GPS for DME?

[52Vincent]

OK, I think I may have put to much thought into this (hence the reason for this post).

While flying an approach that requires DME, and DME is not in the airplane, can you use the GPS in lieu of it? The reason I ask is simple; if DME is used it measures the distance to the end of the runway on the localizer frequency (Correct?). If you use GPS where is the distance calculated from? I would think, the lat. and long. coordinates progammed in the software.

Another way of looking at it; is the distance in the "DIRECT TO" function the same as the distance in the "APPROACH MODE"?

Thanks ahead of time for any clarification.

 

[TonyC]

Originally posted by 52Vincent
if DME is used it measures the distance to the end of the runway on the localizer frequency (Correct?).

If you're talking about the DME associated with a VOR or a TACAN, you can rest assured that the location is the same.

If you're talking about the DME associated with an ILS, you have no idea where it is. It might be at the approach end, or the departure end, or somewhere in between.

 

[pilotman2105]

OK, I think I may have put to much thought into this (hence the reason for this post).

While flying an approach that requires DME, and DME is not in the airplane, can you use the GPS in lieu of it? The reason I ask is simple; if DME is used it measures the distance to the end of the runway on the localizer frequency (Correct?). If you use GPS where is the distance calculated from? I would think, the lat. and long. coordinates progammed in the software.

Another way of looking at it; is the distance in the "DIRECT TO" function the same as the distance in the "APPROACH MODE"?

Thanks ahead of time for any clarification.

Read 1-1-20 in the AIM. There are several sporatic sections referencing DME distance versus GPS distance. In some places it says go for it but be careful of the difference in distances (which you eluded to in your post), but there are other sections that say "no-go." It just depends on the situation I guess.

Read it for yourself and draw your own conclusions. I don't have time to read through 10 pages worth of AIM at the moment.

 

[CDVdriver]

GPS vs DME

DME is obviously colocated with the VOR.
GPS distance is taken from the ARP(=airfield reference point)
which for all GPS purposes is the location of the airport.
It's usually at the intersection of two runways or in the case of a single runway somewhere halfway.
So even if the VOR is located on the airport the difference between DME and GPS can still be in excess of half a mile.

 

[CDVdriver]

Couldn't edit the previous post for some reason,
so you can only use an IFR approved GPS in lieu of a DME.
Get the GPS set up for the approach, even if it's only a monitoring function, and it will display distance to RWY threshold.

 

[Jeff Helgeson]

Use what is on the published approach!

Most GPS units in GA air craft is VFR only. Very few are approved and maintained for IFR use.

Take the time to learne how to use the basic instruments, VOR, DME, NDB/ADF, etc., as well as using the GPS. Many aspiring pilot become "automation slaves" to one magic box and lack basic IFR skills.

The previous posts have also made some valid points.

 

[Jeff Helgeson]

My spell check is on MEL

LOL

 

[A Squared]

It is correct that the distance to the airport that the GPS gives will not be the same as the distance you get when the DME is tuned to the ILS. IT may be off by as much as a mile.


However, you *may* be able to get this distance from your GPS, depending on whether the location of hte localizer DME is in youre GPS database. Try plugging in the identifier for the ILS. If this identifier is in your database, then the distance you get from the GPS will be essentially the same as the DME when tuned to the ILS. I say "essentially" because the GPS gives you a horizontal distance while the DME gives you a slant range. Generally, on an approach, the differnece between the two is inconsequential.

 

[Mmmmmm Burritos]

Yes you can use GPS in lieu of DME as long as you

1) have an IFR certitifed and GPS and the database if current. You absolutely cannot use a handheld GPS

2) set the GPS to the navaid where the DME is based off of. DME will never be based off an airport, doesnt matter if the VOr is based on the field. If it's based off a VOR, set the VOR identifier in the GPS. If it's based off a localizer, set the localizer identification into the GPS. If neither one of them come up, then it is not in the database and it cannot be used. You CANNOT put in your own lat/long coordinates.

 

[ThunderRun]

If your GPS is IFR certified for your aircraft (with a current database), you most certainly can use the GPS in lieu of DME.

Most IFR certifed GPS units include not only VORs in their database but also localizers. Whenever you select a VOR or LOC, the GPS gives you the distance to the nav aid. The only difference from DME distances would be the GPS distances are straight-line not slant, which would be neglible at the altitudes used for approaches. You would have to make allowances for distance differences at higher altitudes.

