Deep technical writeups on the physics, numerics, and algorithms behind the UDTE simulations.
How Venus's position is actually calculated frame by frame, what Kepler drift and Horizons drift measure and why they're fundamentally different, and what happens to accuracy and performance as you change the substep size. Covers the leapfrog integrator, light-delayed gravity, and the parallelisation vs. larger-dt trade-off.
The complete mathematical and algorithmic framework for a relativistic N-body simulator using the 1PN Einstein-Infeld-Hoffmann equations (the same formulation JPL uses for DE440). Covers Mercury's perihelion precession, IAS15 and DOP853 integrators, Barnes-Hut and FMM force computation, SIMD vectorisation, and validation against Horizons data.