What is required for the FMS to calculate a navigation solution?

For the Flight Management System (FMS) to calculate a navigation solution accurately, a minimum of four satellites with acceptable geometry is required. This requirement is vital because the FMS uses the signals from these satellites to triangulate a precise position in three-dimensional space. By receiving signals from four satellites, the system can determine the user's latitude, longitude, altitude, and time, correcting for any errors in the satellite signals.

Using four satellites helps ensure that the geometric arrangement (geometric dilution of precision or GDOP) is such that it minimizes the potential for inaccuracies in the navigation solution. The fourth satellite serves an important role, particularly in providing vertical positioning and correcting for timing errors, which is crucial in aviation contexts where precision and safety are paramount.

The need for four satellites becomes evident when considering that three satellites can determine a location accurately in two dimensions (latitude and longitude) but lack the capability to provide an accurate altitude without the additional reference of a fourth satellite. This ensures that the FMS can compute a reliable and robust navigation solution essential for safe flight operations.