GPS Systems

 How it works.
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers use the relative timing of the signal to triangulate the user's exact location. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.
A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude).
How accurate is GPS?
Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. GARMIN's 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers.
GARMIN GPS receivers are accurate to better than 15 meters in typical usage. Most current hand-held GPS units now incorporate a type of differential GPS using extra satellite systems to increase accuracy to about 3 metres. In the USA WAAS enhancement is available and in Europe EGNOS is now fully live, though with a relatively low powered satellite which may not be receivable from all locations.
GPS and walking navigation.
All Garmin GPS units can be set to display a 10 figure Ordnance Survey grid reference as well as all popular grid systems found on maps world wide.
All handheld units can be used to store Waypoints (i.e. grid references), which can be joined together into routes that you can follow directly, or on a simpler basis to guide the user to a saved point (e.g. mountain summit, river crossing, etc.). The usefulness of a GPS can be greatly expanded when used in conjunction with a computer and appropriate software to create routes and examine track logs. 

All Garmin GPS units sold by us come with a free CD containing coverage of Great Britain from OpenStreetMap which can be used on a PC and loaded on to a mapping-capable GPS.

See our instructions on how to put OpenStreetMap on a Garmin GPS.
Openstreetmap Maps on a Garmin GPS.pdf
These maps can be pre-installed on suitable GPS units. Please use the Special Instructions box of the payment page to request this service.