Assume the abserver and source are moving away from each other with relative velocity v (v is negetive if the abserver and the source are moving towards each other)

$\frac{{\mathrm{f}}_{\mathrm{s}}}{{\mathrm{f}}_{\mathrm{o}}}=\sqrt{\frac{1+\mathrm{\beta }}{1–\mathrm{\beta }}}$           ... (1) 

$\mathrm{\beta }=\frac{\mathrm{v}}{\mathrm{c}}=\frac{\mathrm{C}/2}{\mathrm{C}}=\frac{–1}{2}$    ....(2)

Putting (2) in (1)

f_o = 17.3 GHz