15 Satellite Based Communication

P. Jaisridhar

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1. INTRODUCTION

 

Satellites are specifically made for telecommunication purpose. They are used for mobile applications such as communication to ships, vehicles, planes, hand-held terminals and for TV and radio broadcasting. They are responsible for providing these services to an assigned region (area) on the earth. The power and bandwidth of these satellites depend upon the preferred size of the footprint, complexity of the traffic control protocol schemes and the cost of ground stations. A satellite works most efficiently when the transmissions are focused with a desired area. When the area is focused, then the emissions don’t go outside that designated area and thus minimizing the interference to the other systems. This leads more efficient spectrum usage. Satellite’s antenna patterns play an important role and must be designed to best cover the designated geographical area (which is generally irregular in shape). Satellites should be designed by keeping in mind its usability for short and long term effects throughout its life time. The earth station should be in a position to control the satellite if it drifts from its orbit it is subjected to any kind of drag from the external forces.

 

2. Principles of satellite communication

 

Satellite communications systems relevant to fisheries Monitoring Control and Surveillance of Fishing Vessels (MCS) use satellites that are either geostationary or orbiting. With a geostationary system the satellite remains in a fixed position relative to a given geographical location (the satellite is actually in a fixed orbit and moves in a consistent relationship to the Earth). With this type of system the satellite can, at all times, receive and transmit messages to any transmitter or transceiver that is within the fixed geographical area visible to the satellite. A communications system based on geostationary satellites may have more than one satellite to cover a greater percentage of the Earth’s surface. An orbiting communications satellite moves in an orbit so that it passes above a given geographical location at periodic time intervals. Such a system means that earth bound transmitters or transceivers come into the satellite’s range at these periodic time intervals and transmit or receive only while the satellite is in range or “visible”. The transmitter may store messages until the satellite is in range. When messages are transmitted to the satellite, they may also be stored in the satellite until the satellite comes into range of a receiving earth station.

 

Unlike a geostationary system, a single satellite can feasibly cover the whole of the Earth’s surface. However, there will be time gaps in coverage when the satellite is not in view of given geographical locations. Increasing the number of satellites will increase the coverage of the system by decreasing the time gaps when a satellite is not in view of a given location. In both types of system a fixed or mobile transmitter can be used. Such a transmitter is mounted on a vessel, aircraft, building etc. and uses a radio signal to send a message to the satellite mounted transponder. The message can be stored in the satellite for later forwarding or immediately forwarded to a receiver or transmitter with a receiving capability (transceiver) mounted on another vessel, aircraft, building etc. In some cases the receiving station will be a large fixed station (an “earth station”) which will link to the normal terrestrial telephone system.

 

3.  Application of Satellite

 

1.Weather forecasting: Certain satellites are specifically designed to monitor the climatic conditions of earth. They continuously monitor the assigned areas of earth and predict the weather conditions of that region. This is done by taking images of earth from the satellite. These images are transferred using assigned radio frequency to the earth station. (Earth Station: it’s a radio station located on the earth and used for relaying signals from satellites.) These satellites are exceptionally useful in predicting disasters like hurricanes, and monitor the changes in the Earth’s vegetation, sea state, ocean colour, and ice fields.

 

2.Radio and TV Broadcast: These dedicated satellites are responsible for making 100s of channels across the globe available for everyone. They are also responsible for broadcasting live matches, news, world-wide radio services. These satellites require a 30-40 cm sized dish to make these channels available globally.

 

3.Military satellites: These satellites are often used for gathering intelligence, as a communications satellite used for military purposes, or as a military weapon. A satellite by itself is neither military nor civil. It is the kind of payload it carries that enables one to arrive at a decision regarding its military or civilian character.

 

4.Navigation satellites: The system allows for precise localization world-wide, and with some additional techniques, the precision is in the range of some meters. Ships and aircraft rely on GPS as an addition to traditional navigation systems. Many vehicles come with installed GPS receivers. This system is also used, e.g., for fleet management of trucks or for vehicle localization in case of theft.

 

5. Global telephone: One of the first applications of satellites for communication was the establishment of international telephone backbones. Instead of using cables it was sometimes faster to launch a new satellite. But, fibre optic cables are still replacing satellite communication across long distance as in fibre optic cable, light is used instead of radio frequency, hence making the communication much faster (and of course, reducing the delay caused due to the amount of distance a signal needs to travel before reaching the destination.). Using satellites, to typically reach a distance approximately 10,000 kms away, the signal needs to travel almost 72,000 kms, that is, sending data from ground to satellite and (mostly) from satellite to another location on earth. This cause’s substantial amount of delay and this delay becomes more prominent for users during voice calls.

