Pseudorange

A pseudorange is the fundamental observable of satellite navigation: the apparent distance from receiver to satellite, computed as (signal reception time − transmission time) × speed of light. The satellite stamps its transmission using an onboard atomic clock; the receiver timestamps arrival with its own cheap quartz clock — and therein lies the "pseudo". The receiver clock's unknown offset, even a microsecond (300 m of apparent distance), contaminates every measurement by the same amount, so the geometry becomes a four-unknown problem: latitude, longitude, altitude and clock bias, solved simultaneously from at least four satellites' pseudoranges. That elegant trick — making time itself one of the solved variables — is why GNSS receivers need no precision clock and why every fix is also a time transfer accurate to nanoseconds, the hidden service underpinning telecom and grid synchronisation. Residual errors — ionospheric and tropospheric delay, ephemeris error, multipath reflections — are what correction techniques (dual-frequency, augmentation, RTK) exist to strip, taking raw metre-class pseudorange fixes toward centimetres.