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Autopilots which use GPS to fly an ILS

arj1 wrote:

As far I understood some of the materials, because in that case basic mode is more expensive to implement and has to be more rigorously tested on each airframe, which will make it even more expensive.

That makes sense.

LDZA LDVA, Croatia

wigglyamp wrote:

Many GA autopilots are poor at flying a fully coupled approach as they rely on a Middle Marker signal to reduce system sensitivity as you get closer to the runway.

Can you identify any GA autopilots that use the Middle Marker? Also, my experience is the opposite, the problem I see with most GA autopilots is that the gain is reduced after capture and as one descends, winds change direction and velocity as you near the ground. The low gain allows the tracking to diverge quite a bit before the system reacts and then will S turn as it under achieves the re-intercept and subsequent tracking, so ends up well behind what is actually happening.

KUZA, United States

The KFC225 can use the middle marker signal. This is the TB20GT wiring where it gets it from the PMA7000 intercom

But there are almost no middle markers in Europe.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

But there are almost no middle markers in Europe.

Out of 7 Croatian airports that have ILS, 6 have middle markers.

LDZA LDVA, Croatia

Emir wrote:

I guess it’s what GFC600 (and GFC700) does.

My testing of the GFC700 suggests it does not use GPS on an ILS approach to enhance the tracking. I flew back to back ILS and LPV to the same runway on a day with strong cross winds at FAF crossing altitude. The ILS S turned on final below 500 AGL and never quite captured the localizer before minimums. The LPV was a thing of beauty, absolutely no S turns, continuously adjusting the crab angle to hold the track all the way down to 30 feet AGL where I turned the AP off. At the time I sent a suggestion to Garmin to integrate the track information into the control loop on the ILS localizer. The suggestion was probably ignored, but I think Garmin engineers realized at some point that they could greatly improve the ILS tracking. I may get an opportunity to re-run the test on an ILS vs LPV tracking using the GFC 700 at my home airport in a 2018 G36 with the G1000 NXi.

KUZA, United States

My guess is that the ILS/VOR logic in the GFC500 is primarily using track information to enhance the algorithm for determining the WCA. If so, it primarily needs a consistent GPS position, not necessarily an accurate one.

KUZA, United States

A GPS receiver “always” computes track and horizontal velocity, and these are pretty accurate and stable, even for a crappy GPS.

So yes I reckon they are using the GPS track info, and compare it with the LOC bearing which they know ( = HSI course pointer ).

This still leaves the unanswered Q as to how the GFC600/700, and indeed the KFC225, manage to fly an ILS fine, and I know for a fact the KFC225 knows nuffink about no GPS The answer is probably here and this cannot be anything new since the KFC225, and its immediate “digital” predecessor(s), are mid-1990s.

And then the unanswered Q as to why Garmin decided to need GPS for the GFC500, when actually flying an ILS without GPS and do it well is a well known thing. It can’t be anything to do with it being non-TSO.

Administrator
Shoreham EGKA, United Kingdom

Emir wrote:

Out of 7 Croatian airports that have ILS, 6 have middle markers.

A sample shows that about half of Swedish ILSs have MMs.

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

This still leaves the unanswered Q as to how the GFC600/700, and indeed the KFC225, manage to fly an ILS fine,

Not my experience with the KFC150/200/225 autopilots and the GFC700. The ones I flew all S turn unnecessarily.

KUZA, United States
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