 

[TonyC]

However, you *may* be able to get this distance from your GPS, depending on whether the location of hte localizer DME is in youre GPS database. Try plugging in the identifier for the ILS. If this identifier is in your database, then the distance you get from the GPS will be essentially the same as the DME when tuned to the ILS. I say "essentially" because the GPS gives you a horizontal distance while the DME gives you a slant range. Generally, on an approach, the differnece between the two is inconsequential.

The problem with this idea is the possibility of there being multiple waypoints with the same identifier, even at the same airport.

Allow me to explain:

If you can reference approach plates for Indianapolis (IND) for the ILS Rwy 5L and the ILS Rwy 23R, you will observe that the Frequency for both approaches is the same. On this frequency, then, are:

- the localizer antenna for 23R (located at the departure end of 23R (SW end))

- the glideslope antenna for 23R (located near the approach end of 23R (NE end))

- the localizer antenna for 05L (located at the departure end of 05L (NE end))

- the glideslope antenna for 05L (located near the approach end of 05L (SW end)

- the DME antenna - - where is it???

(Of course, only 3 of these will be transmitting at any one time, but the locations remain the same.)

Well, according to the Jeppesen chart depictions, the DME is located at the NE end of the runway, near the localizer antenna for 05L. (According to the chart for the ILS 23R, it's 0.2 NM NE of the approach end.)

Got all that so far? OK. Now, let's try to use this information in a practical way. Take a look at the INDY TWO DEPARTURE, and let's consider a takeoff on Runway 23R. Departure Instructions require flying a particular heading upon reaching IUZK D2.6, that is, the RWY 23R ILS-DME.

If I tune 111.75 in the radio, I'll get the DME off IUZK, no questions asked. But if I pull up a waypoint called IUZK, a different thing happens. First (in my database, anyway, I'm assuming other GPS databases are similar) I am taken to a list of duplicate waypoints. There are 2 listed under IUZK. Which is which?

Well, as it turns out, one is the ILS-DME, and the other is the localizer antenna. Why, you ask, does that matter? Well, if I choose the incorrect one, the turn I would initiate on the departure would be delayed by about 2.1 NM - - the length of the runway, plus that 0.2 NM. 2 miles on a jet route might be acceptable, but it's far from acceptable in this environment.

Perhaps a more stirring example could be given using the ILS Rwy 23R itself. Now, this one does not require DME, but let's pretend it does. Let's also pretend you're established on the localizer just inside CLAPR (you think) at 4,000'. I said "you think", because you're using your GPS waypoint based on IUZK. Let's also pretend the Glideslope has just been NOTAMed OTS. So, you're using localizer only procedures, no biggie. You'll begin your descent at 9.9DME from IUZK. Then you'll have 4.3 NM to lose 1,300' and arrive at your FAF. (Does it occur to you that your distances might be off by 2.1NM? Not hazardous, so far, because the error, if you made it, would only mean you're descending late - - no risk of hitting any obstacles, anyway.)

So, you get to the FAF - WINOT (just because? ) - and begin your descent to the LOC (GS out) MDA - - you've got 1,520' to lose in 5.8 NM... oh, wait - - forgot to subtract out the VDP... hmmm I hate when I get busy right here, I'll have to start planning ahead better - - next New Year's, for sure - - OK, quick public math here, 397' HAT for this MDA, that means the VDP should be about 1.3 NM from the runway, OK, hmmm... WINOT to IUZK DME is 5.6, and IUZK DME to the runway is another 0.2... add those together, WINOT to MAP is 5.8. OK, now subtract 1.3 from 5.8, that gives me 4.5NM to descend, and I've just used up 2 of 'em... I'm behind, better ease the nose over... and, you know it's my luck, the ceiling is right above the MDA... I'll break out with just seconds to acquire the runway... what was that VVI I wanted, anyhow? Oh, well, we'll see what happens. Besides, this thing will stop on a dime, no big deal if I land long...

By the way, back there on final (before you got to WINOT) tower called you to see if you were up on the frequency (they had that tone in their voice like "Why have you waited so long to call?") and then they cleared you to land. If you hadn't been so busy you might have recognized a little flag pointing to trouble. Anyway, now you're starting to make out some features on the ground, and you realize it's the runway.... but the wrong end of it. In disbelief, you stare at the fixed distance markers for the departure end disappear under the nose, and you finally convince yourself you've got to go missed. Being as you didn't study THAT procedure at length, you're forced to go back to the approach plate and figure out where you're supposed to be going. You're showing 0.2 on your distance to IUZK now, which in your mind means you ought to be at the approach end, and the procedure says the MAP is 0.2 NM beyond that. You're really confused, but you force yourself to read the MAP procedure - - OH, it's turn right direct Brickyard (VHP) VOR and hold. Gee, I hope it's not a problem making this 120 degree right turn 2 miles late. I guess I better get on the radio and called the missed.

