Different "types" of HF Radios"

[ExSimGuy]

Over on R&N, in the Air France aircraft loss thread, someone was talking about the "ancient HF" in the S. Atlantic,and the "modern HF" over Australia (I can't quote, as I read all the way through in the hope of someone picking it up and I could never find the posts there again!)

Is there a different technology over Australia, that would help HF comms get through all the noise of an electrical storm? To my knowledge, there's basically only FM, AM, and SSB (unless aviation HF radios have gone digital??).

In addition to my interest in aviation (BA's Cranebank Centre for many years, as well as building sims with Link in Lancing), my main hobby is radio, and the comments there intrigued me. Can anyone tell me if the Aussie HF systems are significantly different to the African/S.American?

[spannersatcx]

unless aviation HF radios have gone digital

Indeed they have, the 340-600 the enhanced 340 and enhanced 330 have digital VHF and HF.

[seejonfly]

HF is still HF which because of the characteristics of that frequency range there are inherent limitations in bandwidth. Calculating the range of an HF frequency is still at best a swag (scientific wild ass guess) because there are so many factors that influence HF propogation. SSB and some of the other modulation techniques do offer advantages but usually these advantages are limited.

WRT the missing AF flight I have to wonder how the data package was transmitted/received. I would expect via SATCOM because HF would not be reliable nor would it have much bandwidth. The lack of bandwidth means you either send a message with fewer characters (not what I would expect for an aircraft status message) or you have a longer transmission time.

Best to all and sympathies to those whose loved ones were on the flight or associated with the flight in any way.

Jon

[JRBarrett]

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

[ExSimGuy]

Thanks JR,

It was digimodes that I had in mind for advanced HF voice comms. I haven't tried that route yet, though I see from RSGB's RadComm that it is becoming popular in amateur circles. and would appear to have serious advantages in "getting through", at least with marginal path loss situations - how this handles high-noise situations such as storms I'm not sure.

Maybe I can look again into hooking my FT857's audio to my computer when I get back from my current trip abroad (if I can keep the computer's broad-spectrum noise out of the radio!)

TKS & 73s

G3OCR

[AerocatS2A]

I use the Australian HF daily. I believe they recently went through an upgrade of the ground equipment but I couldn't tell you what it is. The comms quality in the aircraft doesn't seem to have changed much, but perhaps it is better for the ground operators. It is still a battle to use and sometimes you can spend 15 minutes trying to get through to someone. Partly this can be because they are busy with other aircraft that you can't hear.

They certainly do have multiple transmitter sites, though they had that prior to the recent upgrade. You can hear the effect of the multiple sites when you hear Brisbane talking to another aircraft and you can hardly hear the transmission, then they talk to you, switch to a more appropriate transmitter for your area, and they come through loud and clear. Or the opposite might happen, you hear them clearly talking to someone else then it goes all crappy when they talk to you, so you tell them "had you strength 4 when you were talking to QF123 but only strength 2 now", they'll change transmitters and come in strong again.

[Cpt_Pugwash]

I think the original comment re. "modern" HF may be referring to the use of Automatic Link Establishment (ALE) equipment. See here (http://hflink.com/automaticlinkestablishment/) and here (http://hflink.com/) for info., albeit in the amateur field.

[Treat]

I am an amatuer radio enthusiast (advanced qual), not a pilot, so my comments are restricted to the underlying HF technology, not current flight applications.

HF is HF, as regards propagation. However, there are two distinct areas of digital innovation with the potential to affect long-distance, weak-signal comms in different ways.

Digital mode, in contrast to analog mode, means signals are converted from analog (voice) into digitized levels and transmited. The mode has the advantage of high fidelity, with the disadvantage that the signal can completely "drop out" under weak signal conditions.

Digital signal processing is something else again, where the signal, which may be transmitted as analog, is detected and filtered using digital processing. DSP is very useful for weak signal work, as the operator can (depending on equipment) create extremely narrow custom filters on-the-fly to exclude interference. (The Flex-Radio Software Defined Radio is probably the best of breed available in the amatuer category).

[bjornhall]

The main problems with HF are partly the low bandwidth (follows from the low frequency, unless you are prepared to gobble up the entire HF range for one user), partly that the channel is dominated by fading. Traditionally, the 'solution' to the second problem is to shrug one's shoulders and say "HF is tricky", while reducing the bandwidth another couple orders of magnitude to get through the crappy channel. :)

Cellular phones also use channels dominated by fading. Enormous effort has been made to achieve truly breathtaking performance on such channels by using tremendously sophisticated modulation and signal processing methods. That effort has been driven and funded by the desire for higher capacity mobile networks, but the results could be applied elsewhere.

By comparison, even state of the art HF technology is primitive. DSP on an SSB signal is a remarkable improvement, but all you are really getting, as far as I know, is steeper IF filters, lower noise figures, sharper notches when needed, etc; it's still just an SSB signal.

Adapting cellular phone technology to the HF channel, such as sophisticated coding and modulation schemes, maybe Rake receivers, perhaps OFDM and so on, could take HF communication to a completely new level.

It would take some effort for sure, since all relevant parameters are orders of magnitude different; not just the frequency (a few MHz vs. a couple GHz), but fading times etc. Also, a lot of the effort in cellular phone technology is aimed at solving the multiple access problem, scheduling and so on, which it is not obvious how it would apply to the HF world. But it sounds like an interesting problem! :8

(Not having radios as a hobby, I only design them 'cuz someone pays me to...)