As free inertial pedestrian navigation systems with foot mounted inertial mobile unit mature, it becomes feasible to conceive a sufficiently accurate derived solution for assessing other indoor navigation systems and assisting research activities. The design of this reference solution and the remaining challenges are at the heart of this paper. A new filter with a quaternion based state vector exploits signals (acceleration and magnetic field) and motions of opportunity for estimating the navigation solution. Quasi Static Field (QSF) updates, Magnetic Angular Rate Updates (MARU) and Angular Gradient Update (AGU) are frequently applied for mitigating the low-cost inertial sensors errors. Experimental tests performed using a post-processed GPS differential solution as reference show an average accuracy of 1.2 m over 500 m walking path: 0.5 %. Discussions about the data acquisition protocol for meeting a 1 meter horizontal accuracy within two standard deviations of the mean (95.45%) is conducted.