In this paper, we describe the image data compressor designed and built for the SPIRIT III radiometer which will be used to provide a real-time "quick-look" capability for the instrument. A a brief description of elements of the radiometer design which determine the data compression requirements is given, followed by a description of the VQ-based hybrid data compression algorithm. Highlights of the hardware design are also discussed.

The performance of the compressor is presented. Tests made using data obtained during cold tests of the instrument indicate an RMS error of 6.26 counts for the 10:1 compression mode and 5.83 counts for the 4:1 mode out of a maximum of 4096 counts. The impact of focal plane nonuniformities on compression performance is also discussed.

The compressor is unique in that the VQ codebooks and Huffman codewords can be changed using an uplink capability. This capability provides the opportunity of improving the performance of the compressor over the lifetime of the spacecraft.