# Why don't aircraft have DME that can automatically convert the slant range to ground range?

Knowing that $$a^2 = b^2 + c^2$$ where $$a = \text{slant range},\ b= \text{ground range},\ c = \text{height}$$.

Why can't the DME equipment which calculates the slant range then just convert it to the ground range? Surely for navigation and position fixing knowing the actual ground range would be more useful? Is there a practical reason why the slant range has stuck?

• Unless you're really close to the station, the difference isn't big enough to matter. The DME signal is very simple, and simple systems are inherently more reliable. Mar 15, 2020 at 17:05
• This simply is not mathematically possible (as well as being not KISS, and not in number domain). Mar 16, 2020 at 18:33
• If you're in a plane flying at 5,000 feet and the DME indicates you're 25 miles from the station (assumed to be at sea level), your actual distance to the station is 24.98 miles. How critical is that 100 feet? Mar 16, 2020 at 19:24

## 2 Answers

There's no accurate and simple way to know the height above the station in order to do the calculation with the existing equipment. Altitude is not an indicator as the DME station may be above sea level. If your airplane is at 15,000ft and the station is at sea level you'll get one answer, if the station is at 10,000ft you'd get a completely different result, and mistakes could compromise safety.

The only way your proposed system would work is if the DME device had a complete database of every station height and altimeter readings, which would require upgrading the DME devices at great expense. I don't think anyone is interested in that when you can have GPS for a lot less money.

• The alternative is if each DME station broadcast its elevation. However, its just not necessary: DME is used to inform pilots when they're getting close to a station, the precision of slant-range adjustment just doesn't add much useful information. Mar 15, 2020 at 13:35
• @abelenky Broadcast how, to what? That'd be an entirely different standard.
– Dan
Mar 15, 2020 at 13:49
• @Dan On a subcarrier, same as VORs send their Morse code identifications. To the DME equipment on the airplane, so it could calculate the slant range. It would be a new version of the same standard; existing DME equipment would ignore the subcarrier and continue to function as normal. Mar 15, 2020 at 18:10
• @GdD: Using a subcarrier in a way that's backwards compatible allows gradual rollout with new planes and when ground stations are being replaced anyway, eventually fixing the "problem" for hypothetical DME2.0 capable planes when approaching ground stations which have upgraded to DME2.0 transmitters. Mar 16, 2020 at 10:48
• When are the ground stations being replaced @PeterCordes? Seriously, nobody is going to spend money on that.
– GdD
Mar 17, 2020 at 8:50

You have to realize that the DME is a justified and ancient nav system. When it was concieved, it was chosen to keep it simple, for obvious reasons.

As it is, it does not have to communicate with any other system, making it a very reliable stand alone device.

There is no need to create a "DME 2.0", we already have gps and other more sophisticated systems to tell us how far we are from any given point in space.

• a justified and ancient nav system Fitted to the Last Plane to Transcentral? Mar 16, 2020 at 14:00
• @Graham: It's Trancentral, as in Trance music. But yes, and its departure time is 3 am. Mar 17, 2020 at 9:09
• @PeterCordes Oops, typo. :) All aboard, all aboard... Mar 17, 2020 at 11:43
• Transcendental 🤔 Mar 17, 2020 at 18:59
• In other words, when DME was invented, avionics didn't have computers. And now it's irrelevant. Aug 11, 2021 at 18:29