What is the purpose of a gravity model in navigation?

• A. Provides horizontal velocity updates.
• B. Calculates GNSS satellite positions.
• C. Provides vertical acceleration component and calibrates altitude; errors produce vertical drift.
• D. Adjusts clock biases in GNSS.

Answer: (C)

The main idea is that gravity isn’t the same everywhere, and knowing its vertical component at your location helps you determine and maintain height in a navigation solution. A gravity model provides the expected vertical gravity (how strong gravity pulls you toward the Earth) for a given position and height. This vertical component is used in the vertical dynamics of the navigation system, and it helps calibrate altitude by linking measured accelerations to actual height.

If the gravity value is off, the vertical acceleration used in the height estimate is also off. Since altitude is obtained by integrating vertical acceleration twice, even small errors in gravity can cause the estimated height to drift steadily over time. That’s why the gravity model is key for keeping vertical position accurate. It isn’t about updating horizontal velocity, computing satellite positions, or adjusting clock biases—that’s handled by other parts of the system.