Introduction**

Satellite-borne infrared hyperspectral atmospheric sounders can obtain global meteorological observations with high precision and high spectral resolution and have been frequently applied to retrieving atmospheric temperature and humidity profiles, data assimilation, and climate studies [1]. In addition, quality control of meteorological satellite observations is a pivotal step before using satellite data for assimilation and retrieval, and it is the fundamental basis for building long-term infrared hyperspectral datasets [2].

FY-3E, the world's first early morning polar-orbiting meteorological satellite, was successfully launched on 5 July 2021. It effectively fills a void for global satellite observations

**Citation:** Chen, H.; Guan, L. Assessing FY-3E HIRAS-II Radiance Accuracy Using AHI and MERSI-LL. *Remote Sens.* **2022**, *14*, 4309. https:// doi.org/10.3390/rs14174309

Academic Editors: Jie Cheng and Nicholas R. Nalli

Received: 20 July 2022 Accepted: 23 August 2022 Published: 1 September 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and provides 100% global satellite data coverage for numerical weather prediction (NWP) at 6-h intervals [3]. The HIRAS-II (Hyperspectral Infrared Atmospheric Sounder-II) is a continuation of the infrared hyperspectral instrument HIRAS onboard the FY-3D, and it can provide hyperspectral observations in the thermal infrared band with 3041 contiguous channels. Compared with its predecessor, the field of view (FOV) array within a field of regard (FOR) has changed from 2 × 2 to 3 × 3 with the spatial resolution increased from 16 km to 14 km at the nadir, the sensitivity is enhanced by more than 2 times with the spectral calibration accuracy increased by 30% and the radiometric calibration accuracy increased from 0.7 K to 0.5 K [4]. HIRAS-II is expected to become the reference instrument for infrared remote sensing instruments; therefore, independently assessing its data quality for radiance measurements is of great importance in improving the accuracy and timeliness of global numerical weather prediction.

The combined remote sensing and intercalibration based on the satellite-borne infrared hyperspectral atmospheric sounder and high-spatial-resolution imager has become one of the most effective means to quantify the radiometric calibration accuracy for both types of instruments. Gunshor calibrated the water vapor channels and window channels of five geostationary satellite imagers using the High-Resolution Infrared Radiation Sounder (HIRS) and the Advanced Very-High-Resolution Radiometer (AVHRR) onboard NOAA-14 [5]. Tobin used the Atmospheric Infrared Sounder (AIRS) to evaluate the radiometric accuracy of the Moderate-Resolution Imaging Spectroradiometer (MODIS) carried on the same platform [6]. Wang used the Infrared Atmospheric Sounding Interferometer (IASI) to intercalibrate the water vapor channel of the GOES-11 and GOES-12 [7]. Xu Na et al., using the hyperspectral measurements of IASI as a reference, objectively assessed the on-orbit radiometric calibration accuracy of the FY-3A Medium-Resolution Spectral Imager (MERSI) thermal infrared channel [8]. Gong used the Cross-track Infrared Sounder (CrIS) onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite platform to cross-check the thermal infrared channels of the Visible Infrared Imaging Radiometer Suite (VIIRS) on the same platform [9]. Yang et al. assessed the relative bias of the HIRAS radiometric calibrations using the Metop-A/B IASI observations based on the Simultaneous Nadir Overpass (SNO) intercalibration method [1].

Accuracy assessments of satellite instrument on-orbit calibrations is necessary to ensure product consistency and interoperability, and it is also extremely important for bias correction in data assimilations [10]. However, HIRAS-II—which is on board the first early morning polar-orbiting satellite launched last year—is operational this year, and the quality of its radiance measurements has not yet been reported in the literature. The Advanced Himawari Imager (AHI) mounted on the Japanese geostationary meteorological satellite Himawari-8 is recognized as one of the most accurate imaging instruments in the world. The AHI is greatly improved over those of the MTSAT (Multi-functional Transport Satellite) series in terms of the number of bands, spatial resolution, and temporal frequency; and infrared (IR) band calibration is accurate to within 0.2 K with no significant diurnal variation [11–13]. Therefore, this paper evaluates the quality of the radiance measurements based on the spatially and temporally matched Himawari-8/AHI observations from 15 March to 21 April 2022, and also performs an intercomparison with the thermal infrared observations of MERSI-LL carried out on the same platform.

#### **2. Data Used in the Research**

The HIRAS-II is an interferometric Fourier transform spectrometer carried in a polar orbit 836 km above the ground. HIRAS-II views the ground in the conventional mode through a cross-track rotary scan mirror that provides ±50.4◦ ground coverage every 8 s. Each scan line observes 32 fields of regard (FORs), including 28 continuous Earth targets, 2 cold space targets, and 2 blackbody targets on the satellite. Each field of regard (FOR) includes a 3 × 3 field of view (FOV) with a spatial resolution of 14 km at the nadir. HIRAS-II covers the 3.92–15.38 μm infrared band with 3041 continuous channels at a spectral resolution of 0.625 cm<sup>−</sup>1. The HIRAS-II Level 1 radiance observations from 15 March to 21

April 2022 are used in this paper. The data can be found on the Chinese Feng Yun satellite remote sensing data service network (http://data.nsmc.org.cn accessed on 11 April 2022).

The Moderate-Resolution Spectral Imager-Low Light (MERSI-LL) is an important optical instrument onboard FY-3E with microlight and infrared detection capabilities. It is equipped with one visible channel operable with low-level illumination and six infrared channels. The spatial resolution of the two infrared split-window channels is 250 m, and the remaining channels are 1000 m. The MERSI-LL Level 1 radiance observations with a spatial resolution of 1000 m from 15 to 22 March 2022 are used in this paper and can be downloaded from http://data.nsmc.org.cn (accessed on 11 April 2022).

The AHI on the geostationary satellite Himawari-8 successfully launched in October 2014 and is one of the most advanced spaceborne imagers in the world. It has 16 observation channels (3 visible, 3 near-infrared, and 10 infrared), of which the spatial resolution of the infrared channel is 2 km and the temporal resolution is 10 min. Himawari-8/AHI radiation data obtained from the Japan Earth Observation Data Center (https://www.eorc.jaxa.jp/ ptree/index.html accessed on 11 April 2022) from 15 March to 21 April 2022 are analyzed.

The channel settings and performance of the thermal infrared band covered by HIRAS-II, AHI, and MESI-LL are shown in Table 1. The last row of the table (spectral coverage) specifies the central wavelength and the corresponding peak height of the weighting function (in parentheses) for each channel. The AHI has nine channels that can be completely spectrum matched with the HIRAS-II spectrum, of which channels 8, 9, and 10 are water vapor absorption channels; channels 13, 14, and 15 are window channels; and channels 11, 12, and 16 are SO2, O3, and CO2 absorption channels, respectively. The weighting function peak heights of AHI channels 8, 9, 10, 12, and 16 are 300 hPa, 371 hPa, 532 hPa, 40 hPa, and 863 hPa, respectively. The weighting function heights of the remaining channels are almost near the surface. For MERSI-LL, only channels 4, 5, 6, and 7 can be completely spectrum matched. Channel 4 is a water vapor channel (the peak height of the weighting function is approximately 400 hPa), and channels 5, 6, and 7 are window channel with a central wavelength of 8.55 μm, 10.8 μm, and 12.0 μm, respectively.

**Table 1.** Instrument performance parameters of HIRAS-II, AHI, and MERSI-LL in the longwave infrared band.

