Answer
1.88 m.
Work Step by Step
The angle $\theta=\frac{28 km}{1200km}$.
Use Rayleigh’s criterion, $sin \theta_1=1.22\frac{\lambda}{D}$.
$$D =1.22\frac{\lambda}{ sin \theta_1}$$
The angle is small, so $sin \theta \approx \theta$.
$$D =1.22 (3.6 cm) \frac{1200km}{28 km}=188 cm$$
