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Satellite Communication Systems

Outcomes

This section addresses aspects of the following syllabus outcomes:

H1.1 describes the scope of engineering and critically analyses current innovations

H2.2 analyses and synthesizes engineering applications in specific fields and reports on the importance of these to society

H4.1 investigates the extent of technological change in engineering

H4.3 appreciates social, environmental and cultural implications of technological change in engineering and applies them to the analysis of specific problems

What is a satellite?
Basic Components of Satellites
Types of Satellites
A Brief History of Satellites
Uses of Satellites
Current Innovations

What is a satellite?

An object that revolves around a planet in a circular or elliptical path is termed a satellite. The moon is Earth's original satellite but there are many man-made satellites, mostly closer to Earth.

The path that a satellite follows is termed an orbit. An object, such as a javelin, that is projected horizontally will fall to earth describing a parabolic arc as shown in figure one.

Fig 1. A Javelin Thrower

A bullet fired by the rifleman in figure two is projected at a higher velocity than the javelin so will travel further but must still fall to earth describing a parabolic arc. The radius of the Earth’s itself becomes significant at this magnitude. The bullet through the simple fact that it is travelling around the earth’s curvature gains extra distance.

Fig 2. The radius of the Earth is significant.

If we could, however, fling a rocket with the astounding velocity of 600 kilometres per hour, the object would miss the earth as it falls (see Figure Three). The Earth would still cause a gravitational pull that would have the effect of continuously changing its direction. The object would be in orbit. If a satellite was attached to the rocket it could be released at the appropriate time and also placed into an orbit.

Fig 3. A Rocket can be used to put a satellite into orbit.

Artificial satellites are special, purpose built craft, designed to perform intended functions and are not simply mass-produced. There are some exceptions, including the GPS satellites, with over twenty in orbit, and the Iridium satellites of which there are more than sixty.

Only a few decades ago satellites were considered top-secret, exotic devices for use by the military for activities such as navigation and espionage. In recent years they have become commonplace and essential for our daily living.

A communications satellite, then, is a spacecraft that orbits the Earth and relays radio, telephone and television signals. Earth stations transmit signals to the satellite, which are then amplified and relayed to other ground stations. As a newer form of communication, communications satellites are very useful in bringing the people in the world together. Communication between people across the world in the nineteenth century often took months. In the twentieth century this became days and minutes, but now takes seconds using satellites.

Advantages and Limitations

Satellite communication has a number of advantages:

It saves the cost of intercontinental cable laying and the maintenance that would otherwise be necessary
Heavy usage can be guaranteed, which makes the satellite commercially attractive
They can cover large areas of the Earth making them particularly useful for sparsely populated areas.

Satellite communication is, however, limited by four factors:

The development of large, antennas on satellite platforms is prevented by technological limitations
There is over-crowding of available bandwidths
The massive investment and insurance costs due to the significant probability of failure
The satellite will need to be replaced over time (to upgrade technology, ‘falling’ out of orbit).

Basic Components of Satellites

Information is transmitted from a ground station (uplink) to the satellite, converted to a different frequency and re-transmitted back to Earth (downlink). The downlink may either be to a single ground station or the transmission may be broadcast to a large region via multiple ground stations.

The satellite must have a receiver with a receive antenna, a transmitter with a transmit antenna, an amplifier and prime electrical power to run all of the electronics. The configuration of this equipment will vary according to the satellite design but every communication satellite will have these basic components. They are illustrated in figure four below.

Fig 4. Basic functional equipment.

The effectiveness of a microwave antenna designed either to provide amplification or beam the signal into defined regions of space is dependent on its size, which is in turn limited by cost. By current calculations, doubling the antenna size will result in the satellite cost increasing eight times.

Activity 1

Use the web links provided below, and your own web research skills to;

Outline the role of engineers that are employed in the field of satellite communications systems.

Answer

Types Of Satellites

Geostationary

Science fiction writer, Arthur C. Clarke calculated that if a satellite was placed in orbit at a distance of 36000Km from the Earth it would have an angular velocity equal to the Earth’s own orbital velocity. It would remain directly overhead of the same point on the earth’s surface at all times. The term geostationary applies to satellites in such an orbit. Kepler’s law proves that as the orbit increases in radius, the angular velocity reduces, until it is coincident with the Earth’s at a radius of 36000Km.

