|
Altitude calculation based on Air pressure
|
|
| @737@ | Дата: Воскресенье, 14.08.2011, 06:15 | Сообщение # 26 |
|
Полковник
Группа: Персонал
Сообщений: 104
Награды: 0
Репутация: 0
Замечания: 0%
Статус: Не в сети
| The AIP for Heathrow (EGLL) says the threshold (THR) elevation of, for example, RWY 09L is 79 ft. It also says the THR Geoid Undulation is 151 ft.
My understanding is that GPS measures its elevation above a reference ellipsoid (WGS84). This doesn't fit mean sea-level very well and it's corrected using a geoid (EGM96) which gives a closer representation of mean sea-level. The difference between the geoid and the ellipsoid is called the Geoid Undulation.
1 What's the significance of the difference (79 - 151 ft)?
2 What use would be made of the figure for Geoid Undulation - can it be entered into an aircraft GPS system?
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:15 | Сообщение # 27 |
|
Группа: Удаленные
| Honestly I don't use GPS for track/course/heading I love it when I do not use GPS (Actually I don't),
1 ) It is 72 ft diff .
2) I don't know anything about GPS and Geoid Undulation.
|
| |
| |
| @737@ | Дата: Воскресенье, 14.08.2011, 06:15 | Сообщение # 28 |
|
Полковник
Группа: Персонал
Сообщений: 104
Награды: 0
Репутация: 0
Замечания: 0%
Статус: Не в сети
| You can read many articles about GPS altitude innacuracy (and why).. and that lead me to wonder how a precision WAAS approach can be executed... We've all encountered "terrain warnings", when we knew we were well clear of it.
The best explanation I've gotten, is that altitude information for the approach, is simply a "course" between to points in 3-dimensional space. Whether or not those two points are accurate as far as altitude is concerned is irrelevant; so long as they're consistenttly interpreted as the same two points.. As in.. if point one is ~1600agl and ~5 miles out, and point two is at 0agl at the touchdown point.. it doesn't matter what the GPS displays as the altitude.
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:16 | Сообщение # 29 |
|
Группа: Удаленные
| Quote (@737@) You can read many articles about GPS altitude innacuracy (and why).. and that lead me to wonder how a precision WAAS approach can be executed... We've all encountered "terrain warnings", when we knew we were well clear of it.
The best explanation I've gotten, is that altitude information for the approach, is simply a "course" between to points in 3-dimensional space. Whether or not those two points are accurate as far as altitude is concerned is irrelevant; so long as they're consistenttly interpreted as the same two points.. As in.. if point one is ~1600agl and ~5 miles out, and point two is at 0agl at the touchdown point.. it doesn't matter what the GPS displays as the altitude.
GPS altitude with WAAS (USA) is ............the closest to actual proximity with terrrain, unless the barametric altimeter setting is current for the area. At that point, they'll read the same, or usually close to 10' or less. I can get current altimeter settings from XM weather, and then compare the Garmin 696's "six pac" panel page to the aircraft's altimeter. They'll (guages) exactly follow each other for a few minutes. It's interesting to watch, considering one is computed, and one is mechanical.
There is a thread on another forum about this............I'll just have to find it.
|
| |
| |
| @737@ | Дата: Воскресенье, 14.08.2011, 06:16 | Сообщение # 30 |
|
Полковник
Группа: Персонал
Сообщений: 104
Награды: 0
Репутация: 0
Замечания: 0%
Статус: Не в сети
| There is a lot unwarranted bad press about GPS-WAAS altitude errors but if one looks into scientific literature it is in fact not so bad at all. There is a whole series of studies published by Stanford University on GPS accuracy (most of that very mathematical but there are many graphs that are quite readable) and I recall looking at numbers and yes, altitude errors are about 2-3 times higher than in horizontal plane but it still means your GPS-WAAS altitude will be within 10-20 ft with overwhelming probability.
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:17 | Сообщение # 31 |
|
Группа: Удаленные
| Hm that might be correct but I think a simple linear relation misses the fact of compressibility, and at a couple 1000s ft of altitude it's won't be the same as on the ground. In fact at FL180 0.1" would be around 200 ft if I'm not mistaken.
