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m Fixed typo: the error correction capability of the hadamard code is 7 bits per block rather than per word (which has only 6 bits). |
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An augmented Hadamard code was used during the 1971 [[Mariner 9]] mission to correct for picture transmission errors. The data words used during this mission were 6 bits long, which represented 64 [[grayscale]] values.
Because of limitations of the quality of the alignment of the transmitter at the time (due to Doppler Tracking Loop issues) the maximum useful data length was about 30 bits (the encoded block). Instead of using a [[repetition code]], a [32, 6, 16] Hadamard code was used.
Errors of up to 7 bits per
The circuitry used was called the "Green Machine". It employed the [[fast Fourier transform]] which can increase the decoding speed by a factor of three. Since the 1990s use of this code by space programs has more or less ceased, and the [[NASA Deep Space Network]] does not support this error correction scheme for its dishes that are greater than 26 m.
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