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Joined: Dec 2008
15-02-2009, 12:59 PM
Satellites have been used for years to provide communication network links. Historically, the use of satellites in the Internet can be divided into two generations. In the first generation, satellites were simply used to provide commodity links (e.g., T1) between countries. Internet Protocol (IP) routers were attached to the link endpoints to use the links as single-hop alternatives to multiple terrestrial hops. Two characteristics marked these first-generation systems: they had limited bandwidth, and they had large latencies that were due to the propagation delay to the high orbit position of a geosynchronous satellite.
In the second generation of systems now appearing, intelligence is added at the satellite link endpoints to overcome these characteristics. This intelligence is used as the basis for a system for providing Internet access engineered using a collection or fleet of satellites, rather than operating single satellite channels in isolation. Examples of intelligent control of a fleet include monitoring which documents are delivered over the system to make decisions adaptively on how to schedule satellite time; dynamically creating multicast groups based on monitored data to conserve satellite bandwidth; caching documents at all satellite channel endpoints; and anticipating user demands to hide latency.
This paper examines several key questions arising in the design of a satellite-based system:
Ã‚Â¢ Can international Internet access using a geosynchronous satellite be competitive with today's terrestrial networks?
Ã‚Â¢ What elements constitute an "intelligent control" for a satellite-based Internet link?
Ã‚Â¢ What are the design issues that are critical to the efficient use of satellite channels?
The paper is organized as follows. The next section, Section 2, examines the above questions in enumerating principles for second-generation satellite delivery systems. Section 3 presents a case study of the Internet Delivery System (IDS), which is currently undergoing worldwide field trials.
Issues In Second-Generation Satellite Link Control
Can international Internet access using a geosynchronous satellite be competitive with today's terrestrial networks?
The first question is whether it makes sense today to use geosynchronous satellite links for Internet access. Alternatives include wired terrestrial connections, low earth orbiting (LEO) satellites, and wireless wide area network technologies (such as Local Multipoint Distribution Service or 2.4-GHz radio links in the U.S.).
We see three reasons why geosynchronous satellites will be used for some years to come for international Internet connections. The first reason is obvious: it will be years before terrestrial networks are able to provide adequate bandwidth uniformly around the world, given the explosive growth in Internet bandwidth demand and the amount of the world that is still unwired. Geosynchronous satellites can provide immediate relief. They can improve service to bandwidth-starved regions of the globe where terrestrial networks are insufficient and can supplement terrestrial networks elsewhere.
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18-03-2011, 10:43 AM
THERMOGRAPHY.doc (Size: 986 KB / Downloads: 46)
Thermography is a non-contact, non-destructive test method that utilizes a thermal image to detect, display and record thermal patterns and temperatures across the surface of an object. Thermography is widely used in industry for predictive maintenance, quality assurance and forensic investigations of electrical, mechanical and structural systems. Other applications include, tank and concrete inspection, nondestructive testing, condition monitoring, night vision and medical and veterinary sciences.
Thermography is useful because:
1. It is non-contact
–Uses remote sensing
–Keeps the user out of danger
2. It is two dimensional
- Thermal patterns can be visualized for analysis
-Comparison between areas of the target is possible
3. It is real time
- Enables very fast scanning of stationary targets
- Enables capture of fast moving targets
- Enables capture of fast changing thermal patterns.
Thermal infrared imagers convert the energy in the infrared wavelength into a visible light video display. All objects above 0 Kelvin emit thermal infrared energy. The radiation from the object makes it possible for a thermal camera to display an object’s temperature.
1.1. About the Thermography
Thermal or infrared energy is an energy, not visible because its wavelength is too long for the sensors in our eyes to detect. It is the part of the electromagnetic spectrum that we perceive as heat. Unlike visible light, in the infrared spectrum, everything with a temperature above absolute zero emits infrared electromagnetic energy. Even cold objects such as ice cubes, emit infrared radiation. The higher the temperature of the object, the greater the infrared radiation emitted. The Infrared camera allows us to see what our eyes cannot.
All objects, cold or hot, radiate heat in the form of infrared energy. As an object increases in temperature, it radiates more energy, and the wavelength gets shorter. Infrared radiation, visible light and ultraviolet light are all forms of energy in the electromagnetic spectrum. The only difference is their wavelength or frequency.
1.1.1. What is Thermography?
Infrared Thermography is the technique that uses an infrared imaging and measurement camera to "see" and "measure" invisible infrared energy being emitted from an object.
Thermography is a non-contact, non-destructive test method that utilizes a thermal imager to detect, display and record thermal patterns and temperatures across the surface of an object. Infrared thermography may be applied to any situation where knowledge of thermal profiles and temperatures will provide meaningful data about a system, object or process.
1.1.2. What principle used in Thermography?
Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature; therefore thermography allows one to see variations in temperature. Radiation also originates from the surroundings and is reflected in the object, and the radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere
If the temperature an object gets hot enough however, above 525°C the energy from that object will radiate energy in the visible spectrum and we will see it. This is when we see an object like the burner on an electric stove “glowing” red. In fact any time an object will emit or reflect energy in the same frequency of our eyes we will see it.
Infrared energy is just one part of the electromagnetic spectrum that encompasses radiation from gamma rays, x-rays, ultra violet, a thin region of visible light, infrared, microwaves, and radio waves. All objects emit a certain amount of black body radiation as a function of their temperatures. The higher an object's temperature is the more infrared radiation as black-body radiation it emits. A special camera can detect this radiation in a way similar to an ordinary camera does visible light. It works even in total darkness because ambient light level does not matter. This makes it useful for rescue operations in smoke-filled buildings and underground.
1.1.3. Where Thermography is used?
Thermal imaging photography finds many uses. For example, firefighters use it to see through smoke, find persons, and localize hotspots of fires. With thermal imaging, power line maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Where thermal insulation becomes faulty, building construction technicians can see heat leaks to improve the efficiencies of cooling or heating air-conditioning. Thermal imaging cameras are also installed in some luxury cars to aid the driver. Some physiological activities, particularly responses, in human beings and other warm-blooded animals can also be monitored with thermographic imaging. Cooled infrared cameras can also be found at most major astronomy research telescopes.
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