So you mean the entanglement source and the detectors are in same inertial reference frame, in other words, they are at rest relative to each other? Then say so. Next step: is this a preferred frame of reference? A preferred frame of reference in this context would mean that only in this frame correct conclusions about what physically is happening can be drawn. Next step: according to SR there are no preferred inertial frames of reference. Still, all observers, whatever their spee