GPS distance VS DME distance

[Flysher]

I have a student that has his own plane, with both IFR GPS and DME available. On an approach that requires DME, they are slightly different due to the slant range/GPS method of measuring. Are they both considered correct in this case??

Heres an approach as an example.

http://204.108.4.16/d-tpp/0602/06883VA.PDF

GPS says 8.1 and DME says 8.2 at the MAP

 

[satpak77]

both are physical distance from the station or waypoint. "Slant range" is just a way to remember that as the airplane nears the station, if its high enough, say 12,000 MSL and the VOR in question is at sea level, that "2.0 DME" may actually mean you are physically on top of it, since one NAUTICAL MILE is 6076 feet.

DME and GPS measure actual physical distance, GPS is more accurate in measuring this physical distance.

Think of it that way, "actual physical distance"

your approach plate in question is a VOR approach only, technically you cannot use GPS distance, as it is not "Distance Measuring Equipment"/DME

you could use it as "advisory info" to cross check the VOR/DME but on this approach when you reach VOR-DME MAP, you go missed, period, end of story.

 

[A Squared]

OK, here's the deal on GPS vs DME distances:

It doesn't matter.

Everyone gets wrapped around the axle about slant range vs horizontal distance, but few actually sit down and figure out what the real difference is.

OK for this approach here's the real difference between the slant range and the horizontal distance. FOllow me through here. At the MAP at MDA you are at 1160 ft. now as nearly as I can tell, the VORTAC is at about 1200 ft, so the difference is zero, zip, nada. Even if the VORTAC was at sea level, the difference between the distance measured horizonatally and the distance measured on a slant is 13.5 feet. So, if you're at MDA, the difference you see is *not* due to the difference between slant range and horizontal distance. It's coming from somewhere else. Guarenteed.

OK, so you have a DME reading of 8.2 and a GPS reading of 8.1 NM when you are over the runway, and you're wondering where the differnece is coming from.

Well to begin with, DME ain't all that accurate. The TERPS assigns DME fixes an accuracy of 0.5 nm. I personally have seen DME readouts bounce around 2-3 tenths while I was on the ground, standing still, at a location with good signal coverage (almost but not quite direct line of sight)

GPS is generally much more accurate than DME. Stand alone GPS receivers typically have accuracies of better than 100 meters.

Also consider this, how far apart are the distances displayed, really? For all you know the DME is measuring 8.16 which would round to 8.2 for display, while the GPS is measureing 8.14, which would round to 8.1. but really they difference is 0.02 nm, which ain't much.

 

[schoolio]

deleted

 

[DC8 Flyer]

Nvfm.

 

[Flysher]

both are physical distance from the station or waypoint. "Slant range" is just a way to remember that as the airplane nears the station, if its high enough, say 12,000 MSL and the VOR in question is at sea level, that "2.0 DME" may actually mean you are physically on top of it, since one NAUTICAL MILE is 6076 feet.

DME and GPS measure actual physical distance, GPS is more accurate in measuring this physical distance.

Think of it that way, "actual physical distance"

your approach plate in question is a VOR approach only, technically you cannot use GPS distance, as it is not "Distance Measuring Equipment"/DME

you could use it as "advisory info" to cross check the VOR/DME but on this approach when you reach VOR-DME MAP, you go missed, period, end of story.

I was under the impression that you could use IFR gps as DME substitute as long as the waypoints were all in the database and current??

 

[b350capt]

SatPak is correct. According to the AIM 1-1-19(e)(3), the chart above it in the AIM, and section (f)(1)....only approaches designated as GPS overlay approaches, those whose titles now contain the phrase "or GPS" are authorized to have GPS distance substituded for DME. You are correct in that the database does have to be current for IFR approaches and use in lieu of ADF/DME. You are also allowed to substitute GPS for ADF or DME in the following cases:
Determining position over DME fix above FL240 (satisfies 91.205(e)
Flying a DME arc (if on approach, only for overlay approaches)
Navigating to/from NDB/LOM
Determining position over NDB/LOM
Determining position over fix defined by NDB/LOM and VOR/LOC course
Holding over NDB/LOM
*Theses procedures do not alter the conditions and requirements for existing

 

[b350capt]

approaches as defined in the overlay program......(sorry, sent the first post on by accident)
Hope that clears some things up.

 

[midlifeflyer]

SatPak is correct. According to the AIM 1-1-19(e)(3), the chart above it in the AIM, and section (f)(1)....only approaches designated as GPS overlay approaches, those whose titles now contain the phrase "or GPS" are authorized to have GPS distance substituded for DME.

I don't think that's correct. The overlay program just means that, for example, you can't fly a VOR or NDB approach using GPS as your primary form of navigation without the "or GPS" in the title.

I think 1-1-19.f makes it clear that you can otherwise "use GPS equipment certified for IFR operations in place of ADF and/or DME equipment for en route and terminal operations."

Notice that f.1. says in part that "The ground-based NDB or DME facility may be temporarily out of service during these operations." which makes no sense without the substitution.

Most of the examples in the AIM make no sense without the substitution being legit.

I'm pretty sure Flyfisher is fine flying the approach using GPS to identify the waypoint.

 

[b350capt]

you are correct, however.....en route and terminal operations do NOT cover approaches. Therein lies the difference. Approach approval and terminal ops approval are not the same thing