Jump to content

ADS-B Altitude on FlightAware


Jeff_S

Recommended Posts

Having started this thread and read all the responses, it’s been interesting to contemplate the different measurements we have for altitude. It’s also clear to me that, short of being at a fixed point on the ground where the USGS has done a recent survey, we never know EXACTLY what our altitudes MSL or AGL are. But our systems are designed to mitigate the errors within.

Case in point. The G1000 actually logs three different altitudes every second. The AltB is the readout from the onscreen altimeter, the one you actually use for flying and that gets corrected for baro pressure. But there is an AltGPS data point which is the raw GPS altitude, and there is an AltMSL data point which (I learned from Trek Lawler today) is a calculated altitude based on GPS plus input from the air data computer.

As you can imagine, they are never the same. In my case the AltB measurement is routinely lower than the other two, which I suspect is a calibration issue but it’s within the allowable margin of error. But interestingly, this difference seems greater on the ground, and becomes somewhat less pronounced at cruise levels. I haven’t figured that one out yet. I’ve tried to see if this is more pronounced when flying at different pressure levels (eg standard vs non-standard) but nothing in the data jumped out at me.

Something else to investigate.

Link to comment
Share on other sites

  • 5 months later...

Did a search on the site to understand something about my flight today and found this topic. So here's my question: I used my Foreflight Scout for the first time today,  with my newly installed AXP340 ADS-B out and iPad Mini. Leveled at 6500' and transponder display showing a transmitted altitude of 6500',  on the iPad, there was a target with my ident that indicated 400' below me. Would it be possible that, since I was about 80nm to the closest ADS-B antenna, there was a .40 difference in altimeter, thus showing a lower version of me? 

Link to comment
Share on other sites

I also noticed  same flightaware altitude error (generally 200 feet lower) after  switching to ADSB.   I subsequently did a few ATC altitude checks which always confirmed my attitude was dead on with what ATC was reading which was reassuring. 

Link to comment
Share on other sites

1 hour ago, drapo said:

Did a search on the site to understand something about my flight today and found this topic. So here's my question: I used my Foreflight Scout for the first time today,  with my newly installed AXP340 ADS-B out and iPad Mini. Leveled at 6500' and transponder display showing a transmitted altitude of 6500',  on the iPad, there was a target with my ident that indicated 400' below me. Would it be possible that, since I was about 80nm to the closest ADS-B antenna, there was a .40 difference in altimeter, thus showing a lower version of me? 

What was the source of the data for your iPad?  Bluetooth to the Scout?

I know that if you look at altitudes on FlightAware, it does not correct for altimeter setting.  That is, if you are at 6500' and the altimeter setting is 3022", FlightAware will show you at 6300'.

Link to comment
Share on other sites

What was the source of the data for your iPad?  Bluetooth to the Scout?
I know that if you look at altitudes on FlightAware, it does not correct for altimeter setting.  That is, if you are at 6500' and the altimeter setting is 3022", FlightAware will show you at 6300'.


That is the exact issue. ADS-B is reporting your pressure altitude.


Sent from my iPhone using Tapatalk Pro
Link to comment
Share on other sites

12 hours ago, Bob - S50 said:

What was the source of the data for your iPad?  Bluetooth to the Scout?

I know that if you look at altitudes on FlightAware, it does not correct for altimeter setting.  That is, if you are at 6500' and the altimeter setting is 3022", FlightAware will show you at 6300'.

Data on the Ipad is bluetooth from the scout

51 minutes ago, Marauder said:

 


That is the exact issue. ADS-B is reporting your pressure altitude.


Sent from my iPhone using Tapatalk Pro

 

Sounds like rational explanation, in fact that was my interpretation. Next time I see this, I’ll set 29.92on the altimeter to corroborate.

Thanks guys!

Link to comment
Share on other sites

Sounds like rational explanation, in fact that was my interpretation. Next time I see this, I’ll set 29.92on the altimeter to corroborate.
Thanks guys!


If you have one of the more modern transponders, it will show the pressure altitude on it. That should match what you are seeing on FlightAware.

afe9d8be745536b5e66532f424ec4cf5.jpg


Sent from my iPhone using Tapatalk Pro
Link to comment
Share on other sites

On 10/25/2017 at 8:46 AM, N201MKTurbo said:

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.

That is probably correct...data from my feeder is enclosed...my antenna has an issue with poor reception to the NE caused by my larger 20m beam .The receiver is simply a USB dongle plugged into a raspberry pi located in the attic.

IMG_4473.PNG

IMG_4474.PNG

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.