Polar mount
Encyclopedia
A polar mount is a movable mount for satellite dish
es that allows the dish to be pointed at many geostationary satellites by slewing around one axis. It works by having its slewing axis parallel, or almost parallel, to the Earth
's polar axis so that the attached dish can follow, approximately, the geostationary orbit
, which lies in the plane of the Earth's equator
.
(TVRO) satellite systems where they can be used to access the TV signals from many different geostationary satellites. They are also used in other types of installations such as TV, cable, and telecommunication Earth station
s although those applications usually use more sophisticated altazimuth
or fix angle dedicated mounts. Polar mounts can use a simplified one axis design because geostationary satellite are fixed in the sky relative to the observing dish and their equatorial orbits puts them all in a common line that can be accessed by swinging the satellite dish along a single arc approximately 90 degrees from the mount's polar axis. This also allows them to use a single positioner to move the antenna in the form of a "jackscrew
" or horizon-to-horizon gear drive. Polar mounts work in a similar way to astronomical equatorial mount
s in that they point at objects at fixed hour angle
s that follow the astronomical right ascension
axis. They differ from equatorial mounts in that the objects (satellites) they point at are fixed in position and usually require no tracking, just accurate fixed aiming.
(i.e. they follow the celestial equator
). But when observed from other latitudes the fact that geostationary satellites are at a fixed altitude of 35,786 km (22,236 mi) above the Earth's equator, and vary in distance from the satellite dish due to the dish's position in latitude and longitude, means polar mounts need further adjustments to allow one axis slewing:
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 that allows the dish to be pointed at many geostationary satellites by slewing around one axis. It works by having its slewing axis parallel, or almost parallel, to 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...
's polar axis so that the attached dish can follow, approximately, the geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
, which lies in the plane of the Earth's 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....
.
Description
Polar mounts are popular with home television receive-onlyTelevision receive-only
Television receive-only, TVRO, or Big ugly dish , is a term used in North America to refer to the reception of satellite television from FSS-type satellites, generally on C-band analog; free-to-air and unconnected to a commercial DBS provider...
(TVRO) satellite systems where they can be used to access the TV signals from many different geostationary satellites. They are also used in other types of installations such as TV, cable, and telecommunication Earth station
Earth station
A ground station, earth station, or earth terminal is a terrestrial terminal station designed for extraplanetary telecommunication with spacecraft, and/or reception of radio waves from an astronomical radio source. Ground stations are located either on the surface of the Earth, or within Earth's...
s although those applications usually use more sophisticated altazimuth
Altazimuth mount
An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two mutually perpendicular axes; one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument...
or fix angle dedicated mounts. Polar mounts can use a simplified one axis design because geostationary satellite are fixed in the sky relative to the observing dish and their equatorial orbits puts them all in a common line that can be accessed by swinging the satellite dish along a single arc approximately 90 degrees from the mount's polar axis. This also allows them to use a single positioner to move the antenna in the form of a "jackscrew
Jackscrew
A jackscrew is a type of jack which is operated by turning a leadscrew. In the form of a screw jack it is commonly used to lift heavy weights such as the foundations of houses, or large vehicles.-Advantages:...
" or horizon-to-horizon gear drive. Polar mounts work in a similar way to astronomical equatorial mount
Equatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
s in that they point at objects at fixed hour angle
Hour angle
In astronomy and celestial navigation, the hour angle is one of the coordinates used in the equatorial coordinate system to give the position of a point on the celestial sphere....
s that follow the astronomical right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
axis. They differ from equatorial mounts in that the objects (satellites) they point at are fixed in position and usually require no tracking, just accurate fixed aiming.
Adjustments
When observed from the equator, geostationary satellites follow exactly the imaginary line of the Earth's equatorial plane on the celestial sphereCelestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
(i.e. they follow the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...
). But when observed from other latitudes the fact that geostationary satellites are at a fixed altitude of 35,786 km (22,236 mi) above the Earth's equator, and vary in distance from the satellite dish due to the dish's position in latitude and longitude, means polar mounts need further adjustments to allow one axis slewing:
- Declination angle - The declination angle or just "declination", from the astronomical term declinationDeclinationIn astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
for the vertical value (north/south) on the celestial sphere, is a "tipping down" of the dish on the mount to allow it to observe geostationary satellites. When observed from any latitude other than the equator the observer is actually looking "down" on the satellite making it look as if it is just below the celestial equator, an angle from the celestial equator that increases with latitude. Polar mounts have mechanisms that allow the dish to be tipped down in a permanently fixed angle to match the declination angle. Mounts may also have a variable declination control to allow them to point at geosynchronous satelliteGeosynchronous satelliteA geosynchronous Satellite is a satellite whose orbit on the Earth repeats regularly over points on the Earth over time. If such a satellite's orbit lies over the equator, the orbit is circular and its angular velocity is the same as the earth's, then it is called a geostationary satellite...
s in inclined orbits since those satellites have a constantly changing declination.
- Declination offset - Because satellites toward the Eastern and Western sky are further away from the observing antenna, there is a change in the declination angle: towards the eastern and western limits the satellites get closer to the celestial equator because they are further out along the lines of perspectivePerspective (visual)Perspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...
. To aim at this apparent shift in the arc of geostationary satellites polar mounts incorporate a slight offset in the angle of their polar axis towards the equator, called a declination offset, to more closely follow this arc. Slewing around a fixed axis which is not parallel with the earth's rotation axis causes the dish to aim at a track in the equatorial plane which is (unless the dish is on the equator) an ellipse rather than a circle. Since the geostationary orbit is circular, the mount does not aim precisely at satellites at all longitudes. These slight differences in tracking have negligible effect on home C-band and Ku bandKu bandThe Kμ band is a portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to —in other words, the band directly below the K-band...
TVRO dishes since they have relatively wide-beam designs.