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 with WAAS
I posted earlier about seeing differences of up to 500' between the
altitude shown on my Garmin GPS 196 (even while receiving WAAS) and my altimeter. The GPS would consistently indicate higher than the altimeter. Dan Luke suggested the cause might be non-standard temperatures. After some more experimenting, I think Dan was correct. On a flight last week, the GPS altitude and altimeter matched within 20 feet. The temperature was close to standard, which made me think that temperature might be in fact be the culprit as Dan suspected. I did another trip over the weekend and found the GPS reading high for altitude again, and the temperatures were above standard. Here's what I got.... SATURDAY SUNDAY Indicated Altitude 7500 8500 GPS Altitude 7700 8677 Pressure Altitude 7350 8310 True Altitude 7700 8700 Density Altitude 8200 9100 Altimeter setting 30.07 30.11 Temp (C) +8 +5 Std Temp (C) 0 -2 So it looks like the GPS is showing True Altitude (which makes a lot of sense). True altitude is the actual height above MSL, and that will differ from indicated altitude when the temperature is not standard, or you forget to change your altimeter setting. The true altitude numbers above were calculated using my good old-fashioned E6B. So the GPS is showing the right value. However, that value is different than what you altimeter shows when the temperature is not standard. When temperatures are above standard, the GPS altitude will be higher than indicated and when termperatures are below standard the GPS altitude will show lower than indicated. Does this make sense? I wonder how this will affect the upcoming WAAS LPV approaches. They are going to have a decision altitude based on indicated altitude, not true altitude. How will the difference between true altitude and indicated altitude affect the approach, since the GPS will be giving vertical guidance based on true altitude? Phil www.pfactor.com |
#2
|
|||
|
|||
In article ,
Phil Verghese wrote: So it looks like the GPS is showing True Altitude (which makes a lot of sense). True altitude is the actual height above MSL, More precisely it's showing the height above the datum, which is the mathematical model of the earth ellipsoid the GPS uses to approximate the actual Earth. -- Ben Jackson http://www.ben.com/ |
#3
|
|||
|
|||
Recently, Ben Jackson posted:
In article , Phil Verghese wrote: So it looks like the GPS is showing True Altitude (which makes a lot of sense). True altitude is the actual height above MSL, More precisely it's showing the height above the datum, which is the mathematical model of the earth ellipsoid the GPS uses to approximate the actual Earth. General question: is there are practical difference between the datum and MSL? Neither have any local relevance, AFAICT. Neil |
#4
|
|||
|
|||
Recently, Ben Jackson posted:
More precisely it's showing the height above the datum, which is the mathematical model of the earth ellipsoid the GPS uses to approximate the actual Earth. "Neil Gould" wrote in message k.net... General question: is there are practical difference between the datum and MSL? Neither have any local relevance, AFAICT. The geoid height is typically -100 ft over the US, meaning that the zero of the NAD 83 ellipsoid and mean sea level differ by about 100 ft. Julian Scarfe |
#5
|
|||
|
|||
Recently, Ben Jackson posted:
In article , Phil Verghese wrote: So it looks like the GPS is showing True Altitude (which makes a lot of sense). True altitude is the actual height above MSL, More precisely it's showing the height above the datum, which is the mathematical model of the earth ellipsoid the GPS uses to approximate the actual Earth. The Garmin GPS receivers that I've used all report elevation above the geoid which models Mean Sea Level (MSL), not the ellipsoid. Internally they initially calculate elevation above the ellipsoid, but then they apply values from a lookup table that has corrections for the geoid - ellipsoid difference based on the geographic position. I believe Magellan units and some newer Lowrance units also correct for the geoid-ellipsoid difference so the reported elevations represent height above MSL rather than the ellipsoid. |
#6
|
|||
|
|||
Phil Verghese writes:
I posted earlier about seeing differences of up to 500' between the altitude shown on my Garmin GPS 196 (even while receiving WAAS) and my altimeter. The GPS would consistently indicate higher than the altimeter. Dan Luke suggested the cause might be non-standard temperatures. After some more experimenting, I think Dan was correct. In Canada, learning about temperature errors in the altimeter is a standard part of the PPL curriculum, but I've noticed that it's not so familiar to U.S. pilots (at least not private pilots). We have tables in our AIP and other publications showing what errors to expect, and when flying IFR, we are required to add those errors to all instrument approach altitudes (MDA, DH, etc.) in very cold temperatures. So the GPS is showing the right value. However, that value is different than what you altimeter shows when the temperature is not standard. When temperatures are above standard, the GPS altitude will be higher than indicated and when termperatures are below standard the GPS altitude will show lower than indicated. Does this make sense? Yes. A rough rule of thumb is that your altimeter will be off by 4 feet, per degree Celsius difference from ISA, per thousand feet above the station reporting the altimeter setting. This works, of course, only with the standard lapse rate -- if there's an inversion or any other non-standard lapse rate between you and the field reporting the altimeter setting, this formula won't work. Let's assume that your field is at 1000 ft MSL with 20 degC above standard temperature and a standard lapse rate all the way up. When you're sitting on the field, your altimeter should show your actual elevation: 4 * 20 * 0 = 0 ft error So you'll see 1000 ft on your altimeter when you dial in the field's altimeter setting. At 200 ft AGL (typical ILS DH), your error will will be 4 * 20 * 0.2 = 16 so that when your altimeter reads 1200 ft on short final, you're really around 1216 