At 23 years old, Joshua Williams seems a little young to be in charge of the Global Positioning System. And yet he is responsible for a constellation of 35 satellites, each one of them worth upwards of £40 million (Dh243 million) and vital for the safe passage of billions of people, in cars, ships and aeroplanes all over the world. Williams's official title is payload system operator. It is his job to monitor the signals from each of the satellites and a network of 16 tracking stations around the world and provide a constant supply of timing corrections to ensure the system is perfectly synchronised. But his boss, Jennifer Grant, commander of the 2d Space Operations Squadron, the United States Air Force unit that operates GPS, has another, more informal, name for him. She calls him "Atlas". He is, she says, "carrying the weight of the world on his shoulders".

GPS is now such an integral part of our lives that we hardly give it a second thought. More than 18 million motorists in Britain at present use satnav to get from A to B, and, far from standing slack-jawed in admiration when a machine the size of a pack of playing cards tells us where we are, and where we should go next, we simply turn on the radio, lean back in our seat and follow the directions. If, God forbid, the machine takes more than 30 seconds to plan our route, doesn't know a street is one-way or takes us up a blind alley, we curse its incompetence.

Even the abbreviation "satnav" distances the gadget from the technology, the satellites, that makes the whole thing possible. Pressing a button and finding out where we are has become as unsurprising to us as receiving pictures on a television. Of course, the fact that it has become so unremarkable is testament to its power. GPS was designed, first and foremost, for the US military, and the information the satellites provide is responsible for the precision bombing that has become such a key component of the US war machine over the past 20 years. The system that gets us around is the same one that allows the US and its allies to fire a missile from 3,000 kilometres away and guide it to an area in Iraq, Afghanistan or Libya the size of a kitchen table. Whatever your opinion of America's wars, there is no question that GPS has dramatically reduced the number of civilians who are killed in them.

It has improved many other areas of our lives too. In agriculture, the system enables more accurate sowing, reaping and treatment of crops; in aviation, it has cut flight times and operating costs; and in shipping, it has massively reduced the number of lost or misdirected containers. It has also benefited the environment by cutting journey times and, as a result, the fuel consumption of millions of vehicles every day, and it has saved countless lives by reducing the time it takes paramedics to get to emergencies and aid agencies to reach the victims of natural disasters.

Power firms rely on the free, accurate time signal from GPS satellites to control electricity supplies and financial institutions use it to time payments. Less essential but increasingly pervasive are the hundreds of smartphone apps that use GPS to help us find our nearest cash point machine or the real-time locations of our friends on Facebook or Twitter.

But the GPS signal is also incredibly vulnerable. In a study published earlier this year, Martyn Thomas, a visiting professor in software engineering at the universities of Oxford and Bristol, warned that the world had become too reliant on GPS and explained how the signal could easily be compromised, either in outer space by a violent solar flare or down on Earth by terrorists armed with primitive jammers.

A single 50-watt jammer, positioned somewhere high, could, he warned, take out every GPS-connected service in the whole of southern England, crippling banks, emergency services, power plants and airports. And if a terror group wasn't satisfied with southern England and wanted to bring death and destruction to the entire planet, it could attack GPS HQ.

Schriever Air Force Base, which houses the 2d Space Operations Squadron, or 2 SOPS, takes this threat extremely seriously. A 1,600-hectare complex 21 kilometres east of the city of Colorado Springs, Schriever is also the control centre for 135 other Department of Defence satellites, under the umbrella of the 50th Space Wing, which provide crucial communication and surveillance capabilities to the whole of the US military.

I had to wait a month just to have my visit approved. Then, once at the base, our photographer, his assistant and I had to make our way through several layers of security. In case we still hadn't got the message, once we had arrived, we were shown a DVD underlining Schriever's central role in the US military. The 3,100 military personnel on base include computer systems experts, spacecraft engineers and "operations" crews, who monitor and fly the satellites, but all are "warfighters", as vital to America's defence as their more high-profile colleagues who fight in the field or in the air. "Just ask the guys on the ground if they would want to be in that fight if they didn't have space on their side," said one interviewee to the camera.

Serene office environment

After such a build-up, the Master Control Station, where 2 SOPS flies the GPS satellites, is something of a let-down. It looks more like an open-plan office than a top-level military installation, with operators sitting at desks in front of computer screens (blanked out, for "security reasons", while we are in the room). But three things betray the room's purpose: the army fatigues worn by the eight-man crew; a plasma screen showing a computer graphic of the Earth and its orbiting satellites; and a digital clock on one wall counting the time from the beginning of the year in days, hours, minutes and seconds.

Nowhere in the world will you get a more accurate time check because, at the heart of GPS, are the most precise clocks known to man. Throughout history, battles have been waged over who to credit for some of our most important scientific and technological advances. Today people have similar arguments over the invention of GPS. Both the US Air Force and the Navy lay claim and scientists from each have been honoured for the part they played. But one thing which no one disputes is that the turning point came one day in March 1958 in the office of Frank McClure, the director of the research centre of the Applied Physics Laboratory (APL) at Johns Hopkins University in Laurel, Maryland.

