Pseudorange is the fundamental measurement type in GNSS positioning, representing the apparent distance between a satellite and receiver calculated from the travel time of the navigation signal. The term ‘pseudo’ indicates that this measurement is not a true geometric range but rather includes the combined effects of satellite and receiver clock errors, atmospheric delays, and other biases that must be accounted for in the positioning solution.
The pseudorange measurement is derived by comparing timestamps. Each satellite continuously broadcasts its current time and orbital position. The receiver notes when this signal arrives according to its own clock and multiplies the apparent travel time by the speed of light to calculate distance. However, because neither the satellite clock nor the receiver clock is perfect, and because the signal is delayed by the atmosphere, the computed distance differs from the true geometric range, hence ‘pseudorange.’
In the positioning solution, pseudoranges from at least four satellites are combined to solve for four unknowns: the receiver’s three spatial coordinates (X, Y, Z) and its clock offset from GNSS system time. Additional satellites beyond the minimum four enable over-determined solutions that average measurement noise and support quality monitoring. The accuracy of pseudorange measurements, typically 1-10 meters depending on signal and receiver quality, directly limits standalone GNSS positioning accuracy.
Pseudorange accuracy is affected by several error sources. Satellite clock and orbit errors contribute meters of pseudorange error if broadcast parameters are used; precise products reduce this to centimeters. Ionospheric delay can add 5-15 meters of error, removed through dual-frequency processing or external corrections. Tropospheric delay contributes roughly 2 meters at zenith, more at low elevations. Multipath from reflected signals can add meters of error in challenging environments. Receiver noise contributes typically 0.3-1 meter depending on signal quality. Differential techniques (RTK, DGNSS) and precise correction services address these error sources to improve positioning accuracy beyond standalone pseudorange limits.
