Section 5 - Template 42 : Grid point data - CCSDS recommended lossless compression
Templates for Section 5
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| Octets | Key | Type | Content |
| 12-15 | Reference value (R) (IEEE 32-bit floating-point value) | ||
| 16-17 | Binary scale factor (E) | ||
| 18-19 | Decimal scale factor (D) | ||
| 20 | Number of bits required to hold the resulting scaled and referenced data values (See Note 1) | ||
| 21 | Type of original field values (see Code Table 5.1) | ||
| 22 | CCSDS compression options mask (See Note 3) | ||
| 23 | Block size | ||
| 24-25 | Reference sample interval |
( 1) The intent of this template is to scale the grid point data to obtain the desired precision, if appropriate, and then subtract the reference value from the scaled field, as is done using data representation template 5.0. After this, the resulting grid point field can be treated as a greyscale image and encoded into the CCSDS recommended standard for lossless data compression code stream format. To unpack the data field, the CCSDS recommended standard for lossless data compression code stream is decoded back into an image, and the original field is obtained from the image data as described in regulation 92.9.4, Note 4
( 2) The Consultative Committee for Space Data Systems (CCSDS) recommended standard for lossless data compression is the standard used by space agencies for the compression of scientific data transmitted from satellites and other space instruments. This is a very fast predictive compression algorithm based on the extended-Rice algorithm. It uses Golomb-Rice codes for entropy coding. The sequence of prediction errors is divided into blocks. Each block is compressed using a two-pass algorithm. In the first pass, the best coding method for the whole block is determined. In the second pass, the output of the marker of the selected coding method is encoded as ancillary information along with prediction errors. The coding methods include:
- Golomb-Rice codes of a chosen rank
- Unary code for transformed pairs of prediction errors
- Fixed-length natural binary code if the block is found to be incompressible
- Signalling to the decoder empty block if all prediction errors are zeroes
( 3) Library flags governing data type, and storage and processing parameters. For further information, see Rosenhauer, Mathis. "Flags" libaec - Adaptive Entropy Coding library. German Climate Computing Centre (Deutsches Klimarechenzentrum, DKRZ), 12 May 2016. Web. 13 June 2016. Further details