A Canadian scientist who had won awards for his contribution to ballistics research during the Second World War, in March 1958, McClure was at his desk reviewing the work of two of his junior physicists, William Guier and George Weiffenbach. Like most of the world, Guier and Weiffenbach had been caught by surprise the previous October when the Soviet Union announced it had successfully launched a satellite called Sputnik into orbit. Although little more than a small beacon inside a tin can, it was the first man-made satellite ever to orbit the Earth and the news spread panic throughout the US, as people reassessed the supposed scientific and technological inferiority of their Cold War rival.

But while politicians and journalists called for answers and president Eisenhower tried to control public hysteria, the Johns Hopkins scientists did something enterprising — they rigged up a listening station on the roof of their laboratory and managed to pick up Sputnik's signal. As they listened to the broadcast — a simple beep-beep-beep in A-flat — they noticed that the radio frequency of the satellite's transmitter kept on changing. And they quickly realised that, if they monitored this shift, they could determine the exact position of the satellite in its orbit.

When McClure called them into his office to ask them to explain what they had been doing, McClure's brain went into overdrive. "If you can find out where the satellite is," he began, "then you ought to be able to turn that problem upside down and find out where you are."

This was no small revelation. McClure knew that the navy had faced significant navigational challenges during the war. Ships and submarines had strayed off course during bad weather and navigators had had to rely on guess work and celestial navigation to locate themselves. Now a solution to those problems seemed at hand. McClure hurriedly relayed his idea to his colleague at APL, Richard Kershner, and over the weekend, the two men created the blueprint for a new satellite navigation system which APL, led by Kershner, went on to develop for the US navy. Called Transit, the new system featured five satellites in low polar orbits, about 960 kilometres above the Earth, broadcasting their locations via radio waves. Receivers on the ground then picked up the satellites' signals and worked out their own locations on Earth.

The first satellite — Transit 1B — was launched into space in April 1960 and by 1964 the system was fully operational. It was used, first, on the new fleet of Polaris nuclear submarines, and, as time went by, on thousands of other warships, freighters and private vessels. It lent a crucial advantage to the US naval force during the Cold War but Transit had some significant drawbacks. For a start, the early receivers were enormous; taking up 12 6-foot-high rack cabinets. Secondly, because the five satellites were spread out over the entire globe, vessels normally had to wait between one and two hours before a satellite was in the right position in the sky for their receiver to get a fix. And even then, the receiver, which had to be static, took about 15 minutes to churn through its calculations. Transit also depended on the receiver being at sea level, so it was useless for anything other than maritime expeditions.

The Air Force thought it could do better. As early as 1964, a top-secret programme — 621B — was set up to develop an alternative to Transit and, over the next two years, the pros and cons of 32 possible systems were assessed. One of these, called "triple delta rho", stood out. It promised to collect data not from just one satellite, like Transit, but from four or more simultaneously, allowing a user to determine his position in three dimensions, on land or sea or up in the air, and to do so while moving at great speed any time of the day or night. It worked so fast that it could be used to guide missiles travelling at 2,000mph. But the Global Positioning System (as "triple delta rho" was eventually renamed) was also the hardest to build.

For a start, it required a constellation of at least 24 satellites at a time, when the average lifespan of a satellite was under two years, so the scientists had to think of a way of building longer-lasting satellites.

The team, led by Brad Parkinson, a 37-year-old chief engineer, also had to design a new way of transmitting the satellites' signals so they could all broadcast on the same frequency. They had to design new, affordable receivers (that weren't 6 feet high) and, most importantly, they had to work out a way for the satellites to broadcast extraordinarily precise time checks, because, if the receiver knew the exact time it had taken for each signal to travel through space, it could work out where the satellites were and, therefore, where it was.

To solve these problems, Parkinson assembled a crack team of the best brains the Air Force had to offer. Now 76 and living with his wife in southern California, Parkinson compares the process to running a maze. "My job was to spot when we were on the wrong track and find an alternative path as soon as possible," he says. Just in case the team lost focus, a sign was pinned to a wall at their base in El Segundo, Los Angeles. "The mission of this Program Office," it read, "is to drop five bombs in the same hole and build a cheap set that navigates, and don't you forget it!"

When you consider how GPS has revolutionised our lives, Parkinson and his team must be among the greatest unsung heroes of the 20th century. However, at the time, many senior figures in the Air Force vehemently opposed the project. "The Air Force is populated by pilots," Parkinson says. "They don't see any need for navigators. They were also coming out of a couple of wars in which precision weaponry was a misnomer. They didn't have any."

