What unit is used to express speed at lower altitudes in aviation?

In aviation, speed at lower altitudes is primarily expressed in knots. A knot is defined as one nautical mile per hour, and this unit is commonly used in aviation due to its alignment with the geographical coordinate system and its applicability over both land and sea travel. Since aviation operations often involve navigation with reference to the Earth's curvature, using nautical miles helps pilots accurately calculate distances based on latitude and longitude.

Using knots allows for a standardized method for communication and calculation between different types of aircraft and within flight operations, thereby enhancing safety and coordination in both international and domestic flights. Other speed units like kilometers per hour, feet per second, and miles per hour may be used in certain contexts, but they are not the standard measure for speed in aviation, particularly at lower altitudes where space and navigation concerns are most critical.