Final Glides: GPS or Pressure Altitude?

Lot's of good comments upstream. I think the question itself needs to be clarified. If the issue is "which one will help keep me from running into big hard things that get in the way of my final glide (e.g. a ridgeline, mountaintop, etc.) " then the obvious answer would be GPS. After all, it's referenced to the stuff you're worried about hitting. Even relatively low-cost GPS engines are providing sub-100 foot vertical errors (more typically sub-50 foot).

Pressure altimetry is eferenced to the airmass which as you know is rarely "standard". Especially when flying over the course of a long task in changing conditions (remember - "from high to low or hot to cold look out below"), it's not uncommon to have several hundred feet of change in altitude indication. Unless you have a current altimeter setting from your target airport (or at least one nearby) you've got a big unknown to deal with.

If the question was really "how do the systems in my cockpit work", that's a whole other kettle of fish.

P3


On Wednesday, July 6, 2016 at 1:20:19 PM UTC-4, wrote:
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"

Agree with P3. The question is: given one indicated altitude (based on pressure, whether from a mechanical altimeter or modern device employing a pressure transducer) that is 1,000 feet lower than another indicated altitude (GPS, which is arguably a lot closer to the actual geometric altitude), which one should I use for an aggressive, 50+ mile final glide into the blue where I may not get another chance to climb?

Thanks to Craig Funston for the Mike Borgelt article. Mike's RAS postings are actually one of the sources I'd consulted before I posted but this article is pretty definitive: "your glider cares about GEOMETRIC [edit: GPS] altitude when it comes to the distance you can glide at a certain glide angle."

So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.

So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.

Does that make sense?

Chip Bearden
ASW 24 "JB"

On Thursday, July 7, 2016 at 8:41:54 AM UTC-7, wrote:

So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.

So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.

Does that make sense?

Yes, that is correct. Old-school glide computers used pressure altitude because that's all they had. My homebrew glide software always used GPS altitude, and it was as accurate as could be expected (allowing for localized lift/sink) in western US soaring conditions. With a proper polar, it nailed more than one 75+ mile final glide into still air.

Just to reiterate, GPS altitude is a short term noisy, long term stable measure of geometric altitude (which is what you want). Pressure altitude is a long term noisy measure of pressure altitude (which you don't want). My software actually blends the two, essentially continuously recalibrating pressure altitude against a 2 minute moving average of GPS altitude, producing a stable short term measure of geometric altitude, which (to me) is the ideal. I assume most modern glide software and computers do something similar.

Marc

On Wednesday, July 6, 2016 at 10:20:19 AM UTC-7, wrote:
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"

There are errors in both methods of measuring altitude, as is discussed in:

http://www.xcmag.com/2011/07/gps-ver...nitive-answer/

The differences are substantial, as is discussed in:

http://www.icao.int/APAC/Meetings/20...20Altitude.pdf

One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.

Tom

On Monday, July 18, 2016 at 1:43:19 AM UTC-4, 2G wrote:
There are errors in both methods of measuring altitude, as is discussed in:

http://www.xcmag.com/2011/07/gps-ver...nitive-answer/

The differences are substantial, as is discussed in:

http://www.icao.int/APAC/Meetings/20...20Altitude.pdf

One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.

Tom

Excellent references; Thanks Tom!

On Monday, July 18, 2016 at 1:43:19 AM UTC-4, 2G wrote:

One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.

Tom

Tom,

Having a little difficulty reconciling your point that using pressure altitude is "more conservative." Having wrestled extensively with this topic when we were introducing GPS Position Recorders as an accepted method for documenting badge flights, I can summarize the two papers even more succinctly:

- GPS Altitude and Pressure Altitude are two different frames of reference..
- Airspace and related navigation constructs have been based on Pressure Altitude since the beginning of aviation since that was the only technology available.
- Therefore, you can't mix-and-match and expect things to work out easily (e.g. penalizing pilots for airspace infractions using "just" GPS altitude)..
- You need to know/understand what "altitude" your instruments are displaying and for GPS which earth model they are using.

So much for the background.

The big eye opener for me in looking at hundreds of log files was how much variability there was in Pressure Altitude recording over the course of a day. Especially for long contest and record flights, it's not uncommon to see 8 or even 10 millibar changes in pressure and 20 degrees F in airmass temperature (common during CFROPA). This translates into several hundred feet of change in altitude indication RELATIVE TO THE GROUND over the course of a flight.

"From high to low or hot to cold, look out below" came about for a reason.

So, for my money, GPS Final Glides are more repeatable since they are consistently aligned to the frame of reference I care about most - the hard stuff I'm going to run into when I run out of air underneath me.

My 0.02
Erik Mann (P3)

My take on this is that there are three issues (at least):

1. Accuracy of GPS altitude. Good discussion in the first paper. Thanks, Tom!

2. Remaining clear of controlled/restricted airspace. U.S. FARs are clear in spec'ing pressure altitude.

3. Managing final glides. This was my original focus, in particular in the Western U.S. where both altitudes and temperatures can be high (leading to greater differences between observed pressure altitudes and GPS altitudes) and final glides are often much longer. The latter can increase the odds that, owing to both distance and time, a pilot may transition from an area with lift to an area where there is less or no lift, making it difficult to salvage a final glide that is falling short at the end.

Pressure altitude is conservative in the sense that it often under reads the geometric altitude and--at least out West--will therefore provide a cushion against unforeseen sink. But I already have an explicit arrival height safety margin. Layering that with an uncertain additional cushion isn't where I wanted to go.

