Getting from A to B isn’t always easy, even if you know the start and end points of your journey. That’s why so many people get excited by the idea of GPS receivers, because they love the idea of something (rather than someone) telling them how to get where they want to go.

But if you don’t buy carefully, you could end up being disappointed. There are a lot of products on the market, so it’s worth understanding how they work and what differentiates them. You wouldn’t want to carry a top-of-the-range car GPS system for your car when hiking up a mountain.

To make finding the right GPS receiver easier, we answer the questions you should ask yourself before you buy:

• What’s my budget?
• How does GPS work?
• How am I going to use my GPS receiver?
• What else should I care about?
• Do I still need my map and compass

Click on any of these links to get some valuable advice, and tips on how you can use PriceRunner to find the perfect GPS receiver.

Alternatively, go straight to any of these to read more:

• Receiver type
• Differential GPS (DGPS)
• EGNOS/WAAS
• Waypoints
• Maps
• Acquisition times

What's my Budget?

Before you start to think about what GPS receiver you want, you should fix your budget. This will help you to focus on the features you need and not to get swayed by extra ones that you could do without.

When you are sorting out your budget, remember that you may need to pay for accessories, such as mounts and software. In-car systems should come with a mount, for example, but always check what is included in the price and read the reviews to make sure that the standard equipment will meet your needs. You may have to play around with your figures and, if necessary, consider opting for a model that’s lower in the range so that you can buy a complete package of receiver and accessories.

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How Does GPS Work?

GPS is short for Global Positioning System. The GPS system works by using signals from a network of satellites, called Navstar, that each orbit the Earth twice a day. These satellites are run by the US Department of Defense and there are always at least 24 active ones, plus a number of spares: in January 2007 there were 29 in operation. (Other, competing satellite systems are being set up, but currently all receivers use Navstar.)

Each satellite sends out very low power radio signals that can be picked up by GPS receivers. As the signals travel in straight lines at a known speed, the receiver only has to pick up signals from four satellites to be able to calculate its position on Earth, including its altitude. If you have a rough idea of your altitude, you could enter this figure into your receiver and get your position from just three satellites, but usually there are at least four within range anyway.

There are, though, various factors that can add errors into the calculations, which is why your position can never be calculated right down to the nearest millimetre. These include:

• satellites going slightly out of orbit (although this is constantly monitored and corrected by a master control station)
• the signals being delayed slightly, and inconsistently, by the Earth’s atmosphere on their way to your receiver
• clock errors, as the clock on your receiver will never be as accurate as the very expensive ones on the satellites
• signals being reflected off tall buildings or large rock surfaces, so that the signal’s actual path is longer and not direct
• satellites being very close together or in a line when you are taking a reading – you get a more accurate result when they are at wide angles relative to each other

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Receiver Type

So to get the most accurate reading possible, you need two extra things. The first is to get readings from as many satellites as possible simultaneously, and preferably not just three or four. To do this the receiver needs to have multiple channels to receive as many signals as possible at the same time – this is indicated in the ‘Receiver type’ part of the PriceRunner specifications (but if this isn’t filled in, it doesn’t necessarily mean this feature isn’t built in).

In general, a GPS receiver can tell you your position to within 15 metres: see the GPS accuracy figure in the specs to find this out for each individual model.

Differential GPS

The second thing to look for is something called differential GPS (DGPS). This sounds complicated, but it is actually a very simple way of helping your receiver to get its sums right.

If you place a GPS receiver at a known location, its position can be compared with the one it calculates from the GPS satellites and any error worked out at any time. DGPS ground stations have a very accurate receiver with a transmitter, and broadcast these errors to GPS receivers within range. These can then incorporate the known error into their calculations to work out their own position more accurately.

The extent to which DGPS can improve the accuracy of a GPS receiver’s calculations depends on how close you are to the DGPS reference station (the closer you are, the more accurately the error message will apply to your circumstances). It also depends on your receiver having the right receiver built in.

DGPS is a particularly valuable aid for mariners, and in the UK, there’s a DGPS system in place for them with a number of reference stations in lighthouses around the coast.

EGNOS/WAAS

For landlubbers in Europe there’s an alternative system based on DGPS principles called the Euro Geostationary Navigation Overlay Service (EGNOS). The equivalent system in the US is the Wide Area Augmentation System (WAAS), and you may see some receivers marked as featuring WAAS rather than EGNOS. The advantage of EGNOS/WAAS is that it’s cheaper to implement in receivers than DGPS.

DGPS Ready

The specifications in PriceRunner can show if a GPS receiver has a differential GPS option, and what the ordinary GPS and DGPS accuracy levels are (that is, how close to your actual position the manufacturers claim the receiver can confidently and consistently calculate your position at any time).

However, be careful as the UK DGPS and EGNOS systems are different and you need to buy a GPS receiver that works with the right one. Also, if you see a unit described as ‘DGPS ready’ or ‘EGNOS/WAAS ready’, check if the extra receiver is already built in or whether the manufacturer is just telling you it can be added in future.

