Basics of the Geostationary OrbitBy Dr. T.S. Kelso | May 1998 |
May 1998 | |
Basics of the Geostationary OrbitBy Dr. T.S. Kelso |
Geosynchronous Orbits One application of the law of gravity is to figure out how to put a satellite into orbit so that it remains in the same position relative to ground all the time. A satellite in such a position can be used to relay microwave communication signals from one part of the. A geosynchronous orbit is a geocentric orbit that has the same orbital period as the sidereal rotation period of the Earth. It has a semi-major axis of 42,164 km (26,200 miles). Geostationary orbit is a special case of geosynchronous orbit. A geosynchronous orbit simply has the same 24 hour period as the Earth, however, it is inclined relative to the equator and traces out an ellipse in the sky as seen from the Earth. Geosynchronous orbit. Orbit around Earth of a satellite with an orbital period that matches Earth's rotation on its axis, which takes one sidereal day (about 23 hours, 56 minutes, and 4 seconds). Circular geosynchronous orbit 35786 km above Earth's equator.
Few aspects of the Space Age have had as much impact on our everyday lives as the invention of the communications satellite. In just a few short decades, they have brought together even the most far-flung reaches of the globe in ways that not that long ago were barely imaginable. In fact, today it is possible to talk directly to climbers at the top of Mount Everest or communicate via the Internet with virtually any computer system on the face of the planet—all with the help of communications satellites.
Geo Transfer Orbit
While communications satellites perform their missions in many types of orbits, from near-earth constellations like Iridium and Globalstar to the highly-inclined, eccentric Molniya orbits used by the Russian Federation, one of the more important classes of orbits for these satellites is the geostationary orbit. In this column, I'd like to examine the unique aspects of this class of orbit which make it suitable for not only satellite communications, but early warning and weather observations, too.
History
The concept of the geostationary orbit has been around since the early part of the twentieth century. Apparently, the concept was originated by Russian theorist Konstantin Tsiolkovsky—who wrote numerous science and science-fiction articles on space travel at the turn of the century. In the 1920s, Hermann Oberth and Herman Potocnik—perhaps better known by his pseudonym, Herman Noordung—wrote about space stations which maintained a unique vantage over the earth.1 Each author described an orbit at an altitude of 35,900 kilometers whose period exactly matched the earth's rotational period, making it appear to hover over a fixed point on the earth's equator.
However, the person most widely given credit for the concept of using this orbit for communications is Arthur C. Clarke. In an article he published in Wireless World in October 1945 titled 'Extra-Terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?' Clarke extrapolates from the German rocket research of the time to a day when communications around the world would be possible via a network of three geostationary satellites spaced at equal intervals around the earth's equator (see Figure 1).
Figure 1. Original figure from Clarke's article in the October 1945 edition of Wireless World2
In this article, Clarke not only determines the orbital characteristics necessary for such an orbit, but also discusses the frequencies and power needed for communications to and from space and how to use solar illumination for power—he even calculates the impact of solar eclipses around the vernal and autumnal equinoxes. What makes this article all the more remarkable is that Clarke wrote it more than a dozen years before the first satellite was even launched.
It wasn't until 1963 that NASA set out to test Clarke's concept with the Synchronous Communications Satellite program. Unfortunately, Syncom 1—launched 1963 February 14—while successfully reaching geosynchronous orbit in an inclined, eccentric orbit was unsuccessful due to an electronics failure. Syncom 2—launched 1963 July 26—became the first operational geosynchronous communications satellite. Syncom 3—launched 1964 August 19—became the first geostationary satellite, finally fulfilling the prediction made by Clarke almost twenty years earlier.
Theory
So just what is a geostationary orbit? In general terms, it is a special orbit for which any satellite in that orbit will appear to hover stationary over a point on the earth's surface. Unlike all other classes of orbits, however, where there can be a family of orbits, there is only one geostationary orbit. Let's examine this orbit's unique characteristics.
For any orbit to be geostationary, it must first be geosynchronous. A geosynchronous orbit is any orbit which has a period equal to the earth's rotational period. As we shall soon see, this requirement is not sufficient to ensure a fixed position relative to the earth. While all geostationary orbits must be geosynchronous, not all geosynchronous orbits are geostationary. Unfortunately, these terms are often used interchangeably.
