Definition
A formula used in dead reckoning navigation to calculate groundspeed (GS), where groundspeed equals distance flown (D) divided by the time it took to fly that distance (T). Distance is typically expressed in nautical miles and time in hours, giving groundspeed in knots.
Plain English
If you know how far you have flown over the ground and how long it took, you can find your speed over the ground by dividing the distance by the time.
Context Anchor
Seen in flight planning and navigation calculations when estimating travel time, arrival time, and fuel needs.
Derivation
GS stands for groundspeed -- the aircraft's actual speed across the ground, as opposed to airspeed (speed through the air). The formula itself is the standard speed = distance ÷ time relationship from basic physics, applied to navigation.
Why Pilots Care
It shows true progress toward the destination after wind has affected the aircraft.
Analogy
It works like a road trip calculation: if you drive 120 miles in 2 hours, your speed over the ground is 60 miles per hour.
Intuition Check
Do not treat groundspeed as the same as airspeed. Groundspeed is how fast the airplane moves across the ground, so wind changes it.
Example Sentence 1
After flying 30 nautical miles between two checkpoints in 12 minutes (0.2 hours), the pilot used GS = D/T to calculate a groundspeed of 150 knots.
Example Sentence 2
Using GS equals distance over time on the E6B confirmed the airplane was making good 105 knots over the ground.