Ground speed is a term used in aviation when referring to the velocity of an aircraft, specifically how fast an aircraft is traveling in relationship to the ground. This is not the same as the airspeed of an aircraft, which is how fast it is traveling in relationship to the surrounding air. Ground speed is affected by wind speed and wind direction: flying with the wind increases it, while flying against the wind decreases it. To determine this speed on board an aircraft, various types of equipment such as a global positioning system (GPS), an inertial navigation system, or a specialized flight computer can be used. Ground speed is used to calculate flight times and is often part of the in-flight information provided to passengers traveling on commercial airlines.
The velocity of an aircraft is usually different depending on whether its ground or airspeed is measured. The two velocities are only the same if there is no wind. In flight, it is vital to know the airspeed, because it affects the performance and aerodynamics of an aircraft. To determine the airspeed, one needs to know the ground speed, because airspeed is calculated by subtracting the wind speed from the ground speed, taking the direction of the wind into account. Variations in wind direction usually complicate this calculation, but the formula remains essentially the same.
A plane flying 100 mph (161 kph) with no wind has a ground and airspeed of 100 mph (161 kph). Flying straight into a wind that has a wind speed of 20 mph (32 kph) decreases the ground speed to 80 mph (129 kph), while the same wind speed coming from straight behind the plane increases it to 120 mph (193 kph). The airspeed remains the same in both cases. A plane needs a certain amount of airspeed to lift off the ground. The relationship between ground, wind and airspeed means that a plane traveling into the wind when taking off, needs less ground speed to achieve lift off than it would with the wind behind it.
Ground speed is used to calculate airspeed, and in aviation, there are several types of airspeed. True airspeed (TAS) refers to how fast an aircraft is really traveling relative to the air it is traveling through. Indicated airspeed (IAS) is the velocity as read on an aircraft's on-board instruments, specifically the airspeed indicator. Calibrated airspeed (CAS) is an adjusted and more exact version of the indicated airspeed, tweaking it to allow for various common sources of error.