Geostationary, LEO, MEO, HEO Orbits
Including Polar and Sun-Synchronous Orbits with Example Systems
and a brief section on satellite history
(dish size list) use back button for other lists.
A very good glossary for terms in this section.
(More glossaries in Comments - Interesting Links page.)
(This is not the fastest loading page but it has a lot of good info - and the object is to keep this good info on one page rather than make you hop around from page to page. Thanks)
|Preface: This section provides definition and description of the commonly used satellite earth orbits. It is written to be non technical and to provide sufficient information to understand the subject but not to make one an engineering expert. With the information presented, you will be able to easily perform more internet searches to locate more technical details if you wish. There are many applications in current use for each orbit type and after a general discussion of each orbit type we present a description of a unique orbital application which we think will provide you with a good idea of the maximum potential of that orbit type. This section does not go into the technical construction and operation of satellites, this can be found for geostationary satellites by following the links per satellite in the Western/Eastern Hemisphere Quick-Look sections; and for all types of low earth orbit satellites by referring to Lloyd's Satellite Constellations.|
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
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
A GOES satellite can examine land features to 0.8 km. resolution.
This illustration of
the advantage of a
geo orbit. While a
low altitude orbiting
satellite is 'hidden' for
part of its orbit
around the Earth, a
geo satellite is always
directly visible from
the same earth
day or night. The
field of view of a
satellite in geo orbit
is fixed. A geo orbit
is essential for a
system between two
points and is ideal for
to a fixed geogra-
phical global area.
GEO System for Personal Communication:
The Inmarsat satellite system is
an example of using a geo orbit global system to provide global communication
services to earth users. It currently is the only provider of
global, integrated, personal communication services from geo
orbit. Its complete earth coverage (except for the poles) is
achieved through four geo satellites. Although a disadvantage
of a geostationary satellite in a voice communication system
is the roundtrip delay of approximately 250 milliseconds from
ground to satellite to ground; the advantage is in management
of the system as stationary satellites do not require the passing
of information from satellite to satellite as do LEO systems. The Inmarsat system
is the first personal communication system designed; its technology predated
development of LEO technology. It originally was authorized,
in 1979, to provide maritime users at sea with positioning traffic
monitoring service; it now additionally provides, depending on
the type of terminal used, direct dial-up voice, facsimile, duplex
data transfer, telex, electronic mail, high quality audio, compressed
video and still video pictures, telephoto, slow-scan television,
videoconferencing and telemedicine to any global user including
services to aircraft (some of which are still in development)
and positioning/tracking/monitoring services to terrestrial mobile systems. Its system is accessible from an assortment of devices including
automatic receiving positioning mounted units for ships and trucks,
portable 'flyaway' satellite parabolic antenna units with briefcase
size terminals and the more recent digital laptop computer control
units with built in satellite antenna. Other similar geo systems
for personal communications are in the planning stages from would-be
competitors to Inmarsat; for example, Hughes is developing a
broadband four geo satellite system with emphasis on high speed,
high volume data transfer capability for personal and business
applications - all plans for future systems include capability
for Internet access. The beauty of a personal communication geostationary
satellite system is that four satellites provide global coverage;
except, of course, for the poles.
Polar Satellites: Polar orbits are LEO orbits. Their applications can be to view
only the poles (to fill in gaps of geo coverage) or to view the same place on earth at
the same time each 24hr day. By placing a satellite at an altitude of about 850 km, a
polar orbit period of roughly 100 minutes can be achieved. This will allow the earth
to rotate beneath the satellite sufficiently that one polar satellite be used per
application though for more continuous coverage, more than one polar orbiting
satellite is employed. A special polar orbit that crosses the equator and each latitude
at the same time each day is called a sun-synchronous orbit. Polar satellites may
carry sensors sensitive to both visible light and infrared (IR) radiation and can make
measurements of temperature and humidity in the Earth's atmosphere, record surface
ground and surface sea water temperatures, and monitor cloud cover and water/ice
boundaries. They may have the capability to receive, measure, process, and
retransmit data from balloons, buoys, and remote automatic stations distributed
around the globe. These satellites may also carry search and rescue transponders to
help locate downed airplanes or ships in distress. Polar orbiting satellites provide
many services in communication and observation applications which geo satellites
are not capable of. (more on polar satellites)
This illustration shows
true relative distance
from the Earth of geo-
stationary and LEO
polar orbit satellites.
