What is the minimum number of NAVSTAR GPS satellites required to obtain an accurate 3D position fix?

To achieve an accurate 3D position fix using the NAVSTAR GPS system, a minimum of four satellites is required. This is because determining a precise 3D position involves calculating three spatial dimensions: latitude, longitude, and altitude, along with the need to account for time as well. Each satellite transmits a signal that includes its location and the exact time the signal was sent.

When the GPS receiver locks on to the signals from four satellites, it can perform a process called trilateration. The first three satellites provide enough information to establish a two-dimensional position (latitude and longitude) and the altitude. The fourth satellite is necessary to solve for the receiver's clock error, ensuring that the time calculation is accurate — a crucial factor since GPS relies on the precise timing of the signals.

Without the fourth satellite, the receiver cannot correctly account for its clock discrepancies, which could lead to significant positional errors. Thus, four satellites are essential not only for pinpointing the location accurately in three dimensions but also for achieving a reliable correction for timing errors.