If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
gps altitude accuracy
On Thu, 10 Jul 2003 08:52:52 +1000, Mike Borgelt
wrote: On Wed, 09 Jul 2003 11:00:12 +0100, Martin Gregorie wrote: On Wed, 09 Jul 2003 10:27:00 +1000, Mike Borgelt wrote: ...snippage... For badges and records ballooning and I think the rest of aviation converts pressure readings from barographs etc to geometric altitude. It is long past time we did this in gliding. The Ballooning people have a nice worksheet to do this. FR's would get considerably cheaper if the pressure altitude requirement was dropped. Mike, do you mean that non-gliding FRs and barographs also record ambient air temperature? If that's not the case then surely they can only do what we do and use the calibration chart to correct the FR altitude to the standard pressure altitude without temperature corrections. - Curious of Essex No, the worksheet asks for the mean temperatures in the layer in question by interpolation from met office temperature soundings at two or three nearby stations. Also QNH values at the stations at the time in question. The whole process is done properly with error bands etc and you get credited with the minimum after the errors are accounted for. All very proper and obviously designed by someone who knew what he or she was doing unlike anything official I've seen in soaring. Thanks for your explanation. Pressure altitude in soaring barographs and FR's is a joke. The calibration chart was done in the lab at room temperature and we expect all this to be the same at -40 degrees at 30.000 feet. Lotsaluck! Thanks for that, too. I had wondered if that might be the case. I have to ask, though, does that matter? Unlike the situation in the real atmospheric column the temperature in the chamber can't affect the pressure unless there's a temperature dependency in either the chamber's pressure measurement or (more likely) in the FR's pressure sensor. I'd appreciate your thoughts on this too. GPS altitude will give this directly, the only matter for discussion is what error band we put on it. I'd suggest add 100 feet to the low point and subtract 100 feet from the high point. This is probably conservative in the direction of crediting you with smaller altitude gains. I've no argument with that! BTW, there's been a lot of discussion of the effect of EPE error on height measurements, but are there any systematic GPS errors that don't show up in the EPE figure? What about satellite clock drift and ephemeris errors? I've been looking at http://vancouver-webpages.com/peter/ but it doesn't appear to answer this question although it does give all the error sources and their magnitude. Judging by the EPE figure I usually see, my GPS II+ may only be calculating the EPE from ionospheric and P-code error estimates. -- martin@ : Martin Gregorie gregorie : Harlow, UK demon : co : Zappa fan & glider pilot uk : |
#2
|
|||
|
|||
I have seen GPS altitude at 19,200 when the panel altimeter set to local
pressure said 17,990. I figure that gives me about 1200 more feet of "headroom" to play in below Class A airspace in the USA. In the high mountain country of the western US, GPS altitude gives much better final glide calculations than pressure altitude. Hopefully, the feds won't take away this extra useable attitude by switching to GPS altitude for ATC. Bill Daniels "Mike Borgelt" wrote in message ... On Tue, 08 Jul 2003 02:45:18 GMT, "Peter Kovari" wrote: Last season I find some great discrepancy between my GPS altitude and indicated altitude by my standard mechanical altimeter. The differences were minimal at ground level, 2-300ft at 10,000' and 7-800ft at 17,000msl. I had the altimeter checked by a certified repair station, who certified it within acceptable tolerances, and it is still off. The question I have therefore, how accurate is the GPS altitude? Peter K. Peter, The pressure altimeter measures the difference between some reference(the setting in the subscale window) and the ambient pressure where you are. If you take that layer of atmosphere and heat it the two pressure levels move apart, hence for the same altimeter reading you are actually higher above the reference level. The GPS altitude and pressure altimeter will read the same within instrument and GPS system errors in an ISA standard atmosphere. In soaring we mostly fly in thermals in warmer than standard atmospheres hence the GPS will show a higher than pressure altimeter number. On a really hot day at 10,000 feet you could get an error of 800 feet GPS vs pressure altimeter. i.e.pressure alt 10,000 GPS 10,800 feet. As to why flight computers don't use GPS altitude - the B2000 does. I was about to build the pressure altitude module for it when SA got turned off and the GPS altitude accuracy got to be at least as good and mostly much better than pressure altitude for glider performance purposes.