I've been using AirNav radarbox for several years and recently upgraded my antenna from DPDProductions with an indoor unit. Since then I have been able to see most ground traffic at Chicago Midway Airport and share that with planefinder. A couple months ago I began experimenting with a dongle receiver using ADSB# and adsbscope software with a home built coaxial collinear antenna. I have been running these computers side by side (radarbox and adsbscope) and get mostly the same same traffic on each except for something new within the last few weeks. On the adsbscope program I began seeing an adsb ground unit on the field of Midway airport on occasion and when this transmitter "comes up", I can see more planes on the ground but this information does not show on radarbox or planefinder (both computers upload to planefinder). This "ground transmitter" tracks planes around the field and on takeoff but shortly after takeoff, some planes will freeze on the map but the altitude keeps updating sometimes all the way up to 18000 feet all the time radarbox shows nothing. My question is, does anybody know what this ground adsb unit is and why radarbox is not receiving this information. In the flight information table (adsbscope) it's in the list as "registration <gnd>, ident test1234" and doesn't always show on map plot.