AN INTRODUCTION TO SATELLITE COMMUNICATIONS

Satellite communication technology is already part of our everyday lives often without us realising it. Nowadays many of our telephone calls are routed via satellite and the majority of us watch satellite TV or even communicate on the Internet via satellite.

When considering whether to use satellite broadcast or communications as part of a corporate network or within the consortia involved in a large international project, it is important to be able to appreciate the role that satellite communications (satcoms) can play. This short paper provides a brief introduction to satcoms.

2. Benefits of Satellite Communications.

Satcom systems have a few classical benefits which mean for certain applications / services there is simply no comparable alternative. These cases are covered briefly below.

2.1. Instantaneous point to multipoint broadcast capability

Each satellite has a certain coverage area (footprint) within which any location can communicate with another. This allows a powerful broadcast capability which can allow a company headquarters to simultaneously provide important information, or database updates to all regional offices instead of having to use multiple point to point links (such as leased telephone lines).

2.2. Coverage of remote geographical areas

In some countries the terrain could be mountainous or small isolated groups of population exist. Terrestial communications tend to provide good communications infrastructure for the larger cities but the more remote areas are less well served. Satellite communications is possible anywhere within a particular footprint coverage regardless of topology, hence, satellites can readily provide the required communication services.

2.3. Back-up communications for existing infrastructure

Since satellite communications can be independent of existing infrastructure then one particular benefit is the potential for fast deployment of additional communication networks at times of natural disasters (e.g. earthquakes) when the existing communication infrastructure could be damaged. Alternatively, during times of short term prestigous events, such as during the Olympic Games or when Meetings between Heads of State and Governments take place, there is also a requirement for the rapid deployment of additional communications. Small portable terminals can easily provide the necessary communications via satellite in these cases.

2.4. Group addressing capabilities

One other benefit of satellite communication systems is the possibility to send information to selected groups of individuals. Once a network has been installed in the various project sites or company premises of an organisation, then the Headquarters site can download financial data to one subset of sites, technical data to another group of sites and maintainance data to a third group. Hence, by configuring your network appropriately, the same equipment allows selective broadcast / information exchange.

3. Components of a Satellite System.

As with any technology there are a certain number of concepts that you need to be familiar with to understand how to evaluate your own requirements. Any satellite communications system basically consists of the following components (see also attached Figure - to be incorporated soon!):

a source of data / video / multimedia (your application)
satellite modems at transmit and receive sites
a VSAT (includes antenna) at transmit and receive site
the satellite providing coverage of required area(s)

The source of data / video / multimedia is basically the information you wish to send. Do you want to send data files or e-mail, be able to videoconference, or teach interactively? The format of your information is important and whether delivery of the information must occur in real time and/or securely.

Once the data, or TV signal is in the correct format then the information will be coded by the modem and transmitted by the VSAT (Very Small Aperture Terminal) towards the satellite. The VSAT is basically an antenna, with an amplifier and equipment that transfers your coded signal to a frequency that the satellite is expecting.

Once at the satellite, the signal is re-amplified and transferred to a slightly lower frequency before being transmitted back down to earth. Every satellite has a coverage area or footprint defined by the design of the antenna on board the spacecraft. Within this coverage area the signal can be received at different strengths depending upon whether the receive site is close to the centre or at the extremes of the footprint. Consequently the location of your site within the satellite footprint will affect the dimensions / capability of the equipment required for your service.

At the receive site, the VSAT antenna will receive the signal from the satellite and decode it to retrieve the original data or video. The VSAT at the receive site could be equipped just to receive the signal or alternatively may need to respond with some further information or requests. In this case the VSAT will also be equipped with an amplifier and transmit equipment. A one-way VSAT (receive only) is considerably cheaper than a two-way (transmit and receive) VSAT.

Clearly, for each service or application there will be different critical requirements which will dictate the size, capability and features of each component of the system.

