What is the minimum number of satellites required to achieve a 3D position fix?

To achieve a 3D position fix using Global Navigation Satellite System (GNSS) technology, a minimum of four satellites is required. This is because determining a precise position involves solving for three coordinates: latitude, longitude, and altitude.

When using three satellites, you can only determine a 2D position fix (latitude and longitude) while assuming a known altitude, which does not provide altitude information. The fourth satellite is essential to calculate the altitude accurately and to account for any timing discrepancies in the satellite signals, thereby providing a complete 3D fix.

The process involves trilateration, where the distances from the satellites help pinpoint the receiver's location in three-dimensional space. If the system used also has timing errors, having a fourth satellite is crucial to resolving those inconsistencies, ensuring that the calculated position is as accurate as possible.

In summary, four satellites are necessary because one satellite provides one possible location on its sphere, two create two intersection points, three intersect in two places, and the fourth satellite helps pinpoint the exact location among the possible intersections while correcting timing errors.