Introduction: There are literally an infinite number of possible orbits for an earth satellite. While there are special orbits that are designed for specific purposes, three general classes of orbits have come into wide spread use for observations of the earth and for microwave communication with the earth: Geostationary orbits, low earth orbits, (LEOs) and polar orbits. Note that all polar orbits are subsets of LEO orbits.

Geostationary Orbit: Geostationary orbits are circular orbits that are orientated in the plane of the earth's equator. In a geostationary orbit, the satellite appears stationary, i.e. in a fixed position, to an observer on earth. More technically, a geostationary orbit is a circular prograde orbit in the equatorial plane with an orbital period equal to that of the earth; this is achieved with an orbital radius of 6.6107 (equatorial) earth radii, or an orbital height of 35786 km. A satellite in a geostationary orbit will appear fixed above the surface of the earth, i.e. at a fixed latitude and longitude. In practice, the orbit has small non-zero values for inclination and eccentricity, causing the satellite to trace out a small figure of eight in the sky. The footprint, or service area, of a geostationary satellite covers almost 1/3 of the earth's surface (from about 75 degrees south to about 75 degrees north latitude), so that near-global coverage can be achieved with a minimum of three satellites in orbit. By placing the satellite at an altitude where it's orbital period exactly matches the rotation of the earth (approximately 35,800 km), the satellite thus appears to 'hover' over one spot on the Earth's equator and thus appears to stay stationary over the same point. A geostationary satellite completes one orbit revolution in circular orbit, round the Earth, every 24 hours. If the orbit is in the equatorial plane, and if rotation is in the same direction as the Earth, (rotating at the same angular velocity as the Earth) and it overflies the same point on the globe permanently then the satellite is termed geostationary. Geostationary satellites, however, do not see the poles at all yet geosynchronous satellites do. (For a more technical description of geosynchronous vs. geostationary nomenclature.) In general, all geostationary orbits are geosynchronous, but not all geosynchronous orbits are geostationary. Geosynchronous means that a satellite makes one orbit every 24 hours so that it is 'synchronized' with the rotation period of the earth. As previously stated, this will happen when a satellite is in a circular orbit at a rough distance of 36,000 kilometers above the surface of the Earth, or roughly 42,000 kilometers from the center of the earth. However, to be a geostationary satellite, the geosynchronous satellite must be in orbit in earth's equatorial plane - geostationary is a small subset of orbits that are geosynchronous. The orbital location of geostationary satellites is called the Clarke Belt in honor of Arthur C Clarke who first published the theory of locating geosynchronous satellites in earth's equatorial plane for use in fixed communications purposes(Clarke, Arthur C., 1945, "Extra Terrestrial Relays", Wireless World.). For brevity, the term 'geostationary satellite' is often shortened to 'geo satellite'.

Earth stations transmit a signal to a satellite in orbit, this act is called an uplink. Geostationary satellites receive the uplinked signal, amplify it, shift it to a lower frequency and then couple the outgoing signal to the transmitting array of on-board satellite antenna where the signal is focused into a narrow beam and sent back to earth. The act of sending the signal back to earth is called downlinking. The on-board satellite electronics which receive the uplinked signal, amplify it and shift the frequencies is called a transponder. For instance, USA C-band uplink frequencies are from 5.925GHz to 6.425Ghz and the downlink frequencies are from 3.7GHz to 4.2GHz; uplinks and downlinks are at different frequencies to avoid interference with each other. Ku signals are uplinked in the 14.0-14.5GHz range and downlinked in the 10.95-12.75GHz range. Each transponder is configured to accept a certain bandwidth of frequencies (36MHz, 54MHz and 72MHz are common). The range of frequencies corresponds to a 'channel', the term we use on earth. When you look at the transponder layout, i.e. configuration, for a satellite you will notice that channels are arranged with a band of frequencies between each one, typically 4MHz, this is known as a 'guardband' and its purpose is to provide isolation between each channel. You will also note that there are two sets of frequencies per transponder and they are at opposite polarities; this is known as frequency reuse and is a technique whereby two transponders share the same frequency but by handling signal at opposite polarities the two signals will not interfere with each other so each frequency is 'reused'. (For further discussion on polarization types.) Note to further avoid 'co-channel' interference, each set of polarities is typically offset from each other by an amount equal to one-half of their bandwidth, i.e. the band ends of one set of polarities fall on the center frequencies of the other set of polarities. Nowadays, in the age of digital compression, frequency reuse is further obtained by digitally compressing multiple channels per transponder, i.e. more use of each frequency is obtained by using compression techniques to 'cram' more information into each uplink/downlink signal. On newer satellites, some individual transponders can be reconfigured from earth control stations to be combined to provide a customer with more bandwidth or can be divided to lessen the bandwidth to customer specifications (for instance a 72MHz transponder can be broken into two 36MHz bandwidth units).