From geo altitude, the
entire Earth disk
subtends an angle of
17.4 degrees in
contrast to a typical
polar orbiting satellite,
which sees only a
relatively small portion
of the globe at any one
time. By definition, a
polar satellite has an
inclination of 90
degrees to the equator.
The critical design goal
is to place a polar
satellite in an orbit that
is low enough to allow
a relatively short
orbital period while at
the same time its orbit
altitude is sufficient to
permit observation of
a sufficiently wide path
so that during a single
orbit the Earth will
rotate by less than the
scan swath ability of
the satellite instrumen-
tation. A polar orbit is
fixed in space, and the
earth rotates under-
neath; a polar orbit
travels from north to
south pole. A typical
polar satellite can
cover the entire globe
every 14 days and can
'see', as example, the
entire east coast of the
at one time, from
southern Florida north
to Hudson Bay, and
from the Atlantic to
west of the Great
Lakes.(top of page)
Low Earth Orbits (LEO): LEOs
are either elliptical or (more usual) circular orbits at a height
of less than 2,000 km above the surface of the earth. The orbit
period at these altitudes varies between ninety minutes and two
hours. The radius of the footprint of a communications satellite
in LEO varies from 3000 to 4000 km. The maximum time during which
a satellite in LEO orbit is above the local horizon for an observer
on the earth is up to 20 minutes. (A satellite with an orbiting
altutude less than geostationary travels at a speed faster than
the earth's orbit.) Although there are long periods during which
the satellite is out of view of a particular ground station.
This may be acceptable for a store-and-forward type of communication
system as in an ecological/earth monitoring application. Most
small LEO systems employ polar, or near-polar, orbits. Accessibility
can of course be improved by deploying more than one satellite
and using multiple orbital planes. A complete global coverage
system using LEO orbits requires a large number of satellites,
in multiple orbital planes, in varied inclined orbits. For instance,
the currently in-operation Iridium (Motorola) system, utilizes
66 satellites (plus six in-orbit spares) in six orbital planes
inclined at 86.4 degrees at an orbital height of 780km with an
orbital period of 100 minutes, 28 seconds. Global coverage with
this single system is an astounding 5.9 million square (statute)
The topology of a full service LEO-based communication network is dynamic; the network must continually adapt to changing conditions to achieve the optimal (least delay) connections between terminals. When a satellite serving a particular user moves below the local horizon, it needs to be able to hand over the service to a proximal or succeeding one in the same or adjacent orbit. Depending on the system design, individual satellites may cross-link with one another to relay a signal typically via a rapid packet switching technique (as in Iridium) or may return the signal to an earth terminal for rerouting.
You can see from the Iridium coverage map the enormity and complexity of the global LEO communications task. Whether satellite-to-satellite or satellite-earth terminal-satellite routing schemes are used, messages are typically treated within a duplex (transmit and receive capability) LEO network as streams of short, fixed length packets. Each packet contains a header that includes destination address and sequence information, an error-control section used to verify the integrity of the header, and a payload section that carries the digitally encoded user data (voice, video, data, etc.) and an adaptive packet routing algorithm to achieve low delay and low delay variability across the network. Each node (satellite or earth terminal) automatically and independently selects the least delay route to the target destination. Packets of the same user session may follow different paths through the network. The terminal at the destination buffers and if necessary reorders the received packets to eliminate the effect of timing variations.
Due to the relatively large movement of a satellite in LEO with respect to an observer on the earth, satellite systems using this type of orbit need to be able to cope with large Doppler shifts. (The Doppler affect is commonly heard when a train pass by you when you are by a railway crossing.) Satellites in LEO are also affected by atmospheric drag which causes the orbit to gradually deteriorate; the typical life of a LEO satellite is 5-8 years. However, launches into low earth orbit are much less costly than to geo orbit and due to their much lighter weight, multiple LEO satellites can be launched at one time whereas only two geo satellites can be launched at one time with today's best heavy rocket technology.
