Two satellites $S_1$ and $S_2$ are revolving in circular orbits around a planet with radius $R_I=3200 \mathrm{km}$ and $R_2=800 \mathrm{km}$ respectively. The ratio of speed of satellite $S_1$ to the speed of satellite $S_2$ in their respective orbits would be $\frac{1}{x}$ where $x=$