FY-3C/MERSI pre-launch calibration for reflective solar bands

Abstract

Medium Resolution Spectral Imager (MERSI) is the key imaging sensor on board Fengyun-3 (FY-3), the second generation polar-orbiting meteorological satellites in China, currently operating on both FY-3A, FY-3B and FY-3C satellites. It has 20 spectral bands, including 19 reflective solar bands (RSBs) with center wavelengths from 0.41μm to 2.1μm and 1 thermal emissive band (TEB) with center wavelength 12μm, making observations at two spatial resolutions: 250 m (bands 1-5) and 1km (bands 6-20). The FY-3C has been launched in 23, Sept., 2013. The MERSI doesn't carry on-board calibration standards. To obtain RSBs radiometric responses, pre-launched field radiometric calibration test which is called Solar Radiation Based Calibration(SRBC) was taken in Dali in 27, Feb. to 2, Mar., 2013. For the SRBC measurement which the sun was the source of irradiance, MERSI viewed the reflected solar irradiance from a set of the sixteen reference spectral on panels with different reflective level. The uniformity, reflectivity and BRDF (Bidirectional reflection of distribution function) of sixteen reference panels were tested in advance. There are two kinds of calibration coefficient generation methods used in SRBC. One is similar as the Sea-WiFS pre-launch calibration method by Langley calibration. Besides this, we use a portable spectrometers produced by Analytical Spectral Devices inc. (ASD inc.) named FieldSpec 3 to measure the absolutely reflected radiance simultaneously. The calibrated spectrometers measured radiance could be as the reference radiance and the the calibration coefficient of the MERSI can be calculated. We called this method Calibration Based on Reference Instrument(CBRI). The results of these two methods are comparable. The CBRI results are less then 6% difference with Langley calibration method in most channels except water-vapor channels and channel 15. An non-linear feature of the most FY-3C/MERSI detectors was found for the first time. This phenomenon is even more obvious for the water-vapor channels. The second order coefficient determined by pre-launched calibration is quite useful to improve the on-obit calibration accuracy.