DME from GPS question

[Steve]

If you have an aircraft that is IFR certified but does not have DME or a GPS installed, can you use the DME from a portable GPS? I'm thinking no, but hoping im wrong.

 

[Westwindflyer]

Sorry to wreck your hopes but a portable GPS is no good for IFR.

 

[pilotman2105]

No go on the IFR use of the portable GPS for anything. But some controllers will allow you to use VFR GPS in the IFR environment and consider it a radar vector instead of a GPS direct.

Hopefully some of the ATC folks will chime in here and say how they approach that situation.

 

[DAS at 10/250]

No. You cannot use a Hand held GPS or a VFR Panel mount GPS for any IFR function, other than situational awareness. Reference AIM Table 1-1-8, pg. 463 in 2004 ASA AIM.

Only an approved IFR En Route and Terminal, IFR Oceanic/Remote, or an IFR En Route, Terminal, and Approach GPS with a current database may be used in lieu of an ADF and/or DME. Again, AIM Table 1-1-8.

Further, to conduct ANY GPS operation under IFR requires that:

1. The unit itself must meet TSO C-129 or equivalent. The installation must meet AC 20-138 or AC 20-130A. Equipment approved in accordance with TSO C-115a is NOT approved for IFR.

2. Aircraft using GPS nav equipment under IFR must be equipped with an approved and operational alternate means of nav appropriate to the flight.

3. Procedures must be establised for use in the event that the loss of RAIM capability is predicted to occur.

4. GPS operation must be conducted in accordance with the FAA-approved AFM or supplement.

5. File appropriate RNAV suffix.

6. Pilot must review appropriate NOTAMs. Ref. AIM 1-1-20 e. 1. a-g

Also, remember that you cannot file an IFR alternate that only has a GPS approach.

 

[PHX767]

Also, GPS distance is absolute. DME is slant range.

As far as practical use, if you ask for a 301.2 degree vector to destination the controller will probably clear you as requested and be happy you are going in a straight line.

 

[coolyokeluke]

Also if using GPS distance to an airport (say in lieu of DME to a VOR on the field) unless you have it set the navaid it's calibrated to the geographic centerpoint of the facility.

 

[A Squared]

Also, GPS distance is absolute. DME is slant range.

well, this is true ...but, so what? This is often mentioned when the subject of GPS vs DME is mentioned. Does it really matter? lets take a look. As you know the difference will be greatest when you are close to the station at high altitudes.


Let's say that you're enroute at 12,000 ft, worst case scenario, when you're right over the VOR, there's 2 NM difference between GPS and DME .... and the GPS is more accurate. Plus or minus 2 NM for enroute operations is plenty accurate.

How about during approaches? Let's say you're on a 10 mile DME arc, at 6000 feet, kinda high for 10 miles out, but that aside, the difference between slant range(DME) and horizontal distance (GPS) is 0.05 NM, that's right, half of a tenth of a mile. Do you fly your DME arcs within 0.05 nm? Yeah, me too

What about closer in?, say at the outer marker? Typically, that's 5 nm out and aproximately 1500 ft AGL. What's the difference between GPS and DME at the outer marker? 0.006 NM, or approximately 37 feet. Not much difference at all. Remember, when they design approaches, DME is assumed to only be accurate to +/- 0.5 NM, unless you're really high, and right over the station, the difference between GPS horizontal distance and DME slant range is less (much, much less) than the assumed error of DME.

 

[Vector4fun]

No go on the IFR use of the portable GPS for anything. But some controllers will allow you to use VFR GPS in the IFR environment and consider it a radar vector instead of a GPS direct.

Hopefully some of the ATC folks will chime in here and say how they approach that situation.

Correct. We can allow someone to go direct via VFR GPS, vector, or whim, so long as we can maintain radar contact, and the downstream facilities approve.

Having said that, please understand that whether you have a VFR or IFR GPS on board, certain sectors are going to want most everyone on the preferred route, most often a VOR radial or airway. If you filed /A, and the sector seems slow, it usually doesn't hurt to ask for direct. If it's busy, I wouldn't bother. The controller probably doesn't have time to coordinate, and if he/she did, it's likely the next facility would decline.

