|Practical No. 2
A satellite network is a combination of modes that provide communication from one point on the earth to other. A node in the network can be a satellite an earth station, or any end user terminal or a telephone. Although a real satellite such as the moon can be used as a relaying node in the network, the use of artificial satellites is preferred because we can install electronic equipment on the satellite to regenerate the signal that has lost its energy during travel.
Another restriction of using a natural satellite is their distances from the earth which create a long delay in communication. Satellite networks are like cellular networks in that they divide the planet into large cells. Satellites can provide transmission capability to and from any location on earth no matter how remote it is. This advantage makes high quality communication available to undeveloped parts of the world without requiring a huge investment in ground based infrastructure.
An artificial satellite needs to have an orbit, the path in which it travels around the earth. The orbit can be equatorial, inclined or polar. The period of a satellite that is the time required for a satellite to make a complete trip around the earth is determined by Kepler’s law which defines the period as a function of the distance of the satellite from the centre of the earth. Satellites process microwaves with bi directional antennas that is line of sight. Therefore the signal from a satellite is normally aimed at a specific area called the footprint. The signal power at the centre of the footprint is highest. The power decrease as we move from the foot print centre. The boundary of the foot print is the location where the power reaches a pre defined threshold.
Based on the location of orbits satellites can be divided into three categories that is geosynchronous earth orbit, low earth orbit and medium earth orbit. Location of orbits is the distance of orbits from earth’s surface. One reason for having different reasons is due to the existence of two Van Allen belts. A Van Allen is a layer that contains charged particles. A satellite orbiting in one of these two belts would be totally destroyed by the energetic charged particles. The medium earth orbits are located between these two belts. The frequencies reserved for satellite microwave communication are in gigahertz range. Each satellite sends and receives over two different bands. Transmission from the earth to the satellite is called uplink and transmission from the satellite to the earth is called downlink.
Compared to the fiber optical communication, satellite communication has the advantage that the quality of transmitted signal and location of sending and receiving stations are independent of distances.
The first artificial satellite was placed in orbit by the Russians in 1957. That satellite, called Sputnik, signaled the beginning of an era.
The United States, who was behind the Russians, made an all-out effort to catch up, and launched Score in 1958. That was the first satellite with the primary purpose of communications.
The first regular satellite communications service was used by the Navy in 1960. The moon was used to bounce teletypewriter signals between Hawaii and Washington, D.C. During the early 1960s, the Navy used the moon as a medium for passing messages between ships at sea and shore stations. This method of communications proved reliable when other methods failed.
Military satellite communications technology was at a low level until 1965. At that time high quality voice transmissions were conducted between a satellite and two earth stations. That was the stepping stone to the Initial Defense Communications Satellite Program (IDCSP), which will be covered later in this chapter.
Experience with satellite communications has demonstrated that satellite systems can satisfy many military requirements. They are reliable, survivable, secure, and a cost effective method of telecommunications. You can easily see that satellites are the ideal, if not often the only, solution to problems of communicating with highly mobile forces. Satellites, if properly used, provide much needed options to large, fixed-ground installations.
For the past fifty years, the Navy has used high-frequency (hf) transmissions as the principal method of sending messages. In the 1970s, the hf spectrum was overcrowded and "free" frequencies were at a premium. Hf jamming and electronic countermeasures (ECM) techniques became highly sophisticated during that period. As a result the need for new and advanced long-range transmission methods became apparent.
Communications via satellite is a natural outgrowth of modern technology and of the continuing demand for greater capacity and higher quality in communications.
In the past, the various military branches have had the resources to support their communications needs. Predicted usage indicates that large-scale improvements will have to be made to satisfy future needs of the Department of Defense. These needs will require greater capacity for long-haul communications to previously inaccessible areas. Satellite communications has the most promise for satisfying these future requirements.
Satellite communications basics
When used for communications, a satellite acts as a repeater. Its height above the Earth means that signals can be transmitted over distances that are very much greater than the line of sight. An earth station transmits the signal up to the satellite. This is called the up-link and is transmitted on one frequency. The satellite receives the signal and retransmits it on what is termed the down link which is on another frequency.
Using a satellite for long distance communications
The circuitry in the satellite that acts as the receiver, frequency changer, and transmitter is called a transponder. This basically consists of a low noise amplifier, a frequency changer consisting a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make sure that any out of band signals such as the transponder output are reduced to acceptable levels so that the amplifier is not overloaded. Similarly the output from the amplifiers is filtered to make sure that spurious signals are reduced to acceptable levels. Figures used in here are the same as those mentioned earlier, and are only given as an example. The signal is received and amplified to a suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a superheterodyne radio receiver. As a result the communications satellite receives in one band of frequencies and transmits in another.
