User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-2253300-20130130083731/@comment-2253300-20130206224213

An the fact that it is difficult to determine your own position in the middle of space. You are talking about pinpoint accuracy. At a certain range a ship has nothing to measure its own position by unless it is near a celestial body. There is no way to determine one's exact position within empty space.