TEG916
Senior Member
Be careful with ADS-B in altitude. I was flying today at 2500', and there was an aircraft opposite direction showing 2000'. My first thought was, we will have 500' clearance, so I should be careful to spot him, but should be OK.
Then I had a thought crossed my mind. The indicated altitude for ADS-B is probably pressure altitude. This is what a transponder sends to ATC. Pressure altitude is corrected on the other end by ATC, as your trasponder has no way of knowing if it is a high pressure day, or a low pressure day. It is not corrected by ADS-B in receivers (at least not that I know of).
I looked down at my Ipad, and sure enough, my ADS-B indicated altitude was also 2000'. I didn't have the traffic in sight yet, but we were starting to get close, so I made a slight right turn, and turned on my landing light. As I was in a turn, I spotted him. He had initiated a climbing right turn.
We didn't get close to each-other, but it makes you think. Keep in mind, on a high pressure day, ADS-B indicated altitude for other aircraft will indicate that they are lower than they actually are. On a low pressure day, they will indicate that they are higher than they actually are.
Then I had a thought crossed my mind. The indicated altitude for ADS-B is probably pressure altitude. This is what a transponder sends to ATC. Pressure altitude is corrected on the other end by ATC, as your trasponder has no way of knowing if it is a high pressure day, or a low pressure day. It is not corrected by ADS-B in receivers (at least not that I know of).
I looked down at my Ipad, and sure enough, my ADS-B indicated altitude was also 2000'. I didn't have the traffic in sight yet, but we were starting to get close, so I made a slight right turn, and turned on my landing light. As I was in a turn, I spotted him. He had initiated a climbing right turn.
We didn't get close to each-other, but it makes you think. Keep in mind, on a high pressure day, ADS-B indicated altitude for other aircraft will indicate that they are lower than they actually are. On a low pressure day, they will indicate that they are higher than they actually are.