The beauty of a LEO personal communication system is it provides a direct satellite link for both incoming and outgoing communications in remote areas, poorly covered regions, and locations outside terrestrial networks whether it be with a hand held unit, from any public telephone in the world, or simply through a unidirectional alpha numeric pager. And it allows the user to bypass, i.e. 'roam', across multiple wireless and national telephone systems using a single telephone number and receiving one telephone bill for calls made anywhere on earth. However, not all LEO personal communications schemes aim at global coverage. Some current LEO proposals market only to specific global latitudes where high density potential market centers are located and others will remain more regional by offering service to limited geographical and market segments.
Early regional, i.e. not global, LEO systems were designed to offer remote monitoring and information transfer to national trucking companies and mobile units. Using a vehicle, or desktop mounted transceiver with a short, flexible antenna, the system is capable to send and receive two-way alphanumeric information packets, similar to two-way paging or e-mail. These systems have been expanded to include hand held devices for personal messaging for individual users. The sender's message goes to the nearest in-view satellite where it is linked to the local earth station gateway for validation and optimal routing to the recipient's transceiver unit. (Satellite-earth terminal-satellite transfer method.) If necessary, gateway earth stations relay messages between satellites for faster delivery. Transmitted information packets can also be frequency encoded for security and for user uniqueness identification. This type routing is not as glorious as satellite-to-satellite technology and relies on a combination of microwave systems in its communication scheme but is very efficient for the application. Although the 'big' LEO global schemes currently in place, or in proposal, receive much more attention than the 'little' LEO schemes, these less intricate LEO schemes have been in place for quite a few years and have made the trucking industry much more efficient and have also had the extra affect to add a safety factor to its drivers by providing a method to always be in contact with the main base.
Regardless of the target market, with vast tracts of the world covered by desert, rugged mountain ranges and impenetrable jungles and with over two thirds of the world covered by water, the value and practicability of all types of LEO communications systems will be increasingly recognized as we enter the twenty-first century. Although geostationary satellites have the advantage over LEO satellites of providing more frequent images per target area and provide a permanent two-way communications link, the era of disposable, light weight satellites as used in LEO systems is upon us and here to stay. (top of page)
Intermediate Circular Orbits (ICO), or Medium Earth Orbits (MEO): ICOs are circular orbits at an altitude of around 10,000 km. Their orbit period measures about 6 hours. The maximum time during which a satellite in MEO orbit is above the local horizon for an observer on the earth is in the order of a few hours. A global communications system using this type of orbit, requires a modest number of satellites in 2 to 3 orbital planes to achieve global coverage. ICO satellite are operated in a similar way to LEO systems. However, compared to a LEO system, transfer of information from one satellite to another is less frequent, and propagation delay and free space loss are greater.
The United States Navistar Global Positioning System (GPS) is a prime example of a MEO system. It is a space-based triangulation system using satellites and computers to measure positions anywhere on earth. GPS is a radio navigation system developed and operated by the U.S. Department of Defense. It allows land, sea, and airborne users to determine their three dimensional position, velocity, and time, 24 hours a day, in all weather, anywhere in the world. The uniqueness of this navigational system is that it avoids the limitations of other land-based systems such as limited geographic coverage, lack of continuous 24-hour coverage, and the limited accuracies of other related navigational instruments.
Each satellite (twenty-one active plus three spares) weighs 844 kilograms and has a design life of 7.5 years and is about the size of a large van with each solar panel covering a surface area of 7.2square metres. They contain two rubidium and two cesium atomic clocks, and three nickel cadmium batteries which provide energy during eclipse periods. The satellites operate in six orbital planes (55 degrees to the equator) with four satellites per plane and have an orbital period of 12 hours such that they complete 2 orbital revolutions within a 24 hour period while the earth rotates 360 degrees. As typical of all LEO systems, this results in a trace of the satellite orbit on the earth's surface which will repeat itself regularly, in this case the repitition is daily. Thus, the positions of the satellites in the sky at any location can be defined for any particular period of time.