Steady growth

Nonetheless, in February 1978, the first GPS satellite was launched. Over the next seven years, nine more were put into orbit and, when the Gulf War began in 1990, the world witnessed the system's awesome power for the first time as television footage captured missiles flying up streets and disappearing down ventilator shafts. On April 27, 1995, the system was finally declared fully operational.

Keeping it that way is a Herculean task. Back at 2 SOPS's Master Control Station at Schriever, Lieutenant Colonel Jennifer Grant a softly spoken woman who stands no more than 5 feet tall in her Air Force flightsuit, gives me a rundown of the personnel on the operations floor. "We have three space system operators," she says, gesturing towards a row of desks to her left. "One space vehicle operator, one network administrative operator, one payload system operator, one mission chief and one mission commander." These people can expect to have 70 contacts with the satellites every day. A contact can range from the routine — checking the temperature in the fuel tanks or the strength of the on-board battery — to the urgent; when a satellite goes "white" and stops broadcasting a signal.

This can happen for a multitude of reasons. Flying the satellites, after all, is like flying an aeroplane over the Atlantic without a pilot, except that they are 20,117 kilometres up in the air. They have hundreds of moving parts, including an extremely sensitive atomic clock, and must maintain a very precise position, with their antennas pointed at Earth and their solar panels pointed towards the Sun, while moving at somewhere close to 7.2 kilometres per second. If one drifts off course or malfunctions, it is the crew's job to fire the thrusters and either move the satellite back into position or park it in a safe configuration while it is repaired.

"On a bad day," says Joshua Williams, the 23-year-old payload system operator I meet on my tour, "there isn't even time to go to the bathroom." Other members of the 50th Space Wing are working just as hard, out of sight, to protect the various fleets.

The biggest threat to the satellites is not solar flares or any on-board malfunctions but space junk: the thousands of pieces of debris — from small discarded components, such as bolts, to an astronaut's glove — that circulate the planet. They may be small but they are travelling so fast that they can cause enormous damage. A team at Schriever keeps track of about 15,000 of these and is under orders to alert the relevant operations crew if it thinks a collision is imminent. Schriever also maintains a standby power plant to keep the base going in the event of a power cut. The largest standby plant in the Air Force, it is the domain of a former US navy engineer and Robert Shaw lookalike called John Paulson, who sports a checked shirt, jeans and a moustache down to below his chin. One of 5,000 civilian employees on the base, Paulson has been supervising the facility for ten years and has yet to have a "major incident".

The base also regularly practises a series of emergency procedures designed to protect the mission during a natural disaster, such as an earthquake or a tornado. The site is not prone to the former but a tornado hit the town of Ellicot, 8 kilometres away, in 2000. There is also a back-up location in California if disaster were to strike and Schriever was unable to operate. Grant points out that all of this is done to maintain a GPS signal, now accurate to within 3 metres, which the US gives to the rest of the world for free. (China, Russia and the European Union are all developing their own navigation systems but none is fully operational as yet.) "If the Department of Defence had charged just a penny for every time somebody had accessed GPS, we probably wouldn't have a deficit right now," she smiles.

Way cleared for satnav

It wasn't always so generous. Up until May 2000, the Air Force deliberately degraded the civilian signal, a technique known as "selective availability". But this was stopped on the orders of the president at the time, Bill Clinton. It was a decision that cleared the way for satnav and all the other GPS applications that have so changed the world we live in. It also made some people very, very rich. TomTom, which, along with Garmin and Magellan, provides most of the in-car satnavs in the world, saw its revenues go from €39 million (Dh206 million) in 2003 to €192 million the following year, to €720 million the year after that, to €1.5 billion in 2010.

Harold Goddijn, the CEO, and his wife Corinne, who together own 24 per cent of the company, are today worth about £235 million. But, if you talk to Goddijn, a Dutch man with a passion for sailing, he will remind you that his company, which started off making applications for mobile phones, was far from an overnight success. "TomTom was founded in 1991," he says. "And we were already a respected, moderately successful company [by 2000]." So, when Clinton "flicked the switch", TomTom already had much of the expertise to take advantage. "We knew something was there," he says. "In the early days of hyper growth, we just couldn't make enough of them." Nevertheless, like all GPS businesses, the company remains at the mercy of the men and women at Schriever.

Martyn Thomas believes a human error at the base, which knocked the accuracy of the system by a fraction, would actually be more disastrous than a "complete failure" because it would produce information that was wrong, but not obviously so.

Aeroplanes, cars and ships could all crash as a result. Grant waves away such concerns. "Our focus here is to provide as accurate a signal as we can, and, in fact, the signal today is the best it's ever been. GPS sets the gold standard." There will always be problems and dangers, she adds, but she is confident they will be overcome. Thirty-three years after Brad Parkinson and his team sent the first satellite into orbit, a new team of unsung heroes continues to innovate and experiment: testing different solutions, running the maze.

- The Telegraph Group Limited, London 2011