4. Key Parameters to Define a Satellite Communications System.

Clearly to a potential user of a satellite communications system the cost of the system is a critical issue - both in terms of initial investment costs and also operating costs of the system per user on a daily basis. Many parameters affect both these costs and need to be defined in order to provide even an approximate figure for the costs of a satcomm system.

4.1. Network Service & Topology.

The number of sites and their locations involved in the network or service is a key factor in defining your optimal system. The location of the sites defines which satellite can be used and how strong the receive signal from the satellite will be.

Satellite communication links are degraded by the weather, particularly rainfall. Internationally agreed values exist that predict the expected rainfall for any location and thus in turn the extra power needed for any satellite communications system to overcome this degradation can be calculated. This very simple process allows your network to be designed (sized) to overcome this effect, however, it does mean that the location of the sites must be defined as early as possible.

Another important parameter to define is the required data rate and whether the service is one-way or two-way. As with any form of communications system, the higher the data rate, the more bandwidth that is required and the higher the cost.

4.2. Availability of Service.

Since the safety margin allowed for degradation by weather is a statistical calculation, the satellite operator will only guarantee a specified quality of performance for a certain percentage (availability) of the year - such as 99 % of the year (i.e. potentially up to 88 hours within a complete year when the link quality cannot be guaranteed). Since a higher availability (such as 99.9%) can become very expensive, it is important to define:

is there a time constraint on the delivery of the information?
what are the consequences of the system not being operational at any moment?
what error rate is acceptable in the data

For example: a service to allow a weekly videoconference session could accept a lower availability (e.g. 99 %) than if financial data were being transferred (99.99% prefered).

4.3. Power.

Satellite communications system design is simply a question of achieving a certain performance by balancing the power transmitted by antennas (both on earth and on the spacecraft) with the degradation to be expected on the signal (by travelling through space, or equipment inefficiencies). Essentially, the more powerful the equipment on the ground, the less power needed from the spacecraft. Satellite capacity is charged according to the satellite resources (i.e. power and bandwidth) used, consequently it becomes a trade-off between a one-off capital cost for the equipment on the ground or repeated operational costs for use of the satellite resources. This calculation must be performed for each proposed satellite network to determine the optimium approach.

Once the above parameters have been defined, then a preliminary "power link budget" is performed to provide a first guide to the size and power of earth stations needed to provide the service. This normally becomes an iterative process based upon optimising equipment costs, space segment costs and actual service requirements.

4.4. Earth Station Performance.

Once a preliminary outline of the capability of the required earth stations has been defined then it will be necessary to contact equipment suppliers and review the options available on the market.

The earth station can be fixed (i.e. permanently in one location) or mobile. Some stations are classified as portable rather than mobile since although transportable they need to be installed (albeit quickly) before they can be used. True mobile systems allow use while in transit such as in a car, airplane or ship.

When reviewing information about earth stations, it is helpful to understand the following terms:

Gain - this is the amplifying factor the antenna will give to the (transmit and/or receive) signal and is dependant upon the size of the antenna and the signal frequency
EIRP (or Effective Isotropic Radiated Power) - is the power that can be transmitted from the earth station and is derived from a formula linking the gain of the antenna and the size of the amplifier
Solid State Power Amplifier (SSPA) - a compact form of amplifier used to increase the power of the signal being transmitted by the VSAT towards the satellite
G/T (or Gain / Temperature) -is a measure of how well the antenna can receive a signal and is derived from a formula linking the gain of the antenna and the losses within the receive system

5. Conclusions.

There are many factors contributing to the definition of a satellite communications systems. If you are part of the EC Telematics Programme then the SATEMA Project can provide guidance in this process or can put you directly in contact with service providers or satellite operators. Whichever path you choose, in order to evaluate whether a satellite communications system is best for you, you will need to provide enough information to allow the definition of the optimal architecture for your network in the manner described above.

Please feel free to contact the SATEMA Project Team at any time to discuss any ideas you may have. and bear in mind that the possible applications that satcoms systems can support are basically unlimited - so don't be shy!

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