The Earth
The Solar System & Time
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Using Aeronautical Charts
Basics of Navigation
Distance, Speed & Time
Vertical Navigation
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Practical Navigation Techniques
Radio Navigation
Practice Exam

Global Navigation Satellite System (GNSS)

What we commonly call ‘GPS’ is officially known as the Global Navigation Satellite System (GNSS).

Several GNSS systems have been developed across the world:

The Global Positioning System (GPS) developed by the USA
The GLONASS system developed by Russia
The Galileo system developed by the EU
Pictured above is SkyDemon, Europe’s most popular moving map GPS program for VFR and general aviation flying

How GNSS Works

A satellite in Earth’s orbit transmits a signal that includes the time it was sent and the aircraft’s receiver compares the time difference between when the signal was sent and when it was received. This time difference is used to calculate the distance between the satellite and the aircraft.

When these signals are received from 3 satellites, a 2-dimensional position fix is made by the GNSS unit. When a signal from a 4th satellite is received, the aircraft’s altitude can be calculated as well.

Each satellite transmits its position and the time. The receiver measures the time difference from several satellites to calculate a position.

3 satellites are required for a 2-dimensional GNSS position fix
4 satellites are required for a 3-dimensional GNSS position fix

Receiver Autonomous Integrity Monitoring (RAIM)

Receiver Autonomous Integrity Monitoring (RAIM) is a ‘self-checking’ system that detects faults with a satellite’s signal and ignores that signal when a fault is detected.

At least 5 satellites are required for RAIM to work (the 4 needed for a 3-dimensional position fix plus the 1 that RAIM ignores). However, the CAA recommends at least 6 satellites so that RAIM still functions after 1 satellite has been ignored.

5 satellites are required for RAIM to function

GNSS/GPS Best Practice

Learn how to use the GPS unit on the ground before using it in the air
Take another pilot with you when using a GPS in the air for the first time. A second pilot can help maintain a good lookout
Don’t get fixated on the GPS. Maintain your lookout and use the GPS unit to assist your traditional navigation techniques
Always check the NOTAMs for GNSS jamming. Periods of GNSS jamming trials are becoming more and more common
Ensure any portable GPS units are securely mounted and not blocking your visibility outside or any cockpit controls.

Click Here to Start the Radio Navigation Quiz

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