Road signs

Where would we be without it?

E&T navigates its way through the maze of GPS applications.

Taxi drivers have them. Walkers have them. You might even have found one in your Christmas stocking! With electronics and consumer outlets offering ever cheaper and more versatile models, GPS navigation systems are infiltrating themselves into everyday life in a way that other satellite applications, such as satellite telephony, have failed to do.

This is not to say that the global positioning system has been an overnight commercial success, but then that's probably because it wasn't designed to be commercial at all. Operated by the US Department of Defense and originally based on a constellation of 18 Navstar satellites and three in-orbit spares, the GPS system was designed for use by US military services. Although the first 'pre-operational' satellite was launched as long ago as 1978, the inauguration of today's 'operational' constellation of 24 spacecraft (plus spares) dates back 20 years, to February 1989.

In fact, the concept of the satellite-based positioning system has an even longer heritage, which began with the Transit system, a constellation of six satellites established in the early 1960s and operated by the US Navy, primarily to determine the positions of Polaris missile-carrying submarines. A far cry from cabbies and hikers!

The global positioning system itself was brought to widespread public attention through its use in the 1990 Gulf War, known - among the cognoscenti at least - as the 'first space war'. Trade magazines covered stories of generals being delivered by jeep in blinding desert sandstorms direct to the tent flaps, while TV news channels replayed clips of cruise missiles navigating their way through city streets to closely designated targets.

When US soldiers realised that commercial GPS receivers were available in the shops back home, they called suppliers direct from the desert to order them using their credit cards. The scene was set for the global satnav revolution.

Global navigation

The term 'GPS' has become the default term for satellite navigation, like 'Hoover' before it, because for 20-odd years it has been the only game in town.

A less well-known and far less accessible example of a global navigation satellite system, or GNSS, is Russia's Glonass system, which is being refurbished from its dilapidated state following the dissolution of the Soviet Union. Somewhat better publicised is Europe's decision to build a GNSS of its own, the Galileo system, which will allow a long-desired independence from the military-run GPS. Meanwhile, China is developing its Compass/Beidou system with global coverage, and India and Japan are developing regional systems.

These international developments indicate the importance attached to global navigation, as well as the collective concern that the US could take unilateral action and restrict the use of GPS signals to its military users. 

Such distrust is based on the original signal specifications, which featured a military 'P-code' (precision) signal, giving an average position accuracy of 7.5m, a civilian coarse acquisition or 'C/A-code' signal accurate to 15m, and a 'degraded C/A-code' signal limited to an average of 50m.

The ability to degrade the accuracy of the civilian signal, known as selective availability, was deactivated by presidential decree in May 2000, but commentators continue to note that this could easily be revoked.

Certainly, it makes sense to have a back-up system, in case GPS signals are switched off or the constellation suffers a system failure (however unlikely that is). Not because drivers would have to learn how to read map books again, but because so much of society relies on the signals.

Pure A-to-B navigation is the obvious GNSS application. Owners of anything from delivery vehicles to luxury yachts are now used to punching in their destination, relying on their GPS receiver to guide them to their destination and, arguably more importantly, to get them home. For these users, the GPS map is very much a two-dimensional concept. By contrast, using GPS for aircraft navigation necessarily includes a third dimension.

Given that GPS provides total global coverage, the application of its signals to aircraft operations - especially long-haul flights - would appear to be obvious. But while pilots of light aircraft can easily retrofit a portable receiver and use it to enhance or confirm their existing navigational skills, things are not so simple for the large passenger fleets, especially when it comes to final approach and landing.

One of the issues is that terrestrial radar tracking and instrument landing systems already exist and represent 'the devil they know', while GPS has historically been seen as an untried and untested alternative. Another problem was the tangible concern that, once the aviation world had adopted GPS and pensioned-off its heritage systems, the US government would either deactivate or degrade the civilian signal, fail to fund upgrades and replacements, or start charging for the service.

However, with the growing realisation that, by enhancing navigational accuracy, GPS would increase airspace usage by decreasing aircraft separations, and reduce fuel burn and CO2 emissions, the airline industry is slowly beginning to adopt the 'new-fangled' technology. Qantas and Southwest airlines, for example, have instituted procedures at several airports under the so-called Required Navigation Performance (RNP) scheme. 

According to Aviation Week's David Hughes, RNP began in 1996 as "a way for carriers to access mountainous airports safely" and "is already thriving in Alaska, Canada, New Zealand and Tibet" (which is another way of saying it's not yet widely adopted at all!).

Indeed, Hughes' analysis of the industry shows how far it is from signing up to wholesale use of GPS. Apparently, Southwest is only just beginning to migrate from its traditional "round-dial cockpit philosophy", which until recently dissuaded its pilots from using autopilots or autothrottles installed on their 'next-generation' Boeing 737s. 

"Instead," reveals Hughes, "the company standardised on the classic 737 cockpits and even installed metal covers on the switches of equipment it didn't want its pilots to use."

It seems, however, that current global concerns regarding engine emissions and the price of aviation fuel might well galvanise the industry into action.