Like most navigation systems, GPS receivers are hand-held radio-receivers/computers which measure the time that the radio signal takes to travel from a GPS satellite until it arrives at the GPS antenna to figure location. Early on, scientists recognized the principle that, given velocity and the time required for a radio signal to be transmitted between two points, the distance between the two points can be computed. In order to perform the calculation, a precise, synchronized time of departure and measured time of arrival of the radio signal must be obtained. By synchronizing the signal transmission time to two precise clocks, one in a satellite and one at a ground-based receiver, i.e. the handheld unit, the transit (travel) time is measured and then multiplied by the exact speed of light to obtain the distance between the two positions. Using the travel time multiplied by the speed of light provides a calculation of range to each satellite in view; the satellites transmit on two L-band frequencies: L1 @1575.42 MHz and L2 @ 1227.6 MHz. Once distance from four satellites is known, position in three dimensions (latitude, longitude, and altitude) is calculated by triangulation, and velocity in three dimensions is computed from Doppler shift in the received signal. From this and additional information on the satellites' orbit and velocity, the internal GPS receiver software calculates its position through the process of triangulation. Standard Positioning Service (SPS) Receivers are for civilian use and are designed to track the 'coarse' acquisition code broadcast by the satellites; they offer a predicted accuracy of location within100 meters of true earth position within a 340 nanoseconds transfer time from user request to satellite system response. Precise Positioning Service (PPS) is a highly accurate military positioning, velocity and timing service that uses receivers designed to track through the 'precise' code and which provides a predictable positioning accuracy of at least 16 meters and time transfer within 200 nanoseconds. (However, there are methods which can be use to provide accuracies of +/- 5 metres. These methods use a known position, such as surveyed control point, as a reference point to correct the GPS position error. These methods of correcting GPS positions are referred to as Differential GPS or DGPS. DGPS places a GPS stationary receiver at a known location on or near the Earth's surface. This reference station receives satellite signals and adjusts for transmission delays and selective availability, using its own known latitude, longitude, and altitude. The stationary receiver sends out a correction message for any suitably-equipped local receiver. A DGPS-compatible receiver adjusts its position calculations using the correction message. DGPS reference stations are constructed, operated, and maintained by the United States Coast Guard. The DGPS corrections can be applied to GPS data in real-time using data telemetry (radio modems) or can be done later on a personal computer.) PPS was designed primarily for United States military use and is denied to unauthorized users by the use of cryptography. However, PPS is being made available to U.S. federal government users including the foresty service and for limited, non federal government, civil use, both domestic and foreign, on an application basis. Receiver units are available from numerous vendors (three are shown in this section) each with a variety of user convenience features. Prices typically range from several hundred USD to $30,000, reflecting the accuracy and capabilities of the instruments. For the general outdoorsman, a good GPS receiver should have 8 satellite tracking capability and be capable of receiving the GPS satellite signals through heavy forest canopy; for the professional user, a minimum 8 satellite tracking capability, high memory capacity, differential GPS capability, and resistance to signal dampening under forest canopy is essential; for the professional surveyor requiring high level precision and accuracy capability, they should assess the project or application for which the technology is to be used with the help of an unbiased consultant to determine the most cost effective and appropriate instrument. GPS is an excellent tool for survey purposes and if you plan to climb Mt. Everest then apply for PPS authorization. A unique GPS PPS application is the NASA Southern California Earthquake Project where GPS units are permanently emplaced to detect earth movement and to hopefully be useful in earthquake prevention. Private sector GPS LEO/MEO satellite systems have been proposed which would provide PPS or greater accuracy to the individual consumer though they are several years from reality; the Navistar system is currently the only 'game in town'. gps technical info
(top of page)
Highly Elliptical Orbits (HEO): HEO orbits for earth applications were initially exploited by the Russians to provide communications to their northern regions not in coverage by their geo satellite networks. HEOs typically have a perigee at about 500 km above the surface of the earth and an apogee as high as 50,000 km. The orbits are inclined at 63.4 degrees in order to provide communications services to locations at high northern latitudes. The particular inclination value is selected in order to avoid rotation of the apses, i.e. the intersection of a line from earth centre to apogee, and the earth surface will always occur at a latitude of 63.4 degrees North. Orbit period varies from eight to 24 hours. Owing to the high eccentricity of the orbit, a satellite will spend about two thirds of the orbital period near apogee, and during that time it appears to be almost stationary for an observer on the earth (this is referred to as apogee dwell). A well designed HEO system places each apogee to correspond to a service area of interest, i.e. which would cover a major population centre, for example in the Russian Molnya satellite system designed to cover Siberia. After the apogee period of orbit, a switchover needs to occur to another satellite in the same orbit in order to avoid loss of communications to the user. Free space loss and propagation delay for this type of orbit is comparable to that of geostationary satellites. However, due to the relatively large movement of a satellite in HEO with respect to an observer on the earth, satellite systems using this type of orbit need to be able to cope with large Doppler shifts. Examples of HEO systems are the Russian Molnya system, which employs 3 satellites in three12 hour orbits separated by 120 degrees around the earth, with apogee distance at 39,354 km and perigee at 1000 km and the Russian Tundra system, which employs 2 satellites in two 24 hour orbits separated by 180 degrees around the earth, with apogee distance at 53,622 km and perigee at 17,951 km.
