What is the minimum number of GPS satellites required to provide altitude information?

To determine altitude information using GPS, a receiver needs signals from a specific number of satellites to calculate its position accurately in three dimensions: latitude, longitude, and altitude. The minimum number of satellites required for obtaining a precise three-dimensional fix is four.

With only three satellites, a GPS receiver can calculate a position in two dimensions (latitude and longitude) on the Earth's surface, but it wouldn't be able to ascertain altitude. The fourth satellite is crucial because it helps to resolve the time offset in the GPS system's internal clock, allowing the receiver to compute altitude correctly. Thus, having four satellites ensures that the receiver can determine its exact position in three-dimensional space.