Before continuing, it is necessary to clarify what is meant by 'the earth's rotational period.' For most timekeeping, we consider the earth's rotation to be measured relative to the sun's (mean) position. However, since the sun moves relative to the stars (inertial space) as a result of the earth's orbit, one mean solar day is not the rotational period that we're interested in. A geosynchronous satellite completes one orbit around the earth in the same time that it takes the earth to make one rotation in inertial (or fixed) space. This time period is known as one sidereal day and is equivalent to 23h56m04s of mean solar time (for more information, see 'Orbital Coordinate Systems, Part I' in the September/October 1995 issue of Satellite Times). Without any other influences, the earth will be oriented the same way in inertial space each time a satellite with this period returns to a particular point in its orbit.
To ensure that a satellite remains over a particular point on the earth's surface, the orbit must also be circular and have zero inclination. Figure 2 shows the difference between a geostationary orbit (GSO) and a geosynchronous orbit (GEO) with an inclination of 20 degrees. Both are circular orbits. While each satellite will complete its orbit in the same time it takes the earth to rotate once, it should be obvious that the geosynchronous satellite will move north and south of the equator during its orbit while the geostationary satellite will not.
Figure 2. Geostationary and Geosynchronous Orbits
Orbits with non-zero eccentricity (i.e., elliptical rather than circular orbits) will result in drifts east and west as the satellite goes faster or slower at various points in its orbit. Combinations of non-zero inclination and eccentricity will all result in movement relative to a fixed ground point.
Figure 3 shows some typical results. The figure-eight ground track is that of the geosynchronous orbit (GEO) shown in Figure 2. The geostationary satellite (GSO) sits fixed at the crossover point of the figure eight (over the equator). If we now give the geosynchronous satellite an eccentricity of 0.10, the slanted teardrop shape results. Typically, eccentric geosynchronous orbits will result in a slanted figure eight—this one just happens to have the crossover point at the northern apex of the ground track.
Figure 3. Geosynchronous Ground Tracks
It should now be apparent that only satellites which orbit with a period equal to the earth's rotational period and with zero eccentricity and inclination can be geostationary satellites. As such, there is only one geostationary orbit—a belt circling the earth's equator at an altitude of roughly 35,786 kilometers.
It should also be clear that it is not possible to orbit a satellite which is stationary over a point which is not on the equator. This limitation is not serious, however, since most of the earth's surface is visible from geostationary orbit. In fact, a single geostationary satellite can see 42 percent of the earth's surface and a constellation of geostationary satellites—like the one Clarke suggested—can see all of the earth's surface between 81° S and 81° N.
Of course, the advantage of a satellite in a geostationary orbit is that it remains stationary relative to the earth's surface. This makes it an ideal orbit for communications since it will not be necessary to track the satellite to determine where to point an antenna. However, there are some disadvantages. Perhaps the first is the long distance between the satellite and the ground. With sufficient power or a large enough antenna, though, this limitation can be overcome.
The fact that there is only one geostationary orbit presents a more serious limitation. Just as in putting beads on a loop of string, there are only so many slots into which geostationary satellites can be placed. The primary limitation here is spacing satellites along the geostationary belt so that the limited frequencies allocated to this purpose don't result in interference between satellites on uplink or downlink. Of course, we also want to make sure the satellites aren't close enough to run into one another since they will have some small movement.
While new communications satellites may be placed in a true geostationary orbit initially, there are several forces which act to alter their orbits over time. Since the geostationary orbital plane is not coincident with the plane of the earth's orbit (the ecliptic) or that of the moon's orbit, the gravitational attraction of the sun and the moon act to pull the geostationary satellites out of their equatorial orbit, gradually increasing each satellite's orbital inclination. In addition, the noncircular shape of the earth's equator causes these satellites to be slowly drawn to one of two stable equilibrium points along the equator, resulting in an east-west libration (drifting back and forth) about these points.
To counteract these perturbations, sufficient fuel is loaded into all geostationary satellites to periodically correct any changes over the planned lifetime of the satellite. These periodic corrections are known as stationkeeping. North-south stationkeeping corrects the slowly increasing inclination back to zero and east-west stationkeeping keeps the satellite at its assigned position within the geostationary belt. These maneuvers are planned to maintain the geostationary satellite within a small distance of its ideal location (both north-south and east-west). This tolerance is normally designed to ensure the satellite remains within the ground antenna beamwidth without tracking.
Once the satellite has exhausted its fuel, its inclination will begin to grow and it will begin to drift in longitude and may present a threat to other geostationary satellites. Oftentimes, geostationary satellites are boosted into a slightly higher orbit at the end of their planned lifetime to prevent them causing havoc with other geostationary satellites. This final maneuver assumes that no unplanned failure has occurred which would prevent it (such as a power or communications failure).
