Is the magnetic unreliability charted? Where could I find that?
The Jeppesen Atlantic Polar High Altitude en route Chart AT(HI)5 defines the
airspace.
Isn't there a limitation on how far north (or south) you can use some GPS units?
From our FOM: GPS updating may be available up to a latitude of 88º N. SATCOM may be available up to a latitude of 84º N.