I recently recorded some transmissions of the French Air Force (FAF), more precisely traffic between the Boeing E-3F Sentry 202 aircraft (ALE address 202E3F) & its home, ie the E-3F Main Operating Base located at BA702 Avord airport (ALE address MOBE3F). These transmissions were recorded on the frequency of 6745.0 KHz/USB and almost all follow the same "format" visible in Figure 1: MS-141A used for automatic link setup (2G-ALE) followed by MS-110A segments used for sending data in (I think) ARQ mode.
 |
| Fig. 1 - an Air-To-Ground transmission from aircraft 202 & MOB ground base |
The analysis of the bitstreams points out some interesting characteristics:
a) Figure 2 shows that three type of "messages" are used: longer messages such as 1 & 4 of Figure 1 - unlike the others - do not carry data and therefore they could have a sync or signaling function for the receiving side, say a "control" type messages? This type of message is present in all the transmissions I recorded and precedes the real "data" message which in its turn is followed by what I think be an "ACK" type message (for clarity, the bistream in the left part of Figure 2 is time-edited).
b) all message types (control, data, ACK) end with the same string 0x69D3D226 (I added the trailing "00" 100101101100101101001011011001[00] bits to get an hex string).
 |
| Fig. 2 |
c) all data messages have the same length and consist of initial phasing sequence followed by 92 bytes of data (Figure 3). This fixed format could mean sending standard or pre-formatted messages, however it is difficult to venture anything about the content of the messages other than the fact that it is largely Air-To-Ground traffic (from aircraft 202 to MOB).
 |
| Fig. 3 - "data" type messages |
In particular, a Ground-To-Air transmission (from MOB to aircraft 202) turned out to be composed only of the short ACK type messages that apparently make no sense (Figs 4,5), a possible explanation is the failure to receive the messages sent by the aircraft... but it's just a my guess.
 |
| Fig. 4 - a Ground-To-Air transmission |
 |
| Fig. 5 - bitstreams related to MS-110A transmissions of Fig. 4 |
https://disk.yandex.com/d/4Ba30b6k4-9cig
 |
| Armée de l'Air (French Air Force) Boeing E-3F Sentry 202 |
No comments:
Post a Comment