Slant range distance to a ground-based navigation station is calculated by which system?

• A. ILS
• B. VOR
• C. GPS/FMS
• D. DME

Answer: (D)

Distance Measuring Equipment is designed to determine how far the airplane is from a ground-based navigation facility. It works by sending an interrogation to the ground station and measuring the time it takes for the reply to return, then converting that time into a distance. The distance reported is the slant range—the actual line-of-sight distance from aircraft to the station, which is why DME is the system used to calculate distance to a ground-based nav station.

Other systems provide different information: ILS gives localizer and glide slope for approach guidance, not distance to a station; VOR provides a bearing to the station, not distance; GPS/FMS can compute distances to many points, but the direct distance to a ground-based navigation station is specifically provided by DME.