That said, the GPS vs. pressure altitude cushion has the virtue that it tends to increase with altitude. Final glide computers I've used allow entering an arrival (safety) altitude of X feet, without regard to MC setting or altitude or length of glide. There are ways to make it proportional (e.g., % bugs or % risk) but using pressure vs. GPS altitude can do the same thing.. Of course, the differences will vary from day to day so that introduces more uncertainty into an area that already has plenty of it. I'd rather nail the altitude (subject to the uncertainties of GPS determination) and factor in the safety factor(s) explicitly myself.

Thanks for all the input. I learned some interesting things.

Chip Bearden
ASW 24 "JB"

wrote on 7/20/2016 7:53 AM:

3. Managing final glides. This was my original focus, in particular
in the Western U.S. where both altitudes and temperatures can be high
(leading to greater differences between observed pressure altitudes
and GPS altitudes) and final glides are often much longer. The latter
can increase the odds that, owing to both distance and time, a pilot
may transition from an area with lift to an area where there is less
or no lift, making it difficult to salvage a final glide that is
falling short at the end.

Pressure altitude is conservative in the sense that it often under
reads the geometric altitude and--at least out West--will therefore
provide a cushion against unforeseen sink. But I already have an
explicit arrival height safety margin. Layering that with an
uncertain additional cushion isn't where I wanted to go.

That said, the GPS vs. pressure altitude cushion has the virtue that
it tends to increase with altitude. Final glide computers I've used
allow entering an arrival (safety) altitude of X feet, without regard
to MC setting or altitude or length of glide. There are ways to make
it proportional (e.g., % bugs or % risk) but using pressure vs. GPS
altitude can do the same thing. Of course, the differences will vary
from day to day so that introduces more uncertainty into an area that
already has plenty of it. I'd rather nail the altitude (subject to
the uncertainties of GPS determination) and factor in the safety
factor(s) explicitly myself.

Using GPS would remove the altitude uncertainty, but still leaves the
two most important ones: wind and lift/sink.

I really noticed the effect of wind while flying at Parowan in June: my
vario nav page used the vario's "live wind" (updated several times a
minute) it computes from it's sensors, while the flight computer used
the wind obtained while circling. When 30-50 NM from the goal, I saw
variations in "altitude required" of as much as 2000'.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

On Wednesday, July 20, 2016 at 6:07:28 AM UTC-7, Papa3 wrote:
On Monday, July 18, 2016 at 1:43:19 AM UTC-4, 2G wrote:

One tends to think that GPS, being digital, is more accurate, but this is misleading. Myself, I would take the more conservative number (pressure altitude) and consider it an insurance policy. Final glides from that altitude are, by the very nature, long and subject to much error.

Tom

Tom,

Having a little difficulty reconciling your point that using pressure altitude is "more conservative." Having wrestled extensively with this topic when we were introducing GPS Position Recorders as an accepted method for documenting badge flights, I can summarize the two papers even more succinctly:

- GPS Altitude and Pressure Altitude are two different frames of reference.
- Airspace and related navigation constructs have been based on Pressure Altitude since the beginning of aviation since that was the only technology available.
- Therefore, you can't mix-and-match and expect things to work out easily (e.g. penalizing pilots for airspace infractions using "just" GPS altitude).
- You need to know/understand what "altitude" your instruments are displaying and for GPS which earth model they are using.

So much for the background.

The big eye opener for me in looking at hundreds of log files was how much variability there was in Pressure Altitude recording over the course of a day. Especially for long contest and record flights, it's not uncommon to see 8 or even 10 millibar changes in pressure and 20 degrees F in airmass temperature (common during CFROPA). This translates into several hundred feet of change in altitude indication RELATIVE TO THE GROUND over the course of a flight.

"From high to low or hot to cold, look out below" came about for a reason..

So, for my money, GPS Final Glides are more repeatable since they are consistently aligned to the frame of reference I care about most - the hard stuff I'm going to run into when I run out of air underneath me.

My 0.02
Erik Mann (P3)

Erik,

I was only offering my opinion on which to use; you, of course, can chose whatever you feel comfortable with. You can always make your glide computer more conservative; I did it today by specifying 25% bugs when I had virtually none. Viola, the safety altitude dropped by 2,500 ft. (and it matched the margin my Oudie came up with)!

We have been used to using pressure altitude. Now, GPS gives us a new, higher altitude. What a blessing! Now you can arrive at the airport at a reasonable altitude and not waste all that time climbing another 2,000 ft. I think that if you have been getting back safely with pressure altitude why change? Remember, the ground doesn't care which method you use.

Tom

Tom,

Since I raised the original question, I'll jump in again to clarify. I'm used to using pressure altitude, too. And, frankly, back east where I usually fly, it doesn't make much difference. GPS and pressure altitude are normally almost identical at the altitudes where we operate.

Transported to Nephi, I found that GPS altitude was 1,000' or more higher than pressure altitude at 17,000'. What to believe? Which to use?

I played it conservatively and used pressure altitude. And my final glides were, as a result, conservative, perhaps overly so. At a contest, that conservatism can be costly in terms of points. And I like to know what the "raw" number is for required altitude and then adjust that for uncertainty, expected/unexpected sink or lift, changing winds, and other imponderables, including the risk/reward of cutting it too close after an otherwise good flight.

Chip Bearden
ASW 24 "JB"