In regards to earth reception, downlink antennas on a satellite can be configured to downlink in 'global' beams (which can cover approximately forty percent of the earth's surface), be configured to target a region or country (sometimes called zone or 'hemi' (hemisphere) coverage), and/or be configured to target only a small area (called a spot beam). And individual transponders, or groups of transponders, can be configured to cover different parts of the earth from the same satellite; transponders that can be reconfigured from earth control stations to change their coverage are called 'steerable beams'. Satellite coverage of Hawaii is a typical spot beam, satellite coverage of the continental United States is a typical regional coverage beam and satellite coverage by the Intelsat international consortium satellites are typical global beams. The downlinked beam pattern is commonly called a footprint and its mapped pattern is in contours of effective isotropic radiated power (eirp) and calibrated in units of dBw (decibels above one watt) (for more on decibels). EIRP maps are used in equations called link budgets which are used by satellite system designers to determine which size satellite dish is required to receive programming from each satellite. Microwave frequencies are set by international convention and the current designations for satellite use are L, S, C, X (military) and K (which includes Ku and Ka) bands. (For complete frequency spectrum list from 9KHz to 300GHz.)

Geo satellite applications have become to have a major global effect on our daily lives; they are best suited to carry large volumes of communications traffic. Virtually all today's modern financial business is conducted at high speed via satellite example being credit card transactions, corporate retail inventory, shipment tracking and even the newspaper USA Today is typeset and transmitted to printing plants via satellite. Geo satellites rapidly and efficiently link remote areas of the Earth with telephone, television and data information and provide transmission from a single uplink location to provide reception to a regional downlink pattern - this is often called point to multi-point distribution. Satellites utilize transmit/receive signals in the microwave frequency range. They are not deflected by the Earth's atmosphere as lower frequencies are. Basically, they travel in a straight line, known as line of sight communication.

Before the development of geo satellite technology, microwave transmission was accomplished via a series of repeater microwave towers and subsea cables. The obvious limitation of microwave towers is they are a point-to-point technology and limited to line-of-site in regards to their placement - a tower is required every twenty-five miles. Microwave towers provide no regional coverage and cabling is required to reach the end user. The first commercial geo satellites were constructed to augment microwave towers for telephone transmission applications and thus were constructed to be the same frequency as microwave towers which is C-band (centered at four MHz); therefore when satellites were applied to television programming distribution, the first commercial satellite TV equipment was C-band. Think of a geostationary communications satellite as a microwave repeater in the sky - geo satellites overcome the difficulty of long-range communications; if a communications satellite is on a line of sight from two points on the Earth's surface, it can act as a relay between points too widely separated for direct transmission by towers.

