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低功率小卫星的有限数据率通常要求图像在发送之前经过了数据压缩。目前研究出了几种数据压缩技术,并对其做了改进。这些方法包括:矢量量化(VQ)、Lempel—Ziv、分数维编码和诸如JPEG及MPEG的离散余弦变换(DCT)方法。 JPEG(联合照相专家组)是一个静止图像压缩系统。MPEG(活动图像专家组)是一个压缩和通信草案,它为传输几种数据类型定义了一种体系,这些类型包括音频、用户数据和全活动压缩视频。MPEG允许数据率低至1.2Mbit/s的NTSC全活动视频传输,以及数据率低至56kbit/s的会议级视频传输。然而,由于MPEG标准把JPEG作为一部分,所以MPEG也允许压缩过的静止图像的传输。 由于JPEG和MPEG压缩都是有损的(取决于图像内容和所需的压缩比),因此需要给压缩图像一些校正量。通过给出这些校正量,就可能对灵敏传感器图像做到无损(没有误差)可变速率压缩。在该系统中用户能够快速扫描多幅压缩图像,然后请求卫星向那些认为需要更高精确度的图像发出像元校正数据。 MPEG体系有望成为图像传输的国际标准,所以以MPEG为基础的小卫星图像的传输将使低成本地面终端的迅速扩散。同样,通过向有价值的图像发出像元校正量,以MPEG为基础的小卫星成像系统能够以合理的费用获得精确、灵活的传感器图像。
The limited data rate of low-power small satellites typically requires that the image undergo data compression prior to transmission. At present, several data compression techniques have been developed and improved. These include vector quantization (VQ), Lempel-Ziv, fractal coding, and discrete cosine transform (DCT) methods such as JPEG and MPEG. JPEG (Joint Photographic Experts Group) is a still image compression system. MPEG (Moving Picture Experts Group) is a compression and communication protocol that defines a system for transmitting several types of data, including audio, user data and fully active compressed video. MPEG allows NTSC full-motion video transmission at data rates as low as 1.2 Mbit / s and conferencing-class video at data rates as low as 56 kbit / s. However, MPEG also allows the transmission of a compressed still image because of the JPEG standard as a part of it. Since both JPEG and MPEG compression are lossy (depending on the image content and the compression ratio required), some amount of correction to the compressed image is required. By giving these corrections, it is possible to achieve lossless (no error) variable rate compression of sensitive sensor images. In this system, the user can quickly scan multiple compressed images and then ask the satellite to send pixel correction data to those images that are deemed to require more accuracy. The MPEG system is expected to become an international standard for image transmission, so the transmission of small satellite images based on MPEG will allow the rapid spread of low cost terrestrial terminals. Likewise, the MPEG-based small satellite imaging system can obtain accurate and flexible sensor images at a reasonable cost by sending pixel corrections to valuable images.