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N6722C avatar image
N6722C asked FlyingsCool answered

How does ATC get a plane's Altitude ? - and get it Wrong !

How does ATC get a plane's Altitude , because ATC appears to think the plane is at a different Altitude to where it really is.

This is NOT an issue at STD Pressure. (Clear skys)

  • Plane Altimeter indicates correct altitude, set to Std Pressure.
  • AP holds the plane at the desired Altitude.
  • GPS indicates same correct altitude.

    ATC does NOT complain about an incorrect altitude

But if weather is not Clear Sky, and altimeter setting is not STD.

After setting altimeter to correct Value

  • Plane Altimeter indicates correct altitude, and current correct Altimeter setting
  • AP Holds the plane at the desired altitude.
  • GPS indicates that same correct Altitude.

But ATC complains that plane is at Incorrect Altitude

Its almost as if ATC is reading the Transponder which is always calibrated to Std Pressure, and is not taking into account that that altitude needs to be corrected by them to show the True Altitude of the Plane.

This may be associated with the use of the B key to set Altimeters, because currently, the B key ALSO sets the Pressure sensor in the Transponder, so it showns ALTITUDE, and not Flight Level

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This is a known issue to be fixed in World Update 6. Okay, I changed this to a comment, which is what I meant to do anyway. I'm a developer myself (I'm not with Asobo) and can only give a general answer as to why ATC does not see the airplane at the same altitude that the plane's altimeter does. (Basically, the ATC altitude has not been updated to the new altimetry algorithms used for the airplane.) But I don't really see the point in identifying in detail how the ATC altitude algorithm is in error since it will be changed (fixed) in the upcoming update (unless you are a developer of ATC software?).

What I think is even more interesting is the fixes that will need to be applied by VATSIM and other 3rd party ATC services to not only provide ATC controllers with the altitude that will match the airplane's indicated altitude (with baro set correctly), but also to display MSFS and non-MSFS airplanes at the correct relative geometric heights since MSFS will be the only simulator that accounts for non-standard pressure gradients.

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Thanks fro your detailed reply. It does raise a few interesting points.

Mainly, if it is not clearly defined and understood how ATC gets the plane's altitude, how can anyone, (Asobo, pilots, ATC, Vatsim) know what is causing any perceived errors.

Yes, I an talking with my Developer's hat on.

In the real world, it does not matter what the pilot sets his Altimeter setting to, correct or incorrect, it will NOT affect the altitude that the ATC system see, coming form the Plane, as that data, sent to ATC, is always based on a Std Pressure calibrated Baro Sensor.
(lets ignore any detailed instrument information, being set by other methods, for what ever other purposes)

It is up to the ATC computers to adjust that FLIGHT LEVEL, to an Altitude, using the current plane's location altimeter setting, and provide that Processed information to the controller.

This is important to know, along with what data, in the SIM, is being used by ATC, or Vatsim, to give the Controller the Plane's altitude (or FL if above the Transition Altitude).

If not done correctly, soon things get into a real confusing mess. (as they appear to be at the moment)

Example. If you set your Baros (the lazy way) using the B key, it also sets the baro in the Transponder, which, should really remain at a calibrated Std Pressure..

Currently, GA Transponders, like the KAP140, are displaying this B Set baro reading on their display, so they are in fact displaying ALTITUDE, when in fact , they should be displaying FL, as obtained from the Std Pressure calibrated Baro associated with the Transponder. (even below the Transition level) is this is what is being used by ATC, or even worse, the Plane's Pilot adjustable, Altitude !!

Then there is the other side of things, how the sim calculates the Pressure, and how that pressure changes with altitude AND Temperature - which I am assuming is the CORRECTION being made for SU#6.

One can take the approach .. let ASOBO sort it all out, no need to understand what is happening, and just blindly assume they get it all correct, OR take steps to Understand how it should work, how it is working (or not), and thereby be able to make a valued judgement as to what is being done.

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I am not aware of transponder baro settings (and that it can be changed when using the "b" key to set the altimeter baro) within MSFS. What I do know is that there is a simvar for pressure altitude and it is that simvar that is being used for VATSIM FL above the transition altitude. VATSIM uses "plane altitude" below the transition altitude, which was okay until SU5's introduction of including the effect of non-standard pressure gradients on actual airplane height above mean sea level for a given altimeter indicated height.

Rather than having me try to explain my understanding of what all is taking place behind the scenes, I suggest you have a look at this thread in the MSFS forum, and in particular this post

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N6722C answered N6722C commented

Good to know .. do you know if the fix includes

(1) making the exception, and not setting Transponders with the B key ( or any Baro that is meant to stay calibrated to Std Pressure

(2) Defining more precisely and standardizing on the definition of the "altitude_indicator" index number.


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Actually, the Question has NOT been answered :(

How does ATC get a plane's Altitude ? - and get it Wrong !

It will be fixed in the next version is NOT an answer to this question.

It may be an answer to different question, about when will it be fixed, but it does not explain HOW ATC gets the altitude, and why it is currently wrong if not at Std Pressure.

As a Dev, I am much more intereted in understanding what is causing the issue, than vague comments about when it might be fixed.

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FlyingsCool avatar image
FlyingsCool answered

Matt from Working Title is aware of this issue, I explained what's going on and showed him why the Transponder doesn't show pressure altitude (because it's designed to show indicated altitude, rather than pressure altitude as it's supposed to.).

He said they are going to look at the issue in general and address it. What the actual answer is I don't know.

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