How does GPS work in aircraft? And how does WAAS improve GPS work?

What is a GPS?

It’s Global Positioning System in the cockpit of every plane, the receivers provide the pilot with a huge amount of information. Receivers of GPS supply the pilot with two important advantages; the first thing is that it enables the users from knowing their location. The second is that it directs users to any wanted point in its database or any place on the earth. Its computer has a list of NAVAIDs, fixes, and airports, the pilot can punch a few buttons and be pointed directly to anything. 

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How does a GPS work?

The constellation of GPS satellites orbiting the earth are mostly in sight wherever you are, some of them are spares. The constellation of GPS consists of 30 satellites; the GPS receiver of the plane’s cockpit gets every satellite signal separately. The satellites only transmit one of the critical information.

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The system used in the time and navigation technique is known as triangulation. GPS can recognize the time and location of the satellite when it sends the signal, also the period taken from recognizing the signal after releasing it from the satellite to the receiver.

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Due to the signal traveling at the speed of light, we can use the time to calculate distance. For example, if it took 0.0025 seconds for the signal to get from the satellite to the plane, the receiver knows it must be 749,481.1 meters, or 404.7 nautical miles, away from the satellite. This is happening in three dimensions, so the data from one satellite isn’t enough because it still means that the plane could be anywhere on the surface of a sphere with a radius of 404.7 nautical miles. The receiver does the same calculation process several times with other satellites, so the more satellites are seen the more accurate the receiver location is.

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What is WAAS EGNOS?

WAAS means Wide Area Augmentation System, although the FAA operates the WAAS system in the U.S. its signal is available to all users. It’s created by the FAA and the Department of Transportation to support approaches to airports. Without WAAS augmentation GPS receivers aren’t accurate enough to get approaches and terminal environments.

There are ground stations in known places that don’t move. Everyone has a GPS satellite and knows the location through these satellites. In case the satellite derived a position and it does not match, this means that there is an error in the system. The location is sent to another satellite and will be rebroadcasted. The suitable GPS receiver can receive the correction of the error and apply the new location.

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So, the WAAS system is like a regular one but always adds in real-time error corrections. Not all GPSs have this capability. To use WAAS, the receiver must support it and receive the signal from the WAAS satellite.

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Sources

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