# 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. – StephenS Mar 15 '20 at 17:05
• This simply is not mathematically possible (as well as being not KISS, and not in number domain). – Fattie Mar 16 '20 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? – Bob Jarvis - Reinstate Monica Mar 16 '20 at 19:24

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. – abelenky Mar 15 '20 at 13:35
• @abelenky Broadcast how, to what? That'd be an entirely different standard. – Dan Mar 15 '20 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. – HiddenWindshield Mar 15 '20 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. – Peter Cordes Mar 16 '20 at 10:48
• When are the ground stations being replaced @PeterCordes? Seriously, nobody is going to spend money on that. – GdD Mar 17 '20 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? – Graham Mar 16 '20 at 14:00
• @Graham: It's Trancentral, as in Trance music. But yes, and its departure time is 3 am. – Peter Cordes Mar 17 '20 at 9:09
• @PeterCordes Oops, typo. :) All aboard, all aboard... – Graham Mar 17 '20 at 11:43
• Transcendental 🤔 – Jpe61 Mar 17 '20 at 18:59