...

Rather than explain how you might be able to determine which IUZK is which, let me leave it at this. Using a GPS waypoint in lieu of an ILS-DME is risky at best, and more likely just downright dangerous. (Notice I didn't even comment on legality - - I have no idea abou that.)

(I apologize for not linking the free approach pages - - I seem to have misplaced that link. If someone can remind me of the link, I'll update this post with appropriate links to procedures.)

 

[A Squared]

Tony,


Interesting, I wasn't aware that some databases had both the DME and the localizer antenna location. THe one I use has only the DME antenna location. If you enter the identifier for an ILS with a DME you get one option, the DME antenna, if you enter the identifier for an ILS with no DME, there is no waypoint. The times I have dialed up the ILS in the GPS, it matched the DME perfectly

Using a GPS waypoint in lieu of an ILS-DME is risky at best, and more likely just downright dangerous. (Notice I didn't even comment on legality - - I have no idea abou that.)

I have to say that I don't agree with your characterization of "downright dangerous". Yeah there is a gotcha, if you're really not paying attention, but then there's a bunch of those in aviation.

My question is, when you enter the ILS identifier, what do you get on the GPS display? I would assume that it alerts you that there are duplicate identifiers and won't proceed untill you have chosen one of them. I would further assume that one would be clearly labled as the DME antenna location and the other would be labled as the Localizer antenna location. Again, I'm assuming, I don't know, my GPS only gives you the DME antenna, but if that assumption is correct, it would be hard to imagine why someone would not chose the DME antenna when thier intent is to use the GPS in place of DME.

Sure, it might be possible to make the wrong selection when you're way behind the power curve and overloaded on an approach, but there are plenty of other mistakes you might make under the same circumstances which would also end in disaster. That doesn't make instrument flying in general downright dangerous. As for the legality, If recall correctly, hte AIM says hte GPS may be used in lieu of DME, it doesn't specify only DME located at a VOR, so I would have to assume that an ILS DME would be legal also.

 

[TonyC]

Interesting, I wasn't aware that some databases had both the DME and the localizer antenna location. THe one I use has only the DME antenna location. If you enter the identifier for an ILS with a DME you get one option, the DME antenna, if you enter the identifier for an ILS with no DME, there is no waypoint.

...

My question is, when you enter the ILS identifier, what do you get on the GPS display? I would assume that it alerts you that there are duplicate identifiers and won't proceed untill you have chosen one of them. I would further assume that one would be clearly labled as the DME antenna location and the other would be labled as the Localizer antenna location. Again, I'm assuming, I don't know, my GPS only gives you the DME antenna, but if that assumption is correct, it would be hard to imagine why someone would not chose the DME antenna when thier intent is to use the GPS in place of DME.

I must admit I was assuming a bit too; I was assuming some commonalities between GPS databases and FMS databases that might not actually exist. My mistake.

I can speak about the system I know best - - and I'll leave it at that. When I enter the idnetifier for the ILS DME in question into my FMS, it brings up a page called DUPLICATE NAMES. It lists the idnetifier twice, makes no distinction about which is which, except that the frequency is listed beside one. The ORDER in which they appear on the page is based on PROXIMITY to the airplane, NOT type of waypoint or facility.

I have to say that I don't agree with your characterization of "downright dangerous". Yeah there is a gotcha, if you're really not paying attention, but then there's a bunch of those in aviation.

Perhaps it's a bit overstated or dramatic, but not excessively. Refer to the departure procedure I described above, and put rising terrain or obstacles just beyond the INTENDED turn point. Delaying the turn by the difference in distance between the localizer antenna and the DME antenna could be risky.



If indeed all GPS systems (purely GPS, mind you) contain nothing but the ILS DME locations, then I suppose my observations are irrelevant. I may have erred in assuming some of the features of this FMS are representative of a GPS system.

 

[52Vincent]

Thanks for all the input, it sounds like the best option is to load the approach into the unit ( if DME is required) and use the distance displayed for DME. This should assure me the distance being displayed is for the LOC or ILS (or whatever approach is chosen) and not from somewhere else on the airport.

