A radar has a power of 1kW and is operating at a frequency of 10 GHz. It is located on a mountain top of height 500 m. The maximum distance upto which it can detect object located on the surface of the earth (Radius of earth = 6.4 × 106m) is :

A radar has a power of $1 \mathrm{~kW}$ and is operating at a frequency of $10 \mathrm{GHz}$. It is located on a mountain top of height $500 \mathrm{~m}$. The maximum distance upto which it can detect object located on the surface of the earth (Radius of earth $=6.4 \times 10^{6} \mathrm{~m}$ ) is :