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:17 | Сообщение # 32 |
|
Группа: Удаленные
| Hmm, hey guys don't mind me typing this but I do not use GPS I like old style navigation. I do not want to calculate altitude using a GPS . I know there is a simple method for it . I have never learnt about a GPS or a WAAS. I am just using mental exercise (maths during a flight) personally I feel every Pilot will know it and should know it how to calculate stuff during a flight, Well what will happen if your instrument like the GPS fails ? It comes down to a pilot handling of his mental maths and good airman ship. My instructor always told me never chase the instruments during a VFR flight as they are there just for reference,but an IFR is a different ball game which I did not do.
Currently I am still switching between aircrafts I need to pick one to start my practice again. I do not want to fly fighters as I am a bit too old to join an AF though could have joined if my background had maths & science. So help me choose an aircraft with which I can practice like e.g. Cessna 152 II or a Piper . I was on the T-6 previously but now I feel I need to fly PPL.
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:17 | Сообщение # 33 |
|
Группа: Удаленные
| So.. when an altimeter reads something like 1700msl, you're actually quite a bit higher than that ?
I always assumed that the compression, and climbing up through air less an less compressed, IS the phenom that altiters are dased on.
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:17 | Сообщение # 34 |
|
Группа: Удаленные
| The default C172 is pretty good for training and learning basics; even IFR basics.. There are many great payware light GA.. and if you want something a bit more challenging, but still forgiving enough for training (and is freeware),, try my Bonanza.. it was designed for FSX, shared cockpit instrument training (no GPS)..
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:17 | Сообщение # 35 |
|
Группа: Удаленные
| Thanks, well I have the Cessna 152II Carenado and a Piper 180 Carenado, let's a person needs to start PPL what aircraft would he be made to fly ?
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:17 | Сообщение # 36 |
|
Группа: Удаленные
| Generally, it's whatever is available at the club/fbo/school.. I'd personally prefer to train a new pilot in a Piper Tomahawk (for a whole thread's worth of reasons). The C152 is a great trainer, but at 6'3" 225#, I can't fit into one. Warriors and Skyhawks make great trainers... and technically, you can train in just about any light single.. even high-performance models, from a C182, to a Comanche, to a Bonanza.
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:18 | Сообщение # 37 |
|
Группа: Удаленные
| Wow, there are so many to choose from , my aim is to fly the Citation X or the LearJet 45/60 (Hey in the Sim :) ) , after redoing every thing that I have learnt. So I guess I could either take the 152,172 or the 182's with the Comanche or a Bonanza
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:18 | Сообщение # 38 |
|
Группа: Удаленные
| It sounds very much like you're trying to sim realistically, very admirable, keeps the boredom at bay, as there's always some learning involved. If that's the case, don't let yourself anywhere near a jet cockpit, until you've master flight by instrument flight rules... not just for being able to get around in bad weather, but for the extra prescicion and workload that comes with it.
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:18 | Сообщение # 39 |
|
Группа: Удаленные
| Yep and why not :) I do not have enough green right now for a PPL and FS is a great tool for learning yeah it may just give me 60-70% realism but I suppose many RL pilots do use FS now for practice. A jet cockpit do you mean a fighter jet or the Citation's and the learjets ?
And yes it is good for boredom and for concentration. For IFR I still have a long way to go :)
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:18 | Сообщение # 40 |
|
Группа: Удаленные
| Yes.. :wink:
You'd have your Commercial and Instrument ratings in-hand, before climbing into a Lear.. And it's pretty rare that even a multi-engine prop pilot would not be instrument rated.. if for no other reason than he'd have trouble insuring something like a Baron, without an instument rating... or, the owner of the Baron's insurance company would not allow him to rent it to a non-instrument pilot.
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:18 | Сообщение # 41 |
|
Группа: Удаленные
| I did not understand "non-instrument Pilot" do you mean a pilot with NO IFR training ?
