Geosynchronous satellites are satellites in a geosynchronous orbit—a geocentric orbit that has the same orbital period as the rotational period of the Earth. The orbit has a semi-major axis of 42,164 kilometers or 26,200 miles. In a more general case, when the orbit has some inclination or eccentricity, the satellite appears to describe a more or less distorted figure-eight in the sky, and essentially rests above the same spot of the Earth's surface once per sidereal day.
Satellites in geosynchronous orbit appear to be stationary above a particular point, which is due to the synchronization. This type of satellite provides a distinct benefit of fixing the receiving antennas at one place. The higher a satellite is above Earth, the slower it moves because of Earth's gravity; the closer a satellite is to a larger body, the greater the pull of gravity, whereas the further away it is, the weaker the pull.
A benefit of a satellite in geosynchronous orbit is that it is capable of seeing one spot of the planet almost all of the time. For Earth observation, this allows the satellite to look at how much a region changes over months or years. However, the drawback can be that the satellite is locked into a small region, and in the case of an event occurring in a place not already covered by another satellite, the satellite would not be able to move to this new location due to the fuel requirements.
The geosynchronous orbit is also sometimes described as a geostationary orbit. Although there is some difference between the two orbits, they are also related. Both orbits are about 35,786 kilometers above Earth's surface, which puts it in a high Earth orbit category. At any inclination, a geosynchronous orbit synchronizes with the rotation of the Earth.
While a geosynchronous orbit can have any inclination, geostationary orbits lie on the same plane as the equator; this is the key difference between these two types of orbits.
Weather monitoring satellites like GOES are in geostationary orbits because they have a constant view of the same area. In a high Earth orbit, these satellites can also be useful for search and rescue beacons.
The original idea of a geosynchronous communications satellite was published by the British science fiction writer Arthur C. Clarke in the early 1950s. These satellites have since become of major importance in most people's lives—allowing people to receive hundreds of television programs from around the world and giving people the opportunity to talk to relatives around the world. Before GEO communication satellites, long-distance signals had to be relayed via earth-based relay towers, which had to be placed in sight of one another.
NASA launched the first geosynchronous satellite, Syncom I, on February 14, 1963, but it stopped sending signals due to some technical failures encountered seconds before reaching its final orbit. Five months later, Syncom II was launched and demonstrated the viability of the satellite and the orbit. Syncom III was used to transmit live coverage of the 1964 Tokyo Olympic Games to stations in Europe and North America.
Due to the near stationary position of geostationary satellites, they have been used for global communications, television broadcasting, weather forecasting, and defense and intelligence applications.
For communications, a large geostationary satellite can provide a large amount of capacity across up to a third of the Earth's surface, and a network can cover the Earth with only three satellites. Further, these satellites can use multiple bands to allow a single satellite to layer capacities. The satellites also have the ability to move resources within themselves to suit the situation, such that if a customer connectivity needs to be increased in a given location and is greater than in other locations, a satellite's resources can be arranged from less busy beams into the beam or beams covering the events.
For government communications, this can ensure that a network is capable of delivering greater capacity as required and that governments can achieve availability and reliability when moving to a new situation or location at short notice. However, one complication with newer communication systems and GEO satellites is the latency introduced through the distance of the orbit from the terrestrial stations. With GEO satellites, there can be latency up to 0.25 seconds between signal origination and reception. In part, this has driven the popularity of low Earth orbit satellites for emerging communications systems for their lack of latency.
However, with low Earth orbit satellites, a signal has to be swapped every seven minutes, and this switching is considered a possible point of failure or interruption with a signal, while a GEO satellite remains "fixed" and does not suffer from this possible point of friction. In a hybrid satellite communications system, using low Earth orbit and GEO satellites, these satellites can be used to create inter-satellite links and move customers' data across various satellites until they reach a suitable gateway and the information is returned to the customer network. These inter-satellite links, however, add time and distance and therefore latency, and they increase the complexity of those connections.
Geosynchronous satellites have long been a major medium for linking together terrestrial telecommunications networks. Satellite systems share the frequency spectrum with other satellite systems, and in most frequency bands with terrestrial radio systems. The most useful orbit for communication satellites is the geostationary orbit, and the signal channels between satellite and control earth stations required for these functions are carried by radio subsystems. Margin of power is provided in addition to ensure that channel performance targets are reached. Communication satellites are designed to relay several signals simultaneously. And satellite communications are capable of long-distance communications and can be operated with mobile terrestrial terminals.
Broadcasting has been a use case for geosynchronous satellites for almost as long as satellites have been in the geosynchronous orbit. These satellites have been used for direct-broadcast satellite television, in which the satellite sent transmission directly to a home. Previous to the popularity of direct-broadcast television, these satellites were used to transmit signals for conventional broadcasting.
As broadcasting has changed and the use of satellites in broadcasting has continued, hybrid satellite systems have been used to offer radio access systems capable of providing both terrestrial and satellite connectivity. Practical examples of the existing hybrid systems, such as digital video broadcasting-satellite services to handheld devices, have been used. The main focus in these types of satellite systems is in solving spectrum challenges between the terrestrial and satellite components.