It is also important to remember that geo satellites are used for earth observation purposes (remote sensing) in addition to communication applications. Before the advent of geo satellite technology, all weather predictions were based on information from ground reporting stations, reports from pilots of aircraft and from ships at sea, and from high altitude balloons. Can you imagine life today without satellite imagery for hurricane tracking and landfall predictions? Today, satellite images showing the advance of weather fronts are regular elements of the evening news. This meteorological information is also available to anyone with a personal computer and relatively inexpensive hardware and software. In addition to weather, a network of American, European, Japanese, and Russian satellites orbits the earth in various configurations to provide "real-time" monitoring of our environment. Useage of geo imaging satellites for civilian purposes and scientific research include monitoring of ocean temperature, rainfall amounts, desert encroachment, rainforest disappearance, air quality levels, updated maps and, what we take most for granted, daily weather reports.

Brief Satellite History: The world's first manmade artificial satellite (into low earth orbit) was the launch of Sputnik 1, by the USSR (Soviet Union), on October 4, 1957. Prior to that, in 1948, the United States Army Signal Corps had transmitted radar signals to the moon and bounced them back to earth proving relatively low power microwave signals could be transmited into space and that they could be detected on earth. In 1954, the U.S. Navy transmitted voice messages (on microwave carriers) to the moon then detected their reflection back on earth. On January 31, 1958, the U.S. launched their first satellite, Explorer 1, it provided preliminary information on the environment and conditions in space outside earth's atmosphere and resulted in the discovery of the Van Allen radiation belts and orbit the earth more than 58,000 times before re-entering the earth's atmosphere over the south pacific on March 31, 1970.. The same year, the U.S. established NASA to facilitate development of space based technology and launched SCORE into LEO orbit; SCORE received messages at 150MHz, taped them, then retransmitted them back to earth. The first broadcast from space to earth was on December 19, 1958, by U.S. president Eisenhower - it was a Christmas greeting - using the SCORE technology. An important test of the potential of satellites for regional (continental) communications from a single uplink was the 1960 launch, by the U.S., of Echo 1 - a passive 'satellite in reality a one hundred foot, high altitude, balloon made of microthin (5/10,000 inch) Mylar which received, and reflected back to earth, low power, duplex (two-way) telephone conversations.

A very important, and practical development, in the history of satellite communications, was the NASA launch of Tiros I, the world's first weather satellite, launched in April, 1960; the images it sent to earth proved that the 'space race' would have major implications on the daily lives of the average citizen beyond that of national pride in developing the 'best' space program. It carried low resolution television and infrared cameras and was in a circular polar orbit of about 600km. Proving communications via satellite is important for business but using satellites for earth monitoring is important for life - and this is the dual pattern in satellite applications that has been maintained to date. Earth monitoring satellites are not glamorous in that they do not bring the instant gratification to stockholders and television viewers that communications satellites do however their role in our world is much more valuable in that we have only one earth and the earth monitoring satellites assist us to understand our world, its impact on us and our impact on it.

Telstar 1, built by ATT, was the first satellite designed to detect microwaves, amplify them 'on board', and then retransmit them back to earth - the current transponder technology in use today. Although a LEO satellite, it linked Europe and North America, via satellite - live!!!, on television, on July 10, 1962, with a broadcast by Elvis Presley. Telstar 1 was the first satellite able to broadcast TV signals and Elvis was the first entertainer to be broadcast on satellite. The first experimental geosynchronous communications satellites were launched in 1963, Telstar 2 and Syncom 2 (Hughes Aircraft Corp.). Syncom 3 (geo, 1964) linked the1964 Tokyo Olympics to the United States. In 1965, the Soviets launched the first of their Molnya 'twelve hour orbit' satellites (highly elliptical orbit) - a clever engineering feat for providing communications to northern latitudes not reachable by geo satellite coverage. And in 1965 the first international geo satellite, Intelsat 1 (commonly called 'Early Bird'), was launched (built and coordinated by Comsat-USA) and was the first truly global commercial satellite service. The formation of Intelsat (which is vigorously active today), with member countries (see Intelsat signatory list), was specifically to rapidly develop (and distribute the cost of such development) global geo communications services and in turn to provide geo satellite services to (and between) member countries - the kind of thing a single country could not do in the emerging geo satellite science. In 1969, Intelsat completed the first global geo network, i.e. satellite system that covered all the globe - a major achievement in only twelve short years after the Sputnik launch, with their Intelsat 3 series; how fitting, it was completed and operational only days before the first lunar landing, Apollo 11, July 20, 1969.

