GPS is a group of 24 satellites each orbiting the earth at around 11000 miles above the surface. Each Satellite circumnavigates every 12 hours. At all times at least four satellites will be in view at any given point on the globe. GPS has been fully operational since 1990.
As long as a GPS device can see a minimum of 3 satellites then an accurate position of the vehicle can be calculated. GPS detectors generally check their position every second and are able to work out accurately the speed of the vehicle and constantly comparing themselves to a stored database of alerts. When nearing a stored location the unit gives an audible and visual alert.
When a GPS device is first powered it must acquire satellites to know its position. The first start is known as a cold start which can take up to 45 minutes to acquire the minimum three satellites for correct operation. Thereafter, warm starts (when the unit has been switched off overnight) should take a matter of minutes and hot starts (same day) a few seconds.
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