The Global Positioning System

How Your GPS Receiver Tells You Which Satellites It Can See

How GPS Works

Accuracy of the Fix

GPS Blackout

Deliberate Interference

GPS Is Line of Sight

Selective Availability

Differential GPS

Wide Area Augmentation Service Switch-On Delays Measurement of Speed Measurement of Course Measurement of Heading Errors in COG and SOG

T he original global positioning system (GPS) consists of 24 satellites orbiting the Earth at a distance of around 11 000 miles. Each orbits once every 12 hours in six orbital plains, so there will be between five and eight satellites in view at any time, from any point on the Earth's surface. The drawing here shows only three orbital plains for clarity.

There are a number of spare satellites in orbit in case of failure and each satellite has a life expectancy of about 7 years. New satellites are launched by the US military as required.

Fears about the American monopoly of accurate position fixing amongst non-USA countries have lead to the establishment of GLONASS (a Russian system) and the pending establishment of GALLILEO (a European system). They work in a similar manner and new versions of GPS receiver may be able to operate with any system.

Artwork courtesy of US Dent of Defense Simplified GPS satellite constellation

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