Discrete Nonlinear Observers for Inertial Navigation
  We derive an exact deterministic nonlinear observer to compute the continuous state of an inertial navigation system based on partial discrete measurements, the so-called strapdown problem. Nonlinear contraction is used as the main analysis tool, and the hierarchical structure of the system physics is sytematically exploited. The paper also discusses the use of nonlinear measurements, such as distances to time-varying reference points. 
Yong Zhao, Jean-Jacques E. Slotine