Not so fast!
There is the consideration that you might end up 'blocking' other users in the satellite belt with the size antenna that is required -
- and the 'space drag' (from solar particles) on a large antenna may also prevent you from staying on-station in your orbital slot ...
The real answer, I think, to 500 mile 'detection' of aircraft with today's processing/computer power is the use of OTH HF RADAR as I mentioned previously ... then there is the possibility of 'stealth' being used ... RADAR absorbing materials/LO (Low Observables) designs aren't near as likely to work with HF (2 - 30 MHz) RF energy as with RF in the microwave spectrum ...
The price of security. But by the time such a huge antenna were built 1000 miles across, the terrorists would be dead of old age or work-related accidents, and their children would view their parents as hopeless and it might all be over anyway.