How2Day: GPS System Signals Basics

Hand-held GPS Device on Highway Thinkstock Images/Comstock/Getty Images
The Global Positioning System (GPS) has greatly improved our lives. It provides ease and convenience in navigation. But as a new technology, GPS is still being developed and continues to improve gradually. Although GPS can zero in on the approximate location of a GPS transmitter, it is not one hundred percent accurate. It is critical that we understand how GPS works in order to know its weaknesses and use it most effectively.

The GPS refers to a collection of satellites orbiting the Earth that transmit precise information about time and position, which is described in terms of latitude, longitude and altitude. At the most basic level, the GPS determines four variables: longitude, latitude, height (elevation) and time. When a GPS receiver is connected to a device, the GPS determines the exact location of that device. Four satellites are used to determine a position in 3-D space. The calculation is performed by determining the signal transit time between the points of observation and the four satellites. The distance between an observation point and the satellite is determined by multiplying the signal time by the speed of light. The use of three or more satellites, each running its own computer algorithms, helps determine the exact position of the device.

Information provided by the Global Positioning System is used for a wide variety of purposes including hiking, hunting, camping, boating, surveying, aviation, national defense and navigation.

The exact location is accurate within a range of 20 m to approximately 1 mm. Accuracy can be affected by a number of factors including errors in range measurements, corrupt transit time measurement, signal propagation errors (atmospheric interference affects the signals) and receiver errors.

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