[zamalek]

Most likely relative to the gyro's axes. Unless they use an accelerometer to calibrate to Earth's gravity well.

[ISL]

When you're in freefall, an accelerometer tells you nothing :).

http://en.wikipedia.org/wiki/Equivalence_principle

[mikeash]

Two accelerometers could tell you something! They probably aren't sensitive enough to get the job done at that scale, though.

[ISL]

True! Modern gravity gradiometers/differential accelerometers probably are sensitive enough for coarse orientation, particularly if they were deployed at the perimeter of the sail.

To do so would, of course, obliterate LightSail's budget :)

Source: worked on R&D for the LISA inertial reference system.