GPS Overlay Approach

RNAV, GNSS, GPSS. Your place to discuss all aspects pertaining to Instrument Flying.

Moderator: Moderators

User avatar
Hop Harrigan
Frequent AvComer
Posts: 875
Joined: Tue Jun 01, 2010 7:55 pm

GPS Overlay Approach

Unread post by Hop Harrigan » Sat Mar 10, 2018 2:29 pm

Hi,
I understand that in the US many approach charts offer a ‘GPS Overlay’ on a terrestrial-navaid based approach. My understanding of these overlay approaches is that they make it legal to emulate the Nav aid based approach using position data generated from a GPS receiver. For example, using the Garmin G430/530 ‘Obs Mode’ it becomes legal to execute a VOR or NDB approach using the Garmin’s GPS.
So...is my understanding of the ‘Overlay’ correct and
If it’s so easy why have we not done something similar in SA?
Hop
There is no gravity...the Earth sucks
N1 TO/GA
Preflighting
Posts: 21
Joined: Sun Jun 26, 2016 1:51 pm
Closest Airfield: FAJS

Re: GPS Overlay Approach

Unread post by N1 TO/GA » Sat Mar 10, 2018 3:05 pm

Not familiar with the 430 anymore.

Off the top of my head. I think there has to be reference to the “raw data” from the VOR. So you would potentially need two OBI - indicating both the radial information and the other indicating GPS information. CDI sensitivities also need to be set to the required performance for the approach if available on the plate of 0.3 or 0.5. I stand under correction.
User avatar
Hop Harrigan
Frequent AvComer
Posts: 875
Joined: Tue Jun 01, 2010 7:55 pm

Re: GPS Overlay Approach

Unread post by Hop Harrigan » Sat Mar 10, 2018 5:54 pm

Hi,
AFAIK the Overlay approach is the same as a RNAV approach ie GPS only with no reference to ground based Nav aids.
The difference is that the airport doesn’t need to have a published RNAV approach. The Overlay is chart is overlaid on the navaid approach and is flown using the GPS to emulate the Nav aid approach by simulating the navaid eg VOR or NDB.
A navcom such as the G430 can do this emulation in “OBS mode” or Bearing mode.
Without an Overlaid Approach, it is not legal to use GPS position data to emulate a VOR or NDB approach. Apparently the reason for this is the subtle difference in the behavior of the GPS as opposed to a VOR or NDB. However the Overlay takes these differences into account.
Not that I can comprehend any difference between a pure (highly accurate) GPS position and an NDB when executing a simple NDB approach. VOR, there might be a small argument that the VOR becomes more sensitive the closer one gets to the facility as opposed to a GPS where the sensitivity remains constant.
My question then is...why do we not use Overlay approaches in SA which I believe are widespread in the US.
Sorry for the loong explanation but I don’t know how to put it more concisely.
Cheers
Hop
There is no gravity...the Earth sucks
User avatar
tansg
ATC Delay
Posts: 116
Joined: Thu Mar 19, 2009 8:29 pm
Closest Airfield: OMAA

Re: GPS Overlay Approach

Unread post by tansg » Sun Mar 11, 2018 7:13 am

The methodology used by FAA to create "GPS Overlay" approaches has been shown to be seriously flawed by the Norwegians who during their project to create something similar found that the FAA had assumed that because GPS was generally more accurate than conventional nav-aids that the protection areas for the "GPS Overlay" approaches would be smaller. However when the Norwegians recreated the protection areas for the "GPS Overlay" version of the procedure and compared it to the protection areas for the conventional nav-aids it was found that the "GPS Overlay" protection areas were in many cases larger than that of the conventional nav-aids. This is due to the different ways that the protection areas are designed due to the fundamentally different ways the systems work. They further found that in the majority of the procedures that they evaluated for possible "conversion" to "GPS Overlay" format required a higher minima and thus were considered a viable alternative. This is why RNAV (GNSS) procedures are designed the way they are as this format gives best solution for the way that GNSS works. The Norwegians later also admitted that the whole project had cost them more than if they had just designed RNAV (GNSS) procedures instead. Slim vang sy baas.

So on case you did not follow what I was saying above in simple terms if you want to create a "GPS Overlay" for a procedure a full aviation study needs to be performed by a qualified Instrument Flight Procedure Designer (IFPD) whereby the protection areas for the GPS tracks are created and evaluated against the protection areas of the conventional nav-aid and the obstacle database to verify that the controlling obstacle remains the same or is less for each phase of the approach. If the controlling obstacle is found to be higher than for the conventional approach changes to the heights in the procedure will be required. PS this is simple terms for an IFPD. :)

Again so unless it is specifically noted on the chart that a conventional procedure that "Procedure may be flown with GNSS" then it may not be done as the aviation study has not been performed.

In the end this project of "GPS Overlays" turned out to be a huge waste of time and money, not that the FAA would ever admit to it, as each and every procedure had to be verified. It would have been faster and cheaper just to have designed new procedures for each of those runway ends. Again a poor decision was made for political expediencies (to quicken the roll-out of GPS guided procedures) due to promises made by politicians without consulting the subject matter experts as to the implications.

Not everything is bigger and better in the good ole US of A.
User avatar
Hop Harrigan
Frequent AvComer
Posts: 875
Joined: Tue Jun 01, 2010 7:55 pm

Re: GPS Overlay Approach

Unread post by Hop Harrigan » Sun Mar 11, 2018 12:26 pm

Hi tansg,
It’s really great to have someone with professional knowledge take the time to respond...thank you very much.
But while I’ve got your attention, please can you provide some clarity as to why an accurate GPS position is less suitable than,
a) let’s say an old fashioned NDB. In both cases we have an instrument that can point a needle at a point on the earths surface. So what is it the makes the GPS driven needle inferior to the NDB needle (that in any case is trying to point to the nearest cloud/lightning/ music radio station)?
b) a DME distance to a point.
Much obliged to for your input!
Hop
There is no gravity...the Earth sucks
Plettflyer
Tripped over wheel chock
Posts: 26
Joined: Fri Nov 11, 2016 3:45 pm
Closest Airfield: FAPG

Re: GPS Overlay Approach

Unread post by Plettflyer » Tue Mar 13, 2018 3:48 pm

There is a further aspect to this depending upon the equipment Hop. With the triple FMS (with dual GNSS input) equipment on some of the larger business jets, we are able to fly what we refer to as a Reference Approach. For example, the VOR Rwy 16 in Zurich. The approach must be retrieved from the FMS database. Waypoints, tracks, distances, speed and altitude constraints are verified against the chart.

If everything is correct, the approach may be flown using the FMS as the Nav source (rather than the VOR), however, the VOR bearing pointer must be displayed throughout together with any associated DME, so that the pilot can cross-reference his position with reference to raw data. It goes without saying that the radio aid must be identified.

I am not sure if there is an equivalent procedure for the Garmin Avionics package you use.

It should also be stressed that this is an operational (aircraft/avionics) procedure. As far as the underlying Instrument Approach Procedure is concerned, I defer to the superior knowledge of TANSG.

Return to “Instrument Flying”