Pressure Adusted Altimeter Readings -vs- GPS Altimeter Readings

Geico266

Geico266

Touchdown! Greaser!
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Geico
I'm getting one of those headaches trying to figure out why the GPS altimeter does not match the altimeter on the panel. I was flying near KC and wanted to remain clear of "B" space. The altimeter setting was 30.43". When I set both altimeters on the panel both read 2,300' MSL, and life was good. I checked the altimeter reading on the GPS and it was 2,170' MSL (I think), not even close. When I landed all 3 were within 10' of the 1,500' MSL of the airport. What gives?

Is it the cold 25F tempurature?

Clearly, the WAAS GPS should be accurate with 7', but it was not even close. Why?

I know there is someone here that can explain it so my headache goes away. :redface:

Thanks in advance for your explanation.
 
Geico266 said:
I'm getting one of those headaches trying to figure out why the GPS altimeter does not match the altimeter on the panel. I was flying near KC and wanted to remain clear of "B" space. The altimeter setting was 30.43". When I set both altimeters on the panel both read 2,300' MSL, and life was good. I checked the altimeter reading on the GPS and it was 2,170' MSL (I think), not even close. When I landed all 3 were within 10' of the 1,500' MSL of the airport. What gives?

Is it the cold 25F tempurature?

Clearly, the WAAS GPS should be accurate with 7', but it was not even close. Why?

I know there is someone here that can explain it so my headache goes away. :redface:

Thanks in advance for your explanation.
Click to expand...

How much coverage does WAAS actually provide? Does it actually cover the enroute portions of flights?
 
The GPS was absolutely correct within its margin of error.

The difference at altitude was due to the change in barometric pressure not being standard lapse and not linear. The agreement on the ground was because both instruments were using the same base (field elevation).

We fly by the altimeter because it gives us all as common reference.
 
The Old Man said:
We fly by the altimeter because it gives us all as common reference.
Click to expand...

Whoa. So the GPS is actually closer than the altimeters? I guess it would have to be as is triangulating the location.

Is it conceivable that someday we will have to get rid of the old altimeters and use WAAS GPS?

Now I really have a headache.
 
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smigaldi said:
How much coverage does WAAS actually provide? Does it actually cover the enroute portions of flights?
Click to expand...


The WAAS is good no matter where you are at, I think..? It has to do with the equipment being used not the location. I mean, there are some places where WAAS would not work, like over the oceans, but near KC? The 496 was indicating "WAAS Aquired" and it was within 7' accuaracy.

I may be wrong and that is why I'm asking. I'm confused, but I think I'm right. LMAO Does that make sense?
 
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Geico266 said:
Is it the cold 25F tempurature?
Click to expand...
I'm going with this.

Geico266 said:
Clearly, the WAAS GPS should be accurate with 7', but it was not even close. Why?
Click to expand...
The GPS measures your real altitude above sea level which is already corrected for the temperature. The altimeters are only measuring indicated altitude, not corrected for temperature. When the temperature is colder than standard, you are really lower than your altimeters would suggest. This effect is magnified with your height above ground. For instance, on the ground there would be no difference between your GPS altitude and your altimeter altitude. If you are 3,000 AGL the error will be X amount. If you are 6,000 AGL the error will be larger than X.
 
Everskyward said:
The GPS measures your real altitude above sea level which is already corrected for the temperature. The altimeters are only measuring indicated altitude, not corrected for temperature. When the temperature is colder than standard, you are really lower than your altimeters would suggest. This effect is magnified with your height above ground. For instance, on the ground there would be no difference between your GPS altitude and your altimeter altitude. If you are 3,000 AGL the error will be X amount. If you are 6,000 AGL the error will be larger than X.
Click to expand...

This makes sense! The temp was colder than 59F standard. That must be the reason.

Someday, we will all be using GPS's and do away with altimeters? No more in route baro corrections! :eek: Hey, maybe the Feds will buy us all WAAS GPS's? Like they did with the digital TV transition? Not likely.
 
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GPS gives altitude relative to a particular model of the earth. I don't believe errors
with GPS altitude correlate at all with altimeters errors. So, it is quite reasonable to
see the errors add with the altimeter errors that perhaps we understand better then
GPS altitude errors.
 
You are trying to reference two different systems against each other. They are two different types of systems. Depending on your GPS system and how it is corrected will give you one reading. Your Baro Altimeter will give you another.

Baro Altimeters are effected by colder than standard temperatures. At the Altitude and temperature that you were operating at it would be very possible to have the two differ by at least 120 to 150 feet.
 
