When is DME groundspeed information considered accurate?

DME (Distance Measuring Equipment) groundspeed information is considered accurate when tracking directly to or from the station. This is because DME measures the distance between the aircraft and the DME station based on the time it takes for a radio signal to travel to the station and back. When an aircraft is traveling directly towards or away from the station, the relationship between the distance covered and the time taken provides a clear and straightforward calculation of groundspeed.

In other flight scenarios, such as when an aircraft is flying at an angle relative to the DME station, the measurement may not accurately reflect the true groundspeed, since the aircraft's path isn't directly aligned with the measurement line. Therefore, while DME can provide valuable distance data, to obtain reliable groundspeed information, the aircraft's track must be aligned directly to or from the DME facility. This specificity ensures that the calculation is based on a predictable geometry, making the groundspeed reading reliable.