Prelaunch Calibration of Microwave Humidity Sounder on China's FY-3A Meteorological Satellite

China's Feng Yun-3A (FY-3A) meteorological satellite is a second-generation polar-orbiting meteorological satellite launched in May 2008. The Microwave Humidity Sounder (MWHS) is the main payload, designed for atmospheric humidity sounding. Before the launch of MWHS, a series of experiments was conducted in a thermal/vacuum (T/V) chamber. This letter describes the MWHS T/V calibration, in which the flight model currently operating on FY-3A was tested. The calibration procedure and data-processing methods are presented. Calibration results, such as radiometric resolution, receiver nonlinearity, and calibration bias, were obtained. The results meet the specifications on bias and sensitivity of MWHS. Since the instrument temperatures will be approximately 10°C-20°C on orbit, the calibration errors of MWHS over the range of 100-300 K would be less than 0.3 K using the nonlinearity coefficients derived in the T/V test.

I. Introduction to MWHS

The microwave Humidity Sounder (MWHS) is a dual-frequency five-channel millimeter-wave radiometer [1]. It performs cross-track scanning with a swath width of 2700 km from an altitude of 836 km with a local equatorial crossing time of 10:20 a.m. The frequencies of MWHS include two atmospheric window channels near 150 GHz and three channels centered on the 183.31-GHz water vapor line, with frequencies of 183.31 ± 1, 183.31 ± 3, and 183.31 ± 7 GHz, respectively. The polarization of channel 1 (see Table I) is horizontal, while the other four channels are vertically polarized at nadir, defined the same as that for the Advanced Microwave Sounding Unit-B (AMSU-B) [2]. The polarization angle rotates with the scan angle in exactly the same way as it does for AMSU-B. Table I Center Frequencies, Passbands, and Planck Correction Coefficients of MWHS Channels

Channel number	Center frequency of channel (GHz)	Channel passband limits		b0 (K)	b1
		Lower sideband (GHz)	Upper sideband (GHz)
1	150.0 (H)	148.615-149.617	150.383-151.385	−.000392	1.000067
2	150.0 (V)	148.606-149.593	150.407-151.394	−.000381	1.000065
3	183.31±1 (V)	182.048-182.528	184.092-184.572	−.000073	1.000010
4	183.31±3 (V)	179.83-180.863	185.757-186.79	−.000340	1.000048
5	183.31±7 (V)	175.105-177.397	189.229-191.415	−.007791	1.001380