How many satellites must always be observable for accurate GPS usage?

For accurate GPS usage, a minimum of four satellites must always be observable. However, in terms of ensuring reliability and accuracy, five satellites are often referred to in discussions about optimal performance. When five satellites are visible, the GPS receiver can perform better calculations, allowing for more accurate positioning and reducing errors that can arise from signal interference or atmospheric conditions.

The key to GPS accuracy lies in the concept of trilateration, which determines a receiver's position based on its distance from multiple satellites. With at least three satellites, a 2D position can be obtained (latitude and longitude), but the addition of a fourth satellite provides the necessary altitude measurement and improves the reliability of the calculation through redundancy. As more satellites are visible, the accuracy and integrity of the GPS signal improve, allowing for corrections based on data from additional satellite signals. Therefore, five satellites make it possible to ensure that the GPS can account for any potential errors and provide a high level of accuracy.