This means that with respect to element 0, the signal is delayed by $\tau_1$ at antenna 1, $\tau_2$ at antenna 2 and $\tau_3$ at antenna 3 (as a word of caution, these times of arrival are not the ones shown in delay blocks after the array output in the above figure. If the signal arriving at the reference point (antenna element 0) is denoted as $r_0(t)$, how exactly is it related to the signal expression arriving at antenna element 1 and given by $r_1(t)$? From Eq (\ref{equation-s0-s1-delay}), we know that the signal at each antenna $r_n(t)$ is a delayed version of the narrowband signal $r_0(t)$ arriving at the reference antenna.