2020 Volume 63 Issue 5 Pages 212-221
Data transmission rate is one of the biggest limiting factors in space exploration because transmitting images is largely restricted by the bitrate. This issue can be crucial for JAXA's future sample-return mission, such as the Martian Moons eXploration (MMX) mission. High-resolution images are important in selecting the most scientifically interesting and safest landing/sampling sites, although the strategy of rapidly sending each image (> 114 MB) with a limited bitrate (< 32 Kbps) has to be considered. Lossy compression can significantly reduce the file size; however, the highly compressed images can be scientifically worthless. Therefore, determining the best compression ratio is necessary. Here, we develop a method to analyze the influence of image compression using image quality indices, Structural SIMilarity (SSIM) and image entropy. Moreover, by measuring the Cumulative Size Frequency Distribution (CSFD) of regolith grains in images with different amounts of compression, the loss of scientific value of images can be measured. We then carefully confirm that image quality indices can truly determine the best compression ratio. Using this method, the spacecraft can autonomously find the best compression ratio, compress an image and send it to the ground in a few minutes using a limited transmission rate.
