Actual Altitude

[Bruce Fenstermacher]

Miles has described the best how we are taught the workings of the GPS system during our annual FAA required 135 training. I'm sure George didn't mean the GPS actually plots a point on the earths surface, the GPS system has no idea there is an earth. Instead it plots a spot on a sphere which happens to be about the size of the earth and in the same location.

Altitude is the hardest of the calculations because as described previously depending on the satellites, how many satellites, and their positions and resulting angles for the calculation, a minor altitude change is an insignificant change in the calculation. When I say minor change I'm talking about a hundred or more feet. Insignificant in the calculation, but important for the task we may be performing.

When you really understand the calculations taking place and then consider that those calculations are taking place continuously, and on an electronic device that itself only understands power on and off or at it's route level a binary code of ones and zeros, only then can you start to understand the amazing feat taking place.

[ronjenx]

In my experience, mostly with a Garmin 296, the vertical solution seems to be slightly better than the horizontal solution, at least around Tehachapi. Landing on the centerline of runway 29 leaves a track as though I had landed on the taxiway about a hundred feet to the south. I don't recall ever seeing the calculated elevation on the ground being off by more than 20 feet. Not too bad for a relatively inexpensive, "don't-you-dare-use-this-thing-in-the-clouds" unit.

Your "The way I understand it" explanation is a good one.

Can you choose different map datums with your Garmin 296?
That could be the source of the shift to the taxi way, if it is consistent.

[GAHorn]

... I'm sure George didn't mean the GPS actually plots a point on the earths surface, ....

Exactly right. And that's why I said...

..the earth's surface (datum) ....

....referring to WGS84 or whatever OTHER datum the user selects during the SetUP routine of the unit.

...Can you choose different map datums with your Garmin 296?....

[hilltop170]

OK, now for a practical question, will the vertical guidance go all the way to the ground on a certified unit or will it cut off at some altitude like the hand held units?

[Bruce Fenstermacher]

That is a good question Richard and when our dual WAAS equipped helicopter finally gets certified by the FAA I'll tell you first hand, but I've been lead to believe it will.

But while thinking about it they probably won't go to the ground but suspend at the DH just like all the other non=precision approaches do,

[jrenwick]

OK, now for a practical question, will the vertical guidance go all the way to the ground on a certified unit or will it cut off at some altitude like the hand held units?

On the GNS430W, it depends on the approach. It goes down at least as far as the MDA, and though I've never followed it through past MDA (I'm either looking out the window at that point, or going around), I'd bet it goes all the way down to the touchdown point. Next time I'm flying the 170 I'll try to remember to verify this.

According to the GNS430 manual: on an approach with LNAV+V or LPV options, the unit checks its signal integrity 60 seconds prior to reaching the FAF. If it's good enough for vertical guidance, you'll get a synthesized glideslope. Otherwise, the glideslope indicator will be flagged, and you'll get a message advising you to fly to LNAV minima.

On an LNAV or plain GPS approach, the glideslope indication you get is advisory only, of course. One instructor warned me that you still have to observe crossing minimums during the approach. He said that following the simulated glideslope might take you below them, because the simulated glide slope goes to a different point on the runway than the official TDZ for the approach.

[KS170A]

That is a good question Richard and when our dual WAAS equipped helicopter finally gets certified by the FAA I'll tell you first hand, but I've been lead to believe it will.

But while thinking about it they probably won't go to the ground but suspend at the DH just like all the other non=precision approaches do,

The G1000-equipped airplanes I fly will fly you right into the ground if the pilot leaves the autopilot engaged. Hence, the pilot needs to know and observe the limitations contained in the Airplane Flight Manual or applicable AFM supplement (G1000 Cessnas limit A/P use to 200 ft AGL and higher during approach operations). Unlike the glide slope indication on an ILS approach, the glide path indication remains from the previously referenced solution confirmation all the way to the runway at the 1000-foot marks.

[fangzz]

Wow. I'm glad I asked.

I was thinking of starting another, related, thread, but since we seem to have some attention here:

My VNAV and VVI)(separate data presentations which are altitude related) on the 496 seem to be perfect. But, of course, the VNAV only leads me to a height at a distance away from my destination. Very Safe. And, as said, the 496 VVI shows minute changes in vertical position. So.....WHY THE HECK can't it figger out how high it is.......accurately?? This, at this point, is rhetorical. No one out there knows, really. Given the VVI, etc. Garmin could probably shed light if the engineers would talk to me but that's not likely. Maybe someone out there 'knows someone' who can clear this up somewhat. It is somewhat important (understatement intended), and, by virtue of what we've all read here, it is, and has been, an area of some consternation. If it hasn't been, it should have been.

Suffice to say, I may as well remove all altitude "fields" from the various pages/screens available to me on my 496 presentations.. They are just a confusion factor at this point, at least for me, and take up useful space since the VVI agrees with the panel instrument.

