GPS and DME are two distinct systems that calculate two different types of distances. GPS calculates your distance across the ground, while DME measures slant range.
GPS (Ground) Distance
Your GPS display shows the lat/long distance measured between two points, and in most cases, it can be used for IFR flying.
But to use GPS for instrument flight, you need a TSO-C129, TSO-C196, TSO-C145, or TSO-C146 compliant GPS (AC 20-138). The FAA refers to these as “suitable RNAV systems.”
Assuming you have a compliant device, you’ll be able to use it to navigate to and from navaids, airports, and more during all flight phases (more on this later).
Slant Range (DME)
Distance Measuring Equipment (DME) is an analog navigational system that uses UHF and VHF radio frequencies to determine your distance to a point in space. So what type of distance is DME measuring?
DME measures slant range. Slant range is the distance of the hypothetical line you could draw between your aircraft and a DME station. More often than not, DME slant distance is slightly greater than your planned ground distance.
A good example of this is if you were to fly over a DME station at 6,000 feet MSL, your DME equipment would read: 1 mile, while your GPS distance would read: 0 miles.
Using GPS Distance as a Substitute for DME
While there can be a slight difference between DME and GPS distance, the FAA says in AIM paragraph 1-2-3 that “Subject to the operating requirements, operators may use a suitable RNAV system in the following ways.” (see the graphic below)
Additionally, you can fly a DME arc with RNAV as a substitute as well.
AIM 1-2-3 goes on to talk about other RNAV substitutions, but for the purposes of this article, we’ll focus on DME.
If you choose to substitute your DME distance, you should ensure your GPS navigation database is up-to-date. Additionally, if you are using GPS distance or navigation as an alternate means of navigation to an inoperative navigation aid, you might need to coordinate your actions with ATC.
GPS Distances on ‘DME required’ Approaches
Seeing the words ‘DME required’ on an approach chart might seem like a problem if your airplane doesn’t have DME onboard, but it doesn’t have to be. Note 1 under AIM Paragraph 1-2-3 continues to say “The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).”
Let’s take a look at the notes sections for the Thief River Falls Airport (KTVF), ILS approach for runway 31 in Minnesota.
You’ll notice under the approach notes section that DME is listed as a requirement. But in this case, your suitable RNAV equipment on board will allow you to fly this approach without traditional analog DME equipment.
This type of substitution makes sense, because most new aircraft being built with glass panels no longer come with DME radios installed. This is just one more example of how RNAV is changing IFR flying, making it easier to fly in and around airports for a wide variety of aircraft equipment types.
