I don't know any specific details of the Aussie system, but they are likely using equipment which employs modern Digital Signal Processing (DSP) in their ground stations which can make a tremendous improvement in the reception of signals in weak or noisy conditions.
In addition to being a working avionics maintenance engineer, I'm an active amateur radio operator, and can speak from first-hand experience as to the advantages of DSP with marginal HF comm links. I own a Drake R8B receiver - which is a top-quality unit of early 1990's design, but one which uses strictly analog circuitry. My other radio is a new Yaesu FT-950, which employs extensive DSP both for the enhancement of demodulated audio, and (especially) the reduction of atmospheric noise, such as lightning sferics.
There have been many occasions where the Yaesu has achieved perfectly readable reception of a weak, noisy station that was completely inaudible on the Drake, both using the same antenna.
The Australians may well be using multiple receiving/transmitting sites as well, using steerable, narrow bandwidth antenna systems which further improve the reception and transmission of HF signals to aircraft in their areas of responsibility.
I'd imagine that the African HF LDOC ground stations in particular are probably using rather old, strictly analog radio equipment, with fixed antenna arrays.
I too am curious as to the specifc technology used for the automatic transmission of aircraft maintenance status messages to AF operations.
Normally, standard ACARS traffic for aircraft flying on transoceanic routes is routed via Inmarsat - which, as another poster noted in the main thread, requires the use of a steerable dish antenna to maintain its lock on the geostationary satellite, which in turn requires that the aircraft be in a relatively stable flight attitude, with a functioning IRU or GPS+AHARS input to the antenna to keep it pointed in the right direction.
Based on the proposed scenario of extreme turbulence, possible jet upset, and multiple system failures, it would seem that any Inmarsat-based SATCOM system would have dropped off line almost immediately - hard to picture it continuing to function for 4 minutes.
That leads me to believe that the AF data system may use the Iridium satellite constellation. No steerable antenna required - just a small "patch" antenna on the upper fuselage, which would continue to communicate with the the low-orbiting Iridium satellites in almost any aircraft attitude short of rolling inverted.
Iridium satellite-based systems for voice traffic are now as common as fleas on aircraft of all sizes, but I do know that both Avionica and L3 are marketing Iridium-based solutions specifically for real-time data link applications - both for standard ACARS and other uses. Perhaps Air France has equipped their fleet with something of this sort.
Addendum: Just noted a new post on the main thread where it was pointed out that newer Inmarsat installations use a non-steered phased array antenna which (like Iridium) does not require active antenna-tracking to maintain a lock on the satellite - which could kept the data stream going even in an upset situation.
JR Barrett
Last edited by JRBarrett; 3rd Jun 2009 at 01:58.