ISCCP CATALOG OF DATA AND PRODUCTS Section 3.5

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3.5 GMS

The Meteorological Satellite Center of the Japan Meteorological Agency provides B1-data processed from raw data (A-level) received from the GMS spacecraft. These data are provided to the ICA on IBM 3480 cartridges.

A-level data consist of data from four spectral channels: visible (0.55-0.90 microns), infrared (10.5-11.5 and 11.5-12.5 microns) and water vapor (6.5-7.0 microns). The infrared channel data and visible channel data are transmitted quantized in 256 levels (8 bits) and 64 levels (6 bits), respectively. The data sets have a spatial resolution at the sub-satellite point of 1.25 km for the VIS data and 5 km for the IR data.

The infrared data undergo in-flight calibration to enable the conversion of sensor output voltage to count values, and the conversion of the sensor output voltages to physical quantities (radiance, temperature). The former uses a lamp signal inserted in the infrared channel signal path. The latter utilizes two targets (space and the calibration shutter) for the IR sensor.

A normalization procedure is performed on the visible data to reduce "stripes" which occur due to differences between the sensitivities of the different detectors. In this procedure, a reference channel, to which the other channels are normalized, is chosen. The reference channel look-up table is fixed.

GMS takes VISSR images of the earth disk hourly each day. These images are processed at the Data Processing Center to reformat the original image into full earth disk.

For B1-data processing, the image window is defined by 1,100 pixels along 1,100 scan lines, centered at the sub-satellite point. One infrared pixel out of every three pixels along a scan line is extracted from every other scan line (3 x 2) of the original image data. The corresponding 24 visible pixels (6 pixels by 4 scan lines) covering the same location of the infrared pixel elements are averaged and converted into 8-bit data. One line of sampled infrared data followed by one line of averaged visible data are repeatedly stored on magnetic tape as B1-data. The B1 data set has a 10 km resolution.

The B1 data sets, archived on IBM 3480 cartridges consist of data for every three hours, nominally 0000, 0300, 0600, 0900, 1200, 1500 or 1600, 1800, and 2100 UTC. Each cartridge contains approximately four days of data.