Let's consider flat 4D Minkowski spacetime. Is it possible to define (local) coordinate system for any observer (its natural reference frame) so that these coordinates posses interpretation of time and spatial position measured by this observer??? It can be done in two dimensional Minkowski spacetime. One coordinate is time measured by the observer - the length of its world line, the second is the spacetime distance from the world line measured along the line of constant time (for the comoving inertial the observer). I proved that there always exist an open set containing observer world line in which these coordinates have sense. An good example of such coordinates are Rindler coordinates for uniformly accelerated observer. I wonder if it's possible to generalize my construction to four dimensional spacetime.