Communications satellites are launched by rockets or carried into space by the Space Shuttle. Once in space, small engines (thrusters) guide the satellite into orbit and help keep them there. Three correctly placed geostationary satellites can provide complete coverage of the Earth’s surface (Figure five). The three Optus satellites sit in geostationary orbit at around 160° E longitude.

Fig 5. Geostationary satellites providing global coverage

Lower Orbiting Satellites

Lower Earth Orbit (LEO) satellites orbit at about 500 km above the earth and are used for reconnaissance, localized weather and photography of natural resources. The NASA space shuttle is capable of launching and retrieving satellites in this orbit. These satellites are also suitable for personal voice and data communications.

Passive and Active Satellites

The first communications satellites were “passive” in that signals from Earth were merely reflected from the orbiting craft. Modern satellites are termed “active” because they receive signals from Earth, electronically strengthen the signals, and transmit the signals back to Earth.

A Brief History of Satellites

Arthur C. Clarke wrote a short article in 1945 in the Wireless World journal that outlined the use of manned satellites in orbit above the Earth to distribute television programs.
John Pierce of Bell Telephone Laboratories was the first person to carefully evaluate the various technical and financial options in satellite communications. In a 1954 speech and 1955 article he explored the usefulness of a communications “mirror” in space, a medium-orbit “repeater” and a 24-hour-orbit “repeater.”
History changed on October 4, 1957 , when the Soviet Union successfully launched Sputnik I. About the size of a basketball, the world’s first artificial satellite weighed only 183 pounds and took about 98 minutes to orbit the Earth on its elliptical path. That launch ushered in new political, military, technological, and scientific developments since it marked the beginning of the space age and the U.S.-U.S.S.R space race.
On November 3, 1957 , Sputnik 2 was launched carrying the first living creature into space.
USA joined the race when, on February 1, 1958 they launched Explorer 1.
The first satellite to relay messages from one Earth station to another was SCORE (Signal Communicating by Orbiting Relay Equipment) launched December 18, 1958 . Because they were visible from both sides of the Atlantic Ocean simultaneously for only a short time, these satellites could provide only a period of five hours a day for communications.
Explorer 7, launched October 13, 1959 was the world’s first meteorology satellite.
In 1960 the United States launched the Echo satellite, a metallic balloon that reflected the first telephone communication & TV broadcast signals via satellite.
Amateur Radio operators around the globe have built and sent dozens and dozens of Amateur Radio communications and science satellites to orbit since the first, OSCAR-1, was launched on December 12, 1961.
The UK launched its first satellite: Ariel 1 on April 26, 1962
By 1964, two TELSTARs, two RELAYs, and two SYNCOMs had operated successfully in space.
On April 6, 1965 COMSAT’s first commercial geosynchronous satellite, EARLY BIRD, was launched from Cape Canaveral . Global satellite communications had begun in Australia .
November 29, 1967 saw Australia ’s first satellite: Wresat
The first man on the moon occurred on 21 st July 1969
On the 11 th February 1970 Japan launched its first satellite: Ohsumi
Satellites were brought back from space for the first time in 1984.
PanAmSat (1995) was the first private satellite to provide global services.
Measat 1, the first Malaysian satellite was launched in 1996.

Uses of Satellites

Unmanned satellites have six primary functions:

perform tasks in applied technology eg. Communications, observation, and navigation.
investigate the characteristics of the upper atmosphere
study the nature of the earth’s space including energetic particles, interplanetary matter, and electromagnetic radiation.
study celestial objects from a vantage point beyond the atmosphere.
carry out biological experiments.
support of manned operations.

Weather Satellites

Some weather satellites, such as the U.S. “Goes” and European “Meteosat”, circle in geostationary orbit while other satellites, such as the U.S. “NOAA” series, are launched into a polar orbit, over the North and South Poles. These satellites can scan the whole Earth every twelve hours.

The use of these and other weather satellites has revolutionized prediction because they are able to scan the whole Earth and its atmosphere continuously. The complete Earth’s surface is photographed at least once daily for use in weather forecasting. The satellite data provides information about the oceans, deserts, and polar areas of the Earth that were unable to be reported on previously. Weather systems developing anywhere in the world, even where there are no ground stations, can now be shown.

