NOTES
Schulers can be set off by any change in the system's knowledge of the direction of gravity (its alignment) - gravity map errors, or equipment noise. Accumulation of these random errors will cause oscillations that tend to grow with time - not good. There needs to be a way to correct these errors. Position fixes won't work, unless you got them about every hour or two. A continuous velocity reference would help, but if you had that, why do you need an inertial navigator? Submarines use an electromagnetic log as a reference; it is an imperfect reference but it does have the advantage of not having oscillating errors, so it can be used as a reference to damp out Schuler errors.