A pass, in spaceflight
and satellite communications
, is the period in which a satellite
or other spacecraft
is above the local horizon
and available for radio
communication with a particular ground station
, satellite receiver
, or relay satellite (or, in some cases, 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 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).
[ An observer directly on the ground track of the satellite will experience the greatest ground pass duration.] 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] (although longer contacts may be made via relay satellite networks 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, thereby providing continuous coverage. [
Prediction and visibility
A number of web-based and mobile applications produce predictions of passes for known satellites.
[ 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 on the spacecraft. The International Space Station, the largest artificial satellite of Earth, has a maximum apparent magnitude of –5.9,] brighter than the planet Venus.
* Ground track, the path on the surface of the Earth directly below a satellite
* Satellite revisit period, the time elapsed between observations of the same point on Earth by a satellite