LameLefty said:

Hey if Harsh wants to get pedantic, I can put my aerospace and mechanical engineering degree up against his (along with my posting history here) any time.

I've had my share of undergraduate engineering and mathematical simulation and approximation classes. No need to get out the measuring stick.

Of course it's not a literal snapshot, but then again, neither is it a "model."

Poppycock. A TLE is precisely the parameters in a time based mathematical model. The TLE tells you where it should be at any particular time until there is a new TLE. Knowing where a satellite was is of little use in the here and now.

The key to this being a model is the Ballistic Coefficient and Second Derivative of Mean Motion. Ballistic Coefficient is a first derivative and, obviously, the Second Derivative is also a derivative. Snapshots don't have time factors in them and the result of a TLE is obviously dependent on time.

You say that it is a snapshot and then you explain that it isn't. Then you say that it isn't a model but that the information is fed to a model which forecasts the position. It either is a snapshot or it isn't. It either is a model or it isn't.

The epoch date is the time at which the TLE in question became the official predictor. If you look at the epoch dates of most DIRECTV satellites, you'll find that they update them every few days even though the satellites are essentially geosynchonous.