Deep-Space Doppler Tracking

This simulator reproduces the core observable behind Deep Space Network orbit determination: two-way coherent Doppler tracking. A spacecraft coasts on a Kepler orbit — solved exactly frame by frame, not force-integrated — while a rotating ground station measures the line-of-sight range-rate between itself and the craft. That range-rate becomes a real Doppler frequency shift on the X-band or S-band downlink, exactly as it does at Goldstone, Madrid or Canberra. Watch the simulated orbit-determination fix tighten during every visible pass as Doppler data accumulates, then drift again through a blackout arc when the craft sets below the station's horizon — the same lock/blackout rhythm that governs real interplanetary navigation. Adjust the orbit's semi-major axis, eccentricity and inclination, or switch tracking bands, to see how geometry and frequency both shape the tracking signal.