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How am I going to use my GPS Receiver

Now that you understand how GPS receivers work, you need to think about how you want to use yours. This will narrow the field down significantly, as there are major differences between the different types:

• motoring (car navigation and motorcyclists)
• marine
• handheld (including biking devices)
• modules (add-ons for mobile phones, PDAs and so on)

Car navigation

GPS receivers designed for motoring are purpose-built devices designed to make it easier for you to get from A to B. The extent to which they will help you to do this varies according to the features they provide and how easy you find it to use them. Here are some things to look out for:

Car GPS System key features

Other features you may want to look for include:

• Traffic receiver to get information (where available) on traffic and road congestion
• Speed camera warning database
• Integrated control of an iPod
• Integrated MP3 player
• Bluetooth handsfree calling on your mobile
• Remote control

Motorcyclists

Motorcyclists can find specially designed motoring GPS receivers, with glove-friendly touch screens and/or buttons, motorcycle mounts and easy-to-read screens. They should also be water- and fuel-resistant. To make them more useful, some will link into a motorcyclist’s intercom system so that you can hear voice guidance while you drive; others may have a Bluetooth headset. Avoid any with a hard drive, though, as these are not usually very resistant to shocks.

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Marine

Marine units are either handheld or mounted. If you use the former, you should consider having a mount as an option for times when you need your hands free. The mounted receivers are quite like in-car systems. In both cases, you need to look for devices that can withstand potentially bad weather and rougher treatment.

Marine GPS System key features

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Handheld

If you are buying a GPS receiver to use while you are hiking, cycling or otherwise out and about, your requirements will be quite different from sailors and drivers.

Handheld GPS System Key Features

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Other features to look for:

• altimeter
• compass
• heart rate monitor, lap times and average speed measurements for runners
• mains power source
• car-mount option, with cigarette light power source and car antenna

Modules

There are numerous GPS receiver modules available to let you upgrade your mobile phone, PDA, Blackberry or laptop. You’ll need to check compatibility with what you have and will sometimes need Bluetooth to make this work.

Many of the features mentioned above in the section on handheld receivers apply here too, so work through the list given there.

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What else should I care about?

Waypoints

A waypoint is a specific place that you want to store in the memory of your GPS receiver. Details of your home and other places you go to frequently can be stored permanently, while others can be added temporarily for a particular journey. Waypoints needn’t just be the endpoint of your journey, but may also be places that you want to navigate via, particularly if you are cycling or hiking off-road.

If possible, look for a device that can store lots of waypoints, as this will allow you to keep them for routes you plan to reuse. If you can’t find specific figures for this, look for as much memory as you can afford.

Maps

Maps also use up memory. Check the memory requirements for extra maps that go with the system you are considering and consider buying that memory upfront.

Some other questions to ask about maps:

• What maps are included as standard?
• What other ones are available?
• How much do extra maps cost and how are they supplied?
• Will you need to buy extra accessories, such as a PC cable, to get them if they are supplied on a CD?
• What map software, if any, do you get as standard to run on your PC for route planning?
• Does your PC meet the specifications for this software?
• Can you transfer parts of the maps from your PC to your receiver, and how?
• Can you choose different co-ordinate systems to match the maps you are using? Many are based on latitude and longitude, but others use different systems.

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Routes and track logs

A route is a line that joins the waypoints you have entered into your receiver, and shows you where to go. A track log records where you have been, so that you can travel back along the same path, so is useful for anyone going offroad.

Datums

Every different map or chart was created from a starting reference point called a datum. As these datums differ from map to map, it’s important that you can set the datum in your GPS receiver to match the maps you are using or you are likely to find that the positioning information won’t be correct.

The datums listed for any unit just means the receiver can be used with the associated maps, not that those maps are already built into the receiver.

Setting North

You need to decide whether you want to use true North or magnetic North on your receiver. If you are using it alongside a compass you will want to choose magnetic North. Make sure you have this option if you are relying on your receiver when hiking, for example.

Acquisition times

When a GPS receiver is being used it stores information about where the satellites are in the sky in something called an almanac. Because of the speed at which the satellites move, the data in the almanac is still quite accurate if the receiver has been used within the last four to six hours, and so it can find, or ‘acquire’, the signals quite quickly. The speed at which it does this and gives a result is called the ‘warm’ acquisition time.

However, as the satellites are moving in different orbits around the earth, the data in the almanac – its map of the satellites - gradually gets out of date. If the receiver hasn’t been used in the last four to six hours, it has to find the satellites again. This takes longer and is referred to as the ‘cold’ acquisition time.

Cables and connectors

Check what cables and connectors are included as standard, and compare these to what you need.

Cases

Consider how much you will be carrying your GPS receiver around, and under what circumstances, and make sure any case that comes with the unit will be suitable. You may need to budget to buy a more robust case.

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Do I still need my map and compass?

Even though GPS receivers are getting more and more accurate, you should always have backup navigational aids with you. Don’t leave your maps behind just because you now have a brand new GPS receiver, and if you are out in a boat, or cycling or hiking, remember your compass and know how to use it.

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