Inside the aircraft, how does the INS differ from the GPS in terms of altitude information?

• A. INS does not provide altitude information; GPS does.
• B. INS provides altitude information; GPS does not.
• C. Both provide identical altitude information.
• D. Altitude data is not a navigational parameter for either.

Answer: (D)

Altitude information is provided by both INS and GPS. An INS determines altitude by integrating vertical accelerations from the inertial sensors, starting from a known reference; this dead-reckoning method can drift over time without external updates. GPS derives altitude from the geometry of satellite signals, giving height above the reference ellipsoid (and, with conversions, to mean sea level). Because they rely on different sources, their altitude outputs have different reference frames and error characteristics. Therefore, saying that altitude data isn’t available from either is not correct. In practice, aircraft use the barometric altimeter as the primary altitude reference, with INS and GPS providing complementary altitude information to improve accuracy and fault detection.