Love these kind of comments where I get to learn about things that I haven't played with.

> the simulation itself needed to be accurate to 8-9 sig figs with nanosecond resolution

What kind of application would require such demanding tolerances ? My first thought was nuclear fission. But then the fact that you had one in orbit sending data feed every 15 mins imploded my fanciful thinking.

Ahh, I really wish I could go into all of the details because it was a really cool project. The high-level answer (which is also related to the software-defined radio post that's on the front page right now) is that it was simulating the orbit of a satellite such that we could simulate the signals the satellite was transmitting with enough accuracy to implicitly get Doppler shift. That is... we didn't explicitly model Doppler shift in any piece of the system, it just showed up as a result of accurately modelling the orbit of the satellite's position and velocity relative to the receiver on Earth.