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Thread: Instruments for Home Built

  1. #11

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    Apparently there has been some old discussion about this and it seems current software has been corrected to make it more accurate, Just one link http://www.gpspassion.com/forumsen/t...TOPIC_ID=10915 last post in 2012.

  2. #12
    Auburntsts's Avatar
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    Quote Originally Posted by wmgeorge View Post
    Apparently there has been some old discussion about this and it seems current software has been corrected to make it more accurate, Just one link http://www.gpspassion.com/forumsen/t...TOPIC_ID=10915 last post in 2012.

    I can tell you from experience that the error is still very real. However, the bottom line is to fly, you have to do it the FAA way regardless of how you personally feel about it. If you really have to know your AGL altitude, invest in a radar altimeter (I'm kidding). Otherwise, my advice is save the philosophical debates for later and focus on learning what you need to obtain your ticket. There will be plenty of time after that to figure out how to implement the better mouse trap. Good luck!
    Todd Stovall
    PP ASEL - IA
    RV-10 N728TT - Flying
    My builder's log (which is woefully out of date): www.mykitlog.com/auburntsts
    WAR DAMN EAGLE!

  3. #13

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    Quote Originally Posted by Auburntsts View Post
    I can tell you from experience that the error is still very real. However, the bottom line is to fly, you have to do it the FAA way regardless of how you personally feel about it. If you really have to know your AGL altitude, invest in a radar altimeter (I'm kidding). Otherwise, my advice is save the philosophical debates for later and focus on learning what you need to obtain your ticket. There will be plenty of time after that to figure out how to implement the better mouse trap. Good luck!
    Thanks for the help and elightment. Frankly I thought the altitude was calculated from the GPS Horizonal position and was referenced back to topographical satellite mapping? I still like the standard altimeter.

  4. #14
    About the only time actual height above ground is referenced is when flying a precision approach; i.e. an ILS or RNAV/GPS approach with LPV(Localizer Performance) minimums. The Decision Height (DH), typically 200', would be referenced on the radar altimeter, but the minimums on the approach are still referenced to the altimeter and are designated as Decision Altitude (DA). When flying in the low altitude airway structure in the U.S., the altimeter is changed along the route from station to station generally within 100 miles of an aircraft's position. This altimeter setting is called QNH and ensures that all aircraft in your vicinity will be flying the same barometric reference. When the airplane lands, you should read field elevation on the altimeter. Some countries might reference QFE altimeter settings(Russia?), so that when the airplane lands the altimeter reads zero. When aircraft are flying in the high altitude structure in the U.S., altimeters are set to "Standard", which is 29.92" or 1013.2 hPa(hectoPaschals)(millibars) when climbing though 18,000'. Aircraft are now flying in the Flight Levels. This altimetry reference is called QNE. In different parts of the world this transition altitude may be much lower; say 5000'. This is probably way more information than you care about; but at any rate, the GPS altitude is useless data and I don't bother to display it on my iFly 740 GPS.

    Dennis Crenshaw Stinson 108-3

  5. #15

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  6. #16

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    No, all information and help is appreciated. I realize being over 70 is a little late to start but I have always been up to date on computers and technology. Being retired and with a nice home machine shop, a couple laser engraving machines and CNC router I never lack for something to do! Just finishing up on a kitchen remodel so I have a little time on my hands.

  7. #17
    FlyingRon's Avatar
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    You're off on your computation of accuracy as well. WAAS GPS vertical accuracy is on the order of 10 meters (32+ feet) and as RonW points out, the GPS altitude is NOT the same as the barometric reason for two reasons:

    1. GPS altitude is determined based on a stylized model of the shape of the earth. IFR vertical guidance is predicated on the fact that the individual approach in the database has the GPS altitude in it as well (and still you have to tell the GPS about the baro correction).

    2. Everybody else (as Ron W says) is using baro altitude, which may not be accurate but ATC has told them what the common altimeter setting everybody should be using is.

  8. #18

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    And GPS can play games as well.

    This morning's flight, as recorded using GPS data, had my aircraft hovering fifty feet above the ground during landing and while putting her back in the hangar.

    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  9. #19

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    Went out to our local airport flying service this morning to pick up a sectional map, and a young pilot was standing there. Turns out he was CFI and he said there a big changes coming from the FAA in the next few years regarding GPS and navigation in general and not to believe everything you read online.

  10. #20
    Auburntsts's Avatar
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    Quote Originally Posted by wmgeorge View Post
    Went out to our local airport flying service this morning to pick up a sectional map, and a young pilot was standing there. Turns out he was CFI and he said there a big changes coming from the FAA in the next few years regarding GPS and navigation in general and not to believe everything you read online.
    Sigh. Yeah NEXGEN will save us all. Meanwhile I’ll just keep motoring along. The thing of it is I navigate 99% by IFR GPS but I have the skills to navigate without it and I do practice those skills. I also fly behind a 100% glass instrument panel and use an iPad based EFB for all my charts and pubs.

    Best to learn how to fly in today’s airspace using today’s equipment and procedures and not put a lot of stock into what iffing the future. If you end up building then you can worry about that kind of stuff once you get to the point where you are ready to purchase avionics.
    Todd Stovall
    PP ASEL - IA
    RV-10 N728TT - Flying
    My builder's log (which is woefully out of date): www.mykitlog.com/auburntsts
    WAR DAMN EAGLE!

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