How many satellites are required for a 3-dimensional position in GPS?

To determine a 3-dimensional position using GPS, a minimum of four satellites is required. This is because three satellites are used to triangulate the position in three-dimensional space, providing latitude, longitude, and altitude. However, GPS systems also need a fourth satellite to accurately calculate the time variable, which is essential for determining precise positioning. The signal sent from each satellite includes timestamped information, and without the fourth satellite, the receiver cannot resolve the discrepancies arising from signal travel time and atmospheric delays.

While two satellites can provide limited information about 2D positions (latitude and longitude), and three satellites can pinpoint the user's location in a flat plane, they are insufficient for determining altitude accurately. The inclusion of the fourth satellite allows the GPS receiver to synchronize time within the system, enhancing overall accuracy and ensuring that the three-dimensional position is determined correctly. Therefore, four satellites are necessary for comprehensive 3D positional data.