Final Glides: GPS or Pressure Altitude?

Ignoring airspace and start/finish gate implications, which is better for managing final glides: GPS or pressure altitude?

At the Nephi Nationals, I was using a Dell Streak 5 running Top Hat glide computer software. I'd built a wiring harness for my PowerFLARM to successfully input traffic and pressure altitude date to a Kobo Mini but I couldn't get the Dell to work using the same set up. So all of the Dell's final glide info was based on GPS altitude.

GPS altitude at this contest differed dramatically from pressure altitude, from negligible at field elevation (5,000 ft.) to 1,000+ ft. at the top of our operating band (over 17,000 ft.). This was new to me. I just hadn't seen much difference between the two in my flying back east.

I assumed that forces on the glider airfoil were more likely based on density altitude, so I used my old Cambridge GPS/NAV driving GNII/Compaq 1550 for glide path control, since the Dell was telling me I had 1,000 ft. more altitude to play with when starting a long final glide.

The difference was also a factor at the start, where I had to configure the Dell with a 700 ft. higher start gate ceiling than the specified 12,000 MSL limit in order to get reasonably accurate automatic climb-thru-the-top starts.

Now that I've done some reading, I'm not so sure I should have ignored the Dell's GPS-based glide path recommendations, however. I've found several recommendations from knowledgeable authorities that GPS altitude is better for managing final glides. Obviously the GPS altitude is more accurate in an absolute sense, but my question relates to performance gliding through a non-standard atmosphere. Of course, I'm ignoring the IAS/TAS issues in all of this.

Comments?

Chip Bearden
ASW 24 "JB"

Unless the PDA is coupled with a reliable pressure altitude instrument upstream (such as a certified, calibrated logger), the PDAs own pressure altitude is going to be pretty poor in terms of accuracy. PDA or mobile device altimeters, especially and old dell streak, are about as reliable as a cheap altimeter watch. In that case (I also had a dell streak and XCSoar for awhile), I do use GPS as a reference and relied on GPS more than pressure, often cross referencing with my steam gauge altimeter to see how far it's off.. In other words, I flew the "worst" of the two in terms of final glide. If one altitude says I have final glide by 300 feet and the other says 200 below, I assumed the worst. I also turn up the fudge factor (add safety buffer). Also, the best final glide profile is generally to build up the margin early in the final glide and then begin diving it off at 5-10 miles depending on the glider performance and conditions. So when using a low accuracy PDA pressure altitude I would hold off longer to burn off any excess altitude and be weary of the potential benefit of risking a low finish. In other words I often finished fairly high.

Obviously, in US and FAI contests, the scoring software adjusts pressure changes and compensates. So the altimeter setting you launch with is very accurate at the finish assuming you have a fairly reliable, calibrated pressure altitude. A PDA without reliable pressure altitude is going to be fairly poor, to almost useless.

Even with a quality pressure altitude instrument, altitude considerations and learning to trust your final glide accuracy, and to understand and trust exactly what your logger is logging as you get close to min altitude, takes some practice. John Godfrey is excellent at explaining this as is John Good.

Post my XCSoar experiences, I have had an SN10/Oudie and now a ClearNAV2 and LXNAV S10. These instruments are all rock solid in terms of pressure altitude and I have quickly grown to trust them completely. They are both very accurate and reliable in terms of contest final glides and accuracy. I can usually take myself down to within 15 feet of the minimum finish altitude in total confidence. Also, the pressure altitude displayed on the screen is (generally) almost exactly what the logger is showing at that moment. So, if you see 1003 and the min is 1000, your OK, generally. Yes, I know the CN2 is expensive but the SN10 is now pretty cheap and very easy to install. And the SN10 NMEAs nicely to the PDA of your choice if you want a moving map to "supplement" the core SN10 data.

Not sure that really helps, but this is a quick summary of my experience.

S-

On Wednesday, July 6, 2016 at 12:20:19 PM UTC-5, wrote:
Ignoring airspace and start/finish gate implications, which is better for managing final glides: GPS or pressure altitude?

I guess the question is how the glide computer figures final glides? If it is purely geometric altitude divided by the glide angle that matches the Mc you set in (adjusted for wind, wingloading, etc), then theoretically, pure GPS elevation (not altitude) should be more accurate - since it is not affected by pressure or temperature differences, and the GPS error is usually pretty small.

I'm guessing that GPS is better...

Kirk

I'm going with GPS. Static pressure is the hardest pressure to measure, and it is almost never without bias. Dick Johnson has evidence of this too.

GPS has more random errors,_I think_. Especially since your final glide is long. Static pressure might differ along the glide.

To clarify:

The Dell Streak doesn't have a pressure transducer, to my knowledge. So it's using GPS altitude to figure final glides. GPS altitude is quite accurate, AFAIK, certainly more accurate than pressure altitude ("accurate" referring to altitude as measured with an extremely long string, that is).

My old GPS/NAV, LNAV, Compaq Aero/GNII combo has always done a great job of helping manage final glides and continued to do so at Nephi. But...the pressure altitude was 1,000+ feet lower than the GPS altitude when up high. And I was gliding through air that was less dense than a standard atmosphere. So I decided I was better off trusting the pressure altitude (used by the Cambridge system) than GPS altitude (used by the Dell Streak/Top Hat system). At the extreme, if the density altitude were extremely high, the glider would hardly glide at all; it would plummet despite the robust GPS altitude because the wings wouldn't generate much lift. At least that was my reasoning.

