Definition
A pseudorange is the GPS receiver's calculated distance to a satellite, derived from the time it takes a signal to travel from the satellite to the receiver. It is called 'pseudo' because the measurement contains a timing error caused by the receiver's inexpensive clock not being perfectly synchronized with the satellite's atomic clock. The receiver resolves this error by comparing pseudoranges from at least four satellites simultaneously.
Plain English
A pseudorange is the GPS receiver's best guess of how far away a satellite is, based on how long the signal took to arrive. It is only a guess at first because the receiver's clock is not perfectly accurate, so the distance it calculates is slightly off until the receiver compares signals from several satellites and corrects itself.
Context Anchor
Seen in GPS discussions about how a receiver calculates position from satellite signals.
Derivation
From 'pseudo' (Greek pseudes, meaning false or apparent) plus 'range' (distance). It is a 'false range' because the measured distance contains a known error that must be corrected before the position is trustworthy.
Why Pilots Care
Understanding pseudorange explains why a GPS receiver needs at least four satellites in view to give a reliable position, and why losing satellites can degrade accuracy.
Analogy
It is like estimating how far away lightning is by counting the seconds until you hear thunder. If your counting is a little off, your distance estimate is useful, but not exact.
Grounding Statement
A GPS receiver turns very tiny signal timing differences into distance estimates.
Intuition Check
“Pseudo” does not mean the GPS distance is useless or fake. It means the distance is an apparent measurement that still includes correctable errors.
Example Sentence 1
The GPS receiver computed pseudoranges from five satellites and used them to fix the aircraft's position.
Example Sentence 2
Four accurate pseudoranges allow the GPS to solve for latitude, longitude, altitude, and receiver clock error simultaneously.