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There is a problem with Voyager attitude data telemetry.

While the spacecraft continues to return science data and otherwise operate as normal, the mission team is searching for the source of a system data issue.

The engineering team with NASA's Voyager 1 spacecraft is trying to solve a mystery: The interstellar explorer is operating normally, receiving and executing commands from Earth, along with gathering and returning science data. But readouts from the probe's attitude articulation and control system (AACS) don't reflect what's actually happening onboard.

The AACS controls the 45-year-old spacecraft's orientation. Among other tasks, it keeps Voyager 1's high-gain antenna pointed precisely at Earth, enabling it to send data home. All signs suggest the AACS is still working, but the telemetry data it's returning is invalid. For instance, the data may appear to be randomly generated, or does not reflect any possible state the AACS could be in.

Source JPL May 18, 2022

There are three different computers onboard Voyager.

The Flight Data Subsystem (FDS) is responsible for configuring and operating science instruments. The FDS collects engineering and science data and formats the data for downlink transmission. Source

The computer command subsystem (CCS) is used to decode commands and executes fault detection and correction routines.

The Attitude and Articulation Control Subsystem (AACS) controls the spacecraft orientation (its attitude). It keeps the high-gain antenna pointing toward the Earth, controls attitude changes, and points the scan platform.

Source

So the AACS seems to work well in keeping the antenna pointing to Earth. The FDS formats the data for downlink transmission.

D. Other Challenges In addition, the FT also has to work around a lack of a hardware test bed, the limited memory of on-board computers, and antiquated programming languages.

The Capability Demonstration Lab (CDL), the testbed used during the prime mission, could not be maintained and had to be abandoned at the start of VIM. The failures of the testbed were too often, even in pre-VIM, due to aging hardware and disappearing repair expertise. The project had to move to a new location in early VIM and the CDL did not survive the move. There are no simulators for the AACS or FDS and only the CCS has a simulator, i.e., HSSIM. As a result, any FSW changes other than something very simple have to be done in the CCS.

The CCS on each spacecraft has 4K memory in each processor, so 8K in total, but the majority of functions are in both processors for redundancy. To make the best use out of such limited memory spaces, different programmers used all kinds of tricks in maintaining and adding patches over the life of the mission, resulting in extremely unstructured code that is prone for mistakes. It is crucial to validate all the sequence products and any kind of FSW changes thoroughly in HSSIM.

So there is no testbed available for the FDS and only a simulator for the CCS.

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The AACS was writing data to a wrong memory address:

The AACS had started sending the telemetry data through an onboard computer known to have stopped working years ago, and the computer corrupted the information. [...] Engineers don’t yet know why the AACS started routing telemetry data to the incorrect computer, but it likely received a faulty command generated by another onboard computer. If that’s the case, it would indicate there is an issue somewhere else on the spacecraft. The team will continue to search for that underlying issue, but they don’t think it is a threat to the long-term health of Voyager 1.

source

While they figured out what happened, they never figured out the root cause. Instead, a software patch was created an uploaded to prevent the glitch from causing problems if it were to occur again:

In 2022, the onboard computer that orients the Voyager 1 spacecraft with Earth began to send back garbled status reports, despite otherwise continuing to operate normally. It took mission engineers months to pinpoint the issue. The attitude articulation and control system (AACS) was misdirecting commands, writing them into the computer memory instead of carrying them out. One of those missed commands wound up garbling the AACS status report before it could reach engineers on the ground.
The team determined the AACS had entered into an incorrect mode; however, they couldn’t determine the cause and thus aren’t sure if the issue could arise again. The software patch should prevent that.

source

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