What is GPS?

GPS is short for Global Positioning Satellite. The US (and soon the EU, Russia and China) maintain a system of satellites that broadcast information that can be used to derive a 3D position on the surface of the earth. This information is, essentially, a very accurate time signal. The initial use was intended for military weapon system navigation.

Based on the information received from a number of the satellites a GPS receiver (from now on GPS) can determine where it is. Based on a number of readings over time a GPS can determine both where it is and how it is moving - speed and direction. With more satellites height can also be calculated - but as this is not the intended purpose of the system is not as accurate.  Also note that the altitude base for the GPS system is NOT the same as that for Ordnance Survey maps, consequently the altitude indicated may differ significantly from that on a map.

The signal is very low level and so is prone to interference and masking. Water absorbs the signal and so rain, trees and buildings tend to mask satellites from a GPS.

In order to work correctly a GPS needs to:-

  Be able to receive (see) a minimum number of satellites - at least 3, preferably more - distributed over a wide area of sky.

  Know where the satellites are. This information is broadcast by the satellites, but slowly so that a GPS that has not been on for a long time, or has been moved a large distance from where it was last used may take a long time to obtain a good location.

  A GPS can be a dedicated unit, or part of a general purpose product (eg iPhones are GPS equipped)

What is a Walkers GPS?

A walker's GPS provides features optimised for use in, generally, open countryside and on any route. They generally provide displays and measurements in forms that can be related to walker's style maps (eg Ordnance Survey), they rarely provide vehicle navigation capability.

A walker's GPS provides some or all of the following features:
Current position - Tracking - Navigation

The current position is displayed in one of a number of different styles, the most useful in the UK is OS Grid, however if abroad other forms are available - including the simple latitude and longitude style. Many GPS units now come with the option of loading (at extra cost) maps. In these cases the current position can be shown superimposed on the map.

Tracking records where you have been in some form that you can refer to later. Many GPS units allow the user to convert a recorded track into a navigable route that the GPS will allow the user to follow. Tracks can often be transferred from the GPS into a computer to allow programs to show where the GPS has been - programs include Google Earth, GPS maker provided software and other mapping products such as Memory Map.

Navigation capability allows the user to enter some route information and then use the GPS to identify how to get from where the GPS is to the entered location. At its simplest this is simply another, single, location. Many GPSs allow the creation of complex routes (either on the GPS or on software on a computer), which link many locations in a sequence.

Some GPSs include compasses, barometric altitude measurement, temperature sensors, links to heart rate monitors etc. These are not necessary but can be useful if their purpose is understood.

As implied above, many GPSs provide an interface that allows information to be transferred to and from a personal computer. Some units include a suitable lead in their original purchase, with others it is an option, often quite expensive.

A GPS with more features will use more battery power, some GPSs have sealed in batteries, making replacement on a walk impossible. Others use standard AA or AAA batteries.

Things to think about

• Do you want to plan and follow routes?
• Do you need or want to transfer information to/from your computer?
• Do you need mapping?
• Do you need to record tracks? If so for how long (both time and distance).
• Do you need to record tracks over a number of walks?
• How long do you want the batteries to last?
• Do you want the ability to change batteries during a walk? Do you need to use the GPS where charging of batteries is not possible (eg DofE Gold expeditions)

Most importantly, talk to people who use them, decide what you need and then try to 'play' in a retailer or on a walk.

Manufacturers of walking GPSs include (in no particular order):

• Garmin
• Lowrance
• Magellan
• MemoryMap
• Satmap
• Mobile Action Technology

We are aware of people who have used products from Garmin, MemoryMap and Mobile Action Technology. Garmin seem to be the most widely available products, but all makes are available on line.

Warning - A GPS is NOT a substitute for paper maps and compass. They are complex electronic items that can (and probably will) go wrong when you most need them. They require good batteries and reception. The US government reserves the right to withdraw GPS service at any time, with no notice, as they consider it a military asset.

Some GPS units

• Garmin eTrex:- compact units, fairly basic (the eTrex H, eTrex 10), some with mapping (eTrex 20 etc)
• Garmin Oregon:- larger than the eTrex. The 450t is available with OS 1 inch maps loaded.
• i-GotU:- Compact tracker only devices. The GT-120 will last for about a day.

Resources/Links

http://www.gpstraining.co.uk/index.html
http://www.garmin.com/uk/
http://www.ordnancesurvey.co.uk/oswebsite/
http://www.memory-map.co.uk/
GT-120B (mobileaction.com)