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:18 | Сообщение # 42 |
|
Группа: Удаленные
| Yes (kinda).. I mean a pilot who does not have an instrument rating..
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:18 | Сообщение # 43 |
|
Группа: Удаленные
| oh k. But is FS good for IFR training ? I have chosen 172 as the training craft, where could I find a good POH OF COURSE ONE WHICH IS FREE
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:19 | Сообщение # 44 |
|
Группа: Удаленные
| My students love this website for visually working through problems with navigational aids and how the e6b actually works. This is a link to one of the altimetry sections.
http://www.luizmonte.../Altimetry.aspx
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:19 | Сообщение # 45 |
|
Группа: Удаленные
| Hey good site, everyone has been real helpful.
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:19 | Сообщение # 46 |
|
Группа: Удаленные
| Well... sort of. And no, you're not higher (unless temperature gradient does not correspond to ISA but that's really another topic). But that's the difference between fluids and gases. Descending in water you'll always get the same dp/dz (same change of pressure for same change of depth). This is because such a fluid is incompressible (that does NOT mean the pressure stays the same, which is what I believe causes your confusion!). The air though IS compressible. I think ISA defines air pressure to halve every 18000 ft/5500 m. That means ~1000 hPa at sea level, 500 hPa at 18000 ft, 250 hPa at 36000 ft and so on. So as you see it's clearly not linear.
When studying for aviation meteorology you will have to solve questions like "QNH is 995, you are flying at FL210, what's your altitude MSL?". For these questions you'd usually use the .1" = 100 ft, or 8 m = 1 hPa or ~30ft = 1 hPa. This is essentially correct, but only at lower altitudes. At 18000 ft 1 hPa would correspond to about 60 ft, not 30, since the air is way less compressed. Think of the spread between isobars, it increases with altitude. Hope that makes any sense, and anyone correct me if I'm wrong.
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:19 | Сообщение # 47 |
|
Группа: Удаленные
| Hey guys , I know that air pressure decreases the higher you go, hmm well everyone is trying to help me but I think the confusion is caused by the question I posted.
Now I do not know how to put that question properly but let me try.
I was taught a very simple arithmetic calculation ( I am not at all good in maths but my instructor made it very simple for me ) e.g. 29.92 * / maybe the altitude with something something and I use to get the right altitude. I just cannot remember it nor I can find my notes for it.
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:19 | Сообщение # 48 |
|
Группа: Удаленные
| Of course it does, yet I'm still not sure I got my point across. Not only does air pressure decrease, but the decrease per altitude itself changes as well, in a non-linear manner (that's why the actual formulae are of exponential type after all). This effect is crucial at higher altitudes!
Hm, well but if it makes you happy I could make up somthing like this: (29.92" - air pressure[inHg]) ∙ 1000 [ft/inHg] ≈ altitude. That takes into account the relation Brett mentioned above: .1" per 100 ft.
Example: Air pressure is 28.50", what's your altitude?
(29.92" - 28.50") ∙ 1000 [ft/inHg] = 1420 ft.
Using mgh's formula I come up with ~1344 ft. So already a 5%+ error here. Why not plug his formula into an Excel sheet? Gives way better results. :Whistle:
|
| |
| |
| Robert | Дата: Воскресенье, 14.08.2011, 06:20 | Сообщение # 49 |
|
Группа: Удаленные
| I understand the compress-abilty of fluids vs gasses.. It can be demonstrated by what happens when you get air into a hydraulic brake line.
But since we know that altimeter adjustment is mechanically linear.. ala .. turn the setting knob, and displayed altitude changes by 100' per 0.1" .. no matter the altitude .. then at higher altitudes, altimeters become less and less accurate ? I guess we just accept that being in error is OK, so long as we're all experiencing the same error ?
If so, then as far as aviation goes, it IS linear.
|
| |
| |
| IROman | Дата: Воскресенье, 14.08.2011, 06:20 | Сообщение # 50 |
|
Группа: Удаленные
| Aha badderjet you should change your nickname to gooderjet :), I think this is what I was searching for.
|
| |
| |
