Altitude and Altimiter setting question

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ted heck

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Ted
Ok
I'm not a student any more but I sure don't know the answer to this from my training.

When I am flying cross country and I set my altitude by GPS ATC will tell me that I my transponder is reporting about 400 feet lower than my GPS Altitude.
If I set my Altimeter to the setting they give me then the Transponder and my Altimeter will match up. However they don't match my GPS altitude.
So should I be flying at altimeter altitude or at GPS altitude.
And yes I am familiar with GPS altitude being off by plus or minus 75 feet most of the time.
So what should it be?
GPS Altitude or Altimeter altitude?
Or is there some thing wrong with my equipment?

Thanks
 
DONT use GPS altitude.

Use local altimeter setting, when you start up use the altimeter setting or set to field elivation.


And no, nothing is wrong with your equipment.
 
As I understand it, you should set your altimeter to use the barometric pressure you get from ATIS/ASOS/ATC so that you're flying on the same altitude standard as everybody else who's doing that.:yesnod: Otherwise, your GPS-based 5500 may end up overlapping with somebody else's barometric 5000, and that's never good.
 
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GPS altitude is positional absolute altitude. Altimeter is pressure altitude and is what your altimeter should be set at. That's the one you use.
 
Everyone is using the altimeter setting to reference their position.....do not use GPS altitude for anything.
 
ted heck said:
Ok

If I set my Altimeter to the setting they give me
Click to expand...
Why would it ever be set to something else?

GPS altitude is absolutely 100% totally without a doubt not relevant for determining your altitude as part of the air traffic system. It may be more accurate than a barometric altimeter for determining your actual height above the ground, but the system is designed around flying at pressure altitudes (as reported by your altimeter and transponder encoder) and flying a geometric altitude will just put you at the wrong place.

If you are worried, you can get a transponder / static system check done, but the fundamental principal is that you fly pressure altitudes.
 
ted heck said:
Ok
I'm not a student any more but I sure don't know the answer to this from my training.

When I am flying cross country and I set my altitude by GPS ATC will tell me that I my transponder is reporting about 400 feet lower than my GPS Altitude.
If I set my Altimeter to the setting they give me then the Transponder and my Altimeter will match up. However they don't match my GPS altitude.
So should I be flying at altimeter altitude or at GPS altitude.
And yes I am familiar with GPS altitude being off by plus or minus 75 feet most of the time.
So what should it be?
GPS Altitude or Altimeter altitude?
Or is there some thing wrong with my equipment?

Thanks
Click to expand...

AIM 1-1-17(3)(b): "Do not use GPS derived altitude due to the large vertical errors that will make the integrity monitoring function invalid."

Bob Gardner
 
1) your encoder won't necessarily match your altimeter. I occasionally ask tower what my Mode C is reporting so I can compare to my altimeter. It's usually different by a couple of hundred feet.
2) your altimeter may have an error. Establish that error and compensate for it. If you need to set it .02" low to properly indicate a known elevation adjust it .02" low during your travels.
 
Ok there we go. Thanks all for your answers and they all seem to correspond.
"Fly by Altimeter altitude using the altimeter settings given by ATC"!

Another question. When we go to ADSB will we switch from flying by Pressure to flying by GPS?
 
ted heck said:
Ok there we go. Thanks all for your answers and they all seem to correspond.
"Fly by Altimeter altitude using the altimeter settings given by ATC"!

Another question. When we go to ADSB will we switch from flying by Pressure to flying by GPS?
Click to expand...

Nope.
 
Also. Transponder reports Pressure Altitude(29.92 duh! :eek:), and not your Altimeter Setting Altitude

Basically, it reports a Flight Level, rather than your Altimeter reading.
 
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genna said:
Also. Transponder reports Pressure Altitude(29.32), and not your Altimeter Setting Altitude

Basically, it reports a Flight Level, rather than your Altimeter reading.
Click to expand...

