Computers in the receiver shave nanoseconds equally off all the apparent readings until a unique solution is attained. In 3-dimensional location exercises, three satellites are needed for location and elevation, and an additional one for clock synchronization. They are above any atmospheric drag effect. They are constantly monitored by DoD. Satellites transmit a data message about their exact location and their system's health. Ionosphere slows the radio signals.
Expensive receivers can compare two parts of the radio signal that have different frequencies.
Different frequencies slow by different amounts. The lower the f, the slower. Water vapor in the atmosphere slows the radio signals. Geometric Dilution of Precision. The wider the angle between satellites, the better the measurement.