Pixhawk altitude accuracy with Reach

Hi there,

In the application that I’m involved with, I need to know the exact absolute altitude of the UAV (within the typical RTK margin of error of course), which would be “alt2” when Reach is configured as 2nd gps and has fix. But would Pixhawk always use this altitude automatically if available?

I recently received my Reach Kit and was able to configure to send corrections through mavlink, obtain fix and centimeter level accuracy as checked with RTKPlot. I am using Pixhawk autopilot, and I have read that for positioning, Pixhawk will automatically use either 1st GPS or 2nd GPS (Reach) depending on which is healthier.

My question is: What altitude does Pixhawk use for flying when RTK is fixed? Does it still use barometric? Does it automatically use RTK altitude if there is fix? Is this configurable?

Thanks in advance for your help.

Hi Juan,

ArduCopter code fuses data from GPS, baro and accelerometers in order to come with altitude estimate. I believe that there are settings in EKF that will make it trust the GPS more. Should be something like decreasing GPS process noise.

Thanks @igor.vereninov, you are right. I found this on ardupilot documentation:

This is the RMS value of noise in the altitude measurement. If you increase this parameter, the filter will think the barometer is more noisy and will place less weighting on its measurements.

If this parameter is set too small, then the filter will constantly react to noise in the barometer measurement which will cause the filter height to be noisy. In copters this will cause the copter to jiggle up and down during altitude hold.

If this parameter is set too high, then the height will tend to wander more and will be more susceptible to GPS vertical velocity glitches.

See the section on interpreting EKF3 log data for more information on using log data to help set this parameter.

I guess i have to increase the EKF_ALT_NOISE parameter. I will have to play with it and test different configurations, I will post my findings.

Do you have any data or information on this? I think that this is something that could be included in the Reach Documentation section, because most people that purchase a Reach Kit will need to face this kind of settings to fine tune it and bring the best out of the solution.

If anyone has some guidance, it would be much appreciated.

Search ek2_alt_source

Set to 2 for gps