Summary
This initial article on geostationary and geosynchronous orbits should give you a basic understanding of some of the fundamental orbital concepts. In our next column, I would like to continue this topic by examining the relationship among the observer, satellite, and the sun to determine a geostationary satellite's longitude, the look angles from a terrestrial observer, and how the position of the sun can affect onboard power management and interference with satellite communications.
As always, if you have any questions, please feel free to write me at [email protected]. Until next time, keep looking up!
Notes
1 Oberth, Hermann. Die Rakete zu den Planetenraumen (The Rocket into Interplanetary Space), 1923. Noordung, Herman. Das Problem der Befahrung des Weltraums (The Problem of Space Travel), 1929.
2 Clarke, Arthur C. 'Extra-Terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?' Wireless World, October 1945, p. 306.
2 Clarke, Arthur C. 'Extra-Terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?' Wireless World, October 1945, p. 306.
Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day).Wikipedia
- Geostationary orbitCircular geosynchronous orbit 35786 km above Earth's equator and following the direction of Earth's rotation. Orbit has an orbital period equal to the Earth's rotational period, one sidereal day, and so to ground observers it appears motionless, in a fixed position in the sky.Wikipedia
- Tundra orbitHighly elliptical geosynchronous orbit with a high inclination (approximately 63.4°), an orbital period of one sidereal day, and a typical eccentricity between 0.2 and 0.3. A satellite placed in this orbit spends most of its time over a chosen area of the Earth, a phenomenon known as apogee dwell, which makes them particularly well suited for communications satellites serving high latitude regions.Wikipedia
- Geocentric orbitA geocentric orbit or Earth orbit involves any object orbiting the Earth, such as the Moon or artificial satellites. In 1997 NASA estimated there were approximately 2,465 artificial satellite payloads orbiting the Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center.Wikipedia
- Geosynchronous satelliteSatellite in geosynchronous orbit, with an orbital period the same as the Earth's rotation period. Typically some form of analemma.Wikipedia
- Low Earth orbitEarth-centred orbit with an altitude of 2000 km or less (approximately one-third of the radius of Earth), or with at least 11.25 periods per day ( an orbital period of 128 minutes or less) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO.Wikipedia
- Supersynchronous orbitEither an orbit with a period greater than that of a synchronous orbit, or just an orbit whose apoapsis is higher than that of a synchronous orbit. A synchronous orbit has a period equal to the rotational period of the body which contains the barycenter of the orbit.Wikipedia
Geosynchronous Orbit Ksp
- SatelliteObject that has been intentionally placed into orbit. These objects are called artificial satellites to distinguish them from natural satellites such as Earth's Moon.Wikipedia
- High Earth orbitGeocentric orbit with an altitude entirely above that of a geosynchronous orbit . Apparent retrograde motion – that is, even if they are in a prograde orbit , their orbital velocity is lower than Earth's rotational speed, causing their ground track to move westward on Earth's surface.Wikipedia
- Polar orbitOne in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. Inclination of about 60 - 90 degrees to the body's equator.Wikipedia
- Semi-synchronous orbitOrbit with a period equal to half the average rotational period of the body being orbited, and in the same direction as that body's rotation. Considered a medium Earth orbit, with a period of just under 12 hours.Wikipedia
- Molniya orbitType of satellite orbit designed to provide communications and remote sensing coverage over high latitudes. Highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of 270 degrees, and an orbital period of approximately half a sidereal day.Wikipedia
- 3753 CruithneQ-type, Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. Asteroid that, relative to Earth, orbits the Sun in a bean-shaped orbit that effectively describes a horseshoe, and that can change into a quasi-satellite orbit.Wikipedia
- Highly elliptical orbitElliptic orbit with high eccentricity, usually referring to one around Earth. Examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used them, and Tundra orbits.Wikipedia
- Areosynchronous orbitThe areosynchronous orbits (ASO) are the synchronous orbits for artificial satellites around the planet Mars. They are the martian equivalent of the geosynchronous orbits (GSO) on the Earth.Wikipedia
