07-07-2015, 02:44 PM
need detailed ppt regarding inter satellite communication and handover issues in LEO.
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optical inter satellite communication ppt
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need detailed ppt regarding inter satellite communication and handover issues in LEO.
08-07-2015, 02:44 PM
optical inter satellite communication ppt
This paper describes the achievements in optical intersatellite communication based on technology developments that started in Europe (European Space Agency) more than 30 years ago. In 2001, the world-first optical intersatellite communication page link was established (between the SPOT-4 and Advanced Relay and TEchnology MIssion Satellite (ARTEMIS) satellites), proving that optical communication technologies can be reliably mastered in space. In 2006, the Japanese Space Agency (JAXA) demonstrated a bidirectional optical page link between its Optical Inter-Orbit Communications Engineering Test Satellite and ARTEMIS, and in 2008, the German Space Agency (DLR) established an intersatellite page link between the near-field infrared experiment and TerraSAR-X satellites already based on the second generation of laser communication technology.
OPTICAL COMMUNICATIONS AND INTERSATELLITE LINKS
Introduction
Space-based, free-space optical communications is a concept that has been around for many years. In the last few years, however, there has been impressive activity to bring the concept to fruition in civilian and government non-classified projects. Today's market for space-based optical communications is primarily intersatellite links (ISLs) which are the main focus of this chapter. There is also a place for high data rate (many Gbps) space-earth links, though propagation effects due to the atmosphere and weather make this a much more difficult link. Some activity in space-earth optical communications will also be covered here.
The usual parameters that system designers want to optimize drive the desire to utilize optical communications onboard a satellite: size, weight and power—and of course, cost. Under ideal assumptions about equivalent efficiency of signal power generation, detectors, and receiving surfaces, page link equations show that optical communications systems with telescope aperture equivalent to that of the antenna of a radio frequency (rf) system could potentially provide tens of dBs of page link efficiency improvement, e.g., data rate, margin, etc. This results strictly from the wavelength difference. These tens of dBs can be traded off against reduced optical aperture size, hence reduced size and weight, and the inefficiencies of optical signal generation/detection, and yet still support increased data rates relative to an rf system.
One significant factor in this trade-off is that the optical system will typically have a much narrower beamwidth than the rf system. This has both a positive and negative side. On the positive side, a narrower beamwidth means that the potential for interference to or from adjacent satellites will be reduced. This is particularly important in large LEO constellations. On the negative side, the requirements for more accurate pointing, acquisition and tracking (PAT) and the impact that this may have on the spacecraft could impose an unwelcome burden. Accurate PAT is critical to the acceptance of optical ISLs.
A secondary, though not unimportant, fact about optical communications is that, unlike the rf spectrum which is regulated by national and international agencies, the optical spectrum is currently unregulated.
Finally, reliability of optical communications systems, particularly their lasers, has been a concern in the past. This issue is being overcome by advances in optical and laser technology but needs documented space validation for wider acceptance.
Applications
Intersatellite communications is used primarily for "networking" a constellation of satellites at data rates up to many Gbps or for data relay purposes from tens of Mbps up to Gbps. These ISLs can be between all the various orbits that one might consider: low earth orbit (LEO), medium earth orbit (MEO), highly elliptical orbit (HEO), and geosynchronous earth orbit (GEO). There are currently systems like Iridium and NASA's Tracking and Data Relay Satellite System (TDRSS) that are using rf ISLs for these purposes. The ill-fated Japanese COMETS was to use rf ISLs. There are planned systems like ESA's ARTEMIS that will use rf and optical ISLs in the future. It is safe to say, however, that for many of the reasons outlined above, the future belongs to the optical ISL. This is evidenced by the fact that most, if not all, of the commercial satellite constellations now being announced, such as Teledesic, will be using optical ISLs. Iridium considered an optical ISL, but did not fly it primarily for business reasons, i.e., the risk perceived by investors.
Space-Earth links have been, and continue to be, primarily rf. Because of the advantages of optical systems related earlier, Japanese, European and U.S. researchers are investigating optical space-earth links from LEO as well as the far reaches of outer space. Optical links face a severe disadvantage due to the effects of the atmosphere and weather. Solutions include adaptive optics, spatial diversity, and onboard storage with burst transmission under good conditions. The first applications are likely to be in scientific satellites but as operational methodologies are developed, space-earth optical links will work their way into commercial systems.
