Research: Spent a morning I'll never get back, after wondering why my Sentry can't see a tower near my home drome. . .duh. . .it's because there isn't coverage below 1,500' at my place. How do I know, you ask? I don't KNOW, really, but looking at a Goole Earth file/layers from the FAA, it appears that's the case. FWIW it's worth, and your own entertainment, the file is at: https://www.faa.gov/air_traffic/technology/equipadsb/research/airspace About half-way down the page - you gotta muck with the layers a bit, but there is a "how to" embedded in the file.