- List of satellites in geosynchronous orbitList of satellites in geosynchronous orbit . These satellites are commonly used for communication purposes, such as radio and television networks, back-haul, and direct broadcast.Wikipedia
- Earth observation satelliteSatellite used or designed for Earth observation from orbit, including spy satellites and similar ones intended for non-military uses such as environmental monitoring, meteorology, cartography and others. The most common type are Earth imaging satellites, that take satellite images, analogous to aerial photographs; some EO satellites may perform remote sensing without forming pictures, such as in GNSS radio occultation.Wikipedia
- Gliese 876 cExoplanet orbiting the red dwarf Gliese 876, taking about 30 days to complete an orbit. Discovered in April 2001 and is the second planet in order of increasing distance from its star.Wikipedia
- EquinoxCommonly regarded as the instant of time when the plane of Earth's equator passes through the geometric center of the Sun's disk. This occurs twice each year, around 20 March and 23 September.Wikipedia
- 2020 SWTiny near-Earth asteroid discovered by the Mount Lemmon Survey on 18 September 2020, six days before it made its closest approach to Earth. Apollo-type orbit to an Aten-type orbit with a semi-major axis within one astronomical unit from the Sun.Wikipedia
- Orbital periodTime a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars. Often referred to as the sidereal period, determined by a 360° revolution of one celestial body around another, e.g. the Earth orbiting the Sun.Wikipedia
- 67P/Churyumov–GerasimenkoJupiter-family comet, originally from the Kuiper belt, with a current orbital period of 6.45 years, a rotation period of approximately 12.4 hours and a maximum velocity of 135000 km/h. Approximately 4.3 by at its longest and widest dimensions.Wikipedia
- 2010 TK7Sub-kilometer near-Earth asteroid and the first Earth trojan discovered to date; it precedes Earth in its orbit around the Sun. Trojan objects are most easily conceived as orbiting at a Lagrangian point, a dynamically stable location (where the combined gravitational force acts through the Sun's and Earth's barycenter) 60 degrees ahead of or behind a massive orbiting body, in a type of 1:1 orbital resonance.Wikipedia
- MEASAT-3bCommunications satellite which MEASAT and Trans Media operates in geosynchronous orbit at 91.5 degrees east longitude, co-located with MEASAT-3 and MEASAT-3a, with orbital period of 1436.1 minutes and orbital speed of ~3.08 km/s . Built by Astrium , based on the Eurostar spacecraft platform, with an investment of approximately MYR1.25bn ($370m), and the 5th MEASAT satellite in orbit.Wikipedia
- Near-equatorial orbitOrbit that lies close to the equatorial plane of the object orbited. Orbit has an inclination near 0°.Wikipedia
- Vinasat-2Second Vietnamese satellite to be placed in orbit. Launched at 22:13 UTC on May 15, 2012 at the European Spaceport in Kourou in French Guiana in South America and entered orbit 35 minutes later.Wikipedia
- Geostationary transfer orbitType of geocentric orbit. Intermediate step for reaching their final orbit.Wikipedia
- (391211) 2006 HZ51Bright, sub-kilometer asteroid on an eccentric orbit, classified as a near-Earth object and a potentially hazardous asteroid of the Amor group. Discovered on 27 April 2006, by astronomers of the Catalina Sky Survey conducted at Mount Lemmon Observatory, Arizona, United States.Wikipedia
- (10115) 1992 SKStony near-Earth object and potentially hazardous asteroid on an eccentric orbit. It belongs to the group of Apollo asteroids and measures approximately 1 km in diameter.Wikipedia
Sentences forGeosynchronous orbit
- As television became the main market, its demand for simultaneous delivery of relatively few signals of large bandwidth to many receivers being a more precise match for the capabilities of geosynchronous comsats.Communications satellite-Wikipedia
- 3 of the satellites are placed in the Geostationary orbit (GEO) and the remaining 4 in the Geosynchronous orbit (GSO) to have a larger signal footprint and lower number of satellites to map the region.Satellite navigation-Wikipedia
- The Geosynchronous orbit (GEO) communications satellites provide commercial satellite services such as direct-to-home digital television, business data transmission, cable program distribution and wireless communications.Orbital Sciences Corporation-Wikipedia
- Geosynchronous orbit launches historically taking advantage of economies of scales with larger launch vehicles and greater use of the maximum payload capacity of a vehicle vs LEO launches.Space launch market competition-Wikipedia
- For example, a geostationary satellite completes one orbit per day above the equator, or 360 degrees per 24 hours, and has angular velocity ω = (360°)/(24 h) = 15°/h, or (2π rad)/(24 h) ≈ 0.26 rad/h.Angular velocity-Wikipedia
