# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 3 Deep-Space Telemetry
### Pioneer  
The _Pioneer_ program comprised a series of unmanned spacecraft that monitored interplanetary phenomenon well outside the influence of the Earth. The missions designated _Pioneer_ 6, 7, 8, and 9, launched 1965-1968, monitored interplanetary space weather in the inner solar system. _Pioneer_ 10, launched in 1972, performed a Jupiter fly-by before heading into interstellar space. _Pioneer_ 11, launched in 1973, performed a Saturn fly-by before heading into interstellar space. The earliest _Pioneer_ missions did not use any form of channel coding. The _Pioneer_ 6 and 7 telemetry links used 6-bit data words plus a single parity bit to identify errors in the data words [17]. Erroneously received data words were discarded. Based on the studies described in [18], a systematic (\(25,1/2\)) convolutional encoder was installed on _Pioneer_ 9 on an experimental basis [19]. The _Pioneer_ 9 convolutional code was the first code to \"fly in space\" [20]. The convolutional code experiment was so successful that convolutional coding was planned for future _Pioneer_ missions [19]. A more powerful (\(32,1/2\)) convolutional code was used for _Pioneer_ 10 and 11 [20,21]. Figure 4: An encrypted aeronautical mobile telemetry system: (a) the encryption follows encoding; (b) encryption precedes encoding. A simplified block diagram of the digital telemetry unit and transmitter for _Pioneer_ 9, 10, and 11 is shown in Figure 5. There were three broad source categories: digital data, \"bi-level\" (two-state) data, and analog data. The analog data were sampled using a 6-bit A/D converter. The bits representing the quantized samples were combined with the bits from the other sources. Four different data formats and eight different bit rates (16 through 2048 bits/s in powers of 2) were supported depending on the phase of the mission [21]. The combined data formed the input to the convolutional encoder (optional for _Pioneer_ 9). The convolutional code output modulated a subcarrier to produce a PCM/PSK signal. The PCM/PSK signal phase-modulated an S-band carrier at either 2292.03037 MHz or 2292.407407 MHz. The convolutional codes used for _Pioneer_ 9, 10, and 11 had long constraint lengths to achieve the desired coding gains. Given the technology constraints of the late 1960s the only practical decoder for these codes was the sequential decoding algorithm [22]. Consequently, the convolutional codes used in the _Pioneer_ program were designed for sequential decoding. An additional constraint imposed on code design was the \"quick-look-in\" property [23]: the information bits appear explicitly in the encoded output. At the decoder input, the quick-look-in property makes it possible to obtain an estimate of the information bits prior to decoding. The justification for this property seems to be a mistrust of coding: a fear that coding might lose data that would otherwise not be lost [23]. Systematic convolutional codes possess the quick-look-in property by definition. For this reason, the first sequential-decoding-friendly convolutional codes considered for _Pioneer_ 9 were systematic. The encoder for the _Pioneer_ 9 convolutional encoders is shown in Figure 6a. The design criteria for this code are described in Section 3.3. For each input \(b_{n}\), the encoder produced two outputs defined by the polynomials  
\[ \begin{split} G^{(1)}(D)&=1\\ G^{(2)}(D)&=1+D+D^{3}+D^{5}+D^{7}+D^{8}+D^{11}+D^{13 }\\ &\quad+D^{14}+D^{15}+D^{19}+D^{20}+D^{21}+D^{23}+D^{24}\end{split}  \tag{1}\]  
The two coded bits are multiplexed to produce an output clocked at twice the rate as the input. It is well-known that for a fixed constraint length, non-systematic convolutional codes are more powerful than systematic convolutional codes [20]. Consequently, the search for more powerful codes for _Pioneer_ 10 and 11 focused on non-systematic does that possessed the quick-look-in property and that was well suited to sequential decoding. The \((32,1/2)\) convolutional code from the class of codes developed in [23] was selected for _Pioneer_ 10 and 11.