The notion of wind Finslerian structure
Σ
\Sigma
is developed; this is a generalization of Finsler metrics (and Kropina ones) where the indicatrices at the tangent spaces may not contain the zero vector. In the particular case that these indicatrices are ellipsoids, called here wind Riemannian structures, they admit a double interpretation which provides: (a) a model for classical Zermelo’s navigation problem even when the trajectories of the moving objects (planes, ships) are influenced by strong winds or streams, and (b) a natural description of the causal structure of relativistic spacetimes endowed with a non-vanishing Killing vector field
K
K
(SSTK splittings), in terms of Finslerian elements. These elements can be regarded as conformally invariant Killing initial data on a partial Cauchy hypersurface. The links between both interpretations as well as the possibility to improve the results on one of them using the other viewpoint are stressed.
The wind Finslerian structure
Σ
\Sigma
is described in terms of two (conic, pseudo) Finsler metrics,
F
F
and
F
l
F_l
, the former with a convex indicatrix and the latter with a concave one. Notions such as balls and geodesics are extended to
Σ
\Sigma
. Among the applications, we obtain the solution of Zermelo’s navigation with arbitrary time-independent wind, metric-type properties for
Σ
\Sigma
(distance-type arrival function, completeness, existence of minimizing, maximizing or closed geodesics), as well as description of spacetime elements (Cauchy developments, black hole horizons) in terms of Finslerian elements in Killing initial data. A general Fermat’s principle of independent interest for arbitrary spacetimes, as well as its applications to SSTK\xspace spacetimes and Zermelo’s navigation, are also provided.