We've had a few customers report inacurate altitudes, random "terrain alerts" mid flight, and dramatic changes to rate of climb. We've traced this back to the pressure altitude coming from the Echo. It doesn't affect everyone, but it has some. I recently heard something that may explain this: The Echo is passively getting pressure altitude from the transponder, but a transponder doesn't always transmit, it waits for a radar ping, then it replies. So if you are in the middle of nowhere it might be a while between sweeps, so a change of altitude wouldn't be registered for a while.
Thanks, Walter. I hadn't thought about the need for a ping to get the Echo to report altitude. I had a different condition a few days ago, when in a flight of two Seareys, flying at about 100 feet above the James River in VA. The other guy could see me on his EFIS (not an iFly) but I couldn't see him. I did, however, see other traffic about 1,100 feet higher and more.
Flght Aware usually tracks me when I'm above 1,500 or 2,000 feet, and occasionally even when I'm on the ground. FlightRadar24 almost never spots me at all. A uAvionix tech told me that's because they look for 1090 mHz transmissions, but the Echo only transmits on 978. So they see 978 mHz traffic only when the signal is re-transmitted by a 1090 setup.