Data streams from the low frequency instrument on-board the PLANCK satellite: Statistical analysis and compression efficiency

The expected data rate produced by the Low Frequency Instrument (LFI) plann ed to fly on the ESA PLANCK mission in 2007, is over a factor 8 larger than the bandwidth allowed by the spacecraft transmission system to download th e LFI data. We discuss the application of lossless compression to PLANCK-LF I data streams in order to reduce the overall data flow. We perform both th eoretical analysis and experimental tests using realistically simulated dat a streams in order to fix the statistical properties of the signal and the maximal compression rate allowed by several lossless compression algorithms . We studied the influence of signal composition and of acquisition paramet ers on the compression rate C-r and develop a semiempirical formalism to ac count for it. The best performing compressor tested up to now is the arithm etic compression of order 1, designed for optimizing the compression of whi te noise like signals, which allows an overall compression rate (C-r) over bar = 2.65 +/-0.02. We find that such result is not improved by other lossl ess compressors, being the signal almost white noise dominated. Lossless co mpression algorithms alone will not solve the bandwidth problem but needs t o be combined with other techniques.