Also, when I'm busy feeding the finals, it's much simpler and easier to issue vectors to achieve a good feed, than to try to send folks direct to a particular fix on the approach. I get asked about that a lot. Vectors and assigned speeds just generally works better. Even with an IFR GPS, folks going direct sometimes wander a bit. Can't explain why.

The Cessna we bought now has both a King KMD-150 GPS, (VFR) and a KNS-80 RNAV. I know from experience that this GPS is generally more accurate than the RNAV enroute, but only the RNAV is legal for IFR. If the controller offers me direct, I'll set up both, just for backup, but the controller is not going to ask (or care) which one I'm using for primary nav. In fact, I'm using both. Just the same way I can use the RNAV to suppliment the ADF on an NDB approach.

 

[DAS at 10/250]

DME vs. GPS Range

Be careful out west. GPS DME has the potential to be off 41% from DME. A squared + B squared = C squared. NBAA found some approaches (sorry, I don't know which) out west that if using GPS derived DME there was the POSSIBILITY of impacting terrain.

Don't be cheap. Spend the $1300 on a refurb King KNS 80, have GPS, backup RNAV in case of a RAIM alert, and DME.

Be safe. We seem to be ruining a lot of aluminum lately.

 

[FN FAL]

well, this is true ...but, so what? This is often mentioned when the subject of GPS vs DME is mentioned. Does it really matter? lets take a look. As you know the difference will be greatest when you are close to the station at high altitudes.


Let's say that you're enroute at 12,000 ft, worst case scenario, when you're right over the VOR, there's 2 NM difference between GPS and DME .... and the GPS is more accurate. Plus or minus 2 NM for enroute operations is plenty accurate.

How about during approaches? Let's say you're on a 10 mile DME arc, at 6000 feet, kinda high for 10 miles out, but that aside, the difference between slant range(DME) and horizontal distance (GPS) is 0.05 NM, that's right, half of a tenth of a mile. Do you fly your DME arcs within 0.05 nm? Yeah, me too

What about closer in?, say at the outer marker? Typically, that's 5 nm out and aproximately 1500 ft AGL. What's the difference between GPS and DME at the outer marker? 0.006 NM, or approximately 37 feet. Not much difference at all. Remember, when they design approaches, DME is assumed to only be accurate to +/- 0.5 NM, unless you're really high, and right over the station, the difference between GPS horizontal distance and DME slant range is less (much, much less) than the assumed error of DME.

Thanks, my sentiments exactly. I kind of figured that approach slant range distance didn't mean squat...thanks for doing the math.

In an emergency it will do in a pinch.

 

[A Squared]

DME vs. GPS Range

Be careful out west. GPS DME has the potential to be off 41% from DME. A squared + B squared = C squared. NBAA found some approaches (sorry, I don't know which) out west that if using GPS derived DME there was the POSSIBILITY of impacting terrain.

I think you may have misunderstood what the NBAA was saying. In order to have a 41% difference between DME range and GPS range to the same station, you would have to be at a 53 degee elevation angle above the VOR. In other words, if you were 2 NM from the VOR, you would have to be at almost 16,000 ft *above the VOR* (not MSL) in order for the difference between horizontal distance and slant distance to be 41%. Given that maximum descent gradient in an approach is 500 ft/NM, you just aren't going to encounter that sort of range difference. In a previous post, I posted the actual differences you would encounter in a typical approach. They are insignificant. If you diddn'
t see that post you might take a look at it to get a feel for how small the differences really are. I am extremely skeptical that it would be possible to design an approach in which the difference between the GPS and DME range to the same navaid would result in problems.

It may be that they were referring to differences when the gps was giving range to a different location than the DME antenna, which can be a real concern. I would be interested in reading what the NBAA had to say about it, if you have a reference.


By the way, I'm not advocating using non IFR GPS in IFR operations, I'm merely commenting on the mathematical realities of the situation.