Without the magnetometer, the only heading reference is from the GPS and this heading reference can only be used on a platform that has some constant lateral motion. This is the only way GPS can get a good heading. Once a heading reference is obtained (either magnetometer or GPS heading) it can be maintained for a short time (less than 30 seconds typically) with just the gyroscopes. This, of course, would have to be done through a fusion filter external to the -35. The LORD MicroStrain 3DM-GX3-45 product actually has this functionality built-in.