Global Navigation using Inter Satellite Links (ISL) in SATCOM
The overall objective of this paper presentation is to provide a forum for researchers and technologists to present new ideas and contributions in the form of technical papers in the field of satellite communications. SATCOM nowadays requires to bring together various satellite communication systems developers to discuss the current status, fundamental issues, and future services , technical challenges and applications. It has been seen over the past decade, wireless communications has seen an exponential growth and will certainly continue to improve due to the emergence of new interactive multimedia applications and highly integrated systems driven by the rapid growth in information services and microelectronic devices. A key requirement in future wireless system is their ability to provide broadband connectivity with end-to-end Quality of Service (QoS), a high network capacity, and throughput at a low cost.
Satellite systems are the medium for wireless communication and support mobile communication and it can cover larger population and areas to spread mobile communication. Characteristics /Properties of satellite communication include orbit visibility and high transmission quality (QoS). Orbit visibility means that at any point of time, after launching satellite we can determine the position of satellite and can follow it, as the rotation of satellite is synchronous to the rotation of earth surface. The signals which are being transmitted by satellite are always of high quality hence it is used for TV and Radio broadcasting . It is also useful in case of weather forecasting , military operations and global mobile communication.
Satellites have had a major impact on our lives, in both personal areas and in industry. This function has proven to be instrumental in assisting communities in times of strife and has also helped companies establish better information links with their facilities in more remote areas. As the communications market continues to grow at an astronomical rate, the main focus of the industry will be on how improvements can be made in cost, speed, quality, and convenience for consumers. While satellites have been a leading force in providing solutions in most of the areas, such as the reduction in size of receiver antennas to three feet or less making installation and usage much easier on the customer. United States has many systems, especially fibre optic networks, global telecommunication, in many areas, lacks the internal infrastructure to support even the most basic of telephony services . Satellites are an excellent method to establish access to areas with poor terrestrial infrastructures where capacity is either not available or is cost prohibitive.
While satellites enjoy increased accessibility to areas around the world, fibres optics enjoy a much greater capacity and speed potential. Where available future systems should look to mesh these two technologies together. Though meshing these two technologies together would require the development of gateways capable of optimizing the inherent benefits of each, while addressing the differences between the two. The major issue to be addressed will be accommodating the delay or latency due to the slower speeds by satellite; this will cause packets arriving at the fiber network to be dropped. The role of satellite systems in global communications is growing at an unprecedented pace. From direct broadcast satellite (DBS) television, to satellite phones, to credit card transactions, to the Internet, corporations and consumers are embracing these new services and placing demands on bandwidth.
From the late 1950's with the launches of Sputnik and Explorer I, satellites have become a growing part of our world. From various types of platforms many different applications are run through these satellites. Since the first commercial model launched into orbit in 1965, the communications satellite has become the linchpin of global communications. From modest beginnings "the first satellite could only handle 240 voice transmissions at a time" the technology has blossomed to the extent that satellites now carry about one third of the voice traffic between countries and essentially all the television signals between countries. The backbone of this system is the geosynchronous orbit satellite (GEO), these are large communication satellites placed in orbit roughly 36,000 kilometers above the same spot on the earth at all times. Because of this high orbit each satellite is able to see about one quarter of the earth, so only four or so are needed for global coverage. Drawbacks to these satellites are that due to the high altitude it takes a quarter of a second for signals to travel to and from the satellite, delaying the responses during a conversation. Also the higher altitude means a weaker signal so larger antennas are needed to maintain overall coverage.
The next layers of satellites are middle earth orbit (MEO) and low earth orbit (LEO). MEO satellites are generally placed in orbits above 10,000 kilometers, while LEO satellites are generally below 1,500 kilometers. Of course now with the lower altitude the signals are stronger so antenna size can be reduced, but with this also comes less coverage so more satellites are required. In the case of the LEO satellites at least 50 are usually required for full global coverage.
The basic types of satellite systems include geostationary (GEO), Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Highly Elliptical Orbit (HEO) satellites . There are also public and private satellite systems such as Television Receive Only (TVRO), Direct Broadcast Satellite (DBS), Global Positioning System (GPS), and multibeam satellite operations. The Internet is another area where satellites can be utilized to provide increased and enhanced service. As content providers look to push more audio, video, and animation over the Web, satellites may enjoy a unique advantage in the area of webcasting. Also, the inherent broadcast architecture of satellites makes connectivity highly attractive to Internet service providers (ISP) who are experiencing bottlenecks and network management difficulties due to the exponential demand for bandwidth. We can spread the bandwidth using spread spectrum techniques such as DSSS and FHSS.
Geosynchronous satellites orbit the Earth on repeatedly regular points over time. Each GEO satellite is stationary over one spot above the equator and therefore does not need any tracking from receiving and transmitting antennas on the Earth. GEO satellites enable the coverage of weather events. They are especially useful for monitoring severe local storms and tropical cyclones. They are best for television transmission and high-speed data transmission. Low Earth Orbit (LEO) satellite systems fly very closely to the surface of the Earth, up to 1,500 kilometers in altitude. They deliver more significant voice quality over GEOs and transmit signals with a small margin of delay. Some LEO systems are designed for satellite phones or global mobile personal communications systems. These can carry voice traffic among other data formats. The reasoning behind the orbit spacing comes from the presence of the Van Allen Radiation Belt. Originally discovered by Explorer I, the Van Allen Belt is composed of energetic ionized particles, which could damage solar cells and perhaps other solid state components.
