In fog conditions at a range of 100m, which position reference system is not usable?

• A. GPS
• B. Hydroacoustic
• C. Laser
• D. Gyrocompass

Answer: (B)

Fog affects signals that rely on light, so optical methods like a laser-based position reference become unreliable in dense fog, even at a relatively short range like 100 m. The laser beam is scattered and absorbed by the fog, reducing signal strength and distorting measurements, which makes accurate positioning impractical. In contrast, GPS relies on satellite radio signals, a gyrocompass uses inertial and Earth rotation data, and hydroacoustic navigation uses sound in water; these are not drawn down by fog in air and can still function under such conditions (assuming the necessary receivers and transponders are in place). So, the laser-based reference is not usable in fog at 100 m.