Solar transit
Encyclopedia
In astronomy
, a solar transit is when any object passes between the Sun
and the Earth
. This mainly includes the planet
s Mercury
and Venus
. A solar eclipse
is also a solar transit of the Moon
, but technically only if it does not cover the entire disc of the Sun (an annular eclipse), as "transit" counts only objects that are smaller than what they are passing in front of. Solar transit is only one of several types of astronomical transit
.
Solar transit (or a solar outage, sometimes solar fade, sun outage, or sun fade) also occurs to communications satellite
s, which pass in front of the Sun for several minute
s each day
for several days straight for a period in the months around the equinox
es, the exact dates depending on where the satellite is in the sky relative to its earth station. Because the Sun also produces a great deal of microwave
radiation
in addition to sunlight
, it overwhelms the microwave radio
signals coming from the satellite's transponder
s. This enormous electromagnetic interference
causes interruptions in fixed satellite services that use satellite dish
es, including TV networks and radio network
s, as well as VSAT and DBS
.
Only downlinks from the satellite are affected, uplink
s from the Earth are normally not, as the planet
"shade
s" the Earth station when viewed from the satellite. Satellites in geosynchronous orbit
are irregularly affected based on their inclination
. Reception from satellites in other orbit
s are frequently but only momentarily affected by this, and by their nature the same signal is usually repeated
or relay
ed on another satellite, if a tracking
dish is used at all. Satellite radio and other services like GPS are not affected, as they use no receiving dish, and therefore do not concentrate the interference. (GPS and certain satellite radio systems use non-geosynchronous satellites.)
Solar transit begins with only a brief degradation in signal quality for a few moments. At the same time each day, for the next several days, it gets longer and gets worse, until finally gradually improving after several more days. For digital
satellite services, the cliff effect
will eliminate reception entirely at a given threshold. Reception is typically lost for only a few minutes on the worst day, but the beam width of the dish can affect this. Signal strength
also affects this, as does the bandwidth of the signal. If the power is concentrated into a narrower band, there is a higher signal-to-noise ratio
. If the same signal is spread wider, the receiver also gets a wider swath of noise, degrading reception.
The exact days and times of solar transit outage
s, for each satellite and for each receiving point (Earth station) on the Earth, are available at various website
s. For broadcast networks, the network feed must be pre-recorded
, replaced with local programming
, fed via another satellite in a different orbit
al position, or fed via another method entirely during these times.
In the Northern Hemisphere
, solar transit is usually in early March and October. In the Southern Hemisphere
, solar transit is usually in early September and April. The time of day varies mainly with the longitude
of the satellite and receiving station, while the exact days vary mainly with the station's latitude
. Stations along the equator
will experience solar transit right at the equinox
es, as that is where geostationary satellites are located directly over.
Note that with large dishes, it would be possible for the feedhorn to be damaged, which is prevented by the parabolic
dishes being done in a flat (non-glossy
) finish
that does not focus
light
or heat
effectively. Non-parabolic dishes cannot focus in this manner.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, a solar transit is when any object passes between the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. This mainly includes the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
and Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
. A solar eclipse
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...
is also a solar transit of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, but technically only if it does not cover the entire disc of the Sun (an annular eclipse), as "transit" counts only objects that are smaller than what they are passing in front of. Solar transit is only one of several types of astronomical transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
.
Solar transit (or a solar outage, sometimes solar fade, sun outage, or sun fade) also occurs to communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
s, which pass in front of the Sun for several minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...
s each day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
for several days straight for a period in the months around the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
es, the exact dates depending on where the satellite is in the sky relative to its earth station. Because the Sun also produces a great deal of microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
in addition to sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
, it overwhelms the microwave radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
signals coming from the satellite's transponder
Transponder
In telecommunication, the term transponder has the following meanings:...
s. This enormous electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
causes interruptions in fixed satellite services that use satellite dish
Satellite dish
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.-Principle of operation:...
es, including TV networks and radio network
Radio network
There are two types of radio networks currently in use around the world: the one-to-many broadcast type commonly used for public information and mass media entertainment; and the two-way type used more commonly for public safety and public services such as police, fire, taxicabs, and delivery...
s, as well as VSAT and DBS
Direct broadcast satellite
Direct broadcast satellite is a term used to refer to satellite television broadcasts intended for home reception.A designation broader than DBS would be direct-to-home signals, or DTH. This has initially distinguished the transmissions directly intended for home viewers from cable television...
