of the antenna in the whole frequency domain. The detector response is obtained by analyzing the motion of the the emitter, detector and laser light in the conformal gauge and synchronous gauge. Working in the synchronous gauge in which the free motion of test particles (thus e.g. emitters, beam–splitters of a freely falling gravitational wave detectors) can be easily computed is particularly convenient for space–based interferometers where the high frequency domain of the detector response usually plays an important role. Furthermore we show the equivalence of the two gauges by giving the explicit gauge transformation for the massive scalar wave solutions. This result is also generalized to theories in which a scalar field can have modified dispersion relations.

The paper is organized as follows. In Sec. \[s:1\] we recount the massive Branse–Dicke theory. In Sec. \[s:2\] we investigate the detector response in the conformal gauge and in Sec. \[s:3\] in the synchronous gauge. In Sec. \[s:4\] we give the angular and frequency characteristics of the one–arm one–way detector. In the Appendix \[A2\] we derive the vacuum plane wave solution in the linearized massive Brans–Dicke theory working directly in the synchronous gauge; in the Appendix \[A3\] we show the equivalence of 