So it's not which system I trust, or how to manage final glides, or which polar to input, or which computer calculation to rely upon, or even whether the static pressure is measured precisely or has changed slightly since launch. I've been doing final glides since 1968 using a variety of methods starting with a cardboard whiz wheel. The question is which altitude to use. From 50 miles out, 13,000 ft on the altimeter/Cambridge (which agreed closely) gave me a 33 glide ratio to the 5,000 ft Nephi airport (ignoring the finish gate floor). The GPS altitude of 14,000 ft. at the same position yielded a 29.3 glide ratio. My feeling was that the 29 glide ratio was overly ambitious. But I'm eager to hear the discussion, hoping to avoid digging out my old engineering books.

Chip Bearden
ASW 24 "JB"

On Wednesday, July 6, 2016 at 3:11:33 PM UTC-7, wrote:
To clarify:

The Dell Streak doesn't have a pressure transducer, to my knowledge. So it's using GPS altitude to figure final glides. GPS altitude is quite accurate, AFAIK, certainly more accurate than pressure altitude ("accurate" referring to altitude as measured with an extremely long string, that is).

My old GPS/NAV, LNAV, Compaq Aero/GNII combo has always done a great job of helping manage final glides and continued to do so at Nephi. But...the pressure altitude was 1,000+ feet lower than the GPS altitude when up high. And I was gliding through air that was less dense than a standard atmosphere. So I decided I was better off trusting the pressure altitude (used by the Cambridge system) than GPS altitude (used by the Dell Streak/Top Hat system). At the extreme, if the density altitude were extremely high, the glider would hardly glide at all; it would plummet despite the robust GPS altitude because the wings wouldn't generate much lift. At least that was my reasoning.

So it's not which system I trust, or how to manage final glides, or which polar to input, or which computer calculation to rely upon, or even whether the static pressure is measured precisely or has changed slightly since launch. I've been doing final glides since 1968 using a variety of methods starting with a cardboard whiz wheel. The question is which altitude to use. From 50 miles out, 13,000 ft on the altimeter/Cambridge (which agreed closely) gave me a 33 glide ratio to the 5,000 ft Nephi airport (ignoring the finish gate floor). The GPS altitude of 14,000 ft. at the same position yielded a 29.3 glide ratio. My feeling was that the 29 glide ratio was overly ambitious. But I'm eager to hear the discussion, hoping to avoid digging out my old engineering books.

Chip Bearden
ASW 24 "JB"

Mike Borgelt wrote up a nice brief on the difference between GPS and pressure altitude. Nephi this year was the first time I'd really noticed this. Thanks Chip for bringing it back to mind so I could learn more about it.

https://www.google.com/url?sa=t&rct=...ZsbGZ0EI4fykrA

Craig
7Q

It is not unusual at Moriarty to set the altimeter (mechanical) and LX Nav S80 to known field altitude at the launch and have them display altitude after landing as much as 400 ft. higher, and occasionally lower, than field altitude.

I naturally (as a non-competition pilot) avoid the long high speed final glides that are designed to get me to the field just high enough to get the gear down. I like a safety margin, because where I fly, the powerful lift is often matched by heart stopping sink.

Using an altimeter based on differing pressures during the day, both mechanical or electronic instruments can vary widely from actual pressures found on approach. I always check our AWOS several miles out to readjust the Kohlsmann setting to actual field settings. I ignore GPS altitudes, as I have no way of evaluating their accuracy.

Looking out the window is still the best method for judging arrival altitudes, pattern flight and touchdown. We aren't equipped for ILS approaches to landing. Don't rely on non-certificated equipment as if you are a B-747 coming into Heathrow in rain at night.

On Wednesday, July 6, 2016 at 6:26:58 PM UTC-6, wrote:
It is not unusual at Moriarty to set the altimeter (mechanical) and LX Nav S80 to known field altitude at the launch and have them display altitude after landing as much as 400 ft. higher, and occasionally lower, than field altitude.

Mark,
Just to clarify, You set the known field altitude Before TO on the S80 and find this variance? On many of the LxNav instruments setting the field elevation sets a baseline for the GPS altitude for final glide. When I do this the QNH comes up exactly as on the AWOS (This, I am assuming verifies a good level of accuracy) . If you have a 400FT variance something may be up with your instrument. I find my (New $900 Winter) Baro to be off by as much as 100FT at the field but almost never on the GPS.
Looking outside is good too. RE the 747; We could start another thread on uncompensated VNAV systems. That's what WAAS is for.

KM- Now that you mention it, I really was just looking at the mechanical altimeter. It often shows a variance from field elevation after a flight, but I guess I haven't checked the S80. I just assumed it would show a difference if the pressure altitude was not adjusted to the AWOS altimeter setting. I will check it on the next flight.

Mechanical altimeters, especially the 57mm Winter are known for being lazy on a fast descend.
It can differ more than 200m compared to ground just at landing after a flight, wait a few minutes and the needle will smoothly be back at zero (or QNH).

The only explanations for a S80 to show different altitude before start and after landing is:
- Its broken (not likely)
- The pressure (QNH) has change during your flight.
I only have the mechanical altimeters as I have to, the flight computer is my primary indicator of height. I guess the majority of XC-pilots use the computer/vario as primary.