Evaluation of brightness resolution of WEFAX images transmitted through GMS

The Japanese meteorological satellite GMS-4 takes visible or infrared images of the Earth´s surface and transmits to a vast area (including the oceans) the cloud images (called WEFAX: weather facsimile). A ship steaming over the oceans can avail itself of the transmitted images for its navigation. In designing the onboard reception system. It is necessary to analyze the relation between the quality of the image and that of the reception systems. The authors evaluate the quality of the images by observing the amplitude variation of the pixels that constitute the gray scale. It is proved that the 64-leveled gradation can be ensured when the C/N is larger than 10 dB. Then, the system transmits the original images properly so far as the quality of the gray scale is concerned. Next, the authors evaluate the brightness resolution of the infrared cloud image. The distribution of the number of pixels of each amplitude of the signal ranging from 80 to 900 (arbitrary units) should have 64 discrete peaks, if the image is transmitted properly with the 64-leveled gradation. The distribution has discrete peaks in the low brightness region corresponding to the sea surface where the temperature varies gently along the scanning line. On the other hand, it is rather continuous in the higher brightness region corresponding to the summits of clouds where the temperature varies roughly. The frequency of the video signal may exceed the transmitter´s specification there