You should be able to spot anything coming close to Earth by the one deep space tracking station you have. It won't show you everything in the system, but you'll see everything on top of Earth and in its vicinity.
My guess is that you sent massdriver packets to Earth, but didn't have a massdriver there to catch them?
If not, deep space tracking stations act as passive sensors for both EM and TH. Then, you can build PDC with preferably large active sensors for active sensor coverage.
Even a completly cloaked ship is visible with active sensors if it's right on top of Earth.
If you want to see ships as soon as they enter Sol, you'll need multiple deep space sensor stations (if the jump point is far away, 10+) on Earth or jump point pickets. For the latter, just use the search function or read some of the fiction - it's a commonly used strategy.