A proposed concept for these challenges has consisted of a terrestrial 3GPP (the 3rd Generation Partnership Project) LTE (Long-Term Evolution) network in dense populated urban areas and of co-channel satellite LTE cells in low-populated areas. Simulation results of this type of scenario have been presented showing that the satellite interference to the terrestrial network can be kept at an acceptable level.
One of the most well-known use cases of GEO satellites is the satellite-based navigation system, which is used to localize a radio receiving terminal, also referred to as the global positioning system (GPS). All GPS satellites share the same frequency bands, making use of the code division multiple access (CDMA) technique. The transmitted signal on each sub-band is a low-rate binary phase shift keying (BPSK) digital signal containing the so-called navigation (NAV) data that are used by the receiver to perform ranging.
Galileo is the European Global navigation satellite systems (GNSSs) that provides a global positioning service under civilian control. This system has to be interoperable with GPS and GLONASS, the American and Russian GNSSs respectively. Like GPS and Galileo, GLONASS is based on several ground stations that monitor and control the satellites and their signals. The GPS and Galileo stations are spread around the globe in order to guarantee a better control of the constellation and its signals.
Differential GPS (DGPS) has been introduced as an enhancement to GPS that uses a network of fixed, ground-based reference stations to broadcast the difference between the position of a receiver located at each station, as derived instant by instant using the GNSS and the known fixed positions of the stations themselves.
Geosynchronous satellites are also used for weather forecasting. These satellite systems have unique characteristics and are capable of producing different products. These geostationary satellites spin at the same rate of the Earth and are capable of constantly focusing on the same area. This enables the satellite to take pictures of the Earth at the same location every thirty minutes.
Computer processing is then capable of taking these images and related data to create "movie loops" of the data, which forecasters then use as their real-time view from space. Two US geostationary satellites provide imagery over North and South America and the Atlantic and Pacific oceans. During severe weather outbreaks, the geostationary satellites can be commanded to take images every five to fifteen minutes and can focus in on smaller impacted areas.
On some occasions, these GEO satellites can be used to take pictures every minute of a very small area such as a severe weather event. GEO satellites can also take atmospheric profiles of temperature and moisture, but at reduced resolution compared to polar satellites and radiosonde soundings.
Another common use for GEO satellites is for signals intelligence (SIGINT) where these satellites can be used for generating intelligence for military and defense applications. This has included the launch of US spy satellites known as Orion that began in operation in 1995, with a legacy that stretches back to the United States' original CORONA spy satellites launched in the 1960s and 1970s.
New versions of these satellites have increased their capabilities and are capable of monitoring different areas and being positioned to monitor those areas in some cases. Since the end of the Cold War, technology has advanced to the point that most countries have a few SIGINT satellites available to them. And with the amount of SIGINT satellites among military, commercial, and non-governmental satellites, people are being monitored by these satellites at all times. This includes satellites monitoring urban and rural development, agriculture, climate change, road traffic, or people smuggling.
With the popularity and increased deployment of low Earth orbit satellites, the challenge of dealing with debris in orbit has reached an increased awareness. In this increased awareness, geosynchronous orbits have been considered to be largely safe compared to the increasingly cluttered low Earth orbit. However, analysis by Analytical Graphics Inc (AGI) and satellite operators SES and Inmarsat used six different approaches to estimate the risk of orbital debris. The six approaches reached a broad agreement that indicated that the threat posed by space debris is much greater than previously assumed.
The results of these studies indicate that the chances of a collision in GEO are up to four orders of magnitude higher than previous estimates suggested. The researchers further predicted that the population of active GEO satellites can be expected to suffer one potentially mission-terminating impact every four years on average. Because of the popularity of the GEO real estate, the region above the equator has become congested with satellites that are both operational and defunct. Because these satellites can maintain contact without need for active tracking, the satellites which are defunct cannot be tracked any longer. As well, smaller "residential space objects" (RSOs) generated by collisions, explosions, and other fragmentation events means that in addition to the satellites, the region is host to thousands of objects larger than 10 centimeters, and at least tens of thousands larger than 1 centimeter.
Due to the increased collision risk, and the relative importance of these satellites for communication and navigation systems, geosynchronous orbit maintenance has become an increasingly important issue. These satellites maneuver frequently and require the ability to detect unknown maneuvers for target satellites and to allow those satellites to recover an accurate orbit. Studies have been used to determine if angles from ground-based optical tracking to detect maneuvers and recover orbits can be used. One such tool, developed by Analytical Graphics Inc, uses sequential estimation software that uses a parametric study of maneuver size and time required to detect a maneuver. The company has also suggested various methods to recover the orbit after such maneuvers have been detected. This work is important towards developing more automatic methods of detecting maneuvers for a large population of active geosynchronous satellites.
Similarly, DARPA has sought solutions for robotic servicing of GEO satellites. This would involve a robotic program that would be capable of cooperative inspection and servicing in GEO and demonstrating those technologies. This would, under DARPA, include RSGS developed modular toolkits, including hardware and software, that would be joined to a privately developed spacecraft to create a commercially owned and operated robotic servicing vehicle that could essentially make house calls in space.
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