# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 2 Early History and Definitions  
What is today known as aeronautical mobile telemetry originated during World War II (1939-1945) with the introduction of new fixed-wing aircraft and, at the end of the war, ballistic missiles [1; 2]. Before deploying new aircraft or missiles, they had to be tested. Testing an experimental aircraft or missile involved instrumenting it with sensors to obtain in-flight measurements of key parameters. The measurements were radioed to the ground during the test flight. On the ground, flight test engineers monitored the received measurements. The main purpose of the test flight was to produce the measurements. Instrumented aircraft and missiles became known as _test articles_. Over time, test articles were expanded to include helicopters, guided missiles, and even artillery shells. The measurements (the _telemetry_) transmitted to the ground (_telemetered_) included parameters such as structural stresses and strains, force, pressure, and temperature [2]. The sensors for each of these produced continuous-time outputs. One of the first issues in aeronautical mobile telemetry was the multiplexing problem. Instead of applying each sensor output to its own RF carrier, it quickly became obvious that tremendous savings in size, weight, and power could be achieved by using the sensor outputs to modulate _subcarriers_, in the 1-100 kHz range. The sum of the modulated subcarriers modulated the RF carrier. Because there were multiple options for doing this, the following notation was adopted to describe the option [3]:  
_[multiplexing method]/[RF modulation]_  
By the early 1950s, FM/FM was firmly established as the most common method for multiplexing continuous-time measurements and transmitting the multiplexed signal to the ground. Over time, test articles became more sophisticated. More sophisticated test articles required more measurements to test them. Throughout the 1950s, it became increasingly common for the number of measurements to overwhelm the capacity of the FM/FM system. To address this problem, discrete-time multiplexing methods started to be used [3]. By 1960, analog-to-digital converters that quantized both the time axis (sampling) and the amplitude axis using an \(N\)-bit binary code (quantization) were viable for multiplexing sampled and quantized sensor outputs. When each of the \(N\) bits in the binary code is represented by a pulse, the resulting representation of the sample is called pulse code modulation (PCM). In fact, the first edition of the IRIG 106 standards dedicated exclusively to telemetry (published in 1960) contained a PCM standard [4] (see also [5]). An example of a PCM system is illustrated in Figure 1a. A PCM system samples each continuous-time sensor output and represents the sample as an \(N\)-bit word. The words from different sensors are time-multiplexed on a word-by-word basis. Each bit in the \(N\)-bit word is represented in the waveform domain by a pulse. The two most common pulses used in telemetry are the non-return-to-zero (NRZ) pulse and the bi-phase (bi-\(\phi\)) pulse. These pulses are illustrated in Figure 1b. In space telemetry, the NRZ pulse tended to be used when the telemetry data modulated a low-frequency subcarrier; the bi-\(\phi\) pulse tended to be used when the telemetry data directly modulated the RF carrier. Initially, the same was true in aeronautical mobile telemetry, but the additional bandwidth required to accommodate the bi-\(\phi\) pulse rendered its use increasingly rare as spectral congestion placed a premium on bandwidth. In aeronautical mobile telemetry, the time-multiplexed PCM signal replaced the sub-carrier FM signals to produce PCM/FM. A block diagram of a typical PCM/FM system is shown in Figure 2a. The multiplexing step is notional, if the A/D converter was fast enough, a single A/D converter was used with a commutator that switched the various continuous-time signals to the A/D converter input in a round-robin fashion. Over time, some of the inputs became \"digital\" in nature and did not require an A/D converter.