Definition
Small position-error adjustments transmitted to GPS receivers from ground reference stations whose locations are precisely surveyed. By comparing the GPS-calculated position against the known surveyed position, the station measures the current satellite signal errors and broadcasts corrections that nearby aircraft receivers apply to sharpen their own position fixes. This technique is the basis of augmentation systems such as WAAS and LAAS, which provide the accuracy and integrity needed for precision approaches.
Plain English
Tiny fix-up numbers sent to your GPS so it knows where it really is. A ground station with a known location checks how far off the GPS signals are right now, then tells your aircraft how to correct for that error.
Context Anchor
Seen in discussions of satellite-based navigation improvements, especially systems that make GPS guidance accurate enough for instrument approach use.
Derivation
Differential' comes from the Latin differre, meaning to carry apart or differ. Here it refers to the difference between the GPS-calculated position and the true surveyed position at a ground station — that difference is what gets sent to aircraft as a correction.
Why Pilots Care
These corrections raise GPS accuracy enough to support precision approaches down to decision altitudes as low as 200 feet.
Analogy
It is like checking a clock against the correct time, seeing that it is two minutes slow, and then using that known difference to correct other clocks set the same way.
Intuition Check
Do not read differential corrections as general maintenance fixes. Here, the correction is a navigation adjustment based on a measured position error.
Example Sentence 1
WAAS provides differential corrections that allow pilots to fly LPV approaches with vertical guidance down to 200 feet.
Example Sentence 2
Without differential corrections the position error could grow large enough to prevent flying the published glidepath.