(top of page)
Sun-Synchronous Orbit: This orbit is a special case of the polar orbit (which is a special coordinated LEO orbit). In a sun-synchronous (SS) orbit, (also called a helio-synchronous orbit), the satellite passes over the same part of the earth at roughly the same local time each day. This can make communication and various forms of data collection very convenient. For example, a satellite in an SS orbit could measure the air quality of Ottawa, Canada, at noon each and every day. In an SS orbit the angle between the orbital plane and sun remains constant, i.e. a constant node-to-sun angle, and therefor the satellite passage over a certain area occurs at the same time of the day each day. This can be achieved by a careful selection of orbital height, eccentricity and inclination which produces a precession of the orbit (node rotation) of approximately one degree eastward each day, equal to the apparent motion of the sun. This condition can only be achieved for a satellite in a retrograde orbit. Like any polar orbit, the earth moves beneath an SS orbit. A satellite in SS orbit crosses the equator and each latitude at the same time each day. All sun-synchronous orbits are polar orbits but not all polar orbits are sun-synchronous orbits. All polar orbits are LEO/MEO orbits.
A special SS orbit, called a dawn-to-dusk orbit, is where the satellite trails the Earth's shadow. When the sun shines on one side of the Earth, it casts a shadow on the opposite side of the Earth - this shadow is night-time. Because the satellite never moves into this shadow, the sun's light is always on it, i.e. like perpetual daytime. Since the satellite is close to the shadow, the part of the earth the satellite is directly above is always at sunset or sunrise; that is why this kind of orbit is called a dawn-dusk orbit. A dawn-dusk orbit allows the satellite to always have its solar panels in the sun. Radarsat is an example of a satellite in a low sun synchronous orbit. Radarsat is in orbit 798 kilometres above the Earth, at an angle of inclination of 98.6 degrees to the equator as it circles the globe from north pole to south pole. Radarsat relies on its dawn-to-dusk orbit to keep its solar panels facing the sun almost constantly. Radarsat can therefore rely mostly on solar power and not on batteries; its images are useful for agriculture, oceanography, forestry, hydrology, geology, cartography, and meteorology.
An SS low altitude polar orbit is widely used for monitoring the Earth because each day, as the Earth rotates below it, the entire surface is covered and satellite views the same earth location at the same time each 24hr. period. Typically such a satellite moves at an altitude of 1000 km (some go lower but don't last long, because of air friction, each orbit takes about 100 minutes, and scans a path that is about 110 degrees width, about a surface distance roughly of 3000 km. Of course this type of orbit is advantageous for an earth observation satellite as it can provide constant lighting conditions; but also for detection applications as in search and rescue programs, as it allows regular monitoring of the same location from the same satellite. An example of a sun-synchronous search and rescue system is the Cospas-Sarsat network system.
Cospas-Sarsat is an international, humanitarian system that uses near polar orbiting satellites to detect and locate emergency beacons carried by ships, aircraft, or individuals. The system consists of a network of satellites, ground stations, mission control centers, and rescue coordination centers. Acting as communication relays, the Cospas-Sarsat satellites receive radio distress signals from emergency transmitting beacon units. The nominal system configuration comprises four satellites, two Cospas and two Sarsat. Russia supplies two Cospas satellites placed in near polar orbits at 1000 km altitude and equipped with SAR (search and rescue) instrumentation at 121.5 MHz and 406 MHz. The USA supplies two NOAA meteorological Sarsat satellites placed in SS, near-polar orbits at about 850 km altitude, and equipped with SAR instrumentation at 121.5 MHz, 243 MHz, and 406 MHz supplied by Canada and France.
