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Longer constraint length codes are more practically decoded with any of several '''sequential''' decoding algorithms, of which the [[Shannon-Fano_coding|Fano]] algorithm is the best known. Unlike Viterbi decoding, sequential decoding is not maximum likelihood but its complexity increases only slightly with constraint length, allowing the use of strong, long-constraint-length codes. Such codes were used in the [[Pioneer program]] of the early 1970s to Jupiter and Saturn, but gave way to shorter, Viterbi-decoded codes, usually concatenated with large [[Reed-Solomon error correction]] codes that steepen the overall bit-error-rate curve and produce extremely low residual undetected error rates.
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Simple Viterbi-decoded convolutional codes are now giving way to [[turbo code]]s, a new class of iterated short convolutional codes that closely approach the theoretical limits imposed by [[Shannon's theorem]] with much less decoding complexity than the Viterbi algorithm on the long convolutional codes that would be required for the same performance.
Turbo codes have not yet been concatenated with solid (low complexity) [[Reed-Solomon error correction]] codes. However, in the interest of planetary exploration this may someday be done.
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