As will be shown below, space-based optical communications development around the world has been primarily supported by government agencies. The European Space Agency, the Japanese government, and NASA and the DOD in the United States have been the main funding agencies. This is changing as the commercial satellite world integrates optical ISLs, and companies will be willing to form partnerships and invest more of their own independent research and development funds.
Japan
The Japanese have a strong program in optical communications. The Science and Technology Agency has designated the Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications as a Center of Excellence for Optical Communications and Sensing. Thus the government has determined that optical communications and optical technologies, including sensing, are extremely important issues for Japan. As a Center of Excellence, the CRL has gathered researchers from around the world and devoted a lot of money for developments in this area. An overview of the types of links and systems being considered, from ISLs to space-earth links, is shown in Figure 3.9. A comment was made during the site visit to CRL that all ISLs of the future would be optical.
Trends
In general, smaller, better, and faster characterize the next generation systems. The single most identifiable trend is towards speed. This has been a dominating factor in keeping pace with terrestrial fiber systems. Ten Gbps systems will appear within the next few years. Higher power lasers and higher speed laser switching are aiding in achieving this, along with high speed electronics (ASIC and MMIC). There does not appear to be universal agreement concerning wavelength. A lot of the earlier work was done at 0.8 µm but there are now terminals at 1.06 µm. High-volume development associated with terrestrial fiber systems make components like Erbium-doped fiber amplifiers attractive for space-based optical communications, so many of the recent systems are focusing on the 1.55 µm range. Regarding smaller terminals, there is a coalescence of elements in the terminal, making use of the same detectors and a lot of the same electronics for doing multiple functions. Similarly, lighter components will be developed with new materials that will make these systems lighter in general.
Conclusion
In conclusion, Japan and Europe have had very vigorous and open development in optical communications terminals and systems. The U.S. providers have been somewhat hampered by previously classified programs but this is rapidly changing and many U.S. companies are competing with the European and Japanese companies for the growing ISL market. It should be clear to everyone that optical ISLs are coming. When? It should be soon since it is an important application. The first time a Teledesic or some other company deploys an optical ISL in a commercial system may well open the floodgate. Once these systems are in orbit and functioning many others will follow. Space to earth is a little trickier because of the atmospheric effects, and the fact that adaptive optics need to be developed, but there will be commercial applications of high data rate space-earth optical links in the near future.
This paper describes the achievements in optical intersatellite communication based on technology developments that started in Europe (European Space Agency) more than 30 years ago. In 2001, the world-first optical intersatellite communication page link was established (between the SPOT-4 and Advanced Relay and TEchnology MIssion Satellite (ARTEMIS) satellites), proving that optical communication technologies can be reliably mastered in space. In 2006, the Japanese Space Agency (JAXA) demonstrated a bidirectional optical page link between its Optical Inter-Orbit Communications Engineering Test Satellite and ARTEMIS, and in 2008, the German Space Agency (DLR) established an intersatellite page link between the near-field infrared experiment and TerraSAR-X satellites already based on the second generation of laser communication technology.
OPTICAL COMMUNICATIONS AND INTERSATELLITE LINKS
Introduction
Space-based, free-space optical communications is a concept that has been around for many years. In the last few years, however, there has been impressive activity to bring the concept to fruition in civilian and government non-classified projects. Today's market for space-based optical communications is primarily intersatellite links (ISLs) which are the main focus of this chapter. There is also a place for high data rate (many Gbps) space-earth links, though propagation effects due to the atmosphere and weather make this a much more difficult link. Some activity in space-earth optical communications will also be covered here.
The usual parameters that system designers want to optimize drive the desire to utilize optical communications onboard a satellite: size, weight and power—and of course, cost. Under ideal assumptions about equivalent efficiency of signal power generation, detectors, and receiving surfaces, page link equations show that optical communications systems with telescope aperture equivalent to that of the antenna of a radio frequency (rf) system could potentially provide tens of dBs of page link efficiency improvement, e.g., data rate, margin, etc. This results strictly from the wavelength difference. These tens of dBs can be traded off against reduced optical aperture size, hence reduced size and weight, and the inefficiencies of optical signal generation/detection, and yet still support increased data rates relative to an rf system.
