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ADS-B Altitude on FlightAware


Jeff_S

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Hey folks,

I've noticed for the last few months that my altitude readout when looking at a FlightAware Track Log is routinely 300' lower than the altitude I'm showing on the altimeter. On FlightAware, this is shown as an ADS-B data point (best I can tell).  But I've never been notified by ATC of this, and when they read back the altitude they are seeing for me it jives with what I'm seeing in the cockpit.  If I was routinely 300' below the assigned altitude, I would think they would mention it.

Anybody else seen this? I've sent a note into FlightAware to get their take on it, but thought I'd open it up to this crowd as well. Thoughts appreciated...

Thanks!

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https://adsbperformance.faa.gov/PAPRRequest.aspx

I have noticed same since my upgrade to ads-b as well.  You may want to consider letting the FAA run a performance check on your ads-b equipment (the link above should help.  The “off”altitude on Flightaware is what alerted me to get the performance checked.  My installed equipment failed the test due to improperly set configurations within the gps and trans.  That was corrected and the next test was successful.  I still had “off” altitudes on Flightaware.  I contacted our local fsdo and reported the issue.  They were able to confirm the actual flight altitudes on a sample of flights were accurate (not 200-300 ft off).  I then contacted Flightaware to report the issue.  The gentleman I spoke with said they receive their data from the various  reporting stations across the U.S and it is very likely due to the flight data not being corrected/adjusted  before being reported.  This is very likely due to the age of the equipment across the system and the fact that not all of it has been brought up to snuff with the next generation ads-b system.  The real concern I had was the possibility of being off several hundred feet in altitude in a system that is being designed to accommodate tighter aircraft separation standards.

Edited by cbarry
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All transponders ADS-B Mode A/C/S report baro altitude referenced to 29.92" independent of the altimeter setting.. The baro altitude is conveyed to the transponder from an encoding altimeter onboard. The altitude you may be reading on a tablet may be coming from a GPS sensor. Baro altitude is the one used for traffic separation. However for obstacle avoidance or LPV approaches GPS altitude is used. When approaching a remote runway below 3,000ft AGL with no altimeter setting nearby I adjust my baro altimeter to the indicated GPS altitude on the GNS 530W to avoid terrain collision. 

José

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I have noticed the same thing, as well.  Occasionally, my flights are off by 200 ft or so on flightaware.   My G1000 and back-up altimeter both agree that I am on the assigned altitude.   And this only started after my ADSB upgrade.  I agree that it likely has to do with lack of the correction for local altimeter settings on the data that flight aware is receiving.

 

Brad

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I think what is happening is FlightAware is now mostly showing ADS-B data. The ADS-B data is the raw data broadcast from your plane therefore it is pressure altitude. In the past FlightAware got their data from the FAA and the data stream from them had the local altimeter setting applied.

So If you just equipped your plane with ADS-B you will see ADS-B data on FlightAware where before all you would see is the data from the FAA radar.

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I have a Trig-22 transponder which has some handy display modes, one of which shows the reported altitude.   This is nice for not busting Class-B shelves if you're squeezing through somewhere.   Since the reported altitude is pressure alt it is often a couple hundred feet off of the altimeter display.

 

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2 hours ago, Piloto said:

All transponders ADS-B Mode A/C/S report baro altitude referenced to 29.92" independent of the altimeter setting.. The baro altitude is conveyed to the transponder from an encoding altimeter onboard. The altitude you may be reading on a tablet may be coming from a GPS sensor. Baro altitude is the one used for traffic separation. However for obstacle avoidance or LPV approaches GPS altitude is used. When approaching a remote runway below 3,000ft AGL with no altimeter setting nearby I adjust my baro altimeter to the indicated GPS altitude on the GNS 530W to avoid terrain collision. 

José

Once the FAA spends the $ to get themselves compliant for 2020, there wont be much need for baro altitude or squawk codes.

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Bingo! You all mirrored my suspicions that FlightAware was reporting an uncorrected baro altitude, but I was able to confirm this by comparing data from my last cross-countries where I had decent cruise time.  Using the data logs from Savvy Analysis, of which one of my datapoints is the barometric pressure setting on the G1000, I was able to confirm that the ADS-B readout is closer to true when the baro pressure is closer to standard.  For each .10" that baro pressure increased, there was an approximate 100' drop in the ADS-B altitude readout. On one flight where I flew through two significant pressure gradients, I could match up the moment I changed baro pressure setting to the change in the FlightAware readout.

This is the most graphic evidence I think we can get from the old adage "High to Low, Lookout Below; Low to High, Look to the Sky."  (And another fun way I've found to use the Savvy Analysis platform along with my data logs.)

Thanks y'all!

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10 hours ago, mike_elliott said:

Once the FAA spends the $ to get themselves compliant for 2020, there wont be much need for baro altitude or squawk codes.

Baro altitude still more reliable than GPS altitude. GPS altitude relies on a bunch of working satellites and the onboard GPS receiver. While baro altitude relies on atmospheric pressure and a simple pneumatic mechanism that requires no electricity. Squawk codes are not only used to ID the aircraft but also to indicate the type of flight e.g. 1200 VFR, 7700 emergency, 7500 hijack, 7600 COM failure and others.

José

 

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GPS altitude readings from a WAAS receiver are probably more reliable than a mechanical baro altimeter.  

Accuracy of GPS altitude fixes is another matter.  Geometry of the satellite constellation dictates that horizontal accuracy is 2 to 3 times better on average than vertical.  Look up GDOP if you're curious.  

About 80% of the time the vertical error is within +/- 75 feet.  That's for an unobstructed sky view which we do have aloft.   But over 8% of GPS altitude fixes are in error by over 300 feet.  

Ref:GPS Altitude Error Statistics

 

Edited by Jerry 5TJ
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8 hours ago, Piloto said:

Baro altitude still more reliable than GPS altitude. GPS altitude relies on a bunch of working satellites and the onboard GPS receiver. While baro altitude relies on atmospheric pressure and a simple pneumatic mechanism that requires no electricity. Squawk codes are not only used to ID the aircraft but also to indicate the type of flight e.g. 1200 VFR, 7700 emergency, 7500 hijack, 7600 COM failure and others.

José

 

Baro readings rely on both controllers and pilots entering info often transposed incorrectly. GPS altitude would be more consistent, and if some crazy guy shoots down a bunch of satellites, ATC can always revert back to the "way we have always done it" and give a baro pressure. reading to everyone. Since they are heading to GPS for traffic separation anyway in 2020,  it will be imperative that we use GPS altitude. Baro altitudes rely on equipment that has to be calibrated every 2 years that is rather antiquated and in a lot of cases, over 50 years old. That billows can blow anytime.

Squawking one of the 3 emergency codes are still doable, but why would there even be a need to squawk anything else? ATC will "see" you, know all about you and where to send the bill. Squawking VFR would be unnecessary, as would anything other than being hijacked, having a busted radio, or an emergency..

 

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1 hour ago, mike_elliott said:

Baro readings rely on both controllers and pilots entering info often transposed incorrectly.

All transponders transmit pressure altitude and don't require any pilot or controller input.

It is true that the controllers need to enter the local altimeter setting to get true altitude. I assume this is automated but I don't know the answer to that. Maybe someone with more ATC experience can chime in on how that works.

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40 minutes ago, N201MKTurbo said:

All transponders transmit pressure altitude and don't require any pilot or controller input.

It is true that the controllers need to enter the local altimeter setting to get true altitude. I assume this is automated but I don't know the answer to that. Maybe someone with more ATC experience can chime in on how that works.

True, its the altimeter that Pilots (and KAP's use) rely on that they have to input a setting into and with some regularity, get wrong. When you hear "NXXXyy  recycle transponder" that more often than not is code for "Check you altimeter setting, Bozo"  and doesn't involve anyone writing anything on any forms. The transponder could care less what that setting is when it squirts its altitude out, and is another reason for changing "thats the way we have always done it" concerning altitude

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23 hours ago, EricJ said:

I have a Trig-22 transponder which has some handy display modes, one of which shows the reported altitude.   This is nice for not busting Class-B shelves if you're squeezing through somewhere.   Since the reported altitude is pressure alt it is often a couple hundred feet off of the altimeter display.

 

Hmm, if I'm understanding what you're saying, I'm pretty sure it doesn't work that way in regards to airspace altitudes.

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On 10/25/2017 at 6:24 AM, Piloto said:

When approaching a remote runway below 3,000ft AGL with no altimeter setting nearby I adjust my baro altimeter to the indicated GPS altitude on the GNS 530W to avoid terrain collision. 

José

I'd like to hear more about this technique.    Conceptually of course it's not something most people would do;   one would think the possible altimeter errors from not having the local altimeter setting are already built into the approach.   I don't have a 530 or know much about one but I assume you're thinking that WAAS vertical guidance is so good that the derived GPS altitude must be accurate too? Does the 530 report AGL altitudes? You still have to go around at the DA, which is baro altitude-based, right?  What am I missing?   

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11 hours ago, Aviationinfo said:

I'd like to hear more about this technique.    Conceptually of course it's not something most people would do;   one would think the possible altimeter errors from not having the local altimeter setting are already built into the approach.   I don't have a 530 or know much about one but I assume you're thinking that WAAS vertical guidance is so good that the derived GPS altitude must be accurate too? Does the 530 report AGL altitudes? You still have to go around at the DA, which is baro altitude-based, right?  What am I missing?   

Good questions. GPS altitude is always true MSL altitude. When on an airport on the ground the indicated GPS altitude is always the airport MSL elevation irrelevant of barometric pressure. On a baro altimeter it will indicate the MSL elevation of the airport only when set to the correct altimeter setting. Below 3,000ft AGL both GPS and baro altitude will track one to one. So setting the baro altimeter to GPS altitude will result in a correct altitude indication on the ground for the baro altimeter. This procedure is very handy for those remote areas where there is no WX data. Also you can use the Terrain Mode on the GNS 530 that depict terrain AGL and GPS MSL altitude when on approach. If the runway has an LPV GPS approach all you have to is follow the glide slope.

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On 10/26/2017 at 8:47 PM, Piloto said:

Good questions. GPS altitude is always true MSL altitude. When on an airport on the ground the indicated GPS altitude is always the airport MSL elevation irrelevant of barometric pressure. On a baro altimeter it will indicate the MSL elevation of the airport only when set to the correct altimeter setting. Below 3,000ft AGL both GPS and baro altitude will track one to one. So setting the baro altimeter to GPS altitude will result in a correct altitude indication on the ground for the baro altimeter. This procedure is very handy for those remote areas where there is no WX data. Also you can use the Terrain Mode on the GNS 530 that depict terrain AGL and GPS MSL altitude when on approach. If the runway has an LPV GPS approach all you have to is follow the glide slope.

Thanks for the explanation.   I know that my non-WAAS receiver shows a calculated GPS altitude and usually it's off by 100'+/-  from my baro altimeter, so is WAAS correction also applied to GPS derived reported altitude as well as lateral nav and glideslope indications?  This is all new to me as I haven't done any LPV approaches before.   

My concern would be that if you are flying down to minimums of 200' and the GPS altitude happens to be off by 100'+,  that could be bad.   The thing about the glideslope is that your DA is based on an MSL altitude so while you have protection by staying on the glideslope, your missed approach point might not be accurate.   Again though, I suppose if WAAS corrects the GPS altitude readout perhaps it's not an issue.

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On 10/28/2017 at 12:08 AM, Aviationinfo said:

My concern would be that if you are flying down to minimums of 200' and the GPS altitude happens to be off by 100'+,  that could be bad.   The thing about the glideslope is that your DA is based on an MSL altitude so while you have protection by staying on the glideslope, your missed approach point might not be accurate.   Again though, I suppose if WAAS corrects the GPS altitude readout perhaps it's not an issue.

I hope I'm miss-understanding your concern.  Flying an LPV approach gives a GPS derived decent angle, altitudes are referenced by the pilot from the baro-corrected Altimeter.  The WAAS GPS does not set minimums, the pilot does, the WAAS derived glide path just directs you to them.

 

  Sorry if I'm missing your point.

 

Ron

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I'm probably not explaining myself well.  If there's an altimeter error, the altimeter reads potentially higher or lower than what you actually are.  Your decision altitude  will fall along the glide slope someplace based on what your altimeter reads.   If your baro altimeter is reading high then your true altitude will be below minimums at your decision altitude.

This is the part I'm not sure about:  If you set your altimeter to match the GPS altitude and it happens to be in error (because in my experience my non-WAAS GPS reported altitude doesn't appear to be super accurate) you've introduced an altimeter error.    Now like I said, I'm not at all sure about that because I don't know if WAAS corrects the GPS altitude readout and makes it more accurate.  I just know that with my non-WAAS GPS, the GPS altitude readout doesn't always match my baro altimeter when it's properly set to the local pressure.   Sometimes it's off by 150.'   It makes me question my GPS altitude readout.  I do trust my altimeter.

So I'm just asking if you know for sure one way or the other, if WAAS correction is applied to the GPS altitude readout since you are setting your baro altimeter to match it.   It would be really great if it does.

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@Aviationinfo,

 

  Ok,  I'll leave this for the experts as I've never used the given GPS (waas or non-waas) altitude as a fallback to baro-altimeter.  I check the altimeter to field elevation (+/-75') prior to taxi to make sure there's no error to start with at least, but I have always been able to pick up the local altimeter setting for this.

 

Sorry for any confusion.

 

Ron

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