What determines the maximum speed of a satellite downlink?

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An artist's impression of the SES-12 satellite.
An artist’s impression of the SES-12 satellite.

The drive for higher and higher speeds on satellite downlinks is never ending. But what is the physics behind this? And can manufacturers squeeze even more out of the pipe? Steve Nichols takes a look.

The fundamental starting point with a satellite link is the frequency in use – the higher the frequency, the faster data can be passed over it. So, Ka-band (around 30 GHz) should be faster than Ku-band (around 12 GHz), which in turn is faster than L-band (about 1.6GHz).

But life really isn’t that simple.

George Nicola, Inmarsat’s Technical Marketing Manager, GX Aviation, says that the speeds achievable on a satellite link can all explained in the so-called Shannon Theorem, named after Claude Shannon, the American mathematician, electronic engineer, and cryptographer.

In his paper “Rocket Science Made Simple – Satellite Connectivity For Aviation Explained”, jointly authored with Inmarsat’s Michele Franci, Nicola said: “There are only three key parameters that can be manipulated in order to optimise the capacity of a communications link – bandwidth, signal power and channel noise.

“Communication channel providers develop their technologies in order to achieve the optimal link capacity based on their market needs. An increase in the transmit power level results in an increase of the communication link throughput, likewise a decrease in power will result in the opposite effect, reducing the throughput.

“Another way to improve the link throughput would be to increase the size of the receiving antenna in order to have a higher level of energy received at the aircraft. But this is where operational constraints become apparent, as, this would lead to an unfeasible installation for a commercial or business aircraft.”

Nicola added that you could get more power in a satellite beam by making it narrower. But that defeats the object if you are trying to serve a wide area, such as aircraft overflying the Atlantic.

A Boeing-built Inmarsat I-5 satellite for its Ka-band GX Aviation service.
A Boeing-built Inmarsat I-5 satellite for its Ka-band GX Aviation service.

Inmarsat is getting around this with its I-5 GX Ka-band satellites by combining both – the satellites will provide so-called Global Service Beams (GSB), with two degree beam widths, and the High Capacity Payload (HCP) which can independently direct which can independently six narrower 1.2-degree beams where more capacity is needed.

Nicola said: “This spot beam architecture is in contrast to all existing Ku-band satellites that utilise wide beams to provide their regional coverage.

“Our coverage charts show that the Inmarsat-5s will offer a greater global service. It is also clear from the plotted power levels that the average power available for use on the Intelsat Ku wide beams is considerably lower than for Inmarsat-5.”

Plus the Inmarsat-5s operate at a higher frequency too – the Ka-band at around 30 GHz, compared with Ku’s 12 GHz.

Spot beam
In Intelsat’s defence its upcoming EPIC next-generation satellite constellation, operating in Ku-band, with one satellite under construction (IS-29e) and one other planned, will offer spot beam coverage over some of the North Atlantic air routes and overlay a wide beam for the remainder of its coverage region.

So we have looked at the frequency, the power and the beam width as ways of jamming more bandwidth into your connection, but there is another equally important factor known as “contention”.

In satellite terms there is only so much bandwidth available in a beam and this has to be shared among all the users. Just as your hotel internet connection slows down to a crawl at peak times, your airborne satellite connection could do the same if it has too many users.

Another factor is satellite handover if an aircraft moves from one satellite to another. The handover must be quick and seamless if the passengers and crew are to have an unbroken service.

And we mustn’t forget another important link in the chain – the satellite earth station that is feeding the signal to the satellite in the first place. Bad weather (rain) over the station can cause a rapid degradation in the signal and therefore bandwidth, which is why most operators a) try to site them in areas where the weather is better and b) have back-up stations that can take over if and when bad weather does strike.

GX Aviation
Inmarsat’s GX service has a headline figure offering 50 Mbps.

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In the meantime Inmarsat’s L-band SwiftBroadband service offers around 432 kbps to an airliner. This is a global service, but limited to latitudes below around 82 degrees North and South.

It also offers its SB200 (around 200 kbps) service for bizjet users using a tiny low-gain antenna.

But while Inmarsat’s Ka-band offering is yet to leave the launch pad, Ka is already flying with the Viasat Exede service and LiveTV, with customers JetBlue and United Airlines.

Don Buchman, Viasat's Director of Mobile Broadband.
Don Buchman, Viasat’s Director of Mobile Broadband.

Viasat’s Don Buchman said ”All flight testing showed that the system is easily capable of delivering the claimed 12 Mbps bandwidth to each passenger – and often even more.

Viasat’s Bruce Rowe added: “Just like internet to the home it is multiplexed, so it’s not a straight calculation of 12 Mbps X N number of passengers = total data to the plane. But regardless of how many passengers are connected, if each does a speed test, as so many did on the JetBlue test flight, they will see 12 Mbps speeds or more.

“It’s just the same as our Exede Internet service for the home, where 12 Mbps is the advertised – and delivered – speed.”

Dynamic allocation
Viasat is planning the launch of Viasat-2, currently scheduled for 2016. This will be even more technically advanced than Viasat-1, increasing Ka coverage by seven times and doubling usable bandwidth, thanks to dynamic allocation.

Ku-band satellites operating in the 12-18 GHz range. A technical paper by Panasonic argues that the supposed performance advantages of Ka-band have nothing to do with the frequency and everything to do with the spot beam size. Its says narrow-beam Ku can match or even better Ka for throughput.

The paper is full of equations, but concludes that the throughput of a satellite link is largely frequency independent for equal sized spot beams and terminal antennas. That is, the key to higher performance is to use smaller spot beams – not necessarily higher Ka-band frequencies.

Panasonic has struck deals with a number of Ku-band satellites providers including Intelsat and SES.

Todd Hill, Director GCS Product Management and Capacity at Panasonic Avionics, said: “At Panasonic we are striving to get the lower cost per bit to the plane. The driving factor in both speed and cost is the network design.

“The current Ku network is far superior to the Ka network in both coverage and redundancy. Additionally all of the great features touted by the Ka operators can be done equally well in Ku, such as spot beams and frequency re-use where you get the great economics and super high throughputs.

“It is important to note that we have worldwide coverage now that covers 99.6% of all flights hours of the top 50 airlines. Now we are less than two years away from adding a spot beam overlay to our network that will add throughput needed for our customers.

“When our spot beam overlay is complete we will cover over 80% of flight hours with spot beams. The less travelled areas will continue to be served well by traditional Ku beams that today are offering speeds up to 60 Mbps to the plane.

“Our first spot beam satellite, IS-29e, over the US, North Atlantic and Europe will provide up to 80 Mbps to a plane and up to 200 Mbps to the planes in one area. So obviously there is no shortage of Ku spectrum or available slots to build great networks.”

For more information:

Inmarsat: www.inmarsat.com
Panasonic Avionics: www.panasonic.aero
Gogo: www.gogoair.com
Viasat: www.viasat.com
Intelsat: www.intelsat.com
SES: www.ses.com

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