# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 3 Deep-Space Telemetry
### Galileo
The coded bits could modulate either the low-rate subcarrier or the high-rate subcarrier, depending on the selected data rate. The TMU outputs (PCM/PSK-modulated subcarriers) could be applied to phase modulators at S-band (2300 MHz) or X-band (8400 MHz). The telemetry could be downlinked at the S-band alone, at the X-band alone, or both simultaneously. S-band transmission was via a low-gain antenna or a high-gain antenna. X-band transmission was possible only with the high-gain antenna. The high-gain antenna was a 4.8-m parabolic reflector [61]. The reflector closed like an umbrella for storage, transport, and launch. The plan was to unfurl the antenna during the Earth-to-Jupiter segment of the voyage. The highest telemetry data rate (134.4 kbits/s) was only possible at the X-band with the high-gain antenna and the convolutional code. The figure also shows the relay receiving hardware (RRH). This system was used to receive transmissions from the atmospheric probe. The probe transmitted two 128 bits/s streams in parallel at different frequencies and on different polarizations. The probe telemetry data were encoded using the same \((7,1/2)\) convolutional code used by the TMU. The coded bit streams directly modulated L-band carriers at 1387.0 MHz (left-hand circular polarization) and 1387.1 MHz (right-hand circular polarization). The received signal was separated, synchronized, and matched filtered by the RRH. The soft-matched filter outputs (quantized to 3 bits) were stored for later transmission to Earth. Figure 13: A block diagram of the originally planned Galileo orbiter telemetry system. Simplified from [61]. When the October 1989 launch date was announced, the impact on the mission was evaluated. The consequences of the delayed launch data and increased transmit time were profound. The December 1995 rendezvous occurred during the maximum Earth-Jupiter range and a southerly declination of \(-23^{\circ}\) rather than the minimum Earth-Jupiter range with a northerly declination of \(+18^{\circ}\) as originally planned. The maximum Earth-Jupiter range minimized the signal power at the receiving stations. The southerly declination meant most advantageous ground station sites were in the Southern Hemisphere, where there were fewer available ground stations. The output power of the Galileo power source, a radioisotope thermoelectric generator (RTG), decreased with time. By December 1997, the RTG was not capable of producing enough power to run the X-band transmitters at full power. Consequently, the X-band transmitters had to operate in low-power mode. The combined effect was to produce received power levels well below the initially planned power levels. Without changes, the only way to close the link was to reduce the transmitted bit rate to such low rates that the viability of the science missions was threatened. In the two years leading up to the October 1989 launch, major upgrades to the capabilities of the ground stations and orbiters were pursued. The ground station improvements included upgrades to tracking systems, the development and installation of new RF components with lower noise figures, and a provision for arraying multiple ground stations [59]. For the orbiter upgrade, a new, more powerful, convolutional code was adopted, namely, the \((15,1/4)\) code defined by the following polynomials:  
\[ \begin{split} G^{(1)}(D)&=1+D^{3}+D^{4}+D^{7}+D^{8} +D^{10}+D^{14}\\ G^{(2)}(D)&=1+D^{2}+D^{5}+D^{7}+D^{9}+D^{10}+D^{11} +D^{14}\\ G^{(3)}(D)&=1+D+D^{4}+D^{5}+D^{5}+D^{7}+D^{9}+D^{10 }+D^{12}+D^{13}+D^{14}\\ G^{(4)}(D)&=1+D+D^{2}+D^{6}+D^{8}+D^{10}+D^{11}+D^{1 2}+D^{14}.\end{split}  \tag{10}\]  
The design procedure that led to this choice is described in [62]. The hardware for the \((15,1/4)\) convolutional encoder was installed in TMU-B. The encoder was designed for use with high-rate telemetry output at 115.2 or 134.4 kbits/s. The resulting orbiter configuration is shown in Figure 14. In the figure, only TMU-B is shown.