Does anyone know the reason for doubled messages from an aircraft like this one? I took the screenshot after the shadow signal lagged a bit. You can also see the paths are not identical. Does the plane have two independent transponders for some reason? If so, why?
Hi,
In that example, those are two different aircraft. Notice the different “ICAO” mode-s codes.
AE0250 HOBBY90 is full ads-b broadcasting its position.
AE0246 is a mode-s aircraft which is NOT broadcasting its position. The position you are seeing here is a TIS-B rebroadcast, which is when you receive a FAA position derived from radar track or possibly FAA mlat. That is broadcast on 1090 by a FAA ground tower, which you then receive.
The TIS-B positions just like mlat positions are never perfect, which is why they bounce around a bit. They are NOT perfect positions broadcast from the aircraft.
We know the second aircraft is mode-s equipped, because the TIS-B rebroadcast includes an identifying mode-s code. If the aircraft was mode-a/c only, the “ICAO” id would be a pseudo code, something like 295xxx, depending on your area.
The second aircraft could be easily tracked with mlat, but FA is not a very reliable way to do that. FA only returns mlat positions to you that you have contributed to. In areas with many overlapping ground stations, only a small number are used to create the mlat position, so only those receive the position.
Here is an easy way to view both of those mode-s codes at once. This shows the second using mode-s mlat.
TIS-B rebroadcast data can be very interesting when received directly. Also look up ADS-R if you are interested.
Regards,
-Dan
Hi,
Since you’re in Austin, TX, you might be able to receive those TIS-B and ADS-R broadcasts directly. The site linked below is a bit dated, but does show a FAA site on one of the towers north of Austin in your area.
FAA ground sites (only supports http://, not https://):
Here is a Google Maps link to the one close to you.
http://maps.google.com/?q=30.58670,-97.68215
Google StreetView has trees blocking the view, but if you move a bit down south you can clearly see the towers. Many of the FAA sites are on normal communications towers like this one. Some at airports are mounted on their own shorter, dedicated towers. My local one about 3 nm away is on a county communications tower, located next to a normal cell tower.
Regards,
-Dan
From that link I’d say they are both the same aircraft. Probably broadcasting on both ADS-B and mode-s.
I used to see TIS-B data a lot. I don’t see it so much anymore. The easiest way for me to have recognized TIS-B data was when the aircraft were well into Louisiana, out of my range.
Hi,
Negative. It’s a two-ship formation of 2 x C-130J flying from KBIX, Keesler AFB, MS to the west coast.
One is ads-b. The other is mode-s. The TIS-B track is the mode-s one.
You will sometimes see duplicate tracks of a single aircraft with mode-s mlat and TIS-B position, but your sample is not one of those. There are other things that can also cause duplicate tracks of the same aircraft.
Regards,
-Dan
OK. Thanks for looking. Edit: I see now in the link you provided that on the right side I can click from one aircraft to the other. I missed that with my previous comment. I’ll be saving that link.
You are correct in that it is a two-ship but my records show it as a couple of UH-72s. I also double-checked with ADSBExchange and it shows UH-72 as well.
As for the difference in transponder signals, I’ve read that some formations only broadcast ADS-B from the lead aircraft, much like the leader handles ATC comms. I see flights of C-130s from the base next to me that will launch with one ADS-B signal and one or two non-ADS-B aircraft right behind. I have always assumed these are “leader only” for ADS-B.
Where are you seeing UH-72s? I have several screenshots where the speed was 297 knots. That seems way too fast for UH-72s.
I looked up the ICAO codes on ADS-B Exchange and I also have access to a registry of DoD Aircraft. Both of the data sources show those two ICAO codes being UH-72s. But I looked deeper at the screenshot and the altitudes and airspeeds. Those are definitely not a rotorcraft, and they are in ranges where I would expect from a C-130.
I did look both codes up on airframes.org and it doesn’t have any information on either aircraft. This is an interesting mystery.
Ok, mystery solved. When I typed the ICAO codes from the original screenshot I switched two numbers. My bad!
I did look at my dataset for the right codes and here’s what I have.
+----------+----------+-----------------------+----------------+--------+------+---------------------+
| icao_hex | n_number | aircraft_manufacturer | aircraft_model | branch | base | unit_notes |
+----------+----------+-----------------------+----------------+--------+------+---------------------+
| AE0246 | 94-8152 | LOCKHEED | C-130J | USAF | KBIX | 403TH WG / 815TH AS |
| AE0250 | 97-1353 | LOCKHEED | C-130J | AFRC | KBIX | 403AW / 815AS |
+----------+----------+-----------------------+----------------+--------+------+---------------------+
I also realized that I’m not far from you in Abilene. I looked at my system to see if I’d seen those two aircraft and sure enough, they were there. I captured a couple of screenshots that are attached.
It’s fun watching the practice going on near San Antonio, e.g., this pair.
I see T-6s and T-38s out of Wichita Falls and Del Rio quite a bit. Sometimes one will pop up out of Randolph or Columbus. I’m under the pattern for the B-1Bs and C-130s at Dyess. We’re also surrounded by MOAs and Air-to-Air refueling routes. I spend way too much time just seeing what I can see out there.