Jim Varney: >[general vs. special perturbations] Minor nit: technically, the distinction between "general" and "special" perturbations is only in how the problem is solved, not the underlying model. > If you look at the SDP4 algorithm, it features a >numerical integrator to calculate the shifts in position for lunar and >solar perturbations. It also includes 12 and 24hr resonance effects. And actually *looking* at the FORTRAN SDP4 code is not recommended. A most unsettling experience it can be. :-/ The integrator in SDP has an annoying property: it computes perturbations (and derivatives) on an evenly spaced grid in time, and extrapolates (using the derivatives) as needed. The extrapolation isn't perfect -- it rarely mates up with the "next" grid point. This results in discontinuities in the state-vector estimates when you cross a grid-line (in time). A more "formal" approach (using, say, Encke's method) would likely be a little slower, but may result in "smooth" state-vector estimates. Hm.