Hey, maybe that last point will get Garmin's attention (useless, inaccurate, dangerous- if -displayed data). Nahhhh. They already have my money. Ohhh, that sounds cynical, and I'm not. A point here, lest I may seem unfair. As others in this thread have written, on the runway, the gps is on-the-money. Not good enuf. Personally, once the gps senses takeoff, I'd personally like it better if the unreliable data was not visible. I went for some time setting my altimeter by my gps thinking there could be no more accurate information. Glad I asked you guys what your experience was. My tipoff was when atc asked me to disable mode c, in excess of 300 ft in error. Wasn't the mode c.

At the VERY least, I don't feel quite like the lone stranger I once did, getting the mushroom treatment from the Garmin techreps. I thank you for that. I'd say misery loves company, except, as a VFR pilot,and loving every single prop rotation in our beauty, Ive been anything but miserable. The 496 is an amazing aid, and now that I know what to believe, and, more importantly, what I cannot, I'm more safe. I hope all you guys and gals are too.

[jrenwick]

Where I find the altitude readout on my 496 useful is when I'm using it in the car. It's fun to have a readout of my *approximate* elevation.

It's really important to understand that for ATC purposes -- which means keeping aircraft separated from each other -- we're not actually flying at constant altitudes above sea level. Instead, all aircraft are flying at assigned atmospheric pressure levels. Our separation from other aircraft is not actually 500 or 1000 feet of altitude; instead it's measured in inches of mercury: approximately .50 and 1.0 inches, respectively. The only way we can reliably maintain vertical separation is for all aircraft to use accurate barometric altimeters that are set to the same pressure setting.

To put it another way: although altimeters appear to measure height above sea level in feet, they're actually measuring only the atmospheric pressure outside the aircraft. How accurately this corresponds to actual height above sea level depends on how closely the column of air above and below you resembles a "standard atmosphere," in which the barometric pressure decreases in direct proportion to the actual height above sea level (about one inch per thousand feet). A standard atmosphere is unlikely to happen in nature, so there's probably always some unmeasurable discrepancy between an altimeter's reading and the actual height above sea level.

That discrepancy increases as you climb, to the point that above 18000 feet, the altimeter setting as measured on the ground is no longer considered reliable. At that point, all aircraft fly pressure altitude assignments (altimeters all set to 29.92 regardless of what the surface altimeter setting is), and the standard separations are 1000 or 2000 "feet" instead of the 500 "foot" separations between IFR and VFR aircraft at lower altitudes.

Even with a WAAS-augmented, IFR-certified GPS that can determine altitude fairly accurately, to use that "altitude" in place of an accurate altimeter set to the current sea level pressure could put you dangerously close to other aircraft that are flying by their altimeters.

Just remember that altitude in an aircraft is really a constant atmospheric pressure level, not a measurement of feet above sea level. In fact, due to variations in air density (i.e. "weather") your actual height in feet will vary as you cruise along at a constant altitude on the altimeter.

[GAHorn]

... I went for some time setting my altimeter by my gps thinking there could be no more accurate information. ....

Uhmmmn.... this is not a sarcastic question... but, what/who gave you the impression (other than when setting to field elevation) you ever could/should set your altimeter to anything except barometric pressure?

Your practice of setting it to a GPS was very much an individual "taking-of-license", and thankfully you now know it was wrong. There is no authorization to use GPS-generated altitude for setting sensitive, barometric altimeters. (Once again, imagine you had fantastic vision and observed an airplane appearing over the horizon 150 miles away. Although he might be 30,000 feet MSL, he would appear to be only JUST above the ocean! To another much-closer observer, it might appear to be 5,000 feet MSL. An observer directly beneath the aircraft might guess the actual altitude much more accurately. But no satellite would likely have the advantage of that last observer. The closest to that situation would be the exact reciprocal... a satellite directly OVERhead... and would still not be capable of determining the actual altitude of the aircraft since the satellite would not have the advantage of a grounded observer!)

In short, without some other input, GPS is not capable of determining altitude closer than several hundred feet of error. (But even THAT is useful information should your altimeter take a vacation, so I would not remove that display, in my personal opinion.)

[fangzz]

AND NOW…..FOR THE REST OF THE STORY……….


John, thanks for a great little lesson on how some of this fits together. Interesting to me that it seems we all are using an antiquated and somewhat inaccurate system. I say ‘inaccurate’ as baro is compared with true. Truth be told, this is not a fair comparison, because they are simply different measurements. I wonder how many pilots know this, and believe, as I did, until reading your post and then doing some further checking. I incorrectly believed that our baros were a very close approximation of true altitude, our actual altitude above sea level, throughout the atmospheric column. Your post (which made perfect sense, and some other stuff (which made no sense to me) caused me to keep checking.

Being interested in accurate altitude, and believing my gps to portray it (it does) in my ignorance, I inserted my gps altitude into the instrument, along with a mental note to get the instrument calibrated soon. Then, we started this very interesting, to me, thread. Pressure Altitude (baro) doesn’t pretend to be true altitude and never did. It simply is another yardstick for altitude and this is the entire crux of the situation. It’s the final answer to the riddle that was vexing me, as well as all the experienced guys at my little airport, until just now, at the end of my conversation with a Garmin product tech rep.