( there are pads for a socket for that module on the main circuit board) Using a Garmin 35 GPS source set for 3D nav only with no averaging and no dead reckoning I get the very isolated single reading GPS altitude glitch on examination of the flight record. Never noticed in flight. A simple software patch could take these out as they are always totally weird and nothing like the readings either side in time. Having calibrated quite a few IGC approved FR's of various makes the pressure sensor accuracy in them all can be unimpressive and I wouldn't use it for final glides. For badges and records ballooning and I think the rest of aviation converts pressure readings from barographs etc to geometric altitude. It is long past time we did this in gliding. The Ballooning people have a nice worksheet to do this. FR's would get considerably cheaper if the pressure altitude requirement was dropped. Mike Borgelt Borgelt Instruments |
#3
|
|||
|
|||
On Fri, 11 Jul 2003 01:18:39 GMT, "Peter Kovari"
wrote: .I merle find that maybe these tolerances I experienced were excessive, apparently some of you have experienced the same. Over and out. Peter K They weren't. They are about what you would expect and the difference between Pressure altitude and GPS altitude readings is completely explained by the physics of what you are measuring in each case. GPS altitude is accurate to 50 feet most of the time. Allow +/- 100 feet and you have it nearly all the time. Pressure altimeters are of similar order of accuracy but measure PRESSURE ALTITUDE which is a different physical entity from GPS altitude and the numbers you get may or may not be the same as GPS altitude. Mike Borgelt |
#4
|
|||
|
|||
Bill,
GPS altitude gives you absolutely NO more headroom below Class A airspace, since FL180 is a PRESSURE altitude (referenced to 29.92), not an absolute altitude above sea level (which is approximately what GPS altitude indicates). At 17,990 ft you have 9 feet of headroom, you still can't go above 17,999' without an IFR clearance or wave window) regardless of what the GPS is telling you. GPS is probably more accurate for final glides. There is practically no chance that ATC will switch to GPS altitude for airspace control, since it would require ALL aircraft to have WAAS GPS with RAIM and all that kind of fancy "stuff". Whereas a simple pressure altimeter, good for +-75'when set to the local altimeter setting, works fine for traffic separation - and doesn't require an electrical system. Kirk 66 "Bill Daniels" wrote in message ... I have seen GPS altitude at 19,200 when the panel altimeter set to local pressure said 17,990. I figure that gives me about 1200 more feet of "headroom" to play in below Class A airspace in the USA. In the high mountain country of the western US, GPS altitude gives much better final glide calculations than pressure altitude. Hopefully, the feds won't take away this extra useable attitude by switching to GPS altitude for ATC. Bill Daniels |
#5
|
|||
|
|||
It strikes me that with GPS altitude and weather soundings and
measurements, with just the GPS WAAS altitude it should be possible to calculate pressure altitude. If so, and this were reliable, there would be no need for a power-hungry mode C transponder, one could use a GPS based transponder and the ATC computer could simply spit out altitude. This isn't so farfetched since that same computer already compensates for pressure differences. On the other hand, with a reliable automated way to get these soundings to the transponder, the transponder could make these calculations BEFORE spitting out the calculated pressure altitude. I wonder how close the altitudes calculated this way would match the altimeter? I think this would be a matter of how good the soundings are at helping this process. |
#6
|
|||
|
|||
Kirk Stant wrote:
Bill, GPS altitude gives you absolutely NO more headroom below Class A airspace, since FL180 is a PRESSURE altitude (referenced to 29.92), not an absolute altitude above sea level (which is approximately what GPS altitude indicates). At 17,990 ft you have 9 feet of headroom, you still can't go above 17,999' without an IFR clearance or wave window) regardless of what the GPS is telling you. I disagree. If the flight level at which class A begins has a true higher altitude that in standart atmosphere, you have mode room. Here in the vicinity of Paris we are very concerned with this, since in some places class A begins at FL045. As usually soaring is done in good weather associated with high pressures (higher than in the standart atmosphere), FL045 is usually significantly higher than 4500ft AMSL. |