In view of the fact that the receiver and transmitter are operating at the same time and in close proximity, care has to be taken in the design of the satellite that the transmitter does not interfere with the receiver. This might result from spurious signals arising from the transmitter, or the receiver may become de-sensitised by the strong signal being received from the transmitter. The filters already mentioned are used to reduce these effects. Signals transmitted to satellites usually consist of a large number of signals multiplexed onto a main transmission. In this way one transmission from the ground can carry a large number of telephone circuits or even a number of television signals. This approach is operationally far more effective than having a large number of individual transmitters.
Obviously one satellite will be unable to carry all the traffic across the Atlantic. Further capacity can be achieved using several satellites on different bands, or by physically separating them apart from one another. In this way the beamwidth of the antenna can be used to distinguish between different satellites. Normally antennas with very high gains are used, and these have very narrow beamwidths, allowing satellites to be separated by just a few degrees.
Separating satellites by position
Communications satellites are ideally placed to provide telecommunications links between different places across the globe. Traditional telecommunications links used direct "cables" linking different areas. As a result of the cost of installation and maintenance of these cables, satellites were seen as an ideal alternative. While still expensive to put in place, they provided a high bandwidth and were able to operate for many years.
In recent years the bandwidth that can be offered by cables has increased considerably, and this has negated some of the gains of satellites. Additionally the geostationary satellites used for telecommunications links introduce a significant time delay in view of the very large distances involved. This can be a problem for normal telephone calls.
Mobile satellite communications systems
There are many instances where communications need to be maintained over wide areas of the globe. Ships, aircraft and the like, need to be able to communicate from points all around the world. Traditionally HF radio communications ahs been used, but this is unreliable. Satellite communications provide an ideal solution to this problem as satellite communications are much more reliable and they are able to provide interference free stable communications links. As a result, Satellite communications is now fitted as standard to all maritime vessels, and it is becoming increasingly used by aircraft, although it is not yet adopted for Air Traffic management (ATM).
In addition to these users, these services can be sued by many land mobile or land portable radio users. Satellite terminals provide are able to access the satellite and the users is able to achieve communications from almost anywhere on the globe. As these communications satellites are in geostationary orbits, communications is not possible towards the poles as in these regions it is not possible to see the satellites.
Another variant of communications satellites is those used for direct broadcasting. This form of broadcasting has become very popular as it provides very high levels of bandwidth because of the high frequencies used. This means that large numbers of channels can be carried. It also enables large areas of the globe to be covered by one delivery system. For terrestrial broadcasting a large number of high power transmitters are required that are located around the country. Even then coverage may not be good in outlying areas.
These DBS satellites are very similar to ordinary communications satellites in concept. Naturally they require high levels of transmitted power because domestic users do not want very large antennas on their houses to be able to receive the signals. This means that very large arrays of solar cells are required along with large batteries to support the broadcasting in periods of darkness. They also have a number of antenna systems accurately directing the transmitted power to the required areas. Different antennas on the same satellite may have totally different footprints.
Satellite phone systems
Satellites have also been used for cellular style communications. They have not been nearly as successful as initially anticipated because of the enormously rapid growth of terrestrial cellular telecommunications, and its spread into far more countries and areas than predicted when the ideas for satellite personal communications was originally envisaged. Nevertheless these satellite phone systems are now well established and have established a specific market. Accordingly these satellite phone systems are now widely available for mobile communications over wide areas of the globe.
The satellite phone systems that are available have varying degrees of coverage. Some provide true global coverage, although others are restricted to the more densely populated areas of the globe.
The systems that were set up used low earth orbiting satellites, typically with a constellation of around 66 satellites. Handheld phones then communicated directly with the satellites which would then process and relay the signals as required.
Other satellite phone systems use a number of geostationary satellites, although these satellite phone systems generally require the use of a directional antenna in view of the larger distances that need to be covered to and from the satellite. Additionally the levels of latency are higher (i.e. time delay for the signal to travel to and from the satellite) in view of the much higher orbit required. However as the satellites are geostationary, satellite or beam handover is less of a problem.
The main advantage of the satellite system is that it is truly global and communications can be made from ships, in remote locations where there would be no possibility of there being a communications network. However against this the network is expensive to run because of the cost of building and maintaining the satellite network, as well as the more sophisticated and higher power handsets required to operate with the satellite. As a result calls are more expensive than those made over terrestrial mobile phone networks.
Antenna’s used in satellite communication
Although the basics of satellite communications are fairly straightforward, there is a huge investment required in building the satellite and launching it into orbit. Nevertheless many communications satellites exist in orbit around the globe and they are widely used for a variety of applications from providing satellite telecommunications links to direct broadcasting and the use of satellite phone and individual satellite communication links.