Guide to Buying a GPS Navigation unit

There are several reasons why you might want to buy a GPS navigation system. These useful units are used in a variety of professional capacities and are becoming regular features included in new motor vehicles to provide drivers with fingertip trip location finders. Many professions use GPS systems in their day-to-day activities, including geophysical surveyors, farmers, mappers and geologists, fishermen, taxi drivers and household service people.

You may want a GPS system for a professional reason, or you may consider its uses in sport and recreational pursuits, such as golf, hiking, off road adventuring, travel or a variety of other reasons. The good news is that there is one of these units for every conceivable application, and they continue to become more and more affordable. You can search through Myshopping.com.au to find a unit that has the features you need at a price you can afford. But what features are you likely to need?

GPS is an acronym for Global Positioning System. As the name suggests, it is a device that is designed to give you a precise location on the earth's surface. For the purpose of mapping, the earth's surface is divided into a grid of meridian lines called longitude (running from pole to pole) and latitude (running around the earth). These divisions are marked off in degrees. Longitude circles the globe from 0 degrees, which runs through Greenwich in England to 180 degrees West in one direction and 180 degrees East in the other, roughly dividing the world through the Bering straights separating North America and Russia. Latitude rings the globe from 0 degrees at the equator to 90 degrees North and South at each pole. Each degree is further broken down into smaller measures of minutes and seconds, enabling your precise position on the earth's surface to be plotted in relation to the grid.

In order for your GPS unit to tell you where you are standing, it needs to communicate with satellites circling the earth. Currently the GPS system uses 29 satellites (including three spares in case of failure) in intermediate orbits circling the earth twice a day at an altitude of 20,200 kilometres. The satellites are aligned so that at least four are within line of sight from virtually any place on earth. These satellites are monitored from five ground stations and atomic clocks on board are monitored and synchronised to within one millisecond of each.

Your GPS receiver receives a signal from four satellites that are currently in line of sight and accurately calculates the precise distance from each of the satellites by measuring the time delay between when the satellite sent the signal and the local time it was received. The time delay multiplied by the speed of light gives the distance to the satellite. The satellite also sends data that identifies its orbital position and the receiver can, by comparing the satellite's position and range, determine its own location. The local GPS receiver "imagines" the satellite is located on a sphere some 65-85 light-milliseconds distance. When it reads the information from four satellites it is able to locate its positioned based on where the four imaginary spheres intersect. Accuracy of location is determined by some technical calculations that enable the local clock in the GPS receiver to recalibrate according to the extremely accurate time setting in the satellites' atomic clocks.

The time-position information can then be translated to the grid system used to map the earth's surface, and therefore give you a precise location in real time, or calculate distance travelled over time or calculate distance between tow fixed points, including altitude.

In order for a GPS receiver to work however, its aerial must have a clear sky view. Dense bush and tall buildings hamper the effectiveness of a GPS receiver. The use of a GPS receiver may also be limited in areas that are involving military operations. Military operations can locally deny GPS to hostile and suspected hostile users without affecting the rest of the world or its own applications using a technique of local jamming.

MAPS

The key to a GPS unit's usefulness is access to the maps you'll need to in order to identify where you are at a given time, and what direction you need to travel in order to reach a destination. Most portable GPS units have a USB connection to enable you to link to a computer and download the maps you'll need. If you are visiting a foreign city and you want a street map of where you are, you will need to access mapping data through your unit's firmware so you can see a visual reference of your location. Many in-car units, and portable units that are adaptable to vehicle installation, can read data from a CD or DVD player, and you can use map libraries in this form. Websites such as whereis.com also provide downloadable map data.

DISPLAY

It's a good idea to look for a unit with the biggest and brightest display that can be read in all light conditions with relative ease. Larger screens are naturally more expensive, but just as screen performance on computers can vary, so too can portable GPS units. Test the screen in direct sunlight for visibility, and test it from different viewing angles. Try to find a unit that can be viewed from low angles.

VERSATILITY

A critical element of your GPS unit is having its aerial in the right position. You may also want to remove it from one vehicle and use it in another, or use it as a portable hand-held unit. Make sure the unit is capable of being used in different situations and can be shared between vehicles. You may want to use it in your boat, golf buggy, off road vehicle, dirt bike or backpack, in which case you will want to make sure it is easy to remove and install and has a strong enough battery and antenna to meet your needs.

NAVIGATION

Most recreational and domestic GPS systems come with street level maps and voice and text directional prompts. You may want to include a database of key buildings and facilities if you are travelling, such as airports, hospitals, train stations, hotels and other tourist destinations. You will be greatly assisted with automatic re-routing features to keep you on track and variable mapping perspectives such as 3D and bird's eye view. Stylus operated and touch screen technology can be a big time saver instead of keypad data entry. Many devices also offer trip calculation features and distance measurements that can be useful on the hiking trail or golf course.

Most GPS units are ready direct you to where you want to go as soon as you charge the battery and turn it on. Depending on your personal circumstances, accessories you might want to consider might include auxiliary antennas and booster kits that can enhance your signal, protective carry cases, motor cycle mounting kits, GPS maps and software, battery packs and AC power adapters.

Compare different brands and features such as screen size, battery life, portability and accessories by searching Myshopping.com.au You can also compare prices and different vendors' reputations.

Other special features can include altimeters to help calculate your altitude above sea level, and barometers that let you keep an eye on the weather. An inbuilt compass can be a handy feature for times when you are not within a good reception area or you have low battery reserves. Some units feature an mp3 player to keep you entertained. Whatever features you are interested in, you can search for those units that include those features on Myshopping.com.au.

GPS units can be purchased for as little as just over $100. Many mobile phones and PDA devices now include GPS features that can be useful, and they are priced in the $300 - $800 range. Professional and feature rich in-car units can extend into several thousands of dollars. Accessory packs and extended warranties can add value to your purchase, with many vendors offering CDs of maps and data as deal sweeteners.

Search Myshopping.com.au for the GPS unit that's perfect for your needs.