Global Positioning System Information

The Global Positioning System (GPS) is a U.S. space-based global navigation satellite system. It provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis in all weather, day and night, anywhere on or near the Earth which has an unobstructed view of four or more GPS satellites.

GPS is made up of three segments: Space, Control and User. The Space Segment is composed of 24 to 32 satellites in Medium Earth Orbit and also includes the boosters required to launch them into orbit. The Control Segment is composed of a Master Control Station, an Alternate Master Control Station, and a host of dedicated and shared Ground Antennas and Monitor Stations. The User Segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service (see GPS navigation devices). GPS satellites broadcast signals from space that GPS receivers use to provide three-dimensional location (latitude, longitude, and altitude) plus precise time.

GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, tracking and surveillance, and hobbies such as geocaching and waymarking. Also, the precise time reference is used in many applications including the scientific study of earthquakes and as a time synchronization source for cellular network protocols.

GPS has become a mainstay of transportation systems worldwide, providing navigation for aviation, ground, and maritime operations. Disaster relief and emergency services depend upon GPS for location and timing capabilities in their life-saving missions. The accurate timing that GPS provides facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids. Farmers, surveyors, geologists and countless others perform their work more efficiently, safely, economically, and accurately using the free and open GPS signals.

History

The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigatordeveloped in the early 1940s, and used during World War II. In 1956 Friedwardt Winterberg^[1] proposed a test of General Relativity using accurate atomic clocks placed in orbit in artificial satellites. To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks. Additional inspiration for the GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion (see Transit (satellite)).

Summary of satellites

Block	Launch Period	Satellite launches				Currently in orbit and healthy
		Suc- cess	Fail- ure	In prep- aration	Plan- ned
I	1978–1985	10	1	0	0	0
II	1989–1990	9	0	0	0	0
IIA	1990–1997	19	0	0	0	11 of the 19 launched
IIR	1997–2004	12	1	0	0	12 of the 13 launched
IIR-M	2005–2009	8	0	0	0	7 of the 8 launched
IIF	2010–2011	0	0	10	0	0
IIIA	2014–?	0	0	0	12	0
IIIB		0	0	0	8	0
IIIC		0	0	0	16	0
Total		58	2	10	36	30
(Last update: 29 December 2009) PRN 01 from Block IIR-M is unhealthy PRN 25 from Block IIA is unhealthy See the GPS almanac. For a more complete list, see list of GPS satellite launches

Courtesy : Wikipedia

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BPA Linked to Heart Disease, Study Confirms

Plastic food containers are among common products that can contain bisphenol-A, or BPA.

Photograph by Charlie Roy, Getty Images

Bisphenol-A, or BPA—a common, human-made chemical that enters most of our bodies everyday—has been linked to heart disease, a new study says.

The study comes as the U.S. Food and Drug Administration—which has until now maintained that BPA is safe at low levels—announced Friday that the FDA has some concerns about the impacts of BPA on developing fetuses, infants, and children. The agency is also urging consumers to reduce their exposure to the chemical until further studies are completed.

BPA is commonly used in consumer plastics, particularly polycarbonate plastic items such as many sunglasses, reusable bottles, food packaging, and baby bottles. It also lines the inside of food cans.

In a sampling of U.S. adults, those with the highest levels of BPA in their urine were almost twice as likely to suffer from coronary heart disease than those with the lowest concentrations of BPA.

The findings almost perfectly dovetail with a 2008 study on the same topic, said study co-author Tamara Galloway, an ecotoxicologist at the U.K.'s University of Exeter.

"If you see it once, that's interesting," Galloway said.

"If you see it twice in a separate population, it's a strong indication that what you're seeing is not just some chance finding."

Frederick vom Saal, a BPA researcher at the University of Missouri-Columbia, agreed that the two sets of "data are compelling and demonstrate repeatability"—the point at which scientific findings move from preliminary to validated.

Study co-author Galloway cautioned, however, that no direct cause-and-effect had been found between BPA and heart disease. It remains possible that the two may be only indirectly linked, for instance through diet or lifestyle factors.

BPA Mimics Estrogen

The American Chemistry Council, which represents the U.S. plastic industry, says that "minimal" exposure to BPA poses no known risk to human health.

Still, BPA's ability to mimic estrogen—and spur reproductive mutations in the womb—has been well documented, leading some cities in the U.S., Canada, and Europe to ban BPA-containing products.

(Related: "Sex-Changing Chemicals Found in Potomac River.")

Meanwhile, BPA's "effects in adults have largely been overlooked," Galloway said, despite the fact that the chemical is found in more than 90 percent of the U.S. population.

So Galloway and colleagues examined data from the U.S. National Health and Nutrition Examination Survey, the only large-scale data set of adult health and nutrition in the world.

The team examined BPA concentrations in the urine samples of 1,493 adults taken between 2005 and 2006. A quarter of the people had "higher amounts" of BPA, the study says.

"Expand that to six billion"—roughly the world's current population—"and you've got a billion people in harm's way," said the University of Missouri's vom Saal, who was not involved in the new study.

BPA Exposure Decreasing?

The previous study, which had used data from 1,455 U.S. adults tested between 2003 and 2004, found an association between higher rates of BPA and occurrences of heart disease, diabetes, and abnormal liver enzymes.

The new study revealed the same relationship with heart disease, though the diabetes and liver-enzyme links were statistically less strong in the 2005-2006 group, the researchers say.

There was one big difference between the two data sets that surprised study co-author Galloway: The average level of BPA exposure in the 2005-2006 group was a third lower than the level in the earlier group.

The drop in BPA levels may be because more people are steering clear of obvious exposures to the chemical, and because some industries—such as plastic-bottle manufacturers—have voluntarily cut out BPA, experts speculate.

One unknown that requires "urgent" attention is how exactly the chemical might encourage heart disease in the body, according to the study published tomorrow in the journal PLoS One.

Cutting BPA Risks a No-Brainer

Bisphenol-A exposure is certainly not the only factor in heart disease, but reducing at least one possible risk is a "no brainer," the University of Missouri's vom Saal said.

For instance, people can limit their exposure by not microwaving polycarbonate plastic food containers (which normally have number sevens on their undersides), avoiding canned foods, and using BPA-free baby bottles, according to the U.S. National Institute of Environmental Health Sciences.

(Explore an interactive showing toxic chemicals that may be lurking in your home.)

"It's not a pretty picture," vom Saal added.

"This is a bad chemical, and it should not be used in the way it's being used."

Courtesy:

Christine Dell'Amore

National Geographic News

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