Setting QNH/Altimeter after GPS?

[IO540]

Perhaps that is already the case? Would every IFR GPS unit regardless of make report the same height every where in the world if you placed the two units side by side.

Yes, they should.

[Tim Dawson]

It depends on the quality of signal. If you've just turned the thing on, you'll get an imprecise fix. After time, it gets better and potentially very good indeed. This will vary subtly with antenna type and position.

[Fitter2]

My understanding of GPS fixes is that having the altitude assumed to be right gives a quick fix, but once you have a multi-satellite fix it is equally accurate in all directions (so say +/- 50 meters on a bad day - +/-10 on a good day for a non-EGNOS/WAAS receiver). We just perceive 50 meters vertically to be a bigger error than 50 meters laterally (and of course it is if you are trying to avoid terrain ;-)

For good geometrical reasons, the algorithm which determines height above geoid is separate from that which outputs lat/long co-ordinates. Since the primary interest of GPS users is lat/long, the WAAS/EGNOS corrected lat/long is more accurate than height by a factor of roughly 2. You have a 95% probability of being within 10 meters of the horizontal fix, and 20 metres of height above geoid. The lat/long are accurate enough for GPS approaches, but I would hesitate to rely on GPS height for the flare and touchdown.....................

[soay]

Originally Posted by IO540

With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out

Does it make sense to have an altitude encoder without a transponder? I often get asked to state my indicated altitude after being given a squawk code, presumably so ATC can confirm if my transponder is working correctly.

Originally Posted by IO540

GPS altitude seems to be a really easy way to deal with this, especially with EGNOS coming in soon, offering vertical accuracy of the order of a few feet or better.

I wonder why Garmin decided to implement EGNOS in the old GPSMAP 296, but not in the new Aera 500?

[IO540]

I would hesitate to rely on GPS height for the flare and touchdown

AFAIK all autoland systems use a radar altimeter for the flare portion. I guess the autopilot switches over to the radalt at about 150ft.

WAAS (EGNOS in Europe) facilitates GPS approaches with a synthetic glideslope but the DH on these is still about 200ft (no European ones are published since none exist) i.e. no better than Cat 1 ILS.

[Torque Tonight]

By way of experiment I have sat on the ground at a point of known elevation and compared a number of GPS receivers that I have, including a couple specifically intended for aviation use. The altitudes indicated have varied significantly, and we're not just talking about that known constant 200ft error relating to the geoid model in certain chipsets. GPS altitudes varied with time, and varied between units at the same time. The altitude accuracy was not something I would stake my life or licence on.

No doubt in future GPS receivers will be better able to accurately measure true altitude - but at the moment altimetry is not, and is not intended to be, based on true altitude. We have a well established, reliable and consistent system of altimetry based on pressure altitude. The 'receivers' are robust, simple, capable of very good accuracy, instant reponse, require no power for their core function, etc etc. There are a number of very good reasons pressure altimeters are here to stay for the forseeable future.

Back to the OP, I think it would be very unlikely that you would ever not have access to a good enough QNH/RPS such that using GPS altitude was preferable. Blind faith in GPS can stitch you up.

Would I ever refer to GPS alt? Well perhaps if I had a static blockage, no alternate source, no standby alt and I was in IMC, then I would probably let down over the sea until VFR using GPS alt, but I can guarantee that I'd descend the last 1000ft or so at a snail's pace. It would take a lot of bad luck to get into that situation though.

[Fuji Abound]

So here is another one - would you rely on a GPS to judge your speed in say, the approach?

(We all know there are other ways of landing with a failed ASI).

[Tim Dawson]

To judge my groundspeed? Absolutely. But a GPS has no idea of airspeed, which is all that matters in an approach

[Rod1]

Originally Posted by IO540 With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out
“Does it make sense to have an altitude encoder without a transponder? I often get asked to state my indicated altitude after being given a squawk code, presumably so ATC can confirm if my transponder is working correctly.”

It makes no sense to have an encoder without a transponder as the transponder has to transmit the encoder output. When IO said there was no way of knowing if the calibration was correct, he had probably forgotten that you can ask for a mode c check from just about any radar unit every time you fly if you want to. On modern units you can also display the encoder output on the front panel and compare it with your altimeter set on 1013.

Rod1

[Fuji Abound]

Of course it doesnt but you can get the wind from the tower.

How good a guide would you arrive at?

[421C]

There is a lot of discussion of geoid corrections but a point that has been missed is that there is NO standard geoid used in aviation for MSL (unlike the WGS84 standard for lat/lon, which is used both by GPS and in paper charts). Approach plates, for example, may use a "local" MSL datum derived from a different geoid than the EGM96 model used in GPS. I believe these differences can be significant.

Hence, whilst GPS is great for IFR LNAV, barometeric altimetery is needed for IFR VNAV.

brgds
421C

[IO540]

Nevertheless in the 8 years I have been flying behind the old KLN94, I have never seen it display an altitude more than about 30ft (i.e. 1 millibar) different from the published airfield elevation.

That's a pretty good record, even before one starts to compare it against mis-setting the QNH by 10 points (300ft), flying 1000ft too high/low (due to misreading the analog altimeter) etc.

GPS altitude is a lot better than most people think - unless the GPS is a piece of junk from a camping shop, and even the modern ones of those can be very good. My Garmin 496 (OK, not from a camping shop) is within 5-10ft of the airfield elevation and most of the error is probably the height of the unit above the concrete

Of course it doesnt but you can get the wind from the tower.

If there is any real wind, say 10kt+ reported SFC wind, I would be more careful using a GPS for flying the final approach, since there is likely to be a significant wind shear in the last 100ft or so. But it should still be better than nothing.

