NOAA Polar Orbiter Data User's Guide
This section describes two general types of data formats which can be obtained from SSB. The first format is the most commonly used and is called the packed data format. The packed format is the format in which the data are archived by SSB. It takes up the least amount of tape but is more difficult to use because of its' compressed nature. This format is described in Section 2.2.1. The second format is known as the "16-bit unpacked format" and is described in Section 2.2.2. It consists of the video (channel) data being "unpacked" into two 16-bit words in four bytes (32 bits), right justified. When the data is selected by channel, the format will automatically be the "16-bit unpacked" format.
The packed data format is the standard format in which the data are received from NESDIS and in which they are ultimately archived by SSB. It is the default if the user does not request a specific format. Because of the sheer volume of satellite data, the packed data format is preferred for storing large quantities of data. However, due to the method of packing, it is more difficult to write software to handle this data. Basically, the packed data format contains the data arranged with as few spaces or gaps between the data elements as possible. This means that it is usually not possible to directly read the data on word or halfword boundaries.
In the case of the AVHRR data, the video (channel) data are packed as three 10-bit samples in four bytes, right-justified. The first two bits of each four-byte group are zero. The channels are interleaved, so the samples have the following order: scan point 1 (Channels 1, 2, 3, 4, 5), scan point 2 (Channels 1, 2, 3, 4, 5), etc. The detailed packed data format can be found in Section 220.127.116.11 for GAC data and in Section 18.104.22.168 for LAC/HRPT data. Appendix B should be helpful in unpacking this data format. Figure 2.2.1-1 illustrates the packed data format for AVHRR data.
Since the TOVS data are generated by three distinct instruments, their packed data format is more complex and will not be discussed here. For detailed information on the TOVS packed data format, refer to Sections 22.214.171.124, 126.96.36.199, and 188.8.131.52 for the HIRS/2, SSU, and MSU data, respectively.
The 16-bit unpacked data format is an ideal format for the occasional satellite data user with a small area and period of interest. The video data are unpacked into two 16-bit words (INTEGER*2 words) in four bytes using SSB's software. The channel data are contained in the ten least significant bits and the six most significant bits are zero-filled. This unpacked data format requires more storage on magnetic tape but considerably less investment in software development. The specific formats for the 16-bit unpacked data format vary according to data type and are fully described in the "Full Data Set Copy" subsection for each data type in Sections 3 and 4. Figure 2.2.2-1 illustrates the 16-bit unpacked data format for AVHRR data.
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