  Cartesian fibrations were originally defined by Lurie in the context of
quasi-categories and are commonly used in $(\infty,1)$-category theory to study
presheaves valued in $(\infty,1)$-categories. In this work we define and study
fibrations modeling presheaves valued in simplicial spaces and their
localizations. This includes defining a model structure for these fibrations
and giving effective tools to recognize its fibrations and weak equivalences.
This in particular gives us a new method to construct Cartesian fibrations via
complete Segal spaces. In addition to that, it allows us to define and study
fibrations modeling presheaves of Segal spaces.
