With the Voyager 1 attitude correction all over the news (related: How do Voyager 1's Trajectory Control Thrusters differ from its Attitude Control Thrusters?), I wonder with what instrument such a spacecraft can measure its own attitude towards earth in the first place.
How do we know exactly what its attitude is so that the thrusters can accurately be fired?
Voyager 1 and Voyager 2 use the same Attitude and Articulation Control Subsystem (AACS) to maintain the crafts orientation; facing the antenna towards the Earth.
Voyager's AACS system uses a sun sensor for yaw and pitch reference, and a star tracker trained continuously on a bright star at right angles to the sun point for a roll reference.
Links:
https://en.wikipedia.org/wiki/Voyager_1#Spacecraft_components
https://en.wikipedia.org/wiki/Voyager_program#Computers_and_data_processing
