ADSB altitude issues?

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4RNB

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4RNB
IFR student here, my plane. Twice today ATC asked us to confirm altitude, telling us to climb to the altitude we were at. When they did this a second time the transponder pressure altitude read 300 feet lower than we were at.
Garmin 175 GPS
Garmin GTX 327 transponder.
FAR 91.217 says reported altitude has to be within 125 feet 95% of the time.

Researching this turns up lots of questions.
ADSB report obtained, shows failure of NIC.

Have you dealt with this before?
Recalibration of encoder was suggested to me.

"Loss of, or incorrect altitude reporting by the transponder. Improper altitude source installation or configuration. Check the altitude source configuration and verify that the primary and secondary sources are properly configured." Above is from a trouble shooting guide

OF NOTE: This flight was started having not turned WaaS back on from previous flight lesson. We turned it back on in flight but still did not capture LPV minimums and we had to fly LNAV. Much later in the flight the LPV came back on, but this was well after apparent altitude issues. I do not recall if the pressure altitude improved towards the end of the flight.

Any idea as to what is going on?
 
4RNB said:
IFR student here, my plane. Twice today ATC asked us to confirm altitude, telling us to climb to the altitude we were at. When they did this a second time the transponder pressure altitude read 300 feet lower than we were at.
Garmin 175 GPS
Garmin GTX 327 transponder.
FAR 91.217 says reported altitude has to be within 125 feet 95% of the time.

Researching this turns up lots of questions.
ADSB report obtained, shows failure of NIC.

Have you dealt with this before?
Recalibration of encoder was suggested to me.

"Loss of, or incorrect altitude reporting by the transponder. Improper altitude source installation or configuration. Check the altitude source configuration and verify that the primary and secondary sources are properly configured." Above is from a trouble shooting guide

OF NOTE: This flight was started having not turned WaaS back on from previous flight lesson. We turned it back on in flight but still did not capture LPV minimums and we had to fly LNAV. Much later in the flight the LPV came back on, but this was well after apparent altitude issues. I do not recall if the pressure altitude improved towards the end of the flight.

Any idea as to what is going on?
Click to expand...
What are you using for an encoder? Is connected via Gray code? RS232?
 
Is it an ACK-30 encoder?

I have recently had xponder issues as well. There is quite a bit of info out there for ACK-30 drifting as they get older.
 
aterry1067 said:
Is it an ACK-30 encoder?

I have recently had xponder issues as well. There is quite a bit of info out there for ACK-30 drifting as they get older.
Click to expand...
Again, I do not know.
 
I wonder if I was just getting confused. What shows on my transponder is the pressure altitude. I do not know if that is what gets reported, but for most of the day the pressure altitude was 300-400 ft lower than the field elevation. We are going to see how Friday goes.
 
I would bet it's your ACK-30. I have had the same issues after my pitot static inspections in recent years. Something about the guy messing with it and or the system seems to easily change how it encodes the signal. I have used the same guy for my 2 previous inspections and had to take it back for a quick adjustment both times. I only knew because the local approach controller had me 300 ft lower than I was showing on my altimeter.
 
What altitude does ADSB report to the controller?
 
Also.....not sure about your particular model of xponder but I believe (may be wrong) that my Stratus xponder displays pressure alt. If the encoder is not correctly calibrated for pressure alt then the mode S signal sent to ATC will be off. The ATC system takes in your MODE S signal (encoded via your encoder) and does the math to account for the local altimeter setting and displays that altitude on their screen.
 
4RNB said:
What altitude does ADSB report to the controller?
Click to expand...


not sure. that's a good question. I know a lot (maybe all) of ATC still relies on mode S for altitude.. not sure if they are doing any ATC based off of ADSB altitude signals or not.

I believe the updated systems would still need to do the math for ADSB signal received + local altimeter setting. I don't know any controllers.

sounds like a good question for the guys at opposing bases podcast
 
OK, so what altitude does mode S report?
 
Mode C seems to be pressure altitude reporting
 
You have to convert pressure altitude to the current altimeter setting to determine if there is an error from the displayed PA.
 
1SGBrokePilot said:
Also.....not sure about your particular model of xponder but I believe (may be wrong) that my Stratus xponder displays pressure alt. If the encoder is not correctly calibrated for pressure alt then the mode S signal sent to ATC will be off. The ATC system takes in your MODE S signal (encoded via your encoder) and does the math to account for the local altimeter setting and displays that altitude on their screen.
Click to expand...
A GTX327 is not mode S.
 
4RNB said:
What altitude does ADSB report to the controller?
Click to expand...

Both the transponder and the ADS-B Out are required to use the same pressure altitude source so they should correlate with one another. Secondary radar gets your transponder pressure altitude and ADS-B ground stations get your pressure altitude from the ADS-B Out. These are fused into a single display to the controller and the pressure altitude is corrected with the local altimeter setting to an equivalent MSL altitude seen by the controller
 
I really find it silly that all adsb out “devices” still use a form of pressure transducer (with all its inherent errors) instead of actual gps derived altitude. The FAA lost a good opportunity to standardization the whole fleet that converted to adsb into using gps for altitude reporting instead of pressure derived altitude reporting.
 
ahypnoz said:
I really find it silly that all adsb out “devices” still use a form of pressure transducer (with all its inherent errors) instead of actual gps derived altitude. The FAA lost a good opportunity to standardization the whole fleet that converted to adsb into using gps for altitude reporting instead of pressure derived altitude reporting.
Click to expand...
Reporting an altitude other than what’s being flown complicates issues quite a bit.
 
ahypnoz said:
I really find it silly that all adsb out “devices” still use a form of pressure transducer (with all its inherent errors) instead of actual gps derived altitude. The FAA lost a good opportunity to standardization the whole fleet that converted to adsb into using gps for altitude reporting instead of pressure derived altitude reporting.
Click to expand...

First, ADS-B Out broadcasts both pressure altitude and geometric altitude (aka GPS altitude). Pressure altitude must be from the same source that is used by the transponder so the ADS-B target can be correlated with the secondary radar target of the transponder. Second, pressure altitude is universally used as the common denominator for ADS-B out equipped and non equipped aircraft, US and worldwide aircraft, transponder equipped aircraft and non transponder equipped aircraft, TAS and TCAS systems, so it would be downright silly to isolate ADS-B from the system. Pressure altitude can be corrected to MSL so controllers and pilots are on the same wavelength, but GPS altitude can't be.
 
Did you have the correct altimeter setting? Have your altimeter and transponder had their biennial checks? Those are a couple obvious things that could result in your altimeter showing one thing while ATC is showing another.

What is your ADS-B Out equipment? I didn't see it in the OP.
 
As others have mentioned, pressure altitude is not the same as your altimeter reading, which is corrected to the local barometric pressure. If there is a consistent discrepancy between what ATC is seeing and what you are seeing for barometric altitude, and you are using the correct altimeter setting, then you may need to do an altimeter check to identify the problem. When was the last time the 91.411 check was performed?
 
Last checks were July 2020.
I have a Garmin GPS 175
Transponder is Garmin GTX 327.

There was no fussing on altitudes last lesson.
 
Ok...you have a gtx327....check the screen on your gtx327 to verify the pressure altitude that it displays. Before your next flight, set your altimeter to 29.92, then look at the pressure altitude on the gtx327. It should be within limits.. If it is, then you might have an issue with your static source during flight...or if you have an old ACK encoder that has gone wacky, you can purchase a new one.

If you have an alternate static source, make sure it is closed during flight.
 
When I was having a problem like yours, it was the encoder. As I recall, it was a couple hundred dollars to get a new one installe.
 
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