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Right from the start of the space age, it was evident that satellites would be very helpful for the military. After all, you didn't need to be a rocket scientist to realize that they could be used to "spy" on your enemies from orbit. But satellites also had the potential to dramatically transform the way war is fought.

The night after Sputnik's historic launch on Oct. 4, 1957, scientists at the Massachusetts Institute of Technology set out to chart the satellite's precise orbit in the sky by tuning into the "beep-beep-beep" of its radio signal.

As the satellite approach overhead, the frequency of the single increased, and as it got further away, the frequency decreased. Or put another way, the signal seemed to become stronger as satellite got closer and then weaker as the spacecraft receded in the distance- a phenomenon known as the Doppler effect.

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It was readily apparent to the scientists that satellite signals could be used for a sophisticated space-based navigational and guidance system. Just as you could use the Doppler effect to pinpoint the location of a satellite flying overhead, you could also employ satellite signals to find your location on the ground. With such beckons in the sky, ships would always know their precise position at sea, troops could manoeuvre on the ground and bombs could be guided to their targets with deadly accuracy.

That early inspiration eventually led to the creation of the Global Positioning System, or GPS, a constellation of two-dozen satellites developed and maintained by the U.S. military.

"Each satellite orbits the earth every 12 hours in fashion that ensures that every point on the planet will always be in radio contact with at least four satellites," according to a document by the U.S. National Academy of Sciences. "Each satellite continuously broadcasts a radio signal that includes both its own position and the exact time, exact to a billionth of a second."

A GPS receiver on the ground can then use this information to pinpoint its own longitude, latitude and altitude. (It essentially does this calculation by comparing and contrasting its distance from at least four satellites.) However, it took years of research and roughly $14-billion (U.S.) of investment for the military to determine the best way to make this complex navigational system work.

The first GPS satellite was launched in 1978. The system reached full capacity in 1995 when final testing was completed on the 24th satellite put into orbit.

Yet even before it was completed, GPS was put to its first combat test in the Persian Gulf War of 1990-91. It quickly became one of the stars of military campaign. GPS satellites guided a lot of bombs to their target with unprecedented accuracy. About 10 per cent air-dropped bombs were "precision guided" although a lot of these weapons were still directed to their targets by lasers, recall Lieutenant General Michael Hamel, who heads up the U.S. Air Force's Space and Missile Systems Center. And GPS receivers on the ground helped troops maneuver in the near-featureless terrain of the desert.

In the more recent U.S.-led invasion of Iraq of 2002, GPS played an even larger role in military operations. About 70 per cent of the weapons dropped from the air were precision guided - and "the vast majority" of them relied upon GPS, said Lt. Gen Hamel.

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The U.S. military had originally hoped to keep GPS all to itself - or at least limit it's use by others. But in 1983, when Korean Airlines flight 007 strayed off course and was shot down over Soviet territory, U.S. President Ronald Reagan promised that GPS would also be made available for civilian uses once it was completed.

Initially, the military broadcasted in two satellite signals, a less accurate one for civilian purposes and a far more precise one for itself. But big business saw great commercial potential in gaining access to the highly accurate military signals. Successive U.S. administrations bowed to the lobbying. The military was eventually compelled to decode its satellite signals, so everyone in the world can now receive unfettered GPS broadcasts.

Today, a growing number of consumer products rely on GPS. Lost hikers can find their way out of the woods with the aid of a hand-held GSP mapping device. Shipping companies use GSP to track the movement of goods around the world. And an increasing number of new cars have GPS guidance systems built right into the dashboard.

Even so, the U.S. Department of Defence maintains total control over the running of GPS, currently the only fully functional satellite navigation system. (The Russians had created a GPS system, known as GLOSNASS, but didn't have the money to keep it running at fully capacity.) The U.S. military still has ability to curtail access to GPS - it can revert back to coded signals.

The European Space Agency has embarked on a plan to create its own GPS system which would be known as Galileo. But the 4-billion Euro price tag to complete the project has been an issue for the member states.

"Millions of people now rely on GPS," noted Pat Norris, with Logica CMG, a British-based firm that creates computer soft ware to run space probes. "There is a feeling, if it is so important, we can't afford to rely on just one source," he added "It would like there was only one telephone company - that's a monopoly and it makes people nervous."

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