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GPS Altitude with WAAS



 
 
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  #1  
Old September 23rd 03, 04:57 AM
Phil Verghese
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Default 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  
Old September 23rd 03, 07:34 AM
Ben Jackson
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Default

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  
Old September 23rd 03, 01:20 PM
Neil Gould
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Default

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  
Old September 23rd 03, 01:21 PM
David Megginson
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Default

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
  #5  
Old September 23rd 03, 02:51 PM
Julian Scarfe
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Default

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


  #6  
Old September 23rd 03, 03:52 PM
Peter
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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.

  #7  
Old September 23rd 03, 09:03 PM
Michael
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Default

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
  #10  
Old September 23rd 03, 11:48 PM
David Megginson
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Default

"G.R. Patterson III" writes:

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.


Unless you're trying to clear mountains using an altimeter setting
from a low-elevation field.

It would be possibly unsafe for you to set your altimeter accurately
when everyone else is setting it to the broadcast local altimeter
setting.


Absolutely -- no one is suggesting changing the altimeter setting.
You just have to be aware of how inaccurate the altimeter is when
obstacle clearance might be an issue. For example, if you are
planning to clear a ridge by only 1000 ft in the winter, you might
want to think again.


All the best,


David
 




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