How is precise orbit determination performed for navigation satellites?

Navigation satellites (GPS, Galileo) require cm-level orbit knowledge for positioning accuracy. Techniques: Tracking - Global network of ground stations, Pseudorange and carrier phase measurements, Satellite laser ranging (SLR) for validation; Data processing - Weighted least squares or Kalman filter estimation, State includes position, velocity, clock, solar radiation pressure coefficients; Force modeling - Gravity field (high-degree spherical harmonics), Solar radiation pressure (complex spacecraft models), Earth tides, atmospheric density, and relativistic effects; Clocks - On-board atomic clocks modeled (polynomial + stochastic), Clock products distributed with orbits. Products: Broadcast ephemeris (predicted orbits uploaded to satellites), Precise ephemeris (post-processed, cm accuracy), and Real-time precise orbits (for RTK applications). Challenges: Antenna phase center variations, Thermal expansion effects, and Eclipse transitions. Orbit accuracy directly impacts user positioning accuracy.