I can see how you could set yourself up for disaster at a large airport with a lot of square mileage if you fly, say, a LOC approach(DME req.) and the distance selected in the GPS is the distance chosen from the "direct to" feature, not the localizer approach. These positions could feasibly be a mile or more apart, setting up a recipe for disaster.

I have also spent some time refering to the AIM (1-1-21-f7) as referenced by "pilotman", and it reads, when flying a particular procedure, such as an ARC, you must select the facility providing the ARC from the airborne database as the active GPS Waypoint.

"THE ONLY ACCEPTABLE FACILITY IS THE DME FACILITY ON WHICH THE ARC IS BASED. IF THIS FACILITY IS NOT IN YOUR AIRBORNE DATABASE, YOU ARE NOT AUTHORIZED TO PERFORM THIS OPERATION".

 

[mizzouguy]

If you use the GPS for DME on an ILS you must put the GPS direct to the ILS identifier such as I-JQX. And where did Jeff get his info. Most GPS's are IFR certified. I haven't seen too many panel mounted ones any more that arent.

 

[52Vincent]

mizzouguy If you use the GPS for DME on an ILS you must put the GPS direct to the ILS identifier such as I-JQX. And where did Jeff get his info. Most GPS's are IFR certified. I haven't seen too many panel mounted ones any more that arent.

Is there a difference between loading the ILS identifier into the GPS or loading the approach? Would not the DME information be the same?

 

[Thedude]

In short, no you can not use GPS to substitue ILS DME or VOR for that matter.

GPS dist while in the arrival area of an airport is a great tool for situational refrence esp when no DME is available from your NAV-aid

All GPS that are panel mounted are not IFR Certified. That costs extra and is specfic to that aircraft.

Not all GPS will accept I-xxx for a waypoint

 

[52Vincent]

In short, no you can not use GPS to substitue ILS DME or VOR for that matter.

GPS dist while in the arrival area of an airport is a great tool for situational refrence esp when no DME is available from your NAV-aid

All GPS that are panel mounted are not IFR Certified. That costs extra and is specfic to that aircraft.

Not all GPS will accept I-xxx for a waypoint

This is confusing, simply due to the fact that most of the new 172's come equipped with GPS and no DME, therefore flying any approach with DME required is not possible in most 172 SP models, providing the GPS is not IFR certified "I guess????"
When programming the GPS for some precision and non-precision approaches you recieve the reminder indicating that GPS guidance on approaches is strictly for monitoring only-use the VLOC recivers and external CDI (or HSI) for primary navigation.
Does this mean I cannot use the GPS for DME information or just that I cannot monitor the approach on the GPS? If thats the case what do I use for DME?

 

[Singlecoil]

To say the GPS is or is not IFR certified is not quite the whole picture. There are 3 different levels of IFR certification: IFR enroute/terminal, IFR remote/oceanic, and IFR enroute/terminal/approach. It sounds like the 172 has an IFR enroute/terminal installation if it is giving you a warning when you load an approach. However, the AIM clearly states that this installation is legal to use for DME/ADF, that is why there is no other DME or ADF installed.

It was stated above that you have to pull the fix from the database, and the database must be current. However, if the fix is not in the database, the AIM gives you guidance to use one that is, provided it is along the same azimuth from the station.



From the AIM:

(1) To determine the aircraft position over a DME fix:

[a] Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

If the fix is identified by a five letter name which is contained in the GPS airborne database, you may select either the named fix as the active GPS waypoint (WP) or the facility establishing the DME fix as the active GPS WP.

NOTE-
When using a facility as the active WP, the only acceptable facility is the DME facility which is charted as the one used to establish the DME fix. If this facility is not in your airborne database, you are not authorized to use a facility WP for this operation.

[c] If the fix is identified by a five letter name which is not contained in the GPS airborne database, or if the fix is not named, you must select the facility establishing the DME fix or another named DME fix as the active GPS WP.

NOTE-
An alternative, until all DME sources are in the database, is using a named DME fix as the active waypoint to identify unnamed DME fixes on the same course and from the same DME source as the active waypoint.

CAUTION-
Pilots should be extremely careful to ensure that correct distance measurements are used when utilizing this interim method. It is strongly recommended that pilots review distances for DME fixing during preflight preparation.

[d] If you select the named fix as your active GPS WP, you are over the fix when the GPS system indicates you are at the active WP.

[e] If you select the DME providing facility as the active GPS WP, you are over the fix when the GPS distance from the active WP equals the charted DME value and you are on the appropriate bearing or course.

Link to the AIM