 

6.Connecting remote areas: Due to their geographical location many places all over the world do not have direct wired connection to the telephone network or the internet (e.g., researchers on Antarctica) or because of the current state of the infrastructure of a country. Here the satellite provides a complete coverage and (generally) there is one satellite always present across a horizon.

 

7.Global mobile communication: The basic purpose of satellites for mobile communication is to extend the area of coverage. Cellular phone systems, such as CDMA and GSM (and their successors) do not cover all parts of a country. Areas that are not covered usually have low population where it is too expensive to install a base station. With the integration of satellite communication, however, the mobile phone can switch to satellites offering world-wide connectivity to a customer. Satellites cover a certain area on the earth. This area is termed as a „footprint‟ of that satellite.

 

Within the footprint, communication with that satellite is possible for mobile users. These users communicate using a Mobile-User-Link (MUL). The base-stations communicate with satellites using a Gateway-Link (GWL). Sometimes it becomes necessary for satellite to create a communication link between users belonging to two different footprints. Here the satellites send signals to each other and this is done using Inter-Satellite-Link (ISL).

 

4. Types of satellites based on orbits

 

4.1. Geostationary or geosynchronous earth orbit (GEO):

 

GEO satellites are synchronous with respect to earth. Looking from a fixed point from Earth, these satellites appear to be stationary. These satellites are placed in the space in such a way that only three satellites are sufficient to provide connection throughout the surface of the Earth (that is; their footprint is covering almost 1/3rd of the Earth). The orbit of these satellites is circular. There are three conditions which lead to geostationary satellites. Lifetime expectancy of these satellites is 15 years.

  • The satellite should be placed 37,786 kms (approximated to 36,000 kms) above the surface of the earth.
  • These satellites must travel in the rotational speed of earth, and in the direction of motion of earth, that is eastward.
  • The inclination of satellite with respect to earth must be 00.
  • Geostationary satellite in practical is termed as geosynchronous as there are multiple factors which make these satellites shift from the ideal geostationary condition
  • Gravitational pull of sun and moon makes these satellites deviate from their orbit. Over the period of time, they go through a drag. (Earth’s gravitational force has no effect on these satellites due to their distance from the surface of the Earth.)
  • These satellites experience the centrifugal force due to the rotation of Earth, making them deviate from their orbit.
  • The non-circular shape of the earth leads to continuous adjustment of speed of satellite from the earth station.

 

These satellites are used for TV and radio broadcast, weather forecast and also, these satellites are operating as backbones for the telephone networks.

 

4.1.1. Disadvantages of GEO: Northern or southern regions of the Earth (poles) have more problems receiving these satellites due to the low elevation above a latitude of 60°, i.e., larger antennas are needed in this case. Shading of the signals is seen in cities due to high buildings and the low elevation further away from the equator limit transmission quality. The transmit power needed is relatively high which causes problems for battery powered devices. These satellites cannot be used for small mobile phones. The biggest problem for voice and also data communication is the high latency as without having any handovers, the signal has to at least travel 72,000 kms. Due to the large footprint, either frequencies cannot be reused or the GEO satellite needs special antennas focusing on a smaller footprint. Transferring a GEO into orbit is very expensive.

 

4.2. Low Earth Orbit (LEO) satellites

 

These satellites are placed 500-1500 kms above the surface of the earth. As LEOs circulate on a lower orbit, hence they exhibit a much shorter period that is 95 to 120 minutes. LEO systems try to ensure a high elevation for every spot on earth to provide a high quality communication link. Each LEO satellite will only be visible from the earth for around ten minutes. Using advanced compression schemes, transmission rates of about 2,400 bit/s can be enough for voice communication. LEOs even provide this bandwidth for mobile terminals with Omni-directional antennas using low transmit power in the range of 1W. The delay for packets delivered via a LEO is relatively low (approx. 10 ms). The delay is comparable to long-distance wired connections (about 5–10 ms). Smaller footprints of LEOs allow for better frequency reuse, similar to the concepts used for cellular networks. LEOs can provide a much higher elevation in Polar Regions and so better global coverage. These satellites are mainly used in remote sensing and providing mobile communication services (due to lower latency).

