Callsign is delayed

[MelK]

Hello,

I have been recording flight paths using RadarBox for a transport studies research project. I have software that accesses the RB data via port 7879 and saves them to a tab delimited text file. One of the problems I currently have is that for some flights the transmission of the callsign seems to be delayed. I receive HEX code, altitude, lat, lon and other data, but no callsign. After some seconds I also record a callsign, resulting in a table that looks like that when sorted by Mode S:

Date                 Time                 Mode S    Callsign   Altitude    Latitude    Longitude
27/04/2012   13:32:19 PM   400FE2   0           37025   54.022   -3.0964
27/04/2012   13:32:24 PM   400FE2   0           37025   54.022   -3.0964
27/04/2012   13:32:28 PM   400FE2   0           37000   54.022   -3.0964
27/04/2012   13:32:33 PM   400FE2   0           37000   54.022   -3.0964
27/04/2012   13:32:37 PM   400FE2   0           37000   54.022   -3.0964
27/04/2012   13:32:42 PM   400FE2   0           37000   54.022   -3.0964
27/04/2012   13:32:46 PM   400FE2   0           37000   54.022   -3.0964
27/04/2012   13:32:51 PM   400FE2   0           37000   54.022   -3.0964
27/04/2012   13:32:56 PM   400FE2   0           37000   54.022   -3.0964
27/04/2012   13:33:00 PM   400FE2   EZY23AU   37000   54.022   -3.0964
27/04/2012   13:33:05 PM   400FE2   EZY23AU   37000   54.022   -3.0964
27/04/2012   13:33:09 PM   400FE2   EZY23AU   36975   54.022   -3.0964
27/04/2012   13:33:14 PM   400FE2   EZY23AU   36975   54.022   -3.0964
27/04/2012   13:33:18 PM   400FE2   EZY23AU   36975   54.022   -3.0964
27/04/2012   13:33:23 PM   400FE2   EZY23AU   36975   54.022   -3.0964
27/04/2012   13:33:27 PM   400FE2   EZY23AU   36975   54.022   -3.0964
27/04/2012   13:33:32 PM   400FE2   EZY23AU   36975   54.022   -3.0964
27/04/2012   13:33:36 PM   400FE2   EZY23AU   36975   54.022   -3.0964
27/04/2012   13:33:41 PM   400FE2   EZY23AU   36975   54.022   -3.0964

After reading in this forum I am aware that callsigns sometimes get mixed up (happened in my data as well), but what are the reasons for the delay in receiving the callsign? It can not be my software. I have checked the data in the original xml file and the callsigns are missing there as well. It does not happen to all flights, but to a significant number across all airlines. Anyone else had this problem before? Your help is very much appreciated.

P.S. I am using RadarBox 3D, version 4.03, running on Windows7 64 bit, if that is any help.

[chewycanes]

Hi

I don't know the answer to you question but i see similar things.
I have alerts set up for 'Modes' codes and 'Callsigns' and quite often i get the Modes alert with no callsign and then a while after i get the Callsign alert with all the previous data.

Brian

[RodBearden]

Hi MelK

I'm no ATC techie, but I believe that the answer lies in the difference between Mode-S signals and ADS-B signals.

ADS-B is squitted (transmitted) twice a second automatically, and each message includes the hex code of the aircraft, but not the callsign (Flight ID).

Mode-S signals are only transmitted by the aircraft on the request of an ATC system's Secondary Surveillance Radar (SSR),  which may only be occasionally, perhaps once every 30 seconds. These do contain the callsign.

Hence the potential for lag in picking up the callsign.

Hope that helps.

Rod

[ACW367]

This eurocontol PDF hosted on a Turkish Govt website may be of use
www.ssd.dhmi.gov.tr/getBinaryFile.aspx?Type=3&dosyaID=123

[RodBearden]

Sorry, ACW , it wasn't!

By brain fuse blew at slide 5, and I think slide 15 may have been in Turkish. I'm obviously not close to Dummy level yet!

Thanks anyway.

Rod

[ACW367]

The main thing to note in relation to the query of the original poster, with slide 15 is the transmission rate.  It says the airborne velocity squitter is broadcast twice a second (TX rate=2/Sec).  But the aircraft identification squitter containing the callsign is only broadcast every five seconds (TX rate=0.2/Sec).

[RodBearden]

Thanks for the translation, ACW!

Rod

[MelK]

Thank you all for your quick and helpful replies.I still try to understand all the technical background of these things. Your explanations sound reasonable and I will have a more detailed look into how the data is transmitted.
Thanks

Melanie