Updated: Dec 8, 2020

Navigation Aids: GPS Lesson by wifiCFI

GPS (PHAK C16) (AIM 1-1-17)

GPS is a means of satellite navigation provided to aircraft in flight.

GPS stands for Global Positioning System.

Let’s talk about how it works.

The Numbers

The GPS System consists of:

24 Satellites that Orbit the Earth around

6 different Orbital Planes.

How it Works

These GPS Satellites relay 3 pieces of information to the planes on-board GPS Equipment. 

They are:

Their ID (number/name)

Their Position (lat/long)

Time Code (atomic clock)

From this information, our GPS Equipment can begin to eliminate areas that we could not be.

The more information we have from the GPS Satellites, the more accurate our position.

How accurate does our GPS need to be?

To calculate our 2D position in space (LAT/LONG) = Minimum 3 Satellites in View

To calculate our 3D position in space (LAT/LONG and Altitude) = Minimum 4 Satellites in View

RAIM = Minimum 5 Satellites in View

Or 4 Satellites in View and Barometric Altimeter Input

FDE = Minimum 6 Satellites in View

Or 5 Satellites in View and Barometric Altimeter Input

What is RAIM?

RAIM stands for “Receiver Autonomous Integrity Monitoring.”

RAIM is the capability of a GPS receiver to perform integrity monitoring on itself by ensuring available satellite signals meet the integrity requirements for a given phase of flight. 

Without RAIM, the pilot has no assurance of the GPS position integrity. 

RAIM provides immediate feedback to the pilot. 

This fault detection is critical.

RAIM Prediction can be checked before flight at:

What is FDE?

FDE stands for “Fault Detection and Exclusion.”

It is the ability of a GPS receiver to exclude faulty satellite information.


Let’s say our GPS Receiver is receiving signals from 10 GPS Satellites.

Now, let’s assume one of those 10 satellites begins giving faulty information.

At this point, without FDE, we would get an INTEG Error message on our GPS and not be able to trust the information.

Even though we are still receiving 9 satellites and the requirement for RAIM is only 5.

However, with FDE, the faulty satellite would be excluded and we would not receive the INTEG Error message.

Sensitivity Modes

Aircraft GPS Typically Have 3 Modes of Sensitivity.

They are:


When more than 30nm from either the departure or destination airports.

Senstivity = 2nm (either side of course until CDI needle goes full deflection).


When within 30nm from either the departure or destination airports.

Sensitivity = 1nm (either side of course until CDI Needle goes full deflection).


When within 2nm of the Final Approach Fix (FAF).

The FAF will be discussed more in instrument training.

Sensitivity = 0.3nm (either side of course until Needle goes full deflection).

GPS Errors

As good as GPS has become in the modern age, it can still experience errors of up to 300 feet.

Errors come from 1 or more of 3 main sources:

Atomic Clock Errors

Satellite Position Errors

Atmospheric Interference

Atomic Clock Errors

Atomic Clocks are accurate to the billionth of a second.

Light travels 1 foot in a billionth of a second.

Therefore, if a satellites atomic clock is off by 5 billionths of a second then our GPS position can be off by up to 5 feet.

Satellite Position Errors

There is a lot of space junk orbiting the earth.

If a piece of this space junk knocks a satellite out of it’s position, then its time code information will be relayed to the airplane in a delayed fashion.

This will cause a position error.

Atmospheric Interference

The Ionoshpere and Troposhere can bend and delay radio signals from GPS satellites.

A delayed time code could result in a GPS position error.

Wide Area Augmentation System (WAAS)

To combat these errors and provide more reliable GPS information to pilots, the FAA came up with WAAS.

WAAS is a system that monitors the data being generated by the GPS satellites.

This system monitors the GPS satellites from fixed ground based stations, makes necessary mathematical corrections for errors detected, uploads the corrected information to one of two Geo-Stationary Satellites, which then relay the corrected information to our on-board GPS unit.

WAAS greatly increases the accuracy and reliability of GPS Navigation with errors from 6-9 feet.

Local Area Augmentation System (LAAS)

In recent years the FAA has taken WAAS one step further.

Ground stations (essentially ground based satellites) are able to measure an aircraft’s altitude above the earth.

This makes the vertical guidance of GPS very accurate!

FAA Sources Used for this Lesson

Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 16

Aeronautical Information Manual (AIM) Chapter 1

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