What GPS Can’t Do

Take time to collect the best GPS data you can. 

Although GPS can make life easier in many ways, it also has its limitations. Errors can creep in and lessen the accuracy of your GPS data. Some causes of error include:

  • Signal interference. Large structures—including buildings, rock surfaces and even dense foliage—can get in the way of a satellite’s signal, causing the receiver to get less accurate information or no information at all. That’s why some GPS devices have trouble in deep valleys or gorges, and most won’t work indoors, underwater or underground. The satellites’ position in space and in relation to each other can also interfere with accuracy.
  • Electrical interference. Sunspot activity, electrical activity in the atmosphere and signals from other electronic devices can all affect the accuracy of your GPS readings.
  • Orbital errors. Information about a satellite’s location in orbit may be off, and that can lead receivers astray. These errors are also known as ephemeris errors.
  • Timing errors. GPS satellites have extremely accurate atomic clocks, but receivers don’t. Although receivers correct for the discrepancy, there can still be minor errors that hurt your overall accuracy.
  • The atmosphere. Each satellite’s signal slows a bit as it moves through the atmosphere. Although receivers calculate an average amount of delay to partially correct for this, there are still small variations that can introduce error.

The good news is that accuracy is getting better. A system of satellites and ground stations called the Wide Area Augmentation System (WAAS) now provides signal corrections to many GPS receivers. The receivers themselves are improving too, and the U.S. Department of Defense is upgrading the existing network of satellites. GPS errors are shrinking fast, and may one day become a thing of the past.

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