NavSatFix

NavSatFix is a term that refers to the process of obtaining a valid and accurate position fix from a Global Navigation Satellite System (GNSS) receiver. This involves a series of steps that enable the receiver to determine its precise location on Earth.

The process begins with the receiver acquiring signals from multiple GNSS satellites. Each satellite continuously transmits data that includes its orbital position, precise time, and health status. The receiver measures the time it takes for these signals to arrive from at least four different satellites. This time difference, when multiplied by the speed of light, provides the receiver with the pseudorange to each satellite.

Using these pseudoranges and the known positions of the satellites, the receiver performs a trilateration calculation. By intersecting spheres with radii equal to the pseudoranges centered on the satellite positions, the receiver can pinpoint its location in three dimensions (latitude, longitude, and altitude) and also determine the receiver's clock error. A fix requires signals from a minimum of four satellites for a 3D position. Additional satellites can improve accuracy and reliability.

Factors that can affect the quality of a NavSatFix include atmospheric conditions, signal obstruction (e.g., by buildings or terrain), multipath effects where signals bounce off surfaces before reaching the receiver, and the geometry of the satellite constellation (Dilution of Precision or DOP). Modern GNSS receivers employ various techniques, such as differential GPS (DGPS) or Real-Time Kinematic (RTK) positioning, to mitigate these errors and achieve centimeter-level accuracy.