One significant factor in this trade-off is that the optical system will typically have a much narrower beamwidth than the rf system. This has both a positive and negative side. On the positive side, a narrower beamwidth means that the potential for interference to or from adjacent satellites will be reduced. This is particularly important in large LEO constellations. On the negative side, the requirements for more accurate pointing, acquisition and tracking (PAT) and the impact that this may have on the spacecraft could impose an unwelcome burden. Accurate PAT is critical to the acceptance of optical ISLs.
A secondary, though not unimportant, fact about optical communications is that, unlike the rf spectrum which is regulated by national and international agencies, the optical spectrum is currently unregulated.
Finally, reliability of optical communications systems, particularly their lasers, has been a concern in the past. This issue is being overcome by advances in optical and laser technology but needs documented space validation for wider acceptance.
Applications
Intersatellite communications is used primarily for "networking" a constellation of satellites at data rates up to many Gbps or for data relay purposes from tens of Mbps up to Gbps. These ISLs can be between all the various orbits that one might consider: low earth orbit (LEO), medium earth orbit (MEO), highly elliptical orbit (HEO), and geosynchronous earth orbit (GEO). There are currently systems like Iridium and NASA's Tracking and Data Relay Satellite System (TDRSS) that are using rf ISLs for these purposes. The ill-fated Japanese COMETS was to use rf ISLs. There are planned systems like ESA's ARTEMIS that will use rf and optical ISLs in the future. It is safe to say, however, that for many of the reasons outlined above, the future belongs to the optical ISL. This is evidenced by the fact that most, if not all, of the commercial satellite constellations now being announced, such as Teledesic, will be using optical ISLs. Iridium considered an optical ISL, but did not fly it primarily for business reasons, i.e., the risk perceived by investors.
Space-Earth links have been, and continue to be, primarily rf. Because of the advantages of optical systems related earlier, Japanese, European and U.S. researchers are investigating optical space-earth links from LEO as well as the far reaches of outer space. Optical links face a severe disadvantage due to the effects of the atmosphere and weather. Solutions include adaptive optics, spatial diversity, and onboard storage with burst transmission under good conditions. The first applications are likely to be in scientific satellites but as operational methodologies are developed, space-earth optical links will work their way into commercial systems.
As will be shown below, space-based optical communications development around the world has been primarily supported by government agencies. The European Space Agency, the Japanese government, and NASA and the DOD in the United States have been the main funding agencies. This is changing as the commercial satellite world integrates optical ISLs, and companies will be willing to form partnerships and invest more of their own independent research and development funds.
Japan
The Japanese have a strong program in optical communications. The Science and Technology Agency has designated the Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications as a Center of Excellence for Optical Communications and Sensing. Thus the government has determined that optical communications and optical technologies, including sensing, are extremely important issues for Japan. As a Center of Excellence, the CRL has gathered researchers from around the world and devoted a lot of money for developments in this area. An overview of the types of links and systems being considered, from ISLs to space-earth links, is shown in Figure 3.9. A comment was made during the site visit to CRL that all ISLs of the future would be optical.
Trends
In general, smaller, better, and faster characterize the next generation systems. The single most identifiable trend is towards speed. This has been a dominating factor in keeping pace with terrestrial fiber systems. Ten Gbps systems will appear within the next few years. Higher power lasers and higher speed laser switching are aiding in achieving this, along with high speed electronics (ASIC and MMIC). There does not appear to be universal agreement concerning wavelength. A lot of the earlier work was done at 0.8 µm but there are now terminals at 1.06 µm. High-volume development associated with terrestrial fiber systems make components like Erbium-doped fiber amplifiers attractive for space-based optical communications, so many of the recent systems are focusing on the 1.55 µm range. Regarding smaller terminals, there is a coalescence of elements in the terminal, making use of the same detectors and a lot of the same electronics for doing multiple functions. Similarly, lighter components will be developed with new materials that will make these systems lighter in general.
Conclusion
In conclusion, Japan and Europe have had very vigorous and open development in optical communications terminals and systems. The U.S. providers have been somewhat hampered by previously classified programs but this is rapidly changing and many U.S. companies are competing with the European and Japanese companies for the growing ISL market. It should be clear to everyone that optical ISLs are coming. When? It should be soon since it is an important application. The first time a Teledesic or some other company deploys an optical ISL in a commercial system may well open the floodgate. Once these systems are in orbit and functioning many others will follow. Space to earth is a little trickier because of the atmospheric effects, and the fact that adaptive optics need to be developed, but there will be commercial applications of high data rate space-earth optical links in the near future.
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