how do I upgrade trade routes

Posted on Tuesday, June 2, 2015

All of my trade routes are basic trade routes, generating very little credits per turn. How do I upgrade them? I tried sending another trade ship to the same other planet but it tells me I can only have one trade route between any 2 planets, so I know that is not right.

Also how do trade routes generate money? Is it based on distance between the 2 planets, population, production? Seems like I get same credits per turn from my nearby neighbor as I do from someone across the galaxy.