Throughout five months of troubleshooting, Voyager’s ground team continued to receive signals indicating the spacecraft was still alive. But as of Saturday they had no information on specific details about Voyager 1’s status.
“It’s pretty much how we left it,” Spilker said. “We are still in the initial phases of analyzing all the channels and observing their trends. Some of the temperatures have dropped a little bit with this period of time that has passed, but we are seeing pretty much everything we expected. And that is always good news.”
Relocation code
Through their investigation, Voyager’s ground team discovered that a single chip responsible for storing a portion of the FDS memory had stopped working, likely due to a cosmic ray strike or a failure of older hardware. This affected some of the computer’s software code.
“That deleted a section of memory,” Spilker said. “What they have to do is relocate that code to a different portion of memory, and then make sure that anything that uses those codes, those subroutines, knows how to go to the new memory location, to access it and execute it.”
Only about 3 percent of FDS memory was corrupted by the faulty chip, so engineers had to transplant that code to another part of the memory bank. But no location is large enough to contain the entire code section, NASA said.
So the Voyager team divided the code into sections to store in different places on the FDS. This wasn’t just a copy and paste job. Engineers needed to modify some of the code to make sure everything worked together. “Any references to the location of that code elsewhere in FDS memory also needed to be updated,” NASA said in a statement.
NASA’s newest missions have hardware and software simulators on the ground, where engineers can test new procedures to make sure they don’t cause harm when sending commands to the real spacecraft. Due to its age, Voyager has no ground simulators and much of the mission’s original design documentation remains on paper and has not been digitized.
“Seeing the code was really just visual,” Spilker said. “So we had to check it three times. “Everyone was checking and making sure we had all the links together.”
This was just the first step in restoring full functionality to Voyager 1. “We were pretty sure it would work, but until it actually happened, we didn’t know 100 percent,” Spilker said.
“The reason we didn’t do everything in one pass is that there was a very limited amount of memory we could find quickly, so we prioritized one data mode (the engineering data mode) and relocated just the code to restore that way. said Jeff Mellstrom, a JPL engineer who leads the Voyager 1 “tiger team” charged with overcoming this problem.
“The next step, relocating the three remaining actively used science data modes, is essentially the same,” Mellstrom said in a written response to Ars. “The main difference is that the available memory limitation is now even stricter. We have ideas about where we could relocate the code, but we haven’t fully evaluated the options or made a decision yet. These are the first steps we will take this week.”