These particles, stored in a vacuum, react to the Earth's magnetic field.
I work for a company that deals with inertial navigation systems, specifically systems based on mechanical gyroscopes. The reason we use gyroscopes is because testing, running, and updating our tools for the last 30 years has shown us that we are inherently more precise than a magnetic measurement tool that measures the Earth's (local) magnetic field. Contrary to our tools, a magnetic measurement device is easily influenced by outside interference. Events like variations in the solar wind, such as solar flares, can easily interfere with the local magnetic field, which in turn changes your measurement of the field. Of course you can compensate for this with a lot of math, but even then those tools are still not as accurate as the tools we provide. I'd really like to know how they solved that problem, if they actually did.