DME Slant Range

[hgrdrro]

Hi, I have a question regarding the DME distances calculated in FIF.
Are these distances the slant range as classic DME devices show (Altitude, if exactly above the DME antenna) or the real distance to the waypoint, airfield or naviagtion aid?
Best Regards Hans

[RogerF]

All GPS applications that I used so far show the horizontal distance.

You can test it in FIF using the simulation feature: you will see that whatever the altitude is, the DME shows zero when you overfly the point.

Actually, except for didactical purpose, it does not make much sense to program the slant in a GPS application, as this is rather an unwanted side effect of the DME technology.

[hgrdrro]

Thank's for your comment. I do not agree in all points. Because, if flying practice ILS approaches VFR according to the official IFR charts the shown distances are all based upon the slant distance to the DME-locator. But due to the fact that the difference between horizontal and slant distance is getting more and more the same as more I approach the runway and we all FIF users are flying VFR, it makes no real difference at all.
Best Regards Hans

[RogerF]

Correct indeed.

[RogerF]

Concerning the use of GPS DME vs standard DME equipment, I found this information on the http://www.boldmethod.com training web site:

GPS DME vs. Traditional DME

Thanks to GPS, pilots are using traditional DME less and less. If you're flying IFR with an approved GPS, you can use GPS distance to substitute for DME.
For instance, if you're flying on an ILS and the DME for the FAF is 5 miles from the LOC antenna, and you're 2 miles away from the fix, your GPS DME will give you a 2 mile distance to the FAF.
If you want GPS DME to match traditional DME on an ILS approach, you could type in "Direct IXXX" to get distances that match your approach chart.
We don't recommend you do it, however, because you might end up placing a direct-to line on your map, leading to confusion about which course you're following to the runway.

[kitercuda]

I think this is a good point of view I did not realize it. Now FIF DME shows only horizontal part of the distance. If you want to have "real" DME it can be implemented.

There is one question -The 3 letters abbreviation for displaying. Slant distance should be DME and GPS horizontal distance should be ...?
99% FIF users do not need slant distance and if I change "FIF DME" to for example DST (distance) they can be confused.

[RogerF]

All other applications also show horizontal distance in their DME display.
I would not recommend to change FIF to match a limitation of the actual DME system...