DME Flashcards
What is a DME?
Distance Measuring Equipment
A ground based unit that measures your distance to a station in nauticle miles.
Slowly being phased out because of GPS, like pretty much everything else
What three ways can a DME receieve distance information?
From a VORTAC, TACAN or DME installation
What does a DME look like on a map?
Four sided box. The one in the picture below is paired with the Fort Smith VOR on 112.4. If I have a DME in my AC, it will automatically recognize it.
DME paired with an NDB on a Lo Chart
What is important to remember about DMEs that are paired with VORs or NDBs?
If a DME is paired with a VOR you only have to dial in to one station for info from both, as they are able to share a frequency.
If a DME is paired with an NDB, they will be on two seperate frequecies, both of which you will have to dial in to
What does every VORTAC have? (A VOR that also gives distance info via a TACAN)
A paired DME. You will only need to tune in to one frequency for both
Why do you need to tell ATC if you are using a DME for distances?
Because a DME uses slant range and GPS uses distance across the ground. There are differences between the two.
The closer and higher to the station, the greater your slant range becomes and the greater the difference between a GPS position
What is the slant range?
The distance between your position in the sky to the spot on the ground where the DME is.
How do you convert slant range to actual distance over the ground?
- If your ALT is given in ALS, convert it to AGL first (by subtracting the height of the station)
- Convert your hight AGL in feet to NM by dividing 6080 into whatver the altitude is.
- Then use pythagoras triangle equation
What frequency band does the DME operate in
UHF (UHFs are only line of sight)
What does the DME unit look like in the cockpit?
What is the positional error of DMEs?
+/- 0.5NM or 3% of the distance, whichever is greater (as of late 2020)
This may come up on the exam as something to do with allowable error at a great distance
Another, more advanced DME in the cockpit
What is important to remember about the GS given on a DME?
It will only be accurate if we are travelling directly to or from the ground station, because the computer is calculating the delay between the pulsed signals from the station to the AC.
First signal goes out and you are a dist from the station, next pulse goes out and you are a little closer. This translates as how fast you moved between the two points of each signal.
So if you fly directly perpendicular, or in a perfect arc/cicrle to a DME, it will read your GS as zero because you are not getting any closer to the DME.
The dist to the station given on a DME is expressed as…
A slant angle in NM
Why in this picture would the DME in your cockpit tell you you are 1NM out from the DME station when you are actually overtop of it at 6100 feet?
Because 6100f is basically 1NM (6080), then the computer would translate that as a 1NM slant range, because slant is all it can measure.
