BAT Challenger 350 Practice Exam

The main idea is that the FMS builds its navigation solution by solving for position (x, y, z) and the receiver’s clock bias using range measurements to satellites. Each satellite provides one equation, so you need four independent measurements to determine four unknowns. That’s why four satellites with good geometry is the standard path. If you only have three satellites, you can’t resolve all four unknowns unless you bring in external information for one of them. Knowing altitude from air data fixes the vertical position, leaving three unknowns (x, y, and the clock bias) with three equations, which allows a valid solution. So the combination of four satellites with acceptable geometry, or three satellites plus air data altitude, is sufficient. The other options don’t provide enough information on their own: three satellites alone don’t resolve the clock bias; two satellites with delta altitude isn’t enough data; and four satellites alone is typically valid but doesn’t acknowledge the alternative that three satellites plus altitude can also work.