According to Chris Manning, a former chief pilot for Qantas, a 12-month trial at Brisbane, Australia, involving more than 8,000 RNP approaches saved 34 Qantas aircraft "a total of 4,200 minutes of flying time, 200,000kg of fuel and 650,000kg of CO2 emissions". This, and the fact that the aircraft's navigation systems are able to track a GPS course to "within a wingspan" suggests that widespread adoption of GPS techniques is long overdue. The eventual inauguration of Europe's Galileo system and the various regional navigation systems should further boost the application of satellite navigation to commercial airliners.

Accurate positioning

Although we tend to think of GPS as being applied to the navigation of moving vehicles, as its name suggests it is equally important for static positioning. 

In fact, if a GPS receiver remains stationary for a period, it is capable of much greater positional accuracy through a process known as signal integration. For example, geological surveyors use GPS to monitor land movements at geological fault boundaries, where receivers can remain stationary for several minutes to increase the integration time. As a result, it is possible to get accuracies of a few millimetres.

Using GPS to monitor the vertical movement of terrain in volcanic areas helps to indicate the degree of subsurface activity in the region, because an increase in local magma pressure causes uplift. The technique was used on Nevada's Yucca Mountain in the late 1990s, following the area's designation as a potential site for the US long-term repository for high-level nuclear waste. The survey showed that the crust was deforming about ten times faster than expected, which didn't help the proposal that has since become bogged down in legal and financial challenges.

Unsurprisingly, GPS is a cornerstone of the Ordnance Survey (OS), the UK's national mapping agency. In his overview of the organisation's four-year strategy, '2012 Vision', Neil Ackroyd, director of data collection and management, highlights the importance of GNSS. Among other things, the strategy aims to enhance the OS Net Permanent GNSS Network - which currently includes more than 110 stations - to allow the reception of both Glonass and Galileo signals. According to Ackroyd, OS Net exists not only to define the coordinate system in the UK, but also "a backbone for GPS commercial services and a GPS data series for scientific study". 

Essentially, it provides a highly accurate geographic reference for anyone involved with location-based services and measurements. This includes the construction industry, which requires accuracies of up to 2cm, and the utilities sector, which is required under the New Traffic Management Act (TMA) to "spatially record [in 3D]… all road and street-works activity in England and Wales". According to the OS, its surveys are also used by the emergency services to speed up the opening of trunk routes following road traffic accidents, and by agricultural and earth-moving machinery operators to control vehicle tracking.

Meanwhile, for outdoor types, GPS continues to break new ground by helping fishermen return to favourite spots and golfers get back to the clubhouse faster. According to Bernie Friedrich, vice president Golf Operations, Boyne, US: "GPS has speeded up play on the golf course [and] for those in the rough, it tells them how far away they are from the trap". However, he adds: "It tells you the brutal truth. Most people think they hit [the ball] 20 or 30 yards farther than they actually do, but GPS doesn't lie."

GPS technology has also generated entirely new activities, such as 'geocaching', a combination of traditional orienteering and treasure hunting, and 'geodashing', a cross-country race to a predefined GPS coordinate.

In the somewhat gentler realm of photography, serious snappers can now 'geotag' their photos with GPS coordinates to show where they were taken. In 2006, Sony introduced a small device capable of recording location data at 15-second intervals; when the photographer downloads images to a PC, the device can be used to add GPS data to the image metadata.

Then, in 2008, software vendor Geotate introduced an 'auto-geotagging system' comprising an in-camera radio receiver that adds raw data received from the GPS constellation during a 200ms period coincident with the exposure. In this case, when images are downloaded the system goes online to retrieve the actual position data from Geotate's archive. Observers expect high-end SLR cameras to incorporate this or similar geotagging technology within the next couple of years; it is then simply a matter of time before it cascades down to other consumer devices.

It's all in the timing

With all this concentration on navigation and positioning applications, one tends to forget that GNSS is fundamentally a timing service, based on atomic clocks on board the satellites and on the ground, and the accurate timing of signals between transmitters and receivers.

As the US government's GPS website states succinctly: "This enables users to determine the time to within 100 billionths of a second, without the cost of owning and operating atomic clocks."

Among other things, this allows the precise synchronisation of communications systems, power grids and other critical infrastructure, and a consequent increase in efficiency and reliability. For example, wireless telephone and data networks use GPS time to keep their base stations in perfect synchronisation, which allows mobile handsets to share limited radio spectrum more efficiently. Similarly, digital broadcast radio services use GPS time to ensure that the bits from all radio stations arrive together at the receiver, thus decreasing delays in tuning.

Less obviously, perhaps, GPS also makes it possible to add time tags to business and financial transactions, providing a consistent and accurate way to maintain records and ensure their traceability. This has self-evident advantages in terms of account management and fraud prevention. Major investment banks and other businesses use it to synchronise their network around the world, which, considering the general rise of globalisation, is likely to become more rather than less important. So if you want to see a real financial crisis, deactivate GPS!

Where would we be if GPS failed? Certainly, a lack of camera synchronisation would be the least of our worries. Ask the climbers, mariners and aviators whose lives have been saved by GPS and the answer is obvious.

For the rest of us, GPS is a benevolent stealth technology that, largely without fanfare, has become embedded in our social infrastructure. Without a global navigation satellite system, one way or another, we'd all be lost!

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