Doppler Shifting A Chirp Signal
You need to resample the signal to simulate the Doppler induced dilation. The resampling factor is I=Td/Ts
where
Td = Duration of the signal after dilation
Ts = Actual duration of the transmitted signal

In Matlab you can use the function resample(), but you need to find the resampling parameters P and Q from I.

Also, I=1+v/c for signal expansion and I=1-v/c for signal compression. v=speed of movement in m/s and c=speed of wave propagation in the medium. For instance sound underwater c=1500 m/s.

In Matlab, you can find P and Q as [P,Q]=rat(I);