A heretical proposition

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Rich
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A heretical proposition

Post by Rich »

One of the known weaknesses of baro-derived altitude is errors due to significantly non-standard temperatures. This materializes in the "cold-weather" adjustments to be made when on approach.

It occurred to me that those of us with WAAS-certified GPS that the GPS-derived altitude has no such problem. Garmin refers to this as "GSL" altitude and it is available in different displays (Terrain and Satellite Status, e.g. for GNS units). In my setup, the 530W value is also displayable in Foreflight.

So why not use GSL in these circumstances? Yes, it assumes the earth is a globe or whatever but I would remind everyone that this is the basic technique by which surveyors determined the elevation of pretty much all of the Earth. In fact, these days, GPS is itself used for this purpose. And it is GPS-derived slope one flies on LPV approaches.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: A heretical proposition

Post by CFIDave »

Yup, let's also get rid of magnetic north and use GPS with true north for everything!
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pietromarx
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Re: A heretical proposition

Post by pietromarx »

It makes sense. I think it would work well and would certainly save a few lives.

The only issue that I can see is that GPS is so easily jammed and spoofed. Having autonomous and onboard sensing (e.g. barometric) is better security, but worse convenience and accuracy.
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Re: A heretical proposition

Post by Rich »

GPS may be easily locally jammed, but not so easily spoofed. WAAS units have extensive real-time analysis of signal quality and reject signal solutions that depart from the norm. That's what Fault Detection and Exclusion (FDE) is for. I normally have > 10 satellites being used. To produce a situation where I was receiving numerous, consistent and credible but safety-threatening signals to fool both units is vanishingly small. Include the likelihood that someone would have the expertise and interest to try to influence an insignificant small GA aircraft coincident with a need to fly IMC to minimums, the probability of being struck by a meteorite. If one obsesses over these concerns you should never use GPS for navigation again.

Today was interesting. CAVU conditions. I was flying for other purposes, but monitored GSL on the 430W terrain page. At AP preselect altitude of 7500 ft. baro altitude indicated 7540 ft. and the GSL indicated 7600. Keep in mind disagreement is expected. More interesting is after I landed. I noted that baro showed 3260 and GSL 3240 as I taxied back to my hangar. The published elevation here at Prineville is 3251 ft. BUT (you knew it was coming) the area of my hangar is right at the approach end of RWY 29. The published TDZE elevation for RWY 29 is 3240 ft.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: A heretical proposition

Post by ultraturtle »

I suppose it all comes down to specs. WAAS specs allow for over 5 minutes of downtime per year. Mom Nature allows for approximately 0.000000 minutes of downtime per year for the atmosphere.
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Re: A heretical proposition

Post by Rich »

ultraturtle wrote: Sun Oct 24, 2021 7:17 pm I suppose it all comes down to specs. WAAS specs allow for over 5 minutes of downtime per year. Mom Nature allows for approximately 0.000000 minutes of downtime per year for the atmosphere.
1. If I’m on an RNAV GPS is working, otherwise the approach would not be available.
2. The baro altitude instruments have a built-in assumption about lapse rate that is often wrong and has no way to compensate for temperature variation from norms.
3. The reporting facility of the destination airport is subject to being OOS. You wind up having to use a “nearby” reporting station which introduces another source of uncertainty.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: A heretical proposition

Post by ultraturtle »

No approach to low minimums with external glide path guidance that I have ever flown (ILS, PAR, LPV, GLS, etc) has ever relied on on the same technology to define both approach guidance and the decision height/altitude. The decision to go missed is always based on either baro or radio altimeter - completely independent systems. That is a very good thing.

I know - there will be flood of “what about FLS and LP+VNAV?” replies. Chill out. Minimums are much, much higher than similar externally produced glide path guidance approaches - again, for good reason.
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Rich
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Re: A heretical proposition

Post by Rich »

Actually my Glide path is typically being generated/followed using my 530W and GSL is displaying on my 430W. Hence I have two different computers feeding me information. It’s also common for each unit to be using a different set of satellites for position solution.

See why it’s heresy? :D
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Rich
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Re: A heretical proposition

Post by Rich »

I wrote off radar altimeters years ago due to vulnerability to realities of terrain in the latter stages of approaches. Examples: KBJC 30R ILS, KPAE 16R ILS (and their RNAV equivalents).
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: A heretical proposition

Post by ultraturtle »

Rich wrote: Mon Oct 25, 2021 12:54 am I wrote off radar altimeters years ago due to vulnerability to realities of terrain in the latter stages of approaches...
...as did the TERPS folks. That is why they will never create CATII or CATIII approaches to radio altimeter mins at either of these runways. They know what they're doing.

When seeking authority to fly CATII or CATIII mins:
...the operator should assure that at each runway intended for Category III operations, the radar altimeter systems used to define Alert Height or Decision Height provides consistent, reliable, and appropriate readings for determination of Decision Height or Alert Height in the event of irregular terrain underlying the approach path, or an alternate method should be used.
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