Why is the GNSS measured distance referred to as a pseudo-range?

The term "pseudo-range" in the context of GNSS (Global Navigation Satellite System) refers to the measured distance from the receiver to the satellite that incorporates various errors, rather than a precise and true geometric distance. One of the significant factors contributing to this pseudo-range is the clock error associated with the receiver.

Each GNSS satellite has an onboard atomic clock, but the receiver’s clock is typically less precise. This discrepancy causes a constant time bias, which translates into a distance error due to the way range is calculated from the time signal received. As a result, the effective distance measured includes this clock error, leading to the designation of "pseudo-range" rather than a true range.

In contrast, while atmospheric delays and other factors such as satellite clock errors also influence GNSS measurements, they lead to variable rather than constant errors. The term "pseudo-range" specifically highlights the fixed nature of the receiver's clock bias as an inherent limitation in achieving an accurate distance measurement, making it crucial to account for in GNSS applications.