.
Only downlinks from the satellite are affected, uplink
Uplink
A telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites to connect two points on earth.-Uplink:...
s from the Earth are normally not, as the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
"shade
Shade
Shade is the blocking of sunlight by any object, and also the shadow created by that object. Shade also consists of the colors grey, black, white, etc...
s" the Earth station when viewed from the satellite. Satellites in geosynchronous orbit
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
are irregularly affected based on their inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
. Reception from satellites in other orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
s are frequently but only momentarily affected by this, and by their nature the same signal is usually repeated
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
or relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
ed on another satellite, if a tracking
Tracking
Tracking can refer to:*Tracking , separating children into different classes according to their academic ability*Tracking, in computer graphics, a vital part of match moving...
dish is used at all. Satellite radio and other services like GPS are not affected, as they use no receiving dish, and therefore do not concentrate the interference. (GPS and certain satellite radio systems use non-geosynchronous satellites.)
Solar transit begins with only a brief degradation in signal quality for a few moments. At the same time each day, for the next several days, it gets longer and gets worse, until finally gradually improving after several more days. For digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
satellite services, the cliff effect
Cliff effect
In telecommunications, the cliff effect or brickwall effect describes the sudden loss of digital signal reception. Unlike analog signals, which gradually fade when signal strength decreases or electromagnetic interference or multipath increases, a digital signal provides data which is either...
will eliminate reception entirely at a given threshold. Reception is typically lost for only a few minutes on the worst day, but the beam width of the dish can affect this. Signal strength
Signal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
also affects this, as does the bandwidth of the signal. If the power is concentrated into a narrower band, there is a higher signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
. If the same signal is spread wider, the receiver also gets a wider swath of noise, degrading reception.
The exact days and times of solar transit outage
Outage
Outage may refer to:* Network outage* Sun outage* Power outage* Outing...
s, for each satellite and for each receiving point (Earth station) on the Earth, are available at various website
Website
A website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...
s. For broadcast networks, the network feed must be pre-recorded
Video recorder
A video recorder may be any of several related devices:*Digital video recorder ; Personal video recorder *DVD recorder*Videocassette recorder *Video tape recorder...
, replaced with local programming
Local programming
The term Local Programme, Local Programming, Local Content or Local Television refers to a television programme made by a television station or independent television producer for broadcast only within the station's transmission area or television market...
, fed via another satellite in a different orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
al position, or fed via another method entirely during these times.
In the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
, solar transit is usually in early March and October. In the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
, solar transit is usually in early September and April. The time of day varies mainly with the longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
of the satellite and receiving station, while the exact days vary mainly with the station's latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
. Stations along the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
will experience solar transit right at the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
es, as that is where geostationary satellites are located directly over.
Note that with large dishes, it would be possible for the feedhorn to be damaged, which is prevented by the parabolic
Parabolic reflector
A parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...
dishes being done in a flat (non-glossy
Gloss (disambiguation)
Gloss may refer to:*Shininess**Gloss **Gloss , the level of shininess of painted finishes**Lip gloss*An explanation or an interpolation**Gloss, a margin text or note, from modern music, ancient manuscripts, etc....
) finish
Finish
Finish refer to:* Finishing in the distillation of Scotch* Finished good, a good that is completed as to manufacturing but not yet sold or distributed to the end-user* Surface finishing, various industrial processes for modifying a workpiece's surface...
that does not focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
or heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
effectively. Non-parabolic dishes cannot focus in this manner.