GPS Systems Buyers Guide
Whether you’re a motorist, on four wheels, or two, a pedal cyclist, a hiker, a sailor, or even a golfer – and maybe even a combination of some, or all, of these – GPS (“Global Positioning System”) equipment has something, or, better, many things, to offer you.
Road atlases, maps and other printed forms of navigation, and direction, are all very well, in their place, but, if you’ve ever fumbled with an “A-Z” whilst circumnavigating Piccadilly Circus, or found yourself in a similar situation, you’ll know that there must be a better way. Similarly, the time-honoured method, of stopping to ask passers-by for directions, although, perhaps, less hazardous, can be equally frustrating and confusing.
A carefully chosen, and properly configured, GPS allows you to determine exactly where you are – and, perhaps, more importantly, where you’re going – at all times.
GPS Technology
Currently, there are 24, or so, so-called “Navstar” (“Navigation System using Timing and Ranging”) GPS satellites – operated by the U.S. Air Force, with the intention of providing accurate positioning information to military forces on the ground, but also available to commercial GPS products – in “active” orbit around the earth. A GPS satellite continually broadcasts data relating to its orbital position, and a time signal, which can be translated, by a GPS receiver, into the exact distance between the receiver and the satellite. Thus, using the signals received from four different satellites – and the satellites are arranged so that, at least, four are always visible, in the sky above any point of the earth’s surface – by employing a technique known as “3D-trilateration” , a GPS receiver can establish its own, precise, geographical position.
As you might expect of a technology of such complexity, and involving such vast distances, however, there are a number of influential factors, which may introduce errors, of, perhaps, up to between 50 and 100 metres, in the final positional calculation. The most obvious of these is something called “Selective Availability” (SA), which is a deliberate adjustment of the orbital position and time signals by the U.S. Department of Defence. Other errors may be introduced by the differences in transmission speeds between space and the upper layers of the earth’s atmosphere, the precision of the clock in a GPS receiver, and DOP (“Dilution of Precision”), caused by poor geometry of the available satellite constellation. A typical GPS receiver is accurate to around 20 metres, or so, depending on these factors.
GPS Features
Selective Availability errors can be corrected by a “differential” GPS (“dGPS”) receiver, which receives “known error” transmissions, relayed to it by a dGPS ground, or reference, station, from a GPS satellite, and incorporates these errors into its positional calculations to produce a more accurate result. A dGPS ground station is basically a high-performance GPS receiver, situated at a known location, which constantly measures any distortion in the satellite signals received, allowing a dGPS receiver to produce a result that is usually accurate within 5 metres, or better. Although more expensive than a standard GPS receiver, you should consider a dGPS receiver, if this level of accuracy is important to you.
Modern GPS receivers – unlike their predecessors that used a single channel, and switched from satellite to satellite – employ multiple channels, typically 12, to receive as many incoming signals, from satellites and ground stations, as possible, at the same time. More channels mean no switching, no delay, and a more accurate result – the manufacturer’s specification will list the channel capability of any given system.
A GPS receiver holds a rather quaintly named “almanac” (a set of data) regarding the approximate position of each of the satellites in the constellation, which it uses to determine which satellites to track. In ordinary operation, the receiver maintains a relatively accurate map of satellite positions, and can locate, or “acquire” satellite signals fairly quickly, whereas if a system has been inactive for a period of time, this process may take a little longer – these response times are referred to as “warm”, and “cold”, acquisition times, terms that you are likely to come across in any sales literature.
In-Car GPS Considerations
Perhaps the most common application of GPS is in motor vehicles, and, technological capabilities aside, there are a number of key operational features that you should consider before kitting out your car, van or motorcycle.
The ability to calculate your position, down to a few metres, or so, is marvellous, but is of little use if you cannot see, or use, the information or system as you would like, so check, for example, the overall size and resolution of the display screen. A touch screen can make a GPS easier to use, but make sure that it’s big enough to accommodate your fingers, and, if you like the thought of audible directions, make sure that there’s a computer voice that won’t grate on your nerves after the first few miles. The Packard Bell COMPASSEO 500, for example, has a 3.5 inch touch screen, and the Dayton MS3100R Navigation System offers icon and voice guidance, with a choice of 19 voices, in 9 different languages.
Similarly, consider if, and how many, maps are included, how much, and what type of, storage is available (and how you add more, if need be), and, last, but not least, the antenna included with the system – you may well find that the antennae included as standard, in many systems, are inadequate, particularly for urban driving. The Garmin NUVIC510 Satellite Navigation System, for example, includes preloaded European maps, and has an SD (“Secure Digital”) memory expansion slot, while the TomTom GO910 GPS unit offers maps of Europe, the USA and Canada, and hard disk storage of 20 GB.
Conclusion
Try to make a realistic appraisal of what you really need from a GPS, the available technology, and, obviously, your budget, before you set your heart on any particular system. Remember to factor in any additional extras, if they’re not already included, such as mounting equipment, maps and an adequate antenna, if need be, and try not be swayed by sales patter, or glossy brochures, into investing extra money into features that although attractive, in principle, you may rarely, or never, use. There are many, real GPS users, and their opinions, in the form of reviews, and user forums, on the Internet, are an invaluable source of all kinds of information regarding the pros and cons of any system.
|
Vouchers Feed
Compare Prices