Which navigation aids can provide redundancy when GPS is denied or degraded?

• A. Inertial navigation
• B. VOR/ILS
• C. TERCOM
• D. All of the above

Answer: (D)

The main idea is identifying navigation methods that can operate without GPS, providing backup when GPS is denied or degraded. An inertial navigation system relies on internal sensors (accelerometers and gyros) to track motion, so it can continue guiding you without any external signals, though it will drift over time and may need occasional updates to stay accurate. Ground-based radio navigation like VOR and ILS works independently of GPS: VOR gives a bearing from a known station (and with multiple stations you can determine position), while ILS provides precise lateral and vertical guidance for approach and landing, which can be used even when GPS isn’t available. TERCOM uses radar to map and compare terrain against a stored database, allowing position estimation through terrain matching without GPS. Because each method offers a distinct, GPS-free means of navigation, all of these provide usable redundancy, making all of the above the best answer.