FLYER Forums

Paul_Sengupta wrote:I don't think anyone's saying that GPS is inaccurate, it's more that it's easy to interfere with or jam and if that happens you have no way of flying an accurate missed approach.

Unlikely to happen in the UK of course, but there have been occurences of aircraft in the USA carrying out instrument letdowns in hilly or mountainous areas and losing the GPS signal as multiple satellites become shadowed by terrain. As Paul says, the GPS signal received is inherently weak as it comes so far so it doesn't take a lot to interfere with it especially as it will get weaker the lower you go.

chevvron wrote:

Paul_Sengupta wrote:I don't think anyone's saying that GPS is inaccurate, it's more that it's easy to interfere with or jam and if that happens you have no way of flying an accurate missed approach.

Unlikely to happen in the UK of course, but there have been occurences of aircraft in the USA carrying out instrument letdowns in hilly or mountainous areas and losing the GPS signal as multiple satellites become shadowed by terrain. As Paul says, the GPS signal received is inherently weak as it comes so far so it doesn't take a lot to interfere with it especially as it will get weaker the lower you go.

Although the way it is encoded makes it much easier to detect and process than a VOR or NDB signal.

I would be interested to see any actual example of terrain masking of a GPS signal in the approach area. A quick search finds very few examples of loss of signal not associated with user error or NOTAMed issues. (Interestingly, I did find an example of a CAT aircraft that rolled before allowing its GPS to lock on and filed an ASR as a result of the DME/DME position being not sufficiently accurate to fly their RNAV departure.)

IMHO the CAA position on GPS has been shockingly Head in Sand for many years and even still seems to require an overwhelming pile of paper to allow people to officially move from using the NDB (which is manifestly inaccurate) to using certified GPS, which either works, or doesn't work and knows/Annunciates this fact. As a point of interest, in one of the few NDB approaches I have flown outside training, I flew it 'perfectly' (needle dead on track) and realized (when my GPS showed the NDB passing underneath me and the needle still pointing forward) that my ADF had chosen those few moments to have its needle drive motor fail (and that I could have flown any heading and would have been on track)!. The ADF system provides no indication of such a failure until one had reason to turn and see the needle not move.

if you fly the approach according to the plate who would ever know that you're using GPS instead of ADF or DME? Just get on with it...

They will know because your track will resemble the one on the plate. I was told off once at Birmingham for not accurately flying the NDB approach whenever I was spot on according to the needles. I was, however some way off and different to those of everyone else who was using GPS

About the only ADF let downs where you don't have to manoeuvre much after becoming visual occur in simulators where the programmers haven't been told how inaccurate the system really is, hence the very high limits.

for dme substitution note GPS does not use slant range which might be an issue if close and high

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Its approved by the FAA with the thousands of GNSS GPS/LPV in use in for some years USA. If there was a safety case with accidents caused by GPS distance vs DME (comparing same geo. position of course ) we would have heard about it by now. Same goes for using GPS for ADF overlay letdown.
End of story!

Has there been any progress on this?

It would be eminently sensible and pragmatic to allow ADF to be replaced by certified IFR GPS. I hope this happens, even if dual GPSs are mandated or similar.

I hope that no-one is seriously proposing the requirement to have two IFR-certified GPS units just to fly an approach given that there are loads of NDB-based approaches out there, and even a single tablet-based GPS on an average day outperforms any ADF on a good day!

"even if dual GPSs are mandated or similar."

Ironic. " Let's consider gold-plating the solution to gold-plating"

There are two things here:
1 In case of a general GPS failure or jamming, the need of an alternate way of flying a missed approach ... and landing elsewhere. There can be a case by case solution (Radar vectoring where available, or simply accept the risk for low traffic airports for private flights etc.), it's seems to me that it's a real issue only for commercial flights.

2 The distance you read on your DME differs from the distance you read on your GPS for two reasons:
- The slant distance. This is a simple trigonometric problem, and most of the time the precision required is larger than the difference. A simple workaround would be to publish an additional plate with GPS distance so that the user can choose.
- The actual coordinates of the DME antenna is not publised. This is a real issue for precision approaches as ... precision is required. If your DME is down, you have no way to fly an ILS DME approach unless your GPS allows an overlay, but I don't think it's legal. A simple workaround seems possible to me (publish the actual location of the antenna and required GPS distance to that point)

172510 wrote:2 The distance you read on your DME differs from the distance you read on your GPS for two reasons:
- The actual coordinates of the DME antenna is not publised. This is a real issue for precision approaches as ... precision is required. If your DME is down, you have no way to fly an ILS DME approach unless your GPS allows an overlay, but I don't think it's legal. A simple workaround seems possible to me (publish the actual location of the antenna and required GPS distance to that point)

Minor point - most DME associated with ILS have their internal delay set so the cockpit readout shows zero at the runway threshold. It is also often settable for the runway in use.

So it's the runway threshold coordinates that are needed, not those of the DME antenna.

User72 wrote:CAA GA Unit will be looking into permitting the use of GPS in lieu of ADF & DME in the New Year - so said Tony Rapson today.

Has Tony followed up with the results, or was he providing incorrect information?

Katamarino wrote:

User72 wrote:CAA GA Unit will be looking into permitting the use of GPS in lieu of ADF & DME in the New Year - so said Tony Rapson today.

Has Tony followed up with the results, or was he providing incorrect information?

Give it another 6 months and we are only 20 years behind the announcement in the first post!

highfive wrote:

172510 wrote:2 The distance you read on your DME differs from the distance you read on your GPS for two reasons:
- The actual coordinates of the DME antenna is not publised. This is a real issue for precision approaches as ... precision is required. If your DME is down, you have no way to fly an ILS DME approach unless your GPS allows an overlay, but I don't think it's legal. A simple workaround seems possible to me (publish the actual location of the antenna and required GPS distance to that point)

Minor point - most DME associated with ILS have their internal delay set so the cockpit readout shows zero at the runway threshold. It is also often settable for the runway in use.

So it's the runway threshold coordinates that are needed, not those of the DME antenna.

The ILS at Dunsfold was LOC/GP only with no DME. Dunsfold ATC had permission to use it for company aircraft only when the radar was serviceable because they had to pass range checks to the pilot.
The DME at Farnborough is positioned midway between the two thresholds and is 'zeroed' at the thresholds, so if you're using it for the noise abatement departure procedures, you MUST take the reading from the second 'zero' point not the first.