Let ξ(t) be the projection of the vector *P*
_{R}(*t*) − *P*
_{T}(*t* − *D*(*t*)) along the propagation direction i_{k} (oriented from *P*
_{T}(*t* − *D*(*t*)) to *P*
_{R}(*t*)), that is

(7.77)

The delay D(t) experienced by the wavefront transmitted at time t − D(t) in *P*
_{T}(*t* − *D*(*t*)) and received at time t in *P*
_{R}(*t*) depends on the distance ||P_{R}(t) − P_{T}(t − D(t))||. Accordingly with (7.11) we have

As observed in Section 7.1.3, |ξ(t)| is the time-varying distance responsible of the Doppler effect, viz, the time-varying delay D(t), and should not be confused with

(7.79)

Let us assume that the relative radial speed between *P*
_{T}(*t* − *D*(*t*)) and *P*
_{R}(*t*) is constant within the observation interval. Thus, the projection of the vector *P*
_{R}(*t*) − *P*
_{T}(*t* − *D*(*t*)) along the propagation direction i_{k} is a linear function of t. That is,

Furthermore, let us assume that, within the observation interval, RX does not collapse on TX. Thus, ξ(t) does not change sign and it is always (t, D(t)) > 0. Under ...

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