Although today we take for granted the offering of satellite services by private companies, in the beginning, all satellite communications were coordinated between countries through government authorized entities and agencies and were international by design. This changed in 1972, when Telesat (although a government monopoly), Canada, began operation of the world's first domestic communications satellite, Anik 1 (launched by NASA), to provide the vast Canadian continental area with voice and data services and the idea to provide television programming; by default of its footprint 'spread', it was also the first geo satellite to provide service to the U.S. domestic market - RCA immediately leased circuits on Anik until they could launch their own satellite. The first United States domestic communications satellite, and the world's first offered by a private company, was Western Union's WESTAR 1, launched on April 13, 1974, followed by Westar 2 then the RCA satellite, Satcom F-1 which set the standard of using twenty-four channels, however it was the 1975 Intelsat 4A series which first used dual polarization per transponder, i.e. twelve transponders for twenty-four channels. These first satellites were initially designed for voice and data useage, but very quickly television became a major user and commercial supportor. By the end of 1976, there were 120 transponders available over the U.S., each capable of providing 1500 telephone channels or one TV channel; Canada had their own geo system; Intelsat was well on its way to another generation of global satellites; Indonesia had become the third country to have their own satellite - Palapa; and Comsat had launched the first mobile communications satellite, Marisat, in February, 1976, to provide mobile services to the United States Navy and other maritime customers.

In 1975, an east coast (USA) cable company, Home Box Office, began delivering TV programming downlinked from satellites to its cable subscribers. On September 30, 1975, it offered the Muhammad Ali/Joe Frazier boxing match (Thriller in Manila) to its cable viewers - the first precursor to 'pay for view'. In 1976, Ted Turner uplinked his WTBS Atlanta TV station to satellite to create the first 'superstation . . . the rest is history. In a short time, the 'movie channels' and 'super stations' were available to most Americans and TV was on its way to dominate satellite useage and to drive its growth as the dramatic expansion in cable TV would not have been possible without the inexpensive method of distributing video as provided by geo satellites. Throughout the late '70's, satellite applications continued to grow, from the involvement of NOAA in weather satellites and earth observation to the 1979 sponsorship by the United Nations International Maritime Organization of the establishment of the International Maritime Satellite Organization (INMARSAT) in a manner similar to INTELSAT as a method of providing international telephone service and traffic-monitoring services on ships at sea. As the decade of the '80's began, satellite technology had become a technology definitely acceptable to business and science and all the concepts and benefits of GEO, LEO, MEO and HEO satellites were being employed or on the 'drawing board'.

Second generation geo communication satellites were hybrids of C-band and the higher frequency Ku-band (centered at MHz) technology (it was Telsat that launched the first commercial hybrid satellite). Ku-band technology allowed the use of smaller diameter satellite antennas for video reception and opened the market for satellite delivered private data networks called VSATs (very small aperture terminals) and extended the application of SNG (satellite news gathering) services. With Ku technology, it became more affordable and practical to deliver all news via SNG vehicles. Hybrid satellites were followed by dedicated high power Ku-band satellites designed exclusively for the DTH (direct to home) television market with first applications in the European and Asian TV markets where high density urban populations and lack of TV cable networks made the marketing of small dish TV programming, commonly called DBS (direct broadcast services), a practical business endeavor. High power DBS satellites now transmit consumer programming to small dishes to homes in all corners of the globe. The newest satellite technology in commerical application are hybrid Ku/Ka-band satellites where Ka-band (centered around 22MHz) technology allows for even smaller receive satellite antennas. Yet technology does not stand still, research is continuing to theV-band (centered around 50/60MHz) for geo global communications network applications with broader bandwidth that utilize smaller receivers including hand held units, inter-satellite links, on-board processing and high-powered spot beams. brief history of early satellite communications
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