The DL on the 791, yo!

Antenna install.JPG

I love in-flight Internet

Accessing in-flight Internet requires an antenna

Ergo, I love antennas

I was never very good at logic. In fact I received a “D” in the subject in college. I had better things to do at 8am, like sleep off a hangover or watch my boyfriend sleep off a hangover or drink coffee at Eat n’ Park before going to sleep. (Oh for a tiny taste of those halcyon days).

My illogical mind makes me either completely unsuited for writing about in-flight entertainment and connectivity OR perfectly suited for covering this often madcap business, which is my roundabout way of bringing you to the topic at hand – the ARINC 791 standard. What the holy hell is that, Mary, err, Runway, and why should I care?

ARINC 791 defines a K-band satellite data airborne terminal unit, and it’s the spec that’s followed for current Ku-band-supported in-flight connectivity systems. Boeing, for example, has started line-fitting its 777s with Ku, so it is punching holes in its aircraft in the fashion described in the graphic above. 

However, when Inmarsat announced plans to offer a global Ka-band service, dubbed Global Xpress, the London-based satellite giant also promised that Ka antennas could be as small as iPads, which would require a decidedly different spec. Rockwell Collins, which won the exclusive deal to lead the development, production and distribution of user terminals for Global Xpress, has some busy days ahead (they beat out the likes of Thales and Panasonic).

But with an increasing array – ahem – of choices, what should airlines do? Should they fit their aircraft with Ku (and the provisions that come along with that) now, or should they wait for those wee iPad-sized antennas?

Here is what some of the leading industry dudes say about the matter:


Gary Hebb, vice president of strategy, EMS Aviation 

“The reality is that airframe manufacturers will probably want to reuse the radome shapes and sizes that already exist for Ku, and there’s little sense in putting a small antenna into a large radome, so yes, Ka antennas will tend to be the same size as Ku.



“However, Ka can support smaller antennas.

“For the same size antenna, Ka has higher gain than Ku.  This means that it has a narrower, more focused beam.  This narrower beam will interfere less with adjacent satellites. So, you can meet the discrimination spec with a smaller Ka-band antenna.

“It comes as a surprise to many that the higher gain of a given size Ka-band antenna is exactly offset by a higher path loss for Ka-band as it travels from the satellite to the antenna.  So, for a given amount of power from the satellite, you receive the same amount of power in a Ka band antenna as you would in a Ku-band antenna of the same size.  While this might seem to eliminate the advantage of Ka-band, two other factors increase the bandwidth that you can cost-effectively support in a Ka-band antenna:

“Ka-band satellites have spot beams, which focus the power more effectively on each user terminal;

“Ka-band has more spectrum available, and the spot beams multiply the spectrum by allowing “reuse” of frequencies.

“So, if you want to keep the same radome shape as for Ku-band, Ka-band can support higher data rates at a lower cost per bit.  If you are willing to change the radome shape, you can support the same data rates, still meet the discrimination requirement and probably still get lower cost per bit, with a smaller antenna.”

Leo Mondale, managing director of Inmarsat’s Global Xpress

“The variable here is a certified unit, and the easiest way to bring new technology to  market. So, will there be a 791 standard Ka band piece of equipment that fits under the radome designed originally for Ku? Yes.

“The Ku radome needs to be redesigned and re-certified in some cases.

“When doing a retrofit, you may not choose to do 791 and in that way, you’re not constrained by a pre-agreed envelope and in that case we have a trade.

“We believe for the same sized terminal, we’ll get significantly higher throughput for that terminal. We can do comparable throughput to a smaller terminal.

“I think you’ll see both solutions really. For retrofit I think a lot of people may prefer the smaller footprint but Airbus’ perspective isdriven in many ways by new aircraft, like the A350 and Boeing has thesame considerations, so I think you’ll see a mix, even a smallerterminal sitting under a larger radome.”

Meanwhile, all the peeps you’d expect – Panasonic, Row 44, Airbus, Boeing and Lufthansa – have been participating in a group to standardize Ku and Ka connectivity hardware installations. But, lo, so is Delta Air Lines, which clearly aims to toss either a Ku or Ka antenna on its long-haul aircraft (perhaps with partner Gogo, which is quietly testing a Ku antenna but also has an upgrade path to Ka).

It is merely a coincidence that the ‘DL’ in my headline – slang for ‘down low’, yee older fogies – is also Delta’s IATA symbol.

(Graphic above taken from a presentation made by DDDi’s Peter Lemme at the March 2010 APEX single focus workshop.)

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