# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 2 Early History and Definitions
For aeronautical mobile telemetry, it is relatively easy for data samples using a modest number of bits to achieve a higher signal-to-noise ratio than that achieved by FM/FM transmission followed by (RF) FM demodulation, (subcarrier) FM demodulation, and magnetic tape recording of the subcarrier FM demodulator output [10]. The \"digital\" nature of PCM allowed channel coding to be applied in a straightforward way. Because space telemetry adopted PCM in the early 1960s, telemetry with channel coding began in 1967. In contrast, it took over a decade for PCM/FM to replace FM/FM as the dominant multiplexing/transmission method in aeronautical mobile telemetry. This was due in part to the long life of weapons systems--systems designed in the FM/FM days were periodically tested over the next 20 or more years thus compelling ranges to maintain the ability to process FM/FM telemetry transmissions. Full-on adoption of PCM/FM also had to wait for advances in LSI to make PCM multiplexing smaller, faster, and cheaper than subcarrier FM multiplexing [8; 10]. This issue was particularly important to missile systems, where size, weight, and power are limited. In the end, the advantages of PCM over existing techniques used in the 1950s and 1960s compelled telemetry system designers to adapt it for new test programs. By 1974, PCM/FM was the dominant multiplexing/modulation method used in aeronautical mobile telemetry [2]. A fascinating personal account of the transition is documented in [11]. Aeronautical mobile telemetry in Germany [12] and France [13] followed similar, but slightly different paths. While both space and aeronautical mobile telemetry were poised to adopt channel coding at about the same time, channel coding was adopted much earlier in space telemetry. The reasons for the delayed adoption of aeronautical mobile telemetry are complex and nuanced, but at a high level may be attributed to trust, differences between the deep-space channel and the aeronautical mobile telemetry channel, and the role of encryption in aeronautical mobile telemetry. **Trust**. Trust refers to the reluctance on the part of the customer to trust channel coding. This lack of trust derives from multiple sources. First, is the way decoders make errors in challenging environments. Many decoders for block codes produce decoding \"failures\" rather than an incorrect codeword. This feature sometimes leads customers to conclude that the code is \"eliminating\" data. The same thing happens with check bits, usually produced by cyclic redundancy check (CRC) codes: the hardware that performs the check usually deletes the data bits corresponding to the failed check bits. In contrast, a convolutional decoder using maximum likelihood sequence detection (usually performed by the Viterbi algorithm) always produces a sequence at the output. Sometimes, the output sequence is the wrong sequence. Customers often view an incorrect sequence as \"worse\" than the noisy version of an uncoded transmission. Second, customers are remarkably risk-averse. The conservative nature of the aeronautical mobile telemetry community produces an environment where customers are reluctant to approve the use of \"new\" technology in the testing phase of program development. This is one of the reasons why uncoded PCM/FM has remained so common. Trust was an issue in the early days of space telemetry. The lack of trust was the reason the quick look-in property was imposed on the codes used in the _Pioneer_ program (see Section 3.1). Trust remained an issue in aeronautical mobile telemetry for much longer [14]. For example, the customer did not approve the use of a simple CRC-based error-checking system, called the \"frame thrower\", in the late 1990s (see Section 4.1). **Deep-space and aeronautical mobile telemetry channels. ** The deep-space channel and the aeronautical mobile telemetry channel have much in common. But the differences between the two have a bearing on the adoption of coding.