A star tracker is one of its reference points (of absolute orientation). There's also a sun sensor with a similar function. I don't know if Voyagers have others.

- "In flight, the device found and locked on to the star Canopus, providing a reference for guidance and navigation."

https://airandspace.si.edu/collection-objects/star-tracker-a... ("Star Tracker and Hood Assembly Support Equipment, Voyager Spacecraft")

https://airandspace.si.edu/collection-objects/sun-sensor-and... ("Sun Sensor and Hood Assembly Support Equipment, Voyager Spacecraft")

edit: In more detail,

https://space.stackexchange.com/questions/43803/how-did-the-...