Orbital pass
An orbital pass is the period in which a spacecraft is above the local horizon, and thus available for line-of-sight communication with a given ground station, receiver, or relay satellite, or for visual sighting. The beginning of a pass is termed acquisition of signal ; the end of a pass is termed loss of signal. The point at which a spacecraft comes closest to a ground observer is the time of closest approach.
Timing and duration
The timing and duration of passes depends on the characteristics of the orbit a satellite occupies, as well as the ground topography and any occulting objects on the ground, or in space. The longest duration ground pass will be experienced by an observer directly on the ground track of the satellite. Path loss is greatest toward the start and end of a ground pass, as is Doppler shifting for Earth-orbiting satellites.Satellites in geosynchronous orbit may be continuously visible from a single ground station, whereas satellites in low [Earth orbit] only offer short-duration ground passes. Satellite constellations, such as those of satellite navigation systems, may be designed so that a minimum subset of the constellation is always visible from any point on the Earth, thereby providing continuous coverage.