# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 3 Deep-Space Telemetry
### Galileo  
NASA's _Galileo_ mission comprised a sophisticated orbiter and atmospheric entry probe designed to explore Jupiter. The orbiter and probe were launched as a combined unit. The probe separated from the orbiter five months before arriving at Jupiter and was timed to plunge into the Jovian atmosphere just as the orbiter made its initial (and closest) pass by Jupiter. The probe transmitted its telemetry to the orbiter during the orbiter's initial pass. After this, the orbiter maneuvered into orbit around Jupiter to explore Jupiter and its moons. This ambitious program suffered an extraordinary number of setbacks that tested the patience of the team [59]. Because these setbacks form the context for several developments in channel coding, a brief summary of the original plans, the setbacks, and the consequences of those setbacks is described first. Initial planning in the 1970s called for a direct launch from Earth to leverage an advantageous Earth-Jupiter alignment. The direct launch was to be on a Centaur multi-stage rocket. The lower stages powered the upper stage into Earth's orbit. The upper stage, equipped with its own rocket engine, carried the _Galileo_ spacecraft as its payload. In 1975, NASA enacted a policy requiring all planetary missions to use the then-future Space Shuttle (the first launch was in 1981) for placing planetary exploration vehicles into Earth orbit. The Space Shuttle launch date for _Galileo_ was scheduled for January 1982. The Space Shuttle cargo bay was to carry the Centaur upper stage containing the _Galileo_ Spacecraft. The high-energy, liquid hydrogen-fueled Centaur upper stage was powerful enough to transition _Galileo_ from Earth orbit to a direct trajectory to Jupiter; a voyage of 27 months. The launch date was delayed three times to May 1986 for causes not involving the _Galileo_ spacecraft. Each change necessitated a complete redesign of the mission (i.e., the sequence of experiments and the schedule for ground station support). The Space Shuttle _Challenger_ disaster in January 1986 delayed, then canceled, the 1986 launch date. Following the _Challenger_ incident, NASA adopted a series of increased safety measures for the Space Shuttle program, one of which was the cancellation of the Centaur upper stage as a Space Shuttle payload. The Centaur upper stage was replaced by a low-risk, low-energy propulsion system called the inertial upper stage (IUS). Because the IUS did not have enough energy for a direct trajectory to Jupiter, _[PERSON]_'s trip to Jupiter was re-planned, for a fourth time, using a convoluted series of gravity assists, once from Venus and twice from Earth, which increased the journey to Jupiter from 27 months to six years. The new mission was launched in October 1989 and scheduled to reach Jupiter in December 1995. (A description of the IUS-to-Shuttle/orbiter telemetry link is given in [60]). The originally planned telemetry system is summarized by the block diagram in Figure 13. The command data subsystem (CDS) is stored and formatted for transmission engineering and science measurements. The CDS produced two telemetry channels: a low-rate channel at 40 bits/s and a high-rate channel with an adjustable rate from 10 bits/s to 134.4 kbits/s. The use of the 40 bit/s CDS output was for safe-mode operation (when the onboard fault-protection software detects a problem that requires unplanned intervention via the command uplink). Within the CDS, image data were encoded with a (255,241) Reed-Solomon code and block interleaved with an interleaving depth of 4. The encoded image data formed part of the high-rate channel output from the CDS. The two CDS outputs formed the inputs to the telemetry modulation unit (TMU). There were two TMUs, designated TMU-A and TMU-B. Initially, the two were identical. The low-rate channel data modulated the low-rate subcarrier. The high-rate channel data were encoded, using the \((7,1/2)\) convolutional code developed for the _Voyager_ program.