- Western Union became the first American telecommunicationscorporation to maintain its own fleet of geosynchronous communications satellites, starting in 1974.Western Union-Wikipedia
- An early orbit calculation showed that it was orbiting Earth in an extremely elliptical orbit, taking it from within the geosynchronous satellite ring to nearly twice the distance of the Moon.WT1190F-Wikipedia
- The STAR-2 Bus is a fully redundant, flight-proven, spacecraft bus designed for geosynchronous missions.GEOStar-2-Wikipedia
- Iran is also planning to launch satellites into orbits of up to 36,000 km.Iranian Space Agency-Wikipedia
- Moonshine was to be a joint effort with NASA to determine the feasibility of a geosynchronous satellite that could in project light directly down on any desired location.Operation Shed Light-Wikipedia
- The first standardized satellite bus design was the HS-333 geosynchronous (GEO) communication satellite launched in 1972.Satellite-Wikipedia
- propellant residuals, it successfully put the bird on a 250 km orbit above GSO and Nahuel 1A was switched off.Nahuel 1A-Wikipedia
- Specifically, geosynchronous Earth orbit (GEO) may be a synonym for geosynchronous equatorial orbit, or geostationary Earth orbit.List of satellites in geosynchronous orbit-Wikipedia
- In the most recent report, Roman was considered for both geosynchronous and L2 orbits.Nancy Grace Roman Space Telescope-Wikipedia
- It is capable of the assembly, integration and testing of several satellites of type Low Earth orbit (LEO) and Geosynchronous orbit (GEO) with a mass up to five tonnes simultaneously.Turkish Space Systems, Integration and Test Center-Wikipedia
- The primary application for this thruster design is intended for satellite station-keeping, long term LEO to GEO orbit transfers and deep space applications.Helicon double-layer thruster-Wikipedia
- For example, the levels of ionizing radiation and radio interference can vary by factors of hundreds to thousands; and the shape and location of the magnetopause and bow shock wave upstream of it can change by several Earth radii, exposing geosynchronous satellites to the direct solar wind.Solar wind-Wikipedia
- It will be the first ISRO made satellite to move from Geostationary transfer orbit to Geosynchronous orbit using Electric Propulsion.GSAT-20-Wikipedia
- If the orbit is a geosynchronous one, then c=1 and the trace is a Viviani's curve.Clélie-Wikipedia
- A space elevator is a theoretical system using a super-strong ribbon going from the surface of the Earth to a point beyond Geosynchronous orbit.Space elevator competitions-Wikipedia
- But science fiction writers have a fairly good track record in predicting future technologies—for example geosynchronous communications satellites (Arthur C. Clarke) and many aspects of computer technology (Mack Reynolds).Interplanetary spaceflight-Wikipedia
- The Geosynchronous-belt subsynchronous orbital regime is regularly used in spaceflight.Subsynchronous orbit-Wikipedia
- In the orbits nearest to Earth—less than 2000 km orbital altitude, referred to as low-Earth orbit (LEO)— there have traditionally been few 'universal orbits' which keep a number of spacecraft in particular rings (in contrast to GEO, a single orbit that is widely used by over 500 satellites).Space debris-Wikipedia
- In June 2014, the company was contracted by the United States Air Force on a fixed-price basis to build the fifth and sixth Geosynchronous Earth Orbit (GEO) satellites, known as GEO-5 and GEO-6, for the Space-Based Infrared System (SBIRS) at a cost of US$1.86 billion.Lockheed Martin Space-Wikipedia
- ! ...to GTO or GSOHeavy-lift launch vehicle-Wikipedia
- AMSC initially provided GEO based satellite services based on its MSAT satellite.SkyTerra-Wikipedia
- One advantage of GPS is all orbits from Low Earth orbit to Geosynchronous orbit can use GPS for ACS.Astrionics-Wikipedia
- Orbits with mean altitudes higher than LEO (such as Medium Earth orbits (MEO), Geosynchronous orbit/Geostationary orbit (GSO/GEO), and other species) are far from the denser parts of the atmosphere, making full de-orbit burns significantly more impractical.Collision avoidance (spacecraft)-Wikipedia
- One particular supersynchronous orbital regime of significant economic value to Earth commerce is a band of near-circular Geocentric orbits beyond the geosynchronous belt—with perigee altitude above 36100 km, approximately 300 km above synchronous altitudeSupersynchronous orbit-Wikipedia
- Even the lower specific impulse represents a significant increase over that of conventional chemical rockets, however, an increase that can provide substantial payload gains (45 percent for a LEO-to-GEO mission) at the expense of increased trip time (14 days compared to 10 hours).Solar thermal rocket-Wikipedia
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