In the plenitude of ambient radio frequency signals in GNSS-challenged environments, a new paradigm termed collaborative opportunistic navigation (COpNav) is proposed to overcome the limitations of GNSS-based navigation. Receivers extract positioning and timing information from ambient radio frequency signals of opportunity and share information to construct and continuously refine a global signal landscape. The background and the concept of COpNav are introduced in this paper. From the aspect of environment observability, several hot research points in COpNav are discussed. At last, the developments and applications of COpNav system are prospected.