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slant range error

Asked by: 1548 views General Aviation

please describe the slant range error

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1 Answers

  1. Dan Chitty on Jul 28, 2015

    Below is a helpful explanation found from an internet article:

    DME displays distance in nautical miles, groundspeed in knots, and time-to-station in minutes. Beware, however, that DME groundspeed and time-to-station are only accurate when you are flying directly to or from the ground station. If you are flying in any other direction, you will see groundspeed that is erroneously low and time-to-station that is erroneously high.

    If you are flying away from the station, groundspeed will be accurate but “time-to-station” will actually show “time-from-station” and will increase as you get farther and farther from the station.

    Another thing to remember is the DME measures the straight-line distance from the aircraft to the ground station. This is called “slant range” and is slightly more than the actual horizontal distance because of the difference in elevation between the aircraft and the station. The most extreme case of “slant range error” occurs when the aircraft passes directly over the station; instead of reading zero, the DME shows the altitude of the airplane above the station (in nautical miles).

    For example, if the station is at sea level and you’re flying at 9000 feet, the DME will show 1.5 NM as you pass directly over the station. When you’re 2 NM away from the station horizontally, the DME will read 2.5 NM. At greater distances from the station, slant-range error is considered negligible.

    Slant range error also affects groundspeed and time-to-station displays when you’re close to the station. Displayed DME groundspeed drops below actual groundspeed as you approach the station and then climbs back to normal after you pass it. Displayed DME time-to-station may not count all the way down to zero as you fly over the station.

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