Incorrect Altitude being reported in ADS-B Mavlink Msg's

[jwuavionix]

In mavlink 2's ADS-B send_dynamic_out msg, the GNSS Altitude is being reported in decimeters instead of the correct mm's.

Also, the pressure Altitude isn't implemented and is always being sent as invalid.

Arduplane V3.7.1 and Arducopter 3.4.3 on a pixhawk

[magicrub]

Hi Jeff,

Here's the problem code:
https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_ADSB/AP_ADSB.cpp#L472

And
https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_ADSB/AP_ADSB.cpp#L519

The GPS alt unit is an easy fix, my bad! The pressure alt one is less obvious. I did not implement it because it wasn't clear what the data was.

I can fix the GPS alt right away, would you be able to supply the description of the pressure sensor data?

[magicrub]

GPS alt is fixed via 372837b

[jwuavionix]

The pressureAltitude reported should be calculated using a standard reference pressure of 1013.25mbar. I've attached a little doc that describes the calculation which is already referenced to this reference.
pressureAltitude.pdf

[magicrub]

@jwuavionix I just noticed this is still lingering. Please assist on what the data is supposed to be, I want to make sure it's correct.

per the mavlink packet note: Barometric pressure altitude relative to a standard atmosphere of 1013.2 mBar and NOT bar corrected altitude (m * 1E-3). (up +ve). If unknown set to INT32_MAX

So is it feet above sea level?

[jwuavionix]

I left a comment on Dec 20th that attached an equation that is used to calculate the standard pressure altitude from the compensated(temp) baro pressure. This isn't AGL or MSL and should be calculated using the equation attached and just the baro pressure.

[magicrub]

is it OK if it is temperature compensated?

[jwuavionix]

yes, the equation needs to use temperature compensated baro pressure as the input.

[magicrub]

great! Then I think this ends up being easier than we thought. I was thinking into it too much. Thanks @jwuavionix !

[magicrub]

See PR #5761