Definition
Errors in GPS position fixes caused by the slowing of satellite radio signals as they pass through the ionosphere, the electrically charged layer of the upper atmosphere. Because the signal travels slightly slower than it would through a vacuum, the receiver calculates the satellite as being farther away than it actually is, introducing a range error that affects position accuracy.
Plain English
GPS signals slow down a little when they pass through a charged layer high in the atmosphere. That slowdown makes the receiver think the satellite is farther away than it really is, so the position it shows is slightly off.
Context Anchor
Seen in GPS error discussions, especially when learning why satellite navigation is not perfectly accurate under all conditions.
Derivation
Ionospheric comes from ion (an electrically charged particle) and sphere (a layer surrounding the Earth). The ionosphere is the layer where solar radiation strips electrons from atoms, leaving charged particles that interact with radio waves and slow them down.
Why Pilots Care
These delays can shift reported GPS position by several meters, affecting the accuracy of instrument approaches and requiring awareness or mitigation through augmentation systems.
Grounding Statement
A GPS signal from space can be slowed slightly as it passes through the charged upper atmosphere before reaching the airplane.
Intuition Check
Delay does not mean an air traffic or scheduling delay here. It means a tiny slowing of the GPS signal before the receiver uses it to calculate position.
Example Sentence 1
Ionospheric delays are one of the main reasons a basic GPS receiver may show a small position error during periods of high solar activity.
Example Sentence 2
Dual-frequency GPS units correct for ionospheric delays automatically by comparing signals on two different frequencies.