NMSC National Meteorological Satellite Center

COMS Infrared Calibration

COMS MI (Meteorological Imager) has 4 infrared channels and its radiometric calibration is conducted in stage of Level 1A data processing by measuring the radiances of warm and cold targets, blackbody onboard COMS and space-looking, respectively. In addition to onboard calibration, NMSC has examined the inter-calibration as a part of GSICS (Global Space-based Inter-Calibration System) activities. It is helpful to check the quality of operational measurements for COMS and sensor degradation with time. GSICS inter-calibration system is to compare two observation values measured by an instrument we want to calibrate and reference instrument which is known as relatively accurate.

Inter-calibration

The inter-calibration for COMS MI uses two hyper-spectral infrared sounders of polar-orbit satellites for reference measurement. One is AIRS (Atmospheric Infrared Sounder) onboard Aqua satellite launched by NASA and the other is IASI (Infrared Atmospheric Sounding Interferometer) onboard Metop-A launched by EUMETSAT. The inter-calibration is conducted by comparing the true measurements of COMS MI and simulated values from the measurement of reference sensors, in the radiance or brightness temperature (TB) format. Outcomes of the inter-calibration, especially provided in linear regression, are able to be used for correction of COMS observations. This correction might be important process for the retrieval of Essential Climate Variables (ECVs) such as Sea Surface Temperature (SST), Atmospheric motion vector (AMV) and so on.

Algorithm (ATBD)

The inter-calibration system for COMS infrared channels is thoroughly based on the GSICS Coordinate Center (GCC) Algorithm Theoretical Basis Document (ATBD). In order to conduct calibration system, first we should make collocation dataset for measured pixels of COMS and IASI. Then, the several channel measurements of hyper-spectral sounder is converted into a simulated radiance according to spectral response function (SRF) of COMS. It is called as constraint method, generating a super channel consisting of combination of a lot of channels to imitate a broadband channel. More details of the methods for collocation and spectral simulation are given in below paragraph.

Collocation method

Collocation algorithm used in NMSC is determined by GSICS research working group, and specific threshold values to filter out unsuitable pixels for inter-calibration are referred to JMA’s ATBD of GSICS. In following equations, subscript of GEO and LEO mean COMS and AIRS/IASI, respectively.

Collocated in space

Choose GEO pixel geometrically closest to the LEO pixel

Concurrent in time

|TimeLEO – TimeGEO| < 300 s (= 5 min)

Aligned in line-of-sight

|cos(VZALEO) / cos(VZAGEO)− 1| < ε1

Uniformity test for environmental pixels

To reduce the difference of two satellite measurements occurring due to different observational condition, especially time, navigation, optical path and cloud advection, only measurements over the uniform scene are selected for inter-calibration. Uniformity test for environmental pixels is conducted over 9x9 area (called as ENV box) of GEO.

STDDEV(RadGEO,ENV) < ε2

Normality test

Because the observational field-of-view for AIRS/IASI is about 13 km, 3 times larger than COMS MI, the average of COMS measurement for 3x3 pixels (called as FOV box) is used for comparison with AIRS/IASI. COMS FOV box should be representative of environment passed uniformity test, therefore normality test is added.

MEAN(RadGEO,FOV) − MEAN(RadGEO,ENV) |ⅹ9 / STDDEV(RadGEO,FOV) < ε3

Collocation algorithm used in NMSC is determined by GSICS research working group, and specific threshold values to filter out unsuitable pixels for inter-calibration are referred to JMA’s ATBD of GSICS. In following equations, subscript of GEO and LEO mean COMS and AIRS/IASI, respectively.