Radio Satellites

Here’s a startling fact – Amateur radio operators have built more than 70 Orbital Satellites Carrying Amateur Radios (OSCARS) over four decades that take advantage of leftover payload transmission space to get a free launch into orbit. The number is astonishing because these sophisticated and groundbreaking spacecraft are little known outside the ham radio fraternity.

Other communications satellites pinpoint the locations to rescue teams by listening for distress signals from ships and aircraft.

Telephony

Cell phones transmit to a local cell radio, which connects to the public service telephone network through an optic fibre or microwave radio. When the phone leaves the cell radio’s area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use digital encoding.

Satellite phones come in two types: INMARSAT and Iridium. Both types provide worldwide coverage. INMARSAT uses geosynchronous satellites, with aimed high-gain antennas on the vehicles. Iridium provides cell phones, except the cells are satellites in orbit.

Television Broadcasting System

The transmission of television programs is a familiar use of communication satellites. High bandwidths can easily be provided that allow the sending of television broadcasts with ease. In addition, developments in broadcast technology allow different types of transmissions to be sent, taking advantage of sharing the same satellite signal. Using receivers, amplifiers and transmitters these communication satellites can simultaneously relay multiple telephone and television signals .

Video

Television sends the picture as AM, and the sound as FM, on the same radio signal.

Navigation

All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the mathematics.

Current Innovations

In the 50 years since Arthur C. Clarke first invented satellite technology it has developed at a startling rate. Satellite technology is all around us and has become a very useful, everyday phenomenon providing a variety of services including broadband communication systems, satellite-based video, audio, internet and data distribution networks, as well as worldwide customer service and support.

Intelsat Telecommunication is the largest worldwide satellite communication network covering 110 nations, and operating powerful communications satellites, such as Intelsat 6, over the Atlantic , Pacific, and Indian Oceans . Intelsat spans the globe, with domestic satellites such as the USSR ’s Molniya satellites, Western Union ’s Westar, and Canada ’s Anik serving individual countries. The Intelsat V is the latest in its spacecraft series and handles 12,000 telephone circuits and two colour television transmission simultaneously.

In-Car Satellite Navigation

GPS car navigation units have become easy to install and easy to use, not requiring links to a computer nor any technical knowledge. They can be simply mounted on the car dashboard. Just turn it on and access software, maps and colour 3D navigation views, as well as voice-guided instructions in a language of choice.

Fig. 6 An example of an In-Car Navigation Unit

These systems offer the latest street-level maps of each country, including details for towns and cities. They can offer blue-tooth enabled mobile phone connection, providing real time information on traffic congestions, weather conditions and extra downloads such as detailed city maps, additional voices, places of interest and much more.

Low-earth orbiting (LEO) Satellites

Recently there has been interest in low-earth orbiting (LEO) satellites. Here, a satellite placed in a 1000Km orbit has an orbital time of 1 hour. These satellites can be operated in a store-and-forward mode, picking up data at one part of the globe and physically transferring it to another. Because the data-rates and orbit radius are greatly reduced, small, low-cost satellites and ground stations are possible. However, such satellites have yet to demonstrate any commercial success.

Iridium Flares

These are small satellites in low orbit around the earth responsible for worldwide telecommunications. There are over 80 of these satellites in 6 orbit planes at an altitude of about 780km. Each day a number of these satellites become visible (as flares) when they are in a position to reflect the sun just like a mirror.

Activity 2

Research one current innovation in satellite communication systems (such as the spread of broadband internet access or in-car GPS navigation) to determine the following aspects.

What existed before the innovation?
How does the innovation work?
How has the innovation impacted on the lives of people, both good and bad.

Answer

Solutions to Activities

Activity 1
Activity 2

Links

NASA Experimental Communications Satellites
A History of Satellite Communication - SatNews Publishers
Satellite Communication – A Short Course – The Glen Research Centre
Beyond The Horizon – Reinhold Kainhofer
Windows To The Universe
Kid’s Net - Radio
Making The Global Village Possible
St Mary’s Physics Online
Wireless Technology