How many signals must the GPS system receive to determine an aircraft's position and altitude?

To determine an aircraft's position and altitude accurately, the GPS system needs to receive signals from four satellites. Here’s how this process works:

Each GPS satellite transmits a signal that contains the satellite's location and the precise time the signal was sent. When a GPS receiver on an aircraft picks up these signals, it calculates the time it took for each signal to arrive. This time delay is then used to determine the distance from each satellite.

With signals from three satellites, the receiver can calculate a two-dimensional position (latitude and longitude). However, to ascertain altitude as well, a fourth satellite is necessary. The additional satellite provides the extra data required to resolve the ambiguity in altitude, allowing the system to pinpoint the aircraft's exact three-dimensional location.

Thus, four signals are essential for achieving a reliable position fix that includes altitude, making the correct answer four satellites.