Regarding altitude information, which statement is true?

• A. INS does not provide altitude information; GPS does.
• B. GPS does not provide altitude information.
• C. Both provide identical altitude information.
• D. Altitude is not used by either system.

Answer: (A)

Altitude data comes from how the two systems compute position. GPS provides a full three-dimensional position, which includes a vertical component (height above mean sea level or the ellipsoid) as part of its fix. Inertial navigation, on the other hand, estimates position by integrating accelerations over time. The vertical estimate from this integration tends to drift because small biases in the accelerometers accumulate, making the altitude reading unreliable without external updates. Because of that drift, the INS by itself isn’t considered a reliable source of altitude, while GPS does provide altitude information. When both are used together, GPS updates help keep the INS’s altitude accurate.