Pass (spaceflight)

Visible pass of the International Space Station and Space Shuttle Atlantis over Tampa, Florida, on mission STS-132, May 18, 2010 (five-minute exposure)

A pass, in spaceflight and satellite communications, is the period in which a satellite or other spacecraft is visible above the local horizon, and therefore available for radio communication with a particular ground station, satellite receiver, or relay satellite (or, in some cases, for visual observation). The beginning of a pass is termed acquisition of signal; the end of a pass is termed loss of signal.^[1] The point at which a spacecraft comes closest to a ground observer is the time of closest approach.^[1]

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 (such as buildings), or in space (for planetary probes, or for spacecraft using relay satellites).^[2] An observer directly on the ground track of the satellite will experience the greatest ground pass duration.^[3] Path loss is greatest toward the start and end of a ground pass,^[4] as is Doppler shifting for Earth-orbiting satellites.^[5]

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^[3] (although longer contacts may be made via relay satellites such as TDRSS). 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, providing continuous coverage.^[2]

Prediction and visibility

A number of web-based and mobile applications produce predictions of passes for known satellites.^[6] In order to be observed with the naked eye, a spacecraft must reflect sunlight towards the observer; thus, naked-eye observations are generally restricted to twilight hours, during which the spacecraft is in sunlight but the observer is not. A satellite flare occurs when sunlight is reflected by flat surfaces.