Turns out, gps altitude is VERY accurate. It IS True Altitude. Everywhere. It very accurately tells us our true height above mean sea level (within the capability of the particular gps), our True MSL altitude, just not our Pressure altitude. When next you’re flying close to one of those increasingly numerous humongous towers out there, check your gps. You should see something very close to the exact number printed on your sectional for THE TRUE ALTITUDE, MSL. Your altimeter might be off by a hundred or more feet when you glide by the red strobe. Our mode C’s xmit Pressure Altitude, commonly referred to as our height above MSL (atc: “say altitude.” Pilot: “four thousand five hundred MSL.” Technically, not correct) and it better agree with what we see in the panel instrument, adjusted with the kohlsman window entry. The gps shows the bias between True and Pressure every time you compare the two instruments, anytime in flight (on the ground, they will show the same or very close to it). If you monitor it, and your baro crumps, George’s hypothetical, you will be aware of the bias, and thus, you can approximate your pressure altitude until again on terra firma. Valuable info, turns out. Another good reason to keep the data field on the gps and in view. BUT….

Our little thread has seemingly revealed this as an area with a good deal of ‘misunderstanding.’ But, it could be it’s just me.

While on the phone with Garmin, the rep was X-ref’ing several sources of info for my product, the 496 and its use. He found a Q&A in the FAQ I’ve pasted at the end of all this which, in brief, puts this to rest in my mind, finally. It’s not in the Users Manual, and not part of the warnings I have to clear each time I turn on the unit before it will begin functioning (probably unreasonable to expect that it would be). Even their reps are not well enuf versed, across the board, that when I raised my concerns of ‘inaccuracy,’ were the other tech reps familiar enuf with this subject to say ‘hey (dummy), you’re comparing apples to oranges.’ They took my gps back for repair/replacement. Even more startling to me is the following: This point, albeit a fine one, but I think a very important one, is not part of the Private Pilot course offered by a large, successful purveyor of instructional material.

Until this past fall, I hadn’t flown since 1982. Gps arrived in between. My F4 had INS, hi tech at the time, but no gps. Long time between flights, so I thought, since so much is likely to have changed, I’ll go through the Private course offered by said purveyor as a refresher. Did. Whole course. Took all the tests. Not one mention of this interesting thing. Yes, they went through the definitions of the various altitudes, but never connected the dots in this area, and I think they should be. The dots, that is. Gps is too pervasive. In the areas of both True and Pressure Altitudes, Mean Sea Level is used in conjunction with both, but without distinction, at least a noticeable one. There’s probably an ‘allusion’ to a distinction, but in an area this important, and subject to such a misinterpretation, pitch to the lowest common denominator—me. Pick up a two by four to make sure I understand. Please. I’m going to offer them/our experience, for their and future pilots’ benefit, but since they essentially teach the tests, I’ll be surprised if it’s stressed more than a simple mention. Since it’s not one of the test questions.

Below, I have pasted the pertinent Q&A from the Garmin website. Wish I had seen it long ago, but…now’s better than never. John, if you’re reading this, it’ll sound familiar. Hope this helps someone. It has helped me.

Best regards, Drew

Question: Why is the altitude on my GPS different than the altitude on my altimeter?
Answer:
Regarding the altitude discrepancy, keep in mind that your altimeter does not measure altitude. What the altimeter measures is air pressure. Your altimeter is calibrated to display a certain change in altitude based on a change in air pressure. For every inch of mercury pressure change, the altimeter displays a change of approx. 1000 ft. of altitude. Think of it this way. If the plane is sitting on the ramp and shows the correct altitude when the outside air pressure is 30.00, it will show an altitude of ramp+250 if the outside air pressure drops to 29.75. It is still on the ramp, but the altimeter changed because the pressure changed. The standard lapse rate (SLR) for air pressure is 1-inch pressure = 1,000-ft altitude and this is how your altimeter is calibrated (approximately). However, when is anything in nature a perfect standard? What the SLR implies is that 1-inch will USUALLY equal 1,000-ft. depending on altitude. The higher you go, the less accurate this is. However, this is not a problem since everyone uses the same system. If everyone's altimeter is set to the same pressure setting, then they will all be off the same amount and vertical separation will be ensured. The GPS does not use air pressure to determine altitude. As such, it is not as apt to have pressure errors in determining altitude. What this means is that your GPS derived altitude is almost always more accurate than your altimeter. However, since everyone uses a pressure-based system to determine altitude, you need to fly using the pressure altimeter to ensure the required vertical separation.
Last modified on: 08/30/2007

[GAHorn]

Well...kinda.
The definitions are a little loose in our discussion. Pressure altitude is the standard reference-term applicable to indicated altitude with the setting in the Kollsman window of 29.92 in Hg.
And yes, the description of the variations of accuracy with increased altitude is relative. (That's why the recent brou-ha-ha with regard to the changes to RVSM in the last couple of years.)

Anyways... this is a good discussion, despite the Garmin rep's slightly erroneous explanation.