Sarcasm would be more effective against someone who had not already pled ignorance on the subject. But kudos on your knowledge.Most light aircraft don't have the ability to tell AGL height because a radar altimeter is expensive and not particularly necessary.
They use a barometric sensor just like your drone does.
But if you want to get a nice economical radar altimeter for your drone, here's a good deal on a secondhand unit.
With the antenna, it weighs 3 pounds and you will need to find a 28 volt power supply to run it.KRA-10 Radar Altimeter System
We are used avionics specialists for General Aviation.www.bennettavionics.com