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  #1  
Old 08-05-2012, 06:37 PM
Jeff R's Avatar
Jeff R Jeff R is offline
 
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Location: Merritt Island, FL
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Default Altimeter vs GPS altitude

The field elevation at my home airport (Merritt Island) is about 6 feet. My Skyview GPS, along with a back up Garmin Aera GPS, will correctly report the altitude. On my Skyview altimeter, the correct elevation appears with the altimeter adjusted to within .01 to .02" of what the AWOS station provides.

In the air, however, I am seeing significant differences. For example, when I went flying at about 4,000 ft per the altimeter, the GPS units were reporting about 4,300 feet. That is a pretty significant difference!

I witnessed the original airspeed/altimeter/transponder calibration, and the Skyview altimeter was right on all the way up to 18,000 feet, so I don't think there is an error in the Skyview static pressure sensor.

Wondering if my GPS antennas were blocked by the fuselage, I flew N, W, S and E and the altitudes didn't vary at all.

I use the Safe Air static ports, one on each side of the fuselage, connected together and to dual ADHRs sensors in the tail.

I tend to believe the GPS altitudes are being reported correctly and since I know the altimeter has been tested and found accurate, the problem points to the static ports.

Anyone else experience big differences between altimeter and GPS altitudes, and has anyone else with the Safe Air static ports had altimeter errors?
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  #2  
Old 08-05-2012, 06:51 PM
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Smile Here's a reasonable answer

Remember, when you are moving you also introduce some error into the system.

http://docs.controlvision.com/pages/gps_altimetry.php

Hope this helps,
Paul
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  #3  
Old 08-05-2012, 06:53 PM
Sam Staton Sam Staton is offline
 
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Location: Jacksonville, FL
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Default Trust your instruments

Disclaimer - I am not an expert - I don't even play one on TV. GPS altitudes are calculated, and, unless you have an approach certified GPS, not dependable. That is why we have approach certified GPS devices. They use the WAAS signal to derive altitude.
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  #4  
Old 08-05-2012, 06:55 PM
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Over the last few years, I've found that the GPS altitude is the correct one, and the altimeter is varying with barometric pressure. If I get a very current altimeter setting, my Garmin 696's panel page altimeter, and the aircraft's will exactly follow each other for a few minutes. It happens every time I get a current setting for my location. After a few minutes, they drift apart, and can be hundreds of feet difference sometimes. Of course, it's the barometer setting that's important for aircraft separation, and I'll take the GPS measurement to miss a mountain.

L.Adamson
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  #5  
Old 08-05-2012, 06:56 PM
guccidude1 guccidude1 is offline
 
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Default Altimeter vs GPS altitude

I think if you check, you will find that there is a difference between GPS altitude, pressure altitude, density altitude, and our barometric altimeter altitude. The one that is accepted by for flight is the standard altimeter. Remember, it is the only altitude readout that requires biannual certification for contiued flight operations. Dan
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  #6  
Old 08-05-2012, 06:57 PM
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Default

Barometric altitude and GPS altitude are very different and would only correspond occasionally in flight. The GPS measures altitude geometrically and is generally more accurate for showing your actual elevation above sea level. The altimeter shows altitude by sensing pressure and relating this to a model of the atmosphere. If there is a standard atmosphere the two different methods will show close to the same figure. Rarely is there a standard atmosphere. This is why altimeters can not be relied on closely for terrain clearance and is why certain flight levels are not available when conditions are certain way. The two different instruments should be close on the ground if the altimeter has a recent and accurate altimeter setting.

It is necessary to use your altimeter for a reference in flight because everyone else is doing it, but that doesn't mean it is giving the best information for all purposes all the time.
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  #7  
Old 08-05-2012, 07:00 PM
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Quote:
Originally Posted by Sam Staton View Post
Disclaimer - I am not an expert - I don't even play one on TV. GPS altitudes are calculated, and, unless you have an approach certified GPS, not dependable. That is why we have approach certified GPS devices. They use the WAAS signal to derive altitude.
See my last reply. It's a 696 with WAAS, and nails altitude every time, as long as I'm moving. It really IS that good...............as it will exactly correlate with a very current altimeter setting. It's almost weird to see a GPS derived animated altimeter, and the aircraft's follow each other. BTW-- since I do have quite an interest in GPS/WAAS reliability, I've often compared readings to known elevations in numerous mountain areas such as national parks & highway summits. The readings are very close.......and who's to say that the posted elevation is exact.
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  #8  
Old 08-05-2012, 07:11 PM
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When I fly, it is a very rare case for my 696 to agree with my altimeter even when I have current updates for the pressure. Today's cross country was typical. I was making regular updates to the barometric setting (30.10 thru 30.06) on my altimeter as I was flying at 10'500 and the 696 was reporting 500' higher throughout the "at altitude" portion of the flight. My outbound flight a few days ago had a very different barometric pressure (29.89 thru 29.90) and the delta results were the same.

As a complementary reference, my APRS tracker also has a WAAS GPS and it agreed with the 696.

Since ATC traffic reports agree with my altimeter, I defer to it "for the record".
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Last edited by humptybump : 08-05-2012 at 07:13 PM.
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  #9  
Old 08-05-2012, 07:18 PM
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Quote:
Originally Posted by humptybump View Post
When I fly, it is a very rare case for my 696 to agree with my altimeter even when I have current updates for the pressure. Today's cross country was typical. I was making regular updates to the barometric setting (30.10 thru 30.06) on my altimeter as I was flying at 10'500 and the 696 was reporting 500' higher throughout the "at altitude" portion of the flight. My outbound flight a few days ago had a very different barometric pressure (29.89 thru 29.90) and the delta results were the same.

As a complementary reference, my APRS tracker also has a WAAS GPS and it agreed with the 696.

Since ATC traffic reports agree with my altimeter, I defer to it "for the record".
Then, we'll battle it out. Perhaps your current altitmeter setting just isn't close enough for the exact area. ATC traffic reports are using barametric pressure. I make comparisons to known elevations. No way, is the 696 500' off a known elevation. It's usually not even 10' off.
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  #10  
Old 08-05-2012, 07:45 PM
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Going back through some in flight pics, some more observations.

Before using XM weather's altimeter setting function...........I see that my altimeter & GPS reading could easily be 300' difference. Once I had the tendency to update my altimeter, as "shown", as the XM setting changed..........then my altimeter & GPS are very close.

L.Adamson
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