Strange results comparing baro vs. radar altimeter readings

[miguelaco]

For some time I tried to understand and consolidate all altitude readings, in cockpit ones with external view and even that reported by Tacview. Looking only at the readings the cockpit provides (barometric & radar altitudes) I cannot understand some variations and lack of consistency.

 

For example, set up a very simple mission over the sea. Establish level flight and set pressure so that both barometric and radar altimeter give the same reading (4570' AGL = ASL in this case):

 

 

Go down to 500', and look at both altimeters. Barometric reports 70' more than radar altimeter:

 

 

OK, 70' is not a great amount of error, but keep in mind I simplified things a lot. If you add external view altitude reading to the equation which I believe is measuring ASL, you can extend the altitude range and observe a few hundred feet variation between 500' ASL and 20000' ASL.

 

Why are we seeing these variation? Is there some kind of effect or error modeled in the sim that induces it?

[EtherealN]

First off, are you at the exact same position in all cases, and if not - what weather setting do you have? You might have moved sufficiently to have pressures change, and then you will get differences.

 

Also, what is the air temperature?

[miguelaco]

First off, are you at the exact same position in all cases, and if not - what weather setting do you have? You might have moved sufficiently to have pressures change, and then you will get differences.

 

Also, what is the air temperature?

 

I'm not in the same position, but both screenshots are taken over the sea in the same mission. Pressure setting in the mission editor by default and standard weather, so I believe pressure is the same at any point of the map (760 mmHg). All settings in the mission were by default, so 20º C and no wind and turbulences. Actually, it's quite simple to reproduce. Just fire up the mission editor, place your A-10C over the sea and you're ready to test it. That's why I had not provided the .miz in the first post.

[EtherealN]

Still good to add the .miz, just to be sure. ;)

[miguelaco]

Found it! See attachment.

Test ALT.miz

[EtherealN]

What I think is going on is actually quite simple: your test assumes that 5000 MSL and 5000 AGL above the black sea are actually the same thing. Trying to find a resource with the average water level in the black sea. But just as a thought - remember that MSL is the mean sea level for the entire planet. Given the geography and hydrography and other charactreristics of the Black Sea, I'd expect water levels to be higher than MSL - but the question is how much higher. I'll let you know if I find something.

[miguelaco]

Sorry but I think it does not explain why I have the same figures at 4500' and not at 500'. Radar altimeter always measures AGL. I'm flying over the sea, so AGL = ASL in this case. If you manage to equate both (adjusting pressure in barometric alt) at 4500', why aren't they providing the same reading at 500' with that same pressure setting?

[EtherealN]

Because there will always be error indications. If you leave it alone at the start, I see a roughly 1.7% erroneous indication compared to radar reading, which is extremely satisfactory for a pneumatic instrument. (If I could have that kind of precision IRL I'd be extatic. :D )

 

If you descend without touching it, error diminshes as far as actual numbers go, and you get no disturbing error when nearing the ground. However, if you change it according to the radar reading when up there, and have that same error (which you will have), after descending 4000 feet you'll have a likely error on the indication of 4000x0,017 = 68, which happens to be exactly what you are seeing. The "problem" is that the error you get will be relative to the point where you aligned it. If you set it such that it will be correct on the ground (where it matters since the biggest part of flying is avoiding the ground), you'll have your error indication up in the air - and the further up you go the bigger the error will be in actual numbers (though it'll remain roughly constant as a percentage, I except). However, obviously, this doesn't matter since you'll switch to STD anyway once at altitude.

 

Now, as for whether there's a bug or something going on - might be. But the error is so small that I'd suspect that removing it would be the unrealistic thing - there's so many things like temperatures, airspeed effects etcetera that can affect a pneumatic altimeter that this looks very good to me. :)

[miguelaco]

Thx for your answer. I was not taking into account temperature effect on baro altimeter. I had the opportunity to test the same mission changing temperature setting, and the variation increases with lower temps, which I think is consistent with RL behavior.

 

Also, found the following thread that expands on the subject:

 

http://forums.eagle.ru/showthread.php?t=86133

 

Thx again!

[effte]

Five degrees above ISA means the altimeter will underread about 2%. When you are at 5000' MSL, as indicated by your radio or radar altimeter, with a correct QNH for a sea-level reference point you would read around 4900' on your baro altimeter. You compensated for this by adjusting your altimeter setting for the altimeter to match your radalt.

 

At 500' radalt, the same error is reduced to a mere 10'. Your dialed in adjustment is still 100', so you should be 90' high on your baro altimeter.

 

In other words, it's pretty much as it should be, but still a good catch! :thumbup:

 

But just as a thought - remember that MSL is the mean sea level for the entire planet.

 

Actually, no. ;)

 

For MSL altitudes, local geoids are used. That's why you should have the corresponding geoid undulation specified for every MSL altitude given in aeronautical publications.

 

In other words, MSL altitudes are relative to a theoretical model of what the sea level would be at your current location, had the planet been covered with water. If it wasn't, you'd end up with interesting phenomena such as theoretically level water surfaces flowing, as they would not be perpendicular to the local gravitational field.

 

An interesting side effect of this difference between ellipsoid and geoid is that you can find places where you can follow flowing water downstream and read an increasing height, as measured in your GPS. Your GPS probably gives height relative to the WGS84 ellipsoid rather than the geoid, at least on default settings. :)

 

(To actually see this, you'd need survey-grade corrected GPS receiver though... and probably do a fair bit of walking.)

 

Cheers,

Fred