adsb ground transmitter

[Chgo Midway]

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.

 

[Richard Lee]

I think there are some 1090 MHz ground support vehicles around.. Maybe those are what you are seeing?


From AC No: 150/5220-26
"The airport ground vehicle ADS-B squitter unit will utilize a sensor navigation source
capable of providing highly accurate position data as outlined in the specification. The
airport ground vehicle ADS-B squitter units can operate on either the 1090 ES link or the
978 MHz/UAT link; however, due to the 1090 MHz spectrum congestion and use by
numerous other systems, the FAA strongly prefers the use of the 978 MHz/UAT link."


~~~

Re: "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."

I've seen something like that, while using VRS setup for reading the RF signal level. The plane will freeze, but the altitude and signal level keeps updating.
Mostly this happens to planes that are on final approach (to Boston) and get down under 800 feet, where I am blocked by hills.
When the altitude reaches Zero or goes negative, I believe the plane is on the runway and the fake 'Frozen' plane, (still shown a half mile from the airport)
is about to vanish off the display.. (Depending on software settings).

My guess was they were still transmitting standard (short) “squawk” codes because they are being peppered by airport squawk interrogation radar.?.
And the long extended packets with Lat & Long, aren't being heard..

My Dongles hear a lot of those short Squawks, and many of them have VERY strong RF signal levels around 150 to 255(max)..
Which makes me think those old-system transponders, run a lot of power output..

Edit:
I've been watching signal levels on planes at 150-200 miles (testing a new antenna) and just before they drop-off the display, they will 'stop' moving. But, the signal level will update a few times, getting weaker.. Once down to 10 or 15, the updating stops and the plane icon will soon be gone.
But, sometimes if the plane is crossing a 'dead zone', it will reappear on the other side, with strong signals.. :

 

[Chgo Midway]

I think there are some 1090 MHz ground support vehicles around.. Maybe those are what you are seeing?


From AC No: 150/5220-26
"The airport ground vehicle ADS-B squitter unit will utilize a sensor navigation source
capable of providing highly accurate position data as outlined in the specification. The
airport ground vehicle ADS-B squitter units can operate on either the 1090 ES link or the
978 MHz/UAT link; however, due to the 1090 MHz spectrum congestion and use by
numerous other systems, the FAA strongly prefers the use of the 978 MHz/UAT link."


~~~

Re: "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."

I've seen something like that, while using VRS setup for reading the RF signal level. The plane will freeze, but the altitude and signal level keeps updating.
Mostly this happens to planes that are on final approach (to Boston) and get down under 800 feet, where I am blocked by hills.
When the altitude reaches Zero or goes negative, I believe the plane is on the runway and the fake 'Frozen' plane, (still shown a half mile from the airport)
is about to vanish off the display.. (Depending on software settings).

My guess was they were still transmitting standard (short) “squawk” codes because they are being peppered by airport squawk interrogation radar.?.
And the long extended packets with Lat & Long, aren't being heard..

My Dongles hear a lot of those short Squawks, and many of them have VERY strong RF signal levels around 150 to 255(max)..
Which makes me think those old-system transponders, run a lot of power output..

Edit:
I've been watching signal levels on planes at 150-200 miles (testing a new antenna) and just before they drop-off the display, they will 'stop' moving. But, the signal level will update a few times, getting weaker.. Once down to 10 or 15, the updating stops and the plane icon will soon be gone.
But, sometimes if the plane is crossing a 'dead zone', it will reappear on the other side, with strong signals.. :

Hi, thanks for your input. All the planes that freeze on the screen but continue to update altitude are planes taking off not landing and I've seen some of them update to FL240 but the planes still is frozen. This is with the software adsbscope which runs along side airnav radarbox which does not show any of these targets that sdsbscope is picking up. There is some type of ground station on the field that is repeating non-adsb aircraft information that is visible to adsbscope but not airnav radarbox. None of this information appears on the Plane Finder uploaded from adsbscope software.

 

[plane-finder_user]

Hey
When i am on top of the Parkinglot in LOWW i see much more Aircraft then normal.
Its possible that the Airport transmit the Aircraft for the Aircraft on the Ground?

 

[Richard Lee]

If you are in a wide open flat-land area, up high above ground level blockage (nearby buildings & hills),
you will see more AC than normal. Especially if your'e at a busy airport.

I see planes at the local air port transmitting ADS-B while they taxi to the terminal..
Some start transmitting when rolling away from the terminal.