Pass (spaceflight)

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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.

See also

  • Ground track, the path on the surface of the Earth directly below a satellite

References

  1. 1.0 1.1 "AOS, TCA, and LOS". Northern Lights Software Associates. Retrieved 17 November 2015. 
  2. 2.0 2.1 Wood, Lloyd (July 2006). Introduction to satellite constellations: Orbital types, uses and related facts (PDF). ISU Summer Session. Retrieved 17 November 2015. 
  3. 3.0 3.1 Del Re, Encrico; Pierucci, Laura (eds.). Satellite Personal Communications for Future-generation Systems. Springer. p. 19. ISBN 1447101316. Retrieved 17 November 2015. 
  4. Richharia, Madhavendra (2014). Mobile Satellite Communications: Principles and Trends (Second ed.). Wiley. pp. 106–107. ISBN 1118810066. Retrieved 17 November 2015. 
  5. Montenbruck, Oliver; Eberhard, Gill (2012). Satellite Orbits: Models, Methods, and Applications. Springer. p. 229. ISBN 3642583512. Retrieved 17 November 2015. 
  6. Dickinson, David (July 11, 2013). "How to Spot and Track Satellites". Universe Today. Retrieved 17 November 2015.