The rock to which Kara and Mon-El travel is said to be five light-years away, and while the confines of science fiction allow for them to travel that far with J'onn's FTL drive, it would still be impossible for Winn to pick up any sort of signal from them using conventional Earth satellites, as light-speed is the speed limit of all information in the universe. This means that even if he picked up a radio signal from the rock that was travelling at the speed of light, it would still take five years to arrive.