SARSAT is an instrument package flown board the NOAA Series of environmental satellites operated by
NOAA's National Environmental Satellite, Data and Information Service (NESDIS). These SS satellites orbit at an altitude of 528 miles and complete an orbit every 100 minutes. Their orbits are inclined 99 degrees from the equator. Each satellite carries a Search and Rescue Repeater (SARR) which receives and retransmits 121.5 MHz and 243 MHz signals anytime the satellite is in view of a ground station. Also carried is a Search and Rescue Processor (SARP) which receives 406 MHz transmissions, provides measurements of the frequency and time, then retransmits this data in real-time and stores it aboard for later transmission.
The Cospas instrument is carried aboard the Nadezhda navigation satellite orbiting the Earth every 105 minutes at an altitude of 620 miles and an orbital inclination of 83 degrees. The COSPAS instrument was built by the former Soviet Union and continues to be operated by the Russian Federation. The only major difference between Cospas and Sarsat is that Russian satellites do not monitor the 243 MHz distress signals which is the NATO distress frequency. Both systems operate and process received signals to ground stations in similar manner - each satellite makes a complete orbit of the earth around the poles in about 100 minutes, travelling at a velocity of 7 km per second; each satellite views a "swath" of the earth over 4000 km wide as it circles the globe, giving an instantaneous "field of view" about the size of a continent; each satellite, when viewed from earth, crosses the sky in about 15 minutes, depending on the maximum elevation angle of the particular pass, but only the Sarsat system is inclined appropriately to be sun synchronous.
These satellites use Doppler to locate emergency beacons. Doppler is the increase in pitch of the frequency as the satellite approaches the emergency beacon and then the sudden decrease in pitch as the satellite moves away from the beacon. (The Doppler affect is commonly heard when a train pass by you when you are by a railway crossing.). Acting as communication relays, the Cospas-Sarsat satellites receive radio distress signals from marine Emergency Position Indicating Radio Beacons (EPIRBs), and aircraft Emergency Locator Transmitters (ELTs) and Personal Locator Beacons (PLBs). The satellites re-transmit distress information to ground stations called Local User Terminals (LUTs) which process the signal and calculates the position from which it originated. The satellite also stores each signal it receives and continuously downloads this data. If the satellite is in view of a ground station when a 406 MHz signal is received the data is transmitted to the ground station in real-time. If the satellite was not in view of a ground station, and for the other detected frequencies, when it receives a beacon signal, the next ground station that sees that satellite will receive the data. The LUT then relays the information to national Mission Control Centres (MCCs) where it is joined with identification data and other information on that beacon who in turn pass location data and other pertinent information to the appropriate Rescue Coordination Centres (RCCs) based on the geographic location of the beacon. If the location of the beacon is in another country's area of responsibility, then the alert is transmitted to that country's mission control center. Locations can be pinpointed in as little as forty minutes from time of beacon activation. The 406MHZ frequency is much more stable than its 121.5MHz counterpart and is therefore able to give a much more accurate location of about 1 to 5 km. The 121.5/243MHz beacons average 25 km accuracy. Because the satellites are near polar orbiting, coverage is better near the poles and less near the equator (as the earth 'bulges', i.e. near the equator, the satellite coverage path is not as inclusive).
The speed and accuracy of this communications process significantly increases the accident victim's ultimate chance for survival. This provides global coverage for 121.5/406 MHz distress signals though not all regions of the world have installed appropriate receive and process ground stations therefore only 60% of the earth's surface is actually currently being served by Cospas-Sarsat. To provide another option for global coverage, NOAA Geosychronous Operational Environmental Satellites (GOES 8 and 10, 9 is an in-orbit spare) geo satellites have been interconnected to the Cospas-Sarsat receive and process ground stations. They provide near instantaneous 406MHz beacon detection to allow more rapid preparation for response but do not give a Doppler position.
Summary: If there was to be a summary to this page on satellite orbits, by combining the benefits of polar LEO orbits, sun-synchronous orbits and the recently integrated geo orbit, the Cospas-Sarsat system, which provides a tremendous resource for protecting the lives of aviators and mariners that was unthinkable prior to the Space-Age, shows us that all types of earth satellite orbits are important in their own way. The Cospas-Sarsat system uses multiple orbit types to its advantage to create its user application. Constant and new advances in microcircuitry and computer technology will allow a continueing of imagination for applications of satellites in orbit to become reality. As long as there is a perceived notion that advances in communications and improvements in global environmental monitoring are good for industry, society and mankind, then financing will be available for new satellite systems and new applications and we can expect to see satellites play an increasing role in our world. (top of page)