An inertial device for determining an initial condition reference frame for a body is provided. The inertial device includes at least one inertial measurement unit, one or more processing devices, and one or more memory devices. The inertial measurement unit provides rotation information along at least one axis to the one or more processing devices. The one or more processing devices use the rotation information to estimate the current attitude of the body, based on estimates of a total rotation angle experienced by the body from the beginning of body motion to when the inertial measurement unit initialized.