What is ground speed (GS) measured in?

Ground speed (GS) is defined as the horizontal speed of an aircraft relative to the Earth’s surface. It is commonly measured in nautical miles per hour. This unit is particularly relevant in aviation because one nautical mile corresponds closely to one minute of latitude on the Earth's surface, making it easier for pilots to navigate using maps and charts that are based on latitude and longitude. Nautical miles are more suitable for navigation over the ocean and across varied terrains, where the curvature of the Earth comes into play.

Units such as feet per second, knots per hour (not a standard measurement for ground speed), or statute miles per hour may be used in different contexts but do not provide the same relevance for aviation navigation. Therefore, using nautical miles per hour aligns with the conventions in aviation and contributes to the efficiency and safety of flight operations when considering distance and time over maritime and aeronautical navigational systems.