GPS: What it can and can't do

The Global Positioning System (GPS) started out as a U.S. military application for guiding things like bombers and missiles and has become a must-have accessory for family road trips.

These days, GPS technology is an option for most new cars and minivans, and hand-held units sell for less than $100. Given the convenience, price and ease of use of GPS, many drivers are abandoning paper maps entirely in favour of a glowing screen and a voice that politely tells them to "turn left in 300 metres" or "merge onto highway."

Yet it seems that every week, theres another story of someone getting profoundly lost following GPS instructions. The Global Positioning System is gaining a reputation for Getting People Stranded.

Arecent example is the case of Albert and Rita Chretien, the B.C. couple that went missing in Nevada in March. Incredibly, Rita Chretien wasfound alive after 49 days; but her husband, Albert Chretien, who left to search for help, hasyet to be located. Police in Nevada say the Chretiens were likely steered off course by their GPS they abandoned a high-traffic route for a lesser-known rural road and couldnt find their way back.

Its a potentially tragic reminder of the technologys limitations.

The backbone of GPS is a group of 24 satellites orbiting Earth. A GPS receiver locates four or more of these satellites, and by calculating the distance to each, infers its own location on the planet. This capability, paired with mapping software like Google Maps, allows drivers to punch in a destination address and receive directions from an automated navigator, all the while following their progress on a computer screen in real time.

The science is sound, but its not foolproof.

According to the Urban Dictionary an online repository of smart-aleck commentary GPS refers to a device "used by those who have no idea where they are, to assist them in going nowhere in particular, using the shortest possible route." The definition is flippant, to be sure, but it reflects a growing attitude towards this seemingly miraculous technology.

Here are some of the misconceptions about, and limitations of, GPS technology:

Computers dont navigate like people

When you ask a human being for directions, they typically suggest a course they know to be navigable, likely because theyve driven it themselves. Mapping software isnt as intimately familiar with geography, traffic patterns and safe neighbourhoods. Because its directive is to find you the quickest and seemingly most direct route, a GPS unit may prescribe a path that is rough, risky, and potentially downright reckless.

In 2007, a British woman nearly died after she unintentionally drove her GPS-enabled Mercedes SL500 into the River Sence in Sheepy Magna, Leicestershire, after following GPS instructions and ignoring signs stating the track was unsuitable for motor vehicles.

The map software could be outdated

GPS vendors encourage consumers to shell out extra money for map updates. This may seem like blatant up-selling, but theres wisdom to it: updated maps reflect new construction or housing complexes and changes to the road system, thus reducing the risk of becoming marooned on a road that didnt exist two months ago or that doesn't go anywhere now because of a washout or landslide.

Up-to-date map software can still be flawed

Even the latest map update is unlikely to reflect the latest roadwork. The map may show a clear route, while you're parked in a giant construction site. As well, a route that is free and easy in summer could become impassable in winter, as aQuebec womanfound out earlier this year, when she became snowbound on a remote road in near Wayerton, N.B., more than 100 kilometres from her destination, Bathurst.

The satellites cant always see you

A GPS unit needs a clear view of the sky to locate the satellites. It will not work if youre too close to tall buildings or hills that obstruct the horizon. Even being near leafy trees can sometimes hinder its effectiveness.

Accuracy is an abiding concern

In ideal circumstances, a GPS receivers maximum accuracy is estimated to be within 10 feet. Bad weather or other atmospheric disturbances can compromise the satellite signal. And despite our utter faith in modern technology, the fact is, a satellite is quite capable of sending out an erroneous signal from time to time.

Never underestimate the potential of human error

Its unwise to put unconditional trust in an electronic device. Keep your wits about you. If it looks like youre about to steer your SmartCar over a cliff because The Voice told you with steely certainty that you must you might consider pumping the brakes.

A lot of headaches (or worse) can be avoided by starting your journey on the right foot namely, by correctly spelling the name of your destination. In 2009, a Swedish couple intending to visit the Italian island of Capri ended up 650 kilometres north in Carpi as a result of sloppy typing.

And make sure you have the right country. South African touristsMichael and Sunette Adendorff were in Eastbourne, New Zealand in September 2011, navigating with a GPS but unable to find the hotel they had booked and paid for online. They eventually learned that the hotel is in Eastbourne, U.K.!

GPS info cannot be traced

Many people are under the false impression that even if GPS leads you into a swamp, satellite technology can trace your whereabouts. A GPS unit is only a receiver, capturing location info it doesn't send a signal out. (In that sense, its like a TV or radio.) As a result, a GPS unit will not enable authorities to find you. A better bet is a cellphone, but chances are that if youre stranded in a remote spot, youre probably in an area with little or no phone reception, too. So no matter how many high-tech gadgets you have with you on your road trip, it pays to have an emergency kit in the trunk and some extra snacks, just in case.