#7
|
|||
|
|||
No, what he said was he would have more headroom "under the Class A
airspace." Since the Class A would start when his altimeter reads 18'000ft (when set to 2992) regardless of what his GPS says, he would not have any more room. He could very well be higher than 18,000ft above MSL, which is a different thing altogether, and is really what we care about for final glides, etc. It's been a long time since I've done any IFR aviating, guess I need to get back into the AIM again! Kirk "Marc Ramsey" wrote in message news:LcEPa.231 What he is saying that when the pressure altitude is reading lower than the GPS altitude (which it does on typical summer soaring days), you get some extra headroom on things like final glide, as you are that much higher above the terrain. I've seen quite a few days when the altimeter, set to a nearby reporting station, reads just under 18,000 feet, while the GPS altitude is closer to 19,000 feet. BTW, the floor of Class A airspace is 18,000 feet MSL, not FL180... Marc |
#8
|
|||
|
|||
"Kirk Stant" wrote... No, what he said was he would have more headroom "under the Class A airspace." Since the Class A would start when his altimeter reads 18'000ft (when set to 2992) regardless of what his GPS says, he would not have any more room. He could very well be higher than 18,000ft above MSL, which is a different thing altogether, and is really what we care about for final glides, etc. It's been a long time since I've done any IFR aviating, guess I need to get back into the AIM again! Yes, since the AIM is quite clear that Class A starts when the altimeter reads 18,000 feet MSL, set to the nearest reporting station, *not* set to 29.92 (which would be FL180). ATC does not assign FLs to aircraft in Class A which would be low enough to conflict with aircraft flying at 18,000 feet MSL, given the ambient pressure. The "headroom" argument is based on the fact that 18,000 feet MSL pressure altitude (again. set to the nearest reporting station) is almost always more than 18,000 geometric feet above mean sea level on hot summer days, due to the fact that the ambient pressure gradient does not match the International Standard Atmosphere, to which altimeters are calibrated. I often start my final glides into Truckee with the altimeter (set to local pressure) reading 17,900 feet, while the GPS (more closely approximating actual elevation above mean sea level) is reading in the range of 19,000 feet. Since my final glide range is determined by my actual elevation, rather than by what the altimeter happens to be reading, that extra 1000 feet or so is often what makes it possible to get home. That meets the definition of "more headroom below Class A" as far as I'm concerned... Marc |
#9
|
|||
|
|||
"Kirk Stant" wrote in message om... OK, I'll buy that. My interpretation of the original post was that since the GPS altitude showed him lower than the altimeter, he could go up higher before entering Class A airspace (18000ft - I stand corrected). That interpretation would be incorrect, for all the reasons we both have stated. I didn't write that I saw GPS altitude less than the altimeter. I wrote that it was higher. GPS altitude is NEVER less than the barometric altimeter set to a local altimeter setting - at least in my experience. Bill Daniels |
#10
|
|||
|
|||
You are absolutely correct, I went back and reread the original post
and realize my mistake. So we are all in agreement after all. Bottom line - Altimeters for airspace, GPS for glides! Kirk 66 "Bill Daniels" wrote in message news I didn't write that I saw GPS altitude less than the altimeter. I wrote that it was higher. GPS altitude is NEVER less than the barometric altimeter set to a local altimeter setting - at least in my experience. Bill Daniels |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Parachute fails to save SR-22 | Capt.Doug | Piloting | 72 | February 10th 05 05:14 AM |
Pressure Altitude and Terminology | Icebound | Piloting | 0 | November 27th 04 09:14 PM |
What's minimum safe O2 level? | PaulH | Piloting | 29 | November 9th 04 07:35 PM |
GPS Altitude with WAAS | Phil Verghese | Instrument Flight Rules | 42 | October 5th 03 12:39 AM |
GPS Altitude with WAAS | Phil Verghese | Piloting | 38 | October 5th 03 12:39 AM |