VSAT, very small aperture terminal typically refers to a class of Earth stations with a small diameter in the range of 0.95 meters to 2.4 meters. The terminals have both transmitted and receive capability. The majority of traditional VSAT users such as gas stations, convenience stores and banks selected the technology primarily to manage transaction-based applications - point-of-sales credit authorization and inventory control. The only reliable means of communication at the disposal of the American Red Cross was INMARSAT satellite technology. Satellite phones were used for voice and fax transmissions, as well as data transmissions and packet switching, primarily for disaster welfare inquiries and contacting people to let them know their relatives. Satellite receivers initially were prohibitively expensive, beyond most consumers' reach, and the antennas required for receiving signals were so large and unsightly that local governments and homeowner associations barred them from residential property use. As satellite services continue to improve there will be more encouragement for widespread use of wireless communication. Satellite links in satellite communication (SATCOM) are responsible for enabling wireless communication between satellite users.
Routing means to find a efficient and dedicated path between the interested parties. Routing in satellite can be done in 2 different ways. With ISL link and without ISL link, in first case, the system requires only 1 uplink and 1 downlink where as in second case; the overall routing mechanism requires 2 uplinks and 2 downlinks . Addition of uplink and downlink increases the overall maintenance cost of the satellite system. Following satellite links such as ISL, GWL, MUL performs all radio specific tasks in satellite communication, ISL ie inter-satellite link is responsible to enable communication between two different satellites whenever there is no connectivity through base station or two satellite can transmit their signals directly. Through GWL which is gateway link, it connects two or more than two gateways so that users can communicate without using ISL . MUL ie mobile user link is responsible for transmitting signals from user to the satellite and from satellite to user with same transmission capability.
Above fig.  shows the typical satellite system routing mechanism, users in different footprint can transmit their signals by using any one of the path/route and can perform their communication. Footprint is the area covered by the satellite and it is restricted to the are dependent on the type of satellite using for the communication.Routing in satellite system is critical as compared to other wired network as there is no dedicated path from sender to receiver.
Military satellite communications architecture (MILSATCOM) uses techniques such as spread spectrum, nulling, and on-board processing that is intended to provide anti-jamming immunity, low probability of intercept and low probability of exploitation. These features make MILSATCOM more robust than commercial satellite communications (SATCOM) and more suitable for the support of military operations. Present planning (1994), proposes that Department of Defence SATCOM requirements be met by a general purpose SATCOM system (GPSS) provided by commercial space segments as well as a highly survivable SATCOM which would be provided by a special purpose SATCOM system (SPSS) using a military space segment. High frequency radio lacks the reliability and the capacity required for military operations . Line of sight radios have neither the range required nor the ability to operate in all topographical areas. Land lines are not often not available, take a long time to install and are highly vulnerable to disruption. During the 1992 Persian Gulf War, allied forces relied heavily on SATCOM resources for communications. SATCOM is a way to provide reliable, effective high quality and global communications in a timely manner. Other means of communications have inherent limitations from the SATCOM.
Satellites operate on three major frequency bands within the radio spectrum - ultra high frequency (UHF), super high frequency (SHF), and extremely high frequency (SHF). The SHF and EHF bands are further subdivided for the purposes of assigning frequencies to specific functions . The table below shows the various frequencies and their sub-divisions and some of the satellites which operate in these bands. Where frequencies are separated by a slash, these numbers represent uplink and downlink transmissions respectively. Sufficient bandwidth separation has been engineered so that uplink and downlink communications to not cause mutual interference.
Localization In satellites system can be done by means of three registers which stores all the relevant information about the satellite and the satellite users . The three above mentioned registers are HLR,VLR and SUMR.HLR are responsible for storing all the static information about the users and VLR stores the last known location of the user respectively. SUMR performs 2 basic functions, first it does the mapping between the user and the satellite and second it stores all the information about the satellite.
Satellites have had a major impact on our lives, in both personal areas and in industry. The technology has proven its versatility versus other communication formats with its ability to provide service to areas on the globe that are virtual inaccessible to most others technologies. This function has proven to be instrumental in assisting communities in times of strife and has also helped companies establish better information links with their facilities in more remote areas.
As the communications market continues to grow at an astronomical rate, the main focus of the industry will be on how improvements can be made in cost, speed, quality, and convenience for consumers. While satellites have been a leading force in providing solutions in these areas, such as the reduction in size of receiver antennas to three feet or less making installation and usage much easier on the customer. The element that will be key to the future is the ability to give access to remote areas around the globe.
While in the United States has many systems, especially fiber optic networks, global telecommunication, in many areas, lacks the internal infrastructure to support even the most basic of telephony services. Satellites are an excellent method to establish access to areas with poor terrestrial infrastructures where capacity is either not available or is cost prohibitive.
The Internet is another area where satellites can be utilized to provide increased and enhanced service. While now the majority of Internet content resides in the United States, in the very near future increased amounts of content will originate from Asia, Eastern Europe, and South America. Also, the inherent broadcast architecture of satellites makes connectivity highly attractive to Internet service providers (ISP) who are experiencing bottlenecks and network management difficulties due to the exponential demand for bandwidth. As content providers look to push more audio, video, and animation over the Web, satellites may enjoy a unique advantage in the area of webcasting.
While satellites enjoy increased accessibility to areas around the world, fiber optics enjoy a much greater capacity and speed potential. Where available future systems should look to mesh these two technologies together. Though meshing these two technologies together would require the development of gateways capable of optimizing the inherent benefits of each, while addressing the differences between the two. The major issue to be addressed will be accommodating the delay or latency due to the slower speeds by satellite, this will cause packets arriving at the fiber network to be dropped.
Routing and localisation in satellite systems are two critical issues . Inter satellite links (ISL) helps to perform communication between two satellites so that the users of two different satellites can perform their data transfer and can achieve global mobile communication. Localisation is important to locate the position of user as well as the position of satellites which enables mobile communication
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