= Spacecraft tracking and the limits of the escape model
Operational evidence comes from reconstructing a spacecraft's motion. A ground antenna transmits a coded signal; the spacecraft returns it. Corrected round-trip <time> constrains distance, while the <Doppler effect> of a coherent carrier constrains the line-of-sight <relative velocity>. NASA's navigation account describes calibration of onboard turnaround delays before launch, calibration of ground delays before use, and corrections for the <terrestrial atmosphere>. Measurements over multiple stations and epochs constrain the <orbit>; range and <Doppler effect> alone do not directly give all transverse motion. Navigators compare a fitted <spacecraft trajectory> with later tracking and remeasure after propulsion to assess the maneuver. The account identifies <radiation pressure> and imperfect propulsion as disturbances. This provides practical checks of a dynamical model and its corrections, rather than a laboratory measurement of a universal <escape speed>. The simple <inverse-square potential> predicts an unpowered exterior arc; accurate navigation includes additional <forces> and measurement delays.<Basics of Space Flight (2017 edition) by Dave Doody/Chapter 13: Navigation>{ref}
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