When IO said there was no way of knowing if the calibration was correct, he had probably forgotten that you can ask for a mode c check from just about any radar unit every time you fly if you want to. On modern units you can also display the encoder output on the front panel and compare it with your altimeter set on 1013.

I didn't forget; I was referring to low end GA not carrying transponders (for all kinds of reasons; real, artificial, or Civil Liberties driven) but presumably still wishing to carry some form of TCAS. If you want to do TCAS on the cheap, nothing beats GPS (i.e. some form of ADS-B) for low lower, simplicity, and accuracy.

By way of experiment I have sat on the ground at a point of known elevation and compared a number of GPS receivers that I have, including a couple specifically intended for aviation use. The altitudes indicated have varied significantly

What models were these?

[LH2]

Originally Posted by Fitter2

For good geometrical reasons, the algorithm which determines height above geoid is separate from that which outputs lat/long co-ordinates.

Please clarify. I believe you might be referring to the coordinate transformation stage, from ECEF to whichever horizontal/vertical/combined reference systems the user chooses to have. The navigation solution itself is obtained by solving all four unknowns simultaneously and does not even involve the concept of local vertical (and hence altitude). Also the geoid correction does not enter the stage until much latter, if at all.

The reason why the vertical component is less precise than the horizontal component is indeed geometrical however: as you can only see satellites above the horizon, you have no observations at all coming from below (the ones coming from low elevations are also less reliable due to atmospheric effects), which increases the measurement uncertainty in the vertical direction. Note incidentally that this effect diminishes with height above ground due to the lower horizon elevation amongst other things.

The relative "goodness" of the position solution in different planes/components is given by the dimensionless DOP (dilution of precision) indicators.

Since the primary interest of GPS users is lat/long

Do not generalise please. Timing is a major field of use.

Lastly, the geoidal correction bit, which seems to mystify a lot of people is not a major factor in recreational GPS use, aviation included. I have geoidal correction source code at hand, originally from a consumer grade GPS receiver which is less than 7Kb (7072 bytes to be exact ) and computes a "rough" correction based on bilinear interpolation on a 10 degree grid. This achieves accuracy in the order of 2-3m worldwide (with the possible exception of the great mountain ranges, I haven't checked). Clearly, that's unacceptable for geodetic or many engineering uses, and for Cat III autolands (where the radio altimeter controls the flare anyway), but for general aviation it doesn't matter a bugger.

[LH2]

Originally Posted by 421C

Approach plates, for example, may use a "local" MSL datum derived from a different geoid

Geoids and vertical datums are different, orthogonal concepts.

[LH2]

Nevertheless in the 8 years I have been flying behind the old KLN94, I have never seen it display an altitude more than about 30ft (i.e. 1 millibar) different from the published airfield elevation.

Well, post May 2000 you shouldn't expect see differences much bigger than those, and if you do see them it's probably a misprint on the airfield chart or the surveyor got the height wrong (it happens ).

And if still on the tarmac and you have set your altimeter to the QNH provided by the tower, everything should more or less agree.

But what I understand is being discussed here is what happens when you take off, climb to say FL180 and then compare the GPS against the altimeter reading. In that case you can expect to see quite a bit more than 30ft discrepancy between altimeter and GPS. More like 1000ft-1500ft on a random day over Greece

[Spendid Cruiser]

Adam,

It is a jolly good question!

However, I don't really see the point of setting both GPS and altimeter to read the same thing. You should use the most appropriate pressure setting for the altimeter. If not because of the usual mode C and vicinity of controlled airspace arguments, but also because then you have two independent references.

To my mind, two independent references would give you a better sense of accuracy than one. Perhaps you could use significant converging/diverging relationship between the two to give you some information about the environment your flying towards or the accuracy of a fix. I reckon it probably adds to the holy grail of SA.

I suppose, if you're of the mind that the pressure setting is out of date and significantly inaccurate and terrain or some such is a threat then maybe consider separation based on lowest of the two until you have resolved the discrepancy to your satisfaction.

[SNS3Guppy]

With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out, whereas a GPS can easily report if there is something wrong with the accuracy of its 3D fix.

With better equipment one can monitor the encoder output, and one can always ask ATC what is being seen.

Ask yourself why RVSM isn't predicated on GPS, but on QNE. Nearly everything flying in RVSM has advanced GPS capability. Never the less, altitude isn't based on GPS, even in RVSM, but on barometric altimeter. We do use an air data computer to correct altimeter at altitude, but none the less, it's baro altimeter use, not GPS, and the corrected altitude invariably disagrees with what's found in the GPS.

[IO540]

climb to say FL180 and then compare the GPS against the altimeter reading

Well, yes, a baro altimeter is going to be pretty useless for determining actual heights, when at high altitude, due to a virtually permanently nonstandard atmosphere.

I have never seen the inside of a real altimeter but believe it contains some kind of logarithmic correction, but that will be a 1st order one only, with the OAT not being measured, never mind corrected for.

An altimeter reads accurately only on the ground, after you have set it to the known elevation of the ground

Once airborne, it is OK for vertical separation of traffic provided everybody uses the same sort of instrument (which they do).

[SNS3Guppy]

True altitude isn't the same as indicated altitude; that much any student pilot knows.

In advanced systems, the air data computer considers temperature, temperature and airspeed and other factors when applying corrections to instrument indications, engine parameters, etc.

QNH is perfectly adequate and appropriate for use in light airplanes, and it works just fine.

[IO540]

the air data computer considers temperature, temperature and airspeed and other factors when applying corrections to instrument indications

Which brand of barometric altimeter connects to the ADC?