How GPS Works

Thursday, August 26, 2021 at 09:10 AM ET

Verified

Key Takeaways:

GPS determines a precise 3D position by calculating distances from multiple satellites, requiring a minimum of four satellites and accurate timing from their atomic clocks and ephemeris data.
The Wide Area Augmentation System (WAAS) enhances GPS accuracy and reliability, particularly for critical applications, by using ground stations to detect and transmit signal error corrections via geostationary satellites.
For IFR flight, GPS receivers must meet strict certification, permanent installation, current database, cockpit documentation, and functional testing requirements, with specific rules governing their substitution for traditional land-based navaids.

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Back in the bad old days of VOR navigation you could find your position by tuning in two different VOR’s. Each radial from a VOR was a “line of position” from that VOR. Your position could be anywhere on that line. Where the two lines from two different VORs intersected was your location. For accuracy you had to plot the radials on a paper sectional, but it worked.

If you were lucky, you might have had a DME, meaning you only needed to tune one VOR for the line of position and use the DME for distance along that line. Plotting was easier that way. But either way, you needed the paper chart to see where you were and your accuracy was at best within a mile or so. These days, GPS-driven moving maps are ubiquitous—even available on some watches—and instantly show your position within a few meters. How can that be?