Error-Correction Codes Achievements
To control for errors in the reception of the grayscale image data send by Mariner 9 (caused by low signal-to-noise ratio), the data had to be encoded before transmission using a so-called error-correcting code (ECC). Without ECC, noise would have made up roughly a quarter of an received image, while the ECC encoded the data in a redundant way which allowed to reconstruct the sent image data at the reception.
As the flown hardware was rather constrained with regards to weight, power consumption, storage and computing power, some considerations had to be put into choosing an error-correcting code and it was decided to use an hadamard code for Mariner 9. The data words used during this mission were 6 bits long, which represented 64 grayscale values. Because of limitations of the transmitter the maximum useful data length was about 30 bits. Instead of using a repetition code, a Hadamard code was used. Errors of up to 7 bits per word could be corrected using this scheme. Compared to a 5-repetition code, the error correcting properties of this Hadamard code are much better, yet its rate is comparable. The efficient decoding algorithm was an important factor in the decision to use this code. The circuitry used was called the "Green Machine". It employed the fast Fourier transform which can increase the decoding speed by a factor of 3.
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