# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 5 Summary and Conclusions  
The journey through the history of channel coding in deep-space telemetry began with codes for which practical soft-decision decoding algorithms were known: the (32,6) Reed-Muller code for _Mariner_ 6 and the \((25,\
icefrac{{1}}{{2}})\) convolutional code for _Pioneer_ 9. The decoder for the _Pioneer_ 9 convolutional code was a sequential decoder based on the Fano algorithm. Subsequent _Pioneer_ missions used longer convolutional codes designed for decoding using the Fano algorithm to increase coding gain. The invention of the Viterbi algorithm made possible soft-decision maximum likelihood sequence detection of convolutional codes with modest constraint lengths. The Viterbi algorithm with the shorter constraint length convolutional codes achieved higher coding gains than the _Mariner_ convolutional codes with sequential decoding. The \((7,\
icefrac{{1}}{{2}})\) convolutional code was used for the _Voyager_ and _Viking_ programs. The _Voyager_ program introduced concatenated coding as a way to further improve coding gain without the exponential increase in decoder complexity. Both the \((7,\
icefrac{{1}}{{2}})\) convolutional code and its concatenation with the (255,233) Reed-Solomon code became international standards for deep-space telemetry. Creative use of channel coding coupled with an early form of iterative decoding allowed the _Galileo_ mission to achieve 70% of its mission goals despite a series of near-devastating catastrophes. The introduction of turbo codes in the 1990s spawned a new era in channel coding for deep-space telemetry. The turbo code concept was capable of producing low-rate codes with manageable decoding complexity. These codes were essential to deep-space missions to the Kuiper belt and higher data rate missions closer to home. In aeronautical mobile telemetry, the adoption of channel codes took longer. In the beginning, telemetry links used analog modulations on the RF links; digital modulation, a prerequisite for the application of channel coding, was not fully adopted until 15 years after space telemetry had adopted it. Even after the transition to fully digital transmission formats, channel coding was not essential for closing the link in most aeronautical mobile telemetry applications. For test points that produced low-quality data, the test point is usually re-flown, a sort of high-level automatic repeat request (ARQ) system with humans in the loop. Channel coding held the promise of reducing the number of test points that had to be re-flown, but in spectrally congested areas, the bandwidth expansion that accompanies the use of channel coding was seen as an unacceptable cost. There were edge cases where channel coding was needed, and these edge cases kept channel coding \"in the discussion\" until good codes compelled their adoption. By the time channel codes received serious consideration, the reduction in available spectrum coupled with an increase in required data rates pushed spectral efficiency to the top priority. Non-binary modulations were adopted and high-rate codes were sought. Because high-rate block codes tend to perform better than high-rate convolutional codes, a family of high-rate LDPC codes was adopted by the aeronautical mobile telemetry community (this was the same family of LDPC codes adopted by the deep-space telemetry community a few years earlier). The channel coding history illustrates the ingenuity and creativity of dedicated engineers and scientists. The early motivation for coding was based solely on mathematical analysis, not on experience. The fact that channel coding became such an enabler for deep-space telemetry is a testament to the correctness of the mathematical models and the collaborations required to achieve success in extraordinarily complex undertakings. In many ways, deep space was the perfect setting for channel coding to evolve from a mathematical curiosity to an essential component with supporting hardware. To all who worked so hard to apply channel coding to telemetry, the author gives a hearty \"well done\"!