MG23:
If I remember correctly, the aircraft isn't supposed to send anything unless it can sync to transmissions from the satellite at the correct frequency: in fact, I don't think it can since it won't know the correct timebase for the channel.
So I agree: the fact that it was transmitting implies that the aircraft had some kind of location and orientation information (I presume the beam steering unit gets that from the same system the crew would) and wasn't so unstable that the antenna couldn't lock to the satellite within the transmission tolerance.
Thanks MG23 for the Sat correction. It must "see" a lock, then before transmission of data. (Man these techs are great.) (thanks also Grdbrk and others)
And apparently the vender is SITA not ACARs from ARINC like we are used to in the states? But it does essencially the same thing. I could only find this 1973 description of the SITA transmission times:
7.3 Response Time
The average response time for Type A messages, i.e. the time lapse between the Instant an operator presses the transmit key of his terminal to send his query and the instant the first character of the reply appears on the screen (see Figure 6) ranges from 1.4 seconds to 3 seconds, depending on the number of links involved in route. A typical response time distribution is shown in figure 9.
7.4 Satellite Processors
The downtime of SPs is around 16 hours per month including scheduled and unscheduled stops (e.g. preventive maintenance, configuration changes, etc.)
Presently, certain SPs switch up to 6 message blocks per second during peak conditions of traffic.
Switching times are in the order of 5 ms per block for both systems.
So the SITA/ACARS transmission is instant, once it gets a lock.
SITA NETWORK by Chretien, Konig & Rech