French AF AWACS comms utilizing MS-110A

i56578-swl.blogspot.com 7 miesięcy temu

I late recorded any transmissions of the French Air Force (FAF), more precisely traffic between the Boeing E-3F Sentry 202 aircraft (ALE address 202E3F) & its home, i.e. 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 utilized for automatic link setup (2G-ALE) followed by MS-110A segments utilized 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 any interesting characteristics:

a) Figure 2 shows that 3 kind of "messages" are used: longer messages specified as 1 & 4 of Figure 1 - unlike the others - do not carry data and so they could have a sync or signaling function for the receiving side, say a "control" kind messages? This kind 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" kind 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 dimension and consist of first phasing series followed by 92 bytes of data (Figure 3). This fixed format could mean sending standard or pre-formatted messages, nevertheless it is hard to venture anything about the content of the messages another than the fact that it is mostly Air-To-Ground traffic (from aircraft 202 to MOB).

Fig. 3 - "data" kind messages

In particular, a Ground-To-Air transmission (from MOB to aircraft 202) turned out to be composed only of the short ACK kind messages that seemingly 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.