 

4.2.1. Disadvantages: The biggest problem of the LEO concept is the need for many satellites if global coverage is to be reached. Several concepts involve 50–200 or even more satellites in orbit. The short time of visibility with a high elevation requires additional mechanisms for connection handover between different satellites. The high number of satellites combined with the fast movements resulting in a high complexity of the whole satellite system. One general problem of LEOs is the short lifetime of about five to eight years due to atmospheric drag and radiation from the inner Van Allen belt1. Assuming 48 satellites and a lifetime of eight years, a new satellite would be needed every two months. The low latency via a single LEO is only half of the story. Other factors are the need for routing of data packets from satellite to if a user wants to communicate around the world. Due to the large footprint, a GEO typically does not need this type of routing, as senders and receivers are most likely in the same footprint.

 

4.3. Medium Earth Orbit (MEO) Satellites:

 

MEOs can be positioned somewhere between LEOs and GEOs, both in terms of their orbit and due to their advantages and disadvantages. Using orbits around 10,000 km, the system only requires a dozen satellites which is more than a GEO system, but much less than a LEO system. These satellites move more slowly relative to the earth’s rotation allowing a simpler system design (satellite periods are about six hours). Depending on the inclination, a MEO can cover larger populations, so requiring fewer handovers.

 

4.3.1. Disadvantages: Again, due to the larger distance to the earth, delay increases to about 70–80 ms. the satellites need higher transmit power and special antennas for smaller footprints.

 

The above three are the major three categories of satellites, apart from these, the satellites are also classified based on the following types of orbits:

 

4.4. Sun-Synchronous Orbits Satellite:

 

These satellites rise and set with the sun. Their orbit is defined in such a way that they are always facing the sun and hence they never go through an eclipse. For these satellites, the surface illumination angle will be nearly the same every time. Special cases of the sun-synchronous orbit are the noon/midnight orbit, where the local mean solar time of passage for equatorial longitudes is around noon or midnight, and the dawn/dusk orbit, where the local mean solar time of passage for equatorial longitudes is around sunrise or sunset, so that the satellite rides the terminator between day and night.

 

4.5. Hohmann Transfer Orbit:

 

This is an intermediate orbit having a highly elliptical shape. It is used by GEO satellites to reach their final destination orbits. This orbit is connected to the LEO orbit at the point of perigee forming a tangent and is connected to the GEO orbit at the point of apogee again forming a tangent.

 

4.6. Prograde Orbit:

 

This orbit is with an inclination of less than 90°. Its direction is the same as the direction as the rotation of the primary (planet).

 

4.7. Retrograde Orbit:

 

This orbit is with an inclination of more than 90°. Its direction is counter to the direction of rotation of the planet. Only few satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude.

 

4.8. Polar Orbit:

 

This orbit passes above or nearly above both poles (north and South Pole) of the planet on each of its revolutions. Therefore it has an inclination of (or very close to) 90 degrees. These orbits are highly inclined in shape.

 

5. Factors affecting the performance of satellite communication:

 

The performance of a satellite system is primarily related to the type and strength of radio signal used between the vessels mounted transmitter and the satellite. The power available in the satellite and the extent to which the satellite can focus on a geographical area are inter related factors and determine the size and power requirements of the vessel transmitter.

 

The type of radio signal used by transmitters relevant to fisheries MCS is usually within the microwave band and as such is highly reliable and relatively low powered. The signal is not greatly affected by atmospheric conditions.

  1. Conclusion

  The use of satellite technology, particularly in the use of communications satellites has grown rapidly in the past thirty years. Each day more and more uses for the satellites are being discovered. Feeding this is the rapid advancement of technology that allows the quick implementation of these uses. Communications satellites will not only help out a person in distress but allow a person walking the street in Manhasset N.Y. USA to use their cellular phone to speak with someone in China. More and more satellites are being launched each year to support new and growing uses for business, military and communication needs. Satellite communications will continue in the right direction and remains the best utilization for communication due to their speed and other advantages like broadcasting, signal providing in terrestrial uncovered areas on earth and independent information transmission irrespective of distance etc. There are still remarkable technologies still to be developed with more powerful processors, new encoding technologies which can make user systems more mobile, more versatile, more powerful in performance and with low cost.

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Web links

 

  1. www.wikipedia.com
  2. http://www.tech-faq.com/vsat.html
  3. M. Richharia, Mobile Satellite Communication: Principles and Trends, Pearson Education
  4. Rappaort, Wireless Communications Principals and Practices
  5. YI Bing Lin , Wireless and Mobile Network Architectures, John Wiley
  6. P. Nicopolitidis ,Wireless Networks, John Wiley Satellite Communications Dennis Roddy 3rd edition, Mc-Graw Hill publication