We have considered the propagation of gravitational waves (GW) in de Sitter spacetime and how a non-zero value of the cosmological constant might aﬀect their detection
in pulsar timing arrays (PTA). If Λ = 0 waves are anharmonic in Friedmann-Robertson-Walker coordinates and although the amount of this eﬀect is very small it gives noticeable
eﬀects for GW originating in extragalactic sources such as spiraling black hole binaries.
The results indicate that the timing residuals induced by gravitational waves from such
sources in PTA would show a peculiar angular dependence with a marked enhancement
around a particular value of the angle subtended by the source and the pulsars, depending
mainly on the actual value of the cosmological constant and the distance to the source.
The position of the peak could represent a gauge of the value of Λ. The enhancement
that the new eﬀect brings about could facilitate the ﬁrst direct detection of gravitational
waves while representing a local measurement of Λ.