Geico266 said:
The WAAS is good no matter where you are at, I think..? It has to do with the equipment being used not the location.
Click to expand...
Sort of, yes. I suspect that the closer you are to one of the WAAS ground stations, the more your accuracy will improve. There are 20 stations in the continental U.S., so you couldn't ever be that far away from one. Btw., WAAS also works over the ocean as long as you're within the coverage area. There is a station in Hawaii.

-Felix
 
One day - perhaps before each and every member of this board has passed on to that great hangar in the sky - we will be using GPS for all altitude applications - perhaps it will be in the ADS-b implementation, perhaps in a future system that replaces ADS-B before it is ever fully implemented. Until that day I will keep in my mind that (absent radar altimetry) GPS will likely keep me out of the rocks and trees, while other aircraft and ATC will expect me to be at a certain pressure altitude.
 
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Geico266 said:
This makes sense! The temp was colder than 59F standard. That must be the reason.
Click to expand...

Your altimiter is built to indicate 1,000' per inch of mercury. That's a nice average but the actual atmosphere is rarely "average". Both temp and pressure variations can influence it. This is why minimum instrument altitudes are at least 2,000' above terrain in mountainous areas - allows room for the altimiter to be wrong.
 
The GPS altitude is your height above the theoretical spheroid in it's model of the earth. This is NOT the same thing as Mean Sea Level.

Then add in the difference in pressure lapse and temperature lapse (which affect baros but not GPS) and you can have marked differences.

The important thing is that there be a common reference for all aircraft and airports, so you can avoid hitting each other and properly reference your charts.
 
This table shows the correction to add to the approach altitudes for cold wx operations. It demonstrates the error cold temp has on altimeter readings. pretty significant.


20
20
30
30
40
40
50
50
60
90
120
170
230
290
-10
20
30
40
50
60
70
80
90
100
150
200
290
390
490
-20
30
50
60
70
100
100
120
130
140
210
280
430
570
710
-30
40
60
80
100
130
130
150
170
190
280
380
570
760
950
-40
50
80
100
120
170
170
190
220
240
360
480
720
970
1210
-50
60
90
120
150
180
210
240
270
300
450
600
890
1190
1500
Height above Aerodrome->
200
300
400
500
600
700
800
900
1000
1500
2000
3000
4000
5000
 
The Bemdix King KLN90B GPS asks for a baro input when you boot up. You can change it anytime by pushing the ALT button (and the manual encourages you to do this each time you get a different setting from ATC.). Interesting that the Garmins don't do this. Maybe such an input would be counterproductive on a WAAS GPS.
 
Geico266 said:
I'm getting one of those headaches trying to figure out why the GPS altimeter does not match the altimeter on the panel.
Click to expand...

Larry ... my 196 and my altimeter seem to be very close when
it's close to standard conditions.
 
The baro altimeter is only accurate when you are close to the reporting station and at lower altitudes. As stated before, non standard temperature and pressure lapse rate in the actual atmosphere introduce significant errors. The further you are away from the reporting location, the greater possibility for error, especially when the baro setting is changing rapidly. Regardless, baro altitude is used for vertical separation as all aircraft that use the same baro setting will have the same basic error (from temperature, pressure, and distance from the source variations). Other error sources are in the individual altimeter.

GPS altitude in WAAS GPS has a typical error of under 10 feet and does not depend on anything except the satellite geometry and the WAAS correction information. There are no temperature corrections and in fact they are not needed. The GPS determines altitude based on three dimensional triangulation from the distance to each satellite to a spherical model WGS-84 that approximates the surface of sea level thru out the world. In the case of the GNS530W and similar GPS systems, this is corrected using a database that provides for local variations between the WGS-84 model and the actual value.

GPS altitude is useful for terrain avoidance. Baro altitude is useful for vertical separation and terrain avoidance when you are close to the reporting station. There is little difference between the two when on the airport surface where the Baro altitude setting is provided. At 10000 feet, I have seen differences of up to 500 feet.

In many parts of the world and at high altitudes, flight levels are used instead of baro corrections from nearby reporting stations. When you fly a flight level, everyone is supposed to set the altimeter baro setting to 29.92. This provides vertical separation. In the past few years, RVSM has been introduced which allows for 1000 ft vertical separation at certain flight levels and provides more altitude options for complying aircraft. This is primarily achieved by a more rigorous specification for the altimetry system than we use in our piston aircraft.
 
I've really enjoyed this discussion. In trying to convince my bosses at my airline to incorporate more complete procedures, I made a simple video to explain cold weather altimetry. I have also made a list of some resources on the topic. Hope this helps:

http://www.robinmaiden.com/?s=cold+weather

tailwinds, Robin
 
Welcome! Taking a peek at your site, since we seem to occasionally have cold weather around here - not so much in the last few days, knock wood! :)
 
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