Maps & Geography Skills: How GPS Works

Understanding Maps & Geography Skills: How GPS Works

Each satellite sends a repeating radio code that works like a highly accurate time stamp. The message tells the receiver when the code was sent and where the satellite was expected to be in its orbit. The receiver has a copy of the same code.

It lines up the two patterns and measures the tiny delay between them. Since radio waves move at the speed of light, even a delay of a few millionths of a second represents a large distance. The measured distance is called a pseudorange because the receiver clock is not perfectly matched to the satellite clocks.

Satellites use atomic clocks, while a phone uses a much less precise clock. Ground stations watch the satellites and send updated orbit information so receivers can account for small changes in their paths.

The geometry of the satellites matters as much as the timing. One measured range places a receiver somewhere on the surface of an invisible sphere around a satellite. A second range narrows the possible positions to a circle where two spheres meet.

More ranges narrow the result until one location fits all the measurements. The extra satellite measurement is needed to correct the receiver clock error. A receiver works best when satellites are spread widely across the sky.

If several satellites appear close together, small timing errors can shift the calculated position by more. This effect is called poor satellite geometry. It explains why a location may be less reliable in a narrow street, deep valley, or crowded urban area.

Signals do not always travel in a perfectly straight, clear path. The upper atmosphere can slow them slightly, and water vapor in the lower atmosphere causes further delay. Buildings, cliffs, and metal surfaces can reflect a signal before it reaches the receiver.

This is called multipath. The receiver may then treat a longer reflected path as if it were the direct path. Trees, roofs, and the human body can weaken signals too.

Modern receivers reduce these problems by comparing many satellite signals and using mathematical models of the atmosphere. Surveying equipment can use nearby reference stations to make corrections. This can improve accuracy from several metres to centimetres in suitable conditions.

A calculated position must be connected to a map reference system before it can be displayed. Most GPS devices use a global Earth model called WGS 84. Latitude gives north or south position, and longitude gives east or west position.

Height needs extra care. GPS height is measured relative to a smooth mathematical shape for Earth, not necessarily relative to sea level shown on a map. This is why a hiking device may show a different elevation from a local sign.

Map apps combine the position with stored roads, paths, building outlines, and place names. They may use internet data to download maps or traffic information, but the satellite signals themselves do not provide a map.

When learning GPS, focus on the chain of evidence from timing to distance, then from several distances to position. A position is an estimate, not a guaranteed exact point. Notice the accuracy value shown by a device, especially before using it for hiking, boating, or emergency work.

It is useful to distinguish GPS from the wider group called global navigation satellite systems. Phones can receive signals from systems run by several countries, which often improves availability.

Location services can be helpful, yet they can reveal where a person has been. Checking app permissions is part of using geographic technology responsibly.