ft MSL, or 216 ft AGL. At 9000 ft MSL (8000 ft AGL), the error will be much larger: 4 * 20 * 8 = 640 ft So when your altimeter says 9000 ft, you'll really be up at 9640 ft. That's no big deal, but it could be dangerous in cold temperatures, if you were trying to clear (say) an 8000 ft ridge with 1000 ft clearance. If the temperature was 20 degC below standard, and your altimeter setting was from a field at 1000 ft MSL, you'd actually be flying at 8360 ft when your altimeter read 9000 ft, just barely clearing the ridge. I wonder how this will affect the upcoming WAAS LPV approaches. They are going to have a decision altitude based on indicated altitude, not true altitude. How will the difference between true altitude and indicated altitude affect the approach, since the GPS will be giving vertical guidance based on true altitude? The approach will be a safer than approaches based on a barometric altimeter, all things considered. Barometric altimeters are fine for cruise, since everyone sees the same error and flies at the same altitude anyway (just not the one they think they're flying at). All the best, David |
#7
|
|||
|
|||
David Megginson wrote
In Canada, learning about temperature errors in the altimeter is a standard part of the PPL curriculum, but I've noticed that it's not so familiar to U.S. pilots (at least not private pilots). I'm sure that Canada is the promised land as far as aviation education goes, and that no instructor there ever sends a student to a checkride when the student isn't REALLY ready to exercise ALL the privileges of the certificate but is likely to be able to pass the flight test. Unfortunately, this is not the case in the US. Nonetheless, altimeter temperature errors ARE a part of the standard US private curriculum. We have tables in our AIP and other publications showing what errors to expect, I'm sure that in this case you are referring to the ICAO Cold Temperature Error Table, which is part of the AIM. Check it out online: http://www1.faa.gov/ATPubs/AIM/Chap7/aim0702.html#7-2-3 I don't know about you, but I have yet to meet a US flight instructor who does not require his students to have a copy of the AIM and be conversant with it. Thus I have to assume that every US student pilot has seen this table at one time or another. It doesn't particularly surprise me that many don't recall this. I'm sure you learned how to extract a square root manually in primary school, as did everyone else. Can you still do it? Maybe. But I assure you that not only are most adults incapable of doing it, but that many will claim never to have learned. Use it or lose it. For most private flying in the continental US, altimeter errors are not particularly relevant. and when flying IFR, we are required to add those errors to all instrument approach altitudes (MDA, DH, etc.) in very cold temperatures. See, that's the basic difference. We are not REQUIRED to do anything about those tables. The information is made available to us, along with a recommendation to adjust minima as appropriate. It is part of the AIM, which every private pilot candidate is expected to become familiar with. How we choose to use the information is up to us. The altimeter errors are very real - but a correction to MDA or DH is not always necessary. For example, many ILS approaches are equipped with a middle marker. The marker is not affected by altimeter errors. It's not really the MAP, but in most cases it's close enough that it can be used to signal a missed approach in situations where the altimeter is suspected of reading high. Let's not forget that the worst case temperature error at 200 ft and -50C is only 60 ft, while altimeters can be up to 75 ft off in some cases and still be legal for IFR use. There used to be a DH penalty for an inop middle marker (either at the transmitter or receiver end) but this penalty no longer applies. All this ignores the possibility that the pilost has a RADAR altimeter available. In the US, it is up to the pilot to decide whether in his particular situation, given the available equipment and his skills, he should adjust the minima as appropriate based on the expected temperature error. Michael |
#8
|
|||
|
|||
|
#9
|
|||
|
|||
"David Megginson" wrote in message ... (Michael) writes: I'm sure that Canada is the promised land as far as aviation education goes, Not at all -- I've just been surprised at how many U.S. pilots don't seem to know about altimeter temperature errors. On mailing lists, I've actually had violent reactions from otherwise experienced and competent pilots when I casually mentioned that pressure altimeters are routinely off by hundreds of feet at cruise altitude. I think what they teach us is density altitude differences, but that's for performance during takeoff. I wonder if the new RVSM rules/equipment might refocus some discussion in that direction? Tom |
#10
|
|||
|
|||
David Megginson wrote: On mailing lists, I've actually had violent reactions from otherwise experienced and competent pilots when I casually mentioned that pressure altimeters are routinely off by hundreds of feet at cruise altitude. It's pretty simple, really. It doesn't matter at all if your altimeter is off by hundreds of feet at cruise altitude if everybody else at that altitude has the same error. It would be possibly unsafe for you to set your altimeter accurately when everyone else is setting it to the broadcast local altimeter setting. In short, it doesn't matter. Just do it like everyone else. For noise abatement reasons. George Patterson A man who carries a cat by the tail learns something that cannot be learned any other way. Samuel Clemens |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Aerobatics | 28 | January 2nd 09 02:26 PM |
Terminology of New WAAS, VNAV, LPV approach types | Tarver Engineering | Instrument Flight Rules | 2 | August 5th 03 03:50 AM |
Big News -- WAAS GPS is Operational for IFR | Lockheed employee | Instrument Flight Rules | 87 | July 30th 03 02:08 AM |
Garmin Behind the Curve on WAAS GPS VNAV Approaches | Richard Kaplan | Instrument Flight Rules | 24 | July 18th 03 01:43 PM |
High Altitude operations (Turbo charge???) | Andre | Home Built | 68 | July 11th 03 11:59 PM |