Definition
The measured distance between a GPS satellite and a receiver, calculated from the time it takes the satellite's signal to reach the receiver, before that measurement has been corrected for clock errors and other inaccuracies. Because the receiver's clock is not perfectly synchronized with the satellite's atomic clock, this raw distance value contains errors and is therefore called a pseudo-range rather than a true range.
Plain English
A first-pass guess at how far away a GPS satellite is, based on how long its signal took to arrive. It is called a 'pseudo' range because the number still has timing errors in it that have to be cleaned up before it can be trusted as a real distance.
Context Anchor
Seen in LAAS and GPS position discussions, where ground equipment measures GPS signals and sends corrections to improve aircraft guidance.
Derivation
Pseudo' comes from Greek 'pseudes', meaning false or apparent. 'Range' here means distance to the satellite. So a pseudo-range is an 'apparent distance' -- a measurement that looks like a range but is not yet accurate enough to be treated as one.
Why Pilots Care
Raw pseudo-range measurements contain clock-induced errors that LAAS and similar systems correct, directly affecting the accuracy needed for instrument approaches and navigation.
Analogy
It is like measuring distance with a clock that is slightly off. The measurement is useful, but it needs a correction before it can be treated as accurate.
Intuition Check
Do not read pseudo-range as a fake or useless distance. It means an apparent GPS distance that still needs correction before it represents the true distance closely.
Example Sentence 1
The LAAS ground station compares pseudo-ranges from each satellite to known reference values and broadcasts corrections to nearby aircraft.
Example Sentence 2
LAAS ground stations measure pseudo-range errors and broadcast corrections so aircraft receivers can compute a more precise position.