Satnav Secrets: Friends in High Places

‘Satnav’ is a piece of verbal shorthand, for ‘satellite navigation’. Though most people call the unit that helps you navigate, with the aid of satellites, a ‘satnav’, it is more properly called a ‘Portable Navigation Assistant’ or a ‘PNA’ for short. That’s a bit of a mouthful though and unsurprisingly, it is rarely used.

Whatever you call it, and regardless of whether it’s a fixed item or a portable one with a windscreen mount, the unit uses the same satellite network. It can tell you all sorts of things, including where you are, how fast you’re going, how to get to where you’re headed and how long that will take. Should this seem more than a bit clever for a little box or just a dash display, just remember that it has friends in high places.

Looking up into the sky, you’ll be unable to see the satellites that make up the GPS (Global Positioning System) constellation. There are plenty of them – a minimum of 24, though the numbers vary and there are usually three spare satellites. However, they’re a bit too high and travel rather too fast for you to be able to spot them.

Here, ‘too high’ refers to an altitude of around 12,000 miles and ‘too fast’ is about 7,000 mph, fast enough for the satellites to orbit the earth in less than 12 hours. And each weighs about 2,000 pounds and, with its solar panels extended, measures about 17 feet across. Perhaps it’s just as well they aren’t visible from the earth’s surface!

America initiated the GPS system in 1973, and the constellation was complete by 1978. Originally for military use, the GPS network was ultimately made public. The abovementioned box or installation, provided it can ‘see’ at least three GPS satellites, uses information they transmit to locate you anywhere on earth. Civilian satnavs are typically accurate to around 10 metres, or 32 feet.

While it’s all very well for a satnav to know where it is- and therefore where you are – this information isn’t particularly useful unless you or the unit knows some rather advanced geography. So, satnavs have internal ‘maps’, software that knows where places, roads, roundabouts and even, on occasion, speed cameras, are located. Using a combination of this internal knowledge and the information the satellites provide, a satnav can guide you to where you want to be.

The versatility of satnav puts you in control of what information is presented. You can select the fastest or shortest route, and decide whether or not to use or avoid toll roads, minor roads and motorways. You can seek out ‘points of interest’ (POIs), from restaurants to amusement parks, and be guided to the one(s) you choose. The same elements are available abroad, on the Continent and beyond. You can update the internal maps and some units can even play music files or display digital pictures. Moreover, there is no subscription fee for the use of the satellite constellation, and it’s there 24 hours a day, 365 days a year.

A satnav can’t tell you lies but certain situations can make it seem able to. So if it advises you to make a U-turn slap in the middle of the M25…don’t! Chances are its signal has become compromised! Similarly, the accuracy of a satnav’s navigation depends on the accuracy of its mapping. Some are better than others and if the PNA suggests a road doesn’t exist, it ain’t necessarily so. In some locations, the satellites’ signals can become delayed or distorted so the satnav may ask you to turn off some yards after you’ve driven past the turning – in this case, just wait. It’ll find the next turning for you.

The single overarching rule about using a satnav is to remember that it is there to advise you, not to make decisions. If you find yourself trying to drive up a 1-in-3 sheep track, flanked by a 1000-foot drop, it’s reasonable to assume the mapping in the satnav that took you there is doubtful. And you won’t get away with saying, “It’s the satnav’s fault!” It is you, not your satnav, who is in charge of the car.


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