use units of light-years for distance, ( or ) and years for time, ( or ), this would mean light year/year2, equal to about 9.5 meters/second2, while if we use units of light-seconds for distance, ( or ), and seconds for time, ( or ), this would mean light-second/second2, or 299 792 458 meters/second2). Variants of transformation formulas A more general derivation of the transformation formulas is given, when the corresponding Fermi–Walker tetrad is formulated from which the Fermi coordinates or Proper coordinates can be derived. Depending on the choice of origin of these coordinates, one can derive the metric, the time dilation between the time at the origin and at point , and the coordinate light speed (this variable speed of light does not contradict special relativity, because it is only an artifact of the accelerated coordinates employed, while in inertial coordinates it remains constant). Instead of Fermi coordinates, also Radar coordinates can be used, which are obtained by determining the distance using light signals (see section Notions of distance), by which metric, time dilation and speed of light do not depend on the coordinates anymore – in particular, the coordinate speed of light remains identical with the speed of light in inertial frames: The Rindler observers In the new chart () with and , it is natural to take the coframe field which has the dual frame field This defines a local Lorentz frame in the tangent space at each event (in the region covered by our Rindler chart, namely the Rindler wedge). The integral curves of the timelike unit vector field give a timelike congruence, consisting of the world lines of a family of observers called the Rindler observers. In the Rindler chart, these world lines appear as the vertical coordinate lines . Using the coordinate transformation above, we find that these correspond to hyperbolic arcs in the original Cartesian chart. As with any timelike congruence in any Lorentzian manifold, this congruence has a kinematic decomposition (see Raychaudhuri equation). In this case, the expansion and vorticity of the congruence of Rindler observers vanish. The vanishing of the expansion tensor implies that each of our observers maintains constant distance to his neighbors. The vanishing of the vorticity tensor implies that the world lines of our observers are not twisting about each other; this is a kind of local absence of "swirling". The acceleration vector of each observer is