An orbital pass (or simply pass) is the period in which a

spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, ...

is above the local horizon, and thus available for line-of-sight communication with a given

ground station A ground station, Earth station, or Earth terminal is a terrestrial radio station designed for extraplanetary telecommunication with spacecraft (constituting part of the ground segment of the spacecraft system), or reception of radio waves f ...

, receiver, or relay satellite, or for visual sighting. The beginning of a pass is termed ''acquisition of signal'' (AOS); the end of a pass is termed ''loss of signal'' (LOS). The point at which a spacecraft comes closest to a ground observer is the ''time of closest approach'' (TCA).

Timing and duration

The timing and duration of passes depends on the characteristics of the

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...

a satellite occupies, as well as the ground

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...

and any

occulting An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...

objects on the ground (such as buildings), or in space (for planetary probes, or for spacecraft using relay satellites). The longest duration ground pass will be experienced by an observer directly on the

ground track A ground track or ground trace is the path on the surface of a planet directly below an aircraft's or satellite's trajectory. In the case of satellites, it is also known as a suborbital track, and is the vertical projection of the satellite's ...

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 A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbital ...

orbit may be continuously visible from a single ground station, whereas satellites in

low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...

only offer short-duration ground passes (although longer contacts may be made via relay satellite networks such as TDRSS).

Satellite constellation A satellite constellation is a group of artificial satellites working together as a system. Unlike a single satellite, a constellation can provide permanent global or near-global coverage, such that at any time everywhere on Earth at least one s ...

s, such as those of

satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location ( longitude, latitude, and altitude/ elevation) to hig ...

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 Coverage may refer to: Filmmaking * Coverage (lens), the size of the image a lens can produce * Camera coverage, the amount of footage shot and different camera setups used in filming a scene * Script coverage, a short summary of a script, writ ...

Prediction and visibility

A number of web-based and

mobile app A mobile application or app is a computer program or software application designed to run on a mobile device such as a phone, tablet, or watch. Mobile applications often stand in contrast to desktop applications which are designed to run on d ...

lications produce predictions of passes for known satellites. In order to be observed with the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to norma ...

, a spacecraft must reflect sunlight towards the observer; thus, naked-eye observations are generally restricted to

twilight Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this i ...

hours, during which the spacecraft is in sunlight but the observer is not. A satellite flare occurs when sunlight is reflected by flat surfaces on the spacecraft. The

International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...

, the largest artificial satellite of Earth, has a maximum

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

of , brighter than the planet

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

References

{{Spaceflight Astrodynamics Spacecraft communication

Timing and duration

Prediction and visibility

See also

References