29.32? I'm genuinely curious how that typo came about.
 
mkosmo said:
29.32? I'm genuinely curious how that typo came about.
Click to expand...

my bad.. I have no idea to be honest. I must have had a type blackout moment :mad2::rolleyes2:

29.92... I shall correct that
 
write-stuff said:
GPS altitude is positional absolute altitude. Altimeter is pressure altitude and is what your altimeter should be set at. That's the one you use.
Click to expand...

No. GPS altitude is relative to a fictional ellipsoid. It differs from real altitude by up to several hundred feet, unless augmented by ground stations to remove the difference. Those ground stations are really only intended to support approaches, so don't assume they will clear you over arbitrary terrain too precisely.

More importantly, for dodging traffic, if you magically had the "right" altitude and everyone else was off by 500 feet, who do you think would be at fault for a midair? It's rather explicit for Class A, but it's just as important for other airspace.
 
ted heck said:
... If I set my Altimeter to the setting they give me then the Transponder and my Altimeter will match up ...
Click to expand...

Only if your altimeter setting happens to be 29.92 because that is what the encoder for your transponder uses.

As you've already been told you don't use GPS altitude for anything. Where did you get the idea that you should?
 
When set to the ATC provided barometric pressure, your altimeter shouldn't be more than 75 feet off from a known elevation such as somewhere on an airport. The elevation provided in pubs for an airport is the highest point on a runway.
 
Here's some reading on the subject.

http://www.avweb.com/news/avionics/183231-1.html

Here's the relevant stuff:

Altitude reporting

The altitude-reporting capability of your transponder transmits your aircraft's PRESSURE ALTITUDE (rounded off to the nearest 100 feet) whenever it receives a Mode C interrogation and is switched to ALT mode. You might recall from your private pilot groundschool that pressure altitude is what the altimeter reads if you set it to 29.92 In. Hg. Because the transponder reports pressure altitude, the altimeter setting that you dial into your altimeter's Kollsman window has absolutely no effect on your Mode C altitude reports. It is this fact that makes "blind encoders" (which are mounted behind the panel and have no setting knob at all) practical.

Now you might ask, "if my altimeter is set at 30.23 and the Mode C is putting out altitude referenced to 29.92, won't the controller see my altitude incorrectly?" No, because ATC's ground equipment automatically adjusts your Mode C readout for the local altimeter setting (which its computer knows about). That's why it's important always to make sure your altimeter is set to the altimeter setting that ATC gives you from time to time. That way, the controller will be seeing the same altitude that you're seeing.
Click to expand...
 
bobmrg said:
AIM 1-1-17(3)(b): "Do not use GPS derived altitude due to the large vertical errors that will make the integrity monitoring function invalid."

Bob Gardner
Click to expand...

Bob,

The AIM is wrong on the reason they cite, if anything the GPS altitude is more accurate than the barometric altimeter in measuring height above the ground. They are correct that the GPS altitude should not be used in lieu of the Barometric altitude, because we separate from each other vertically by using a common pressure altitude and altimeter setting. GPS altitude is more suited for uses such as terrain or obstacle avoidance.
 
Use local altimeter setting ,so everyone is on the same page.
 
Norman said:
I found this to be informative.

http://gpsinformation.net/main/altitude.htm
Click to expand...

Note that that is before WAAS.

It's OK to use (certified!) GPS altitude to land your small plane IF the approach has been surveyed and contains LPV minima, the database is up to date, the approach plate is accessible, RAIM is functional and not alerting, and you can sight the "runway environment" from the minimum or higher. Otherwise, it's not a good idea. LPV minima go down to 200 AGL in the best case, and it's foolish to F around with any unknowns that close to the ground.

Taking the equipment outside its certification box is effectively test piloting, and in IMC, it can mean surveying the terrain by touch.

I had a portable GPS (not an iPad) on Monday that had a hell of a time finding more than two satellites. The two G1000 GPSs each found 14.
 
Your aircraft altimeter knows the air pressure at its current altitude and location. You get the sea level adjusted barometric pressure (like they dug a hole to sea level and dropped a barometer in it) when ATC gives it to you. This doesn't change much from location to locatin, it only changes with weather changes between you and the station. It doesn't change because the ground below you is lower or higher for example because its a sea level figure, nor does it change because you are at a different altitude.

Now, having those two readings, sea level pressure and pressure at your aircrafts location, you put the barometer ATC gives you into the Kohlsman window and presto! You get the "correct" altitude reading on your planes altimeter. At least its close to being correct and the same as everyone else around you is using. Its the one to use.

As for GPS altitude, that one isn't used for your altimeter for a variety of reasons we use pressure altitudes, not GPS altitudes.
 
John Collins said:
Bob,

The AIM is wrong on the reason they cite, if anything the GPS altitude is more accurate than the barometric altimeter in measuring height above the ground. They are correct that the GPS altitude should not be used in lieu of the Barometric altitude, because we separate from each other vertically by using a common pressure altitude and altimeter setting. GPS altitude is more suited for uses such as terrain or obstacle avoidance.
Click to expand...

"All I know is what I read in the papers." -- Will Rogers

Bob
 
SkyDog58 said:
Yep. Flying 101.

I have never heard of anyone using GPS altitude to set their altimeter. Do some CFIs teach this?
Click to expand...
I sure hope not! There's an issue in fundamentals if so.

Sent from my Nexus 9 using Tapatalk
 
coloradobluesky said:
You get the sea level adjusted barometric pressure (like they dug a hole to sea level and dropped a barometer in it) when ATC gives it to you.
Click to expand...
Not really, because the altimeter setting is also corrected for non-standard temperature up to the reporting station's field elevation. That's why the "SLP" in the METAR often doesn't match the altimeter setting.
 
Raw GPS altitude without correction is an altitude above a fictional ellipsoid that doesn't match the planet. And depending on which datum is used, even that ellipsoid isn't the same. WAAS is the standard way to correct it for aviation use, in other industries there's other ways to "differentially" correct the number so it matches the planet.

And as others have pointed out, it doesn't matter anyway. We fly pressure altitudes in aviation, not true altitudes. Everyone has to reference the same source, or the separation isn't what is desired.
 
RalphInCA said:
So, is GPS altitude useful for anything in aviation?
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I found GPS altitude very useful one pitch black night where I though I might have been caught in a downdraft and was able to cross check my GPS altitude to verify that my instruments were indeed reading correctly and I was not in fact loosing altitude when my indicated airspeed was going through the roof without a power or pitch change.
 
RalphInCA said:
So, is GPS altitude useful for anything in aviation?
Click to expand...

When coupled with WAAS, it is very accurate when dealing with terrain and obstructions, generally better than the barometric altimeter (when away from the airport), since the latter is temperature dependent.
But for traffic separation it's important for everyone to be on the same page, even if that page has errors, since they are common to everyone.
 
Yes to all the above. If you want something simple, set field elevation in the altimeter before you start.
 
RalphInCA said:
So, is GPS altitude useful for anything in aviation?
Click to expand...

It's interesting to see how inaccurate barometric altimetry at altitude. At FL370 the altimeter set to 29.92 is often several thousand feet off.
 
RalphInCA said:
So, is GPS altitude useful for anything in aviation?
Click to expand...

Not really. And on purpose. GPS is not required, and honestly, it doesn't buy you much.

We use it to estimate water vapor overburden in the stratosphere, as our observation durations depend on that exponentially. To a large extent, that depends on how much atmosphere is above you, and what few observations there are are done geometrically (by observing the sun through the earth's limb from orbit).
 
MAKG1 said:
Not really. And on purpose. GPS is not required, and honestly, it doesn't buy you much.
Click to expand...

GPS is used for terrain and obstacle warning. See the terrain page on a 430w for an example. The synthetic vision systems also use GPS data.
 
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