3 ms·

Pressure encoders, as I said. That's what feeds all aviation altitude data... i.e. anytime you see the word 'altitude' and its not qualified with 'GPS altitude'

by dramm 3y ago

Pressure encoders, as I said. That's what feeds all aviation altitude data... i.e. anytime you see the word 'altitude' and its not qualified with 'GPS altitude' which is effectively not normally used. ADS-B Out concurrently transmits GPS height about the ellipsoid data as well as pressure altitude data. No use is normally made of the GPS height data. We are discussing pressure altitude data here, that's what aviation works off of. The accuracy and reliability metrics in the ADS-B broadcast you are referencing refers to the GPS data not the pressure transducer/encoder data. In cases of encoder failure being detected a flag is broadcast and the pressure aka baro altitude data field is set to all 0. ADS-B cannot give information about the pressure altitude accuracy or reliability like it does for GPS metrics. It relies on the encoders being better than their +/- 125' accuracy requirement and that's tested for periodically. ADS-B can in principle broadcast 100' or 25' resolution encoders, that info is in the messages. The ones here will be 25'. (I've got a long background with ADS-B related technology, currently helping the FAA out on some niche stuff). edit: trying to improve clarity/correctness but there is too much to cover here.

jjwiseman 3y ago
Cool, thanks for the info. (BTW I wasn't disagreeing with anything you wrote, I just wanted to mention, since the topic of resolution/accuracy came up, that lots of aircraft broadcast information about the accuracy of some of the sensor data they're reporting, which you can do some pretty cool stuff with. E.g. that's how https://gpsjam.org/ https://gpsjam.org/ works.)
- dramm 3y ago
  Oh sorry I did not think you were disagreeing I was just trying to be clear and more trying to clear up the GPS accuracy/reliability metrics like SIL, NIC, NACp and NACv being broadcast by ADS-B being completely separate to anything to do with pressure derived altitude, I was going down a rat hole and had to edit that several times. And it can be painfully confusing e.g. the failure flag for pressure altitude aka baro altitude in ADS-B speak failure is called NICbaro but has no relationship to the GPS NIC (Navigation Integrity Category) value. And depending on the system some of the GPS accuracy/reliability metrics are just hard coded, and the ones that really matter like NACp are derived from the GPS and SBAS reception. John Wiseman does great stuff with ADS-B Out data. Also for pilots/aircraft owners/A&Ps: The FAA PAPR (Public ADS-B Performance Report) https://adsbperformance.faa.gov/PAPRRequest.aspx https://adsbperformance.faa.gov/PAPRRequest.aspx provide a summary of their aircraft's ADS-B performance, including all the broadcast GPS quality metrics and any reported failure flags etc. The PAPR system will email out the PDF report. The owner/pilot/A&P can reply to that email and request a Google Earth/kmz and Spreadsheet/CSV data for that flight showing all the received ADS-B transmissions including all those accuracy/reliability metrics. Interesting stuff and very useful for diagnosing problems with ADS-B Out installations. So sensitive you'll might see say NACp degrade as an aircraft banks steeply because the GPS antenna now has a view of fewer GPS satellites. Installations in most (non-experimental/non-light sports) aircraft effectively require use of PAPR to formally validate a new installation is working correctly. It's a good thing for owners to also just periodically check their aircraft's ADS-B performance using PAPR. I suggest just before and after each annual inspection for GA/light aircraft.
simonjgreen 3y ago
Also worth noting that the datum used to translate from pressure to altitude switches at the transition threshold, usually 18,000ft, from actual local ground pressure (supplied by ATC in all initial contacts) to 29.92 Hg.

by dramm 3y ago

jjwiseman 3y ago

Cool, thanks for the info. (BTW I wasn't disagreeing with anything you wrote, I just wanted to mention, since the topic of resolution/accuracy came up, that lots of aircraft broadcast information about the accuracy of some of the sensor data they're reporting, which you can do some pretty cool stuff with. E.g. that's how https://gpsjam.org/ https://gpsjam.org/ works.)

dramm 3y ago
Oh sorry I did not think you were disagreeing I was just trying to be clear and more trying to clear up the GPS accuracy/reliability metrics like SIL, NIC, NACp and NACv being broadcast by ADS-B being completely separate to anything to do with pressure derived altitude, I was going down a rat hole and had to edit that several times. And it can be painfully confusing e.g. the failure flag for pressure altitude aka baro altitude in ADS-B speak failure is called NICbaro but has no relationship to the GPS NIC (Navigation Integrity Category) value. And depending on the system some of the GPS accuracy/reliability metrics are just hard coded, and the ones that really matter like NACp are derived from the GPS and SBAS reception. John Wiseman does great stuff with ADS-B Out data. Also for pilots/aircraft owners/A&Ps: The FAA PAPR (Public ADS-B Performance Report) https://adsbperformance.faa.gov/PAPRRequest.aspx https://adsbperformance.faa.gov/PAPRRequest.aspx provide a summary of their aircraft's ADS-B performance, including all the broadcast GPS quality metrics and any reported failure flags etc. The PAPR system will email out the PDF report. The owner/pilot/A&P can reply to that email and request a Google Earth/kmz and Spreadsheet/CSV data for that flight showing all the received ADS-B transmissions including all those accuracy/reliability metrics. Interesting stuff and very useful for diagnosing problems with ADS-B Out installations. So sensitive you'll might see say NACp degrade as an aircraft banks steeply because the GPS antenna now has a view of fewer GPS satellites. Installations in most (non-experimental/non-light sports) aircraft effectively require use of PAPR to formally validate a new installation is working correctly. It's a good thing for owners to also just periodically check their aircraft's ADS-B performance using PAPR. I suggest just before and after each annual inspection for GA/light aircraft.

simonjgreen 3y ago

Also worth noting that the datum used to translate from pressure to altitude switches at the transition threshold, usually 18,000ft, from actual local ground pressure (supplied by ATC in all initial contacts) to 29.92 Hg.