SATELLITE COMMUNICATIONS

Orbits of satellite:

 An orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the centre of a star system, such as the Solar System. Orbits of planets are typically elliptical. But unlike the ellipse followed by a pendulum or an object attached to a spring, the central object is at a focal point of the ellipse and not at its centre.

Current understanding of the mechanics of orbital motion is based on Albert Einstein’s general theory of relativity, which accounts for gravity as due to curvature of space-time, with orbits following geodesics. For ease of calculation, relativity is commonly approximated by the force-based theory of universal gravitation based on Kepler’s laws of planetary motion

Inclined orbit:

A satellite is said to occupy an inclined orbit around the Earth if the orbit exhibits an angle other than zero degrees with the equatorial plane. This angle is called the orbit’sinclination. A planet is said to have an inclined orbit around the Sun if it has an angle other than zero to the plane of the ecliptic.

 Polar orbit:

A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Sun) on each revolution. It therefore has an inclination of (or very close to) 90 degrees to theequator. A satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits.

Polar orbits are often used for earth-mapping, earth observation, capturing the earth as time passes from one point, reconnaissance satellites, as well as for some weather satellites.  The disadvantage to this orbit is that no one spot on the Earth’s surface can be sensed continuously from a satellite in a polar orbit.

Geostationary orbit:

   A circular orbit positioned approximately 35,900 km (22,258 mi) above Earth’s equator and having a period of the same duration and direction as the rotation of the Earth. An object in this orbit will appear stationary relative to the rotating Earth. Communications and weather satellites are usually placed in a geostationary orbit.

Geosynchronous orbit:

A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with an orbital period of one sidereal day, intentionally matching the Earth’s sidereal rotation period (approximately 23 hours 56 minutes and 4 seconds). The synchronization of rotation and orbital period means that, for an observer on the surface of the Earth, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day.

Low Earth orbit:

 A low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilo meters (99 mi) (orbital period of about 88 minutes), and 2,000 kilo meters (1,200 mi) (about 127 minutes). Objects below approximately 160 kilo meters (99 mi) will experience very rapid orbital decay and altitude loss. 

Medium Earth orbit:

Medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), is the region of space around the Earth above low Earth orbit (altitude of 2,000 kilometres (1,243 mi)) and below geostationary orbit (altitude of 35,786 kilometres (22,236 mi)).

Orbit paths:

The orbit paths itself can be circular or elliptical,

Circular motion is a movement of an object along the circumference of a circle or rotation along a circular path. It can be uniform, with constant angular rate of rotation and constant speed, or non-uniform with a changing rate of rotation. The rotation around a fixed axis of a three-dimensional body involves circular motion of its parts. The equations of motion describe the movement of the centre of mass of a body.

Examples of circular motion include: an artificial satellite orbiting the Earth at constant height, a stone which is tied to a rope and is being swung in circles, a car turning through a curve in a race track, an electron moving perpendicular to a uniform magnetic field, and a gear turning inside a mechanism.

Elliptic orbit:

An elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 (thus excluding the circular orbit). In a wider sense it is a Kepler orbit with negative energy. This includes the radial elliptic orbit, with eccentricity equal to 1.

In a gravitational two-body problem with negative energy both bodies follow similar elliptic orbits with the same orbital period around their common barycentre. Also the relative position of one body with respect to the other follows an elliptic orbit.

Height of geostationary orbit:

At height of geostationary orbit, the orbital velocity of satellite equals the velocity of the point on earth`s equator.

Let          M- Mass of the earth

      m- Mass of the satellite

       r- Radius of synchronous circular orbit of the satellite

      R_E– Radius of earth=6370 km

      w- Angular velocity of the satellite

The centripetal force acting on the satellite will be mrw² .The gravitational force on the satellite will be

Geostationary Earth Orbit:

GEO is also called as geosynchronous orbits, which has 24 hour period of revolution but are inclined with respect to equator. Orbits that are below a mean altitude of about 36,000 km have periods of revolution shorter than 24 hours and hence are terms as non GEO.

  GEO satellite has the ability to provide coverage of an entire hemisphere at one time. Satellites are designed to last only about 15 years in orbit, because of the practical inability to service a satellite in GEO.

  A system of three satellites in GEO each separated by 120 degrees of longitude, can receive and send radio signals over almost all the parts of earth.

Doppler shift:

   The effect of Doppler shift of frequency is negligible. Doppler shift in frequency results due to change in relative movement between source and receiver.

       For a geostationary satellite, the angular velocity v , is given by

                       V= r√g/r+h

 Where      r – Average radius of earth =6371 km

        G – Gravitational acceleration=9.81 m/sec

        h – Height of satellite above ground

Advantages/Disadvantages of Geostationary Satellites

• These satellites are placed at high altitude, allowing them to inspect the entire earth’s surface area except for small regions at the south and north geographic poles, which significantly helps in meteorological studies.
• Highly directional dish antennas can reduce signal interventions from earth based sources and other satellites too.
• The orbital sector is a really thin loop in the equator’s plane. Hence, a very small number of satellites can be maintained within this sector without mutual conflicts and collisions.
• A geostationary satellite’s precise hovering location fluctuates a little over each 24-hour period loop. This fluctuation happens due to the gravitational interference among the satellite, the earth, the sun, the moon, and other planets. Radio signals take roughly 1/4th of a second for a two-way trip to the satellite, resulting in a small but major signal wait. This wait raises the trouble of interactive communication like telephonic conversation.

Application of Geostationary Satellites

• Geostationary satellites have modernized and transformed worldwide communications, television broadcasting, and meteorological and weather forecasting. They also have a number of significant defence and intelligence applications.

 Advantages of GEO

• satellites Most communications satellites in use today for commercial purposes are placed in the geostationary orbit, because one satellite can cover almost 1/3 of Earth’s surface, offering a reach far more extensive than what any terrestrial network can achieve.
• The geosynchronous satellites remain stationary over the same orbital location, users can point their satellite dishes in the right direction, without costly tracking activities, making communications reliable and secure GEO satellites are proven, reliable and secure – with a lifespan of 10-15 years.
• GEO systems have significantly greater available bandwidth than the Low Earth Orbit -LEO and Medium Earth Orbit -MEO systems.
• This permits them to provide two-way data, voice and broadband services that may be unpractical for other types of systems. Because of their capacity and configuration, GEOs are often more cost-effective for carrying high-volume traffic, especially over long-term contract arrangements. For example, excess capacity on GEO systems often is reserved in the form of leased circuits for use as a backup to other communications methods.

 Disadvantages of GEO

• Satellites GEO systems, like all other satellite systems, require line-of-sight communication paths between terrestrial antennae and the satellites.
• But, because GEO systems have fewer satellites and these are in a fixed location over the Earth, the opportunities for line of sight communication are fewer than for systems in which the satellites “travel” across the sky.
• This is a significant disadvantage of GEO systems as compared to LEO and MEO systems, especially for mobile applications and in urban areas where tall buildings and other structures may block line-of-sight communication for hand-held mobile terminals.
• There are concerns with the transmission delays associated with GEO systems, particularly for high-speed data.

 However, sophisticated echo